Atmospheric and Potable Water Exposures to Methyltert-Butyl Ether (MTBE)

REGULATORY TOXICOLOGY AND PHARMACOLOGY ARTICLE NO.

25, 256–276 (1997)

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Atmospheric and Potable Water Exposures to Methyl tert-Butyl Ether (MTBE) Stephen L. Brown R2C2, 4700 Grass Valley Road, Oakland, California 94605 Received December 16, 1996

This paper presents information on the ways in which people can be exposed to methyl tert-butyl ether (MTBE) via air and water and on the distribution of doses that can result from those exposures. Data on concentrations of MTBE in air were compiled for 15 different occupational, commuting, or residential exposure categories, and concentrations in potable water were compiled from five states in the MTBE-using areas of the United States. Based on these concentrations and characteristics of the exposed populations, average daily and lifetime average doses were estimated. Both the concentration data and several of the population characteristics were estimated as distributions rather than as point values so that the numbers of people exposed at various levels could be estimated. Arithmetic mean occupational doses via air were in the range of 0.1 to 1.0 mg/kg-day, while doses from residential exposures, commuting, and refueling were in the range of 0.0004 to 0.006 mg/kg-day. Lifetime doses for workers were in the range 0.01 to 0.1 mg/kg-day. The cumulative dose distribution for the entire population of the MTBE-using regions of the United States was estimated by combining the distributions of doses and the numbers of people in each exposure category. In the MTBE-using areas, arithmetic mean doses via air were estimated to be 0.0053 and 0.00185 mg/kg-day for the chronic and lifetime cases, respectively. Approximately 98.5% of the population living in MTBEusing regions uses water with concentrations affected only by atmospheric deposition, if at all, and too low to be detected with current methods (õ2 mg/liter). The remaining population uses water with an estimated geometric mean concentration of 0.36 mg/liter, an arithmetic mean concentration of 49 mg/l, and a 95th percentile concentration of 64 mg/liter. Doses via ingestion, inhalation, and dermal absorption were included. The estimated arithmetic mean dose for the population exposed via water was 1.4 1 1003 mg/kg-day. q 1997

fuels. Concerns have been raised about its possible acute toxicity (Balter, 1996; Borak et al., 1996), chronic noncancer toxicity (Clary, 1996), and carcinogenicity (Rudenko, Wilmarth, and Starr, 1996; Ginevan and Cox, 1996). This paper discusses the exposures to MTBE that may occur via air and potable water that have been affected by releases of MTBE. Direct inhalation is the dominant route of exposure to MTBE released into air. For MTBE released into water, however, direct ingestion of drinking water, inhalation following exchange into air during water use, and dermal absorption during bathing or showering may all be important. Exposures to MTBE can be characterized by understanding the nature of its sources, its transport and fate and the resulting concentrations in the media of exposure, the human activities that cause the exposures, the numbers of people involved, and the characteristics of the people who are exposed. A key feature of this paper is the estimation of distributions of exposure rather than point estimates. Different people are exposed to different concentrations for different times in a variety of situations. They have different physiologic characteristics and different patterns of activity. Therefore, even people from the same exposure category may have markedly different exposures over various time periods of interest. Several techniques have been proposed for analyzing such variations, including Monte Carlo simulation of the distribution of exposures based on input distributions of important parameters of exposure. The simplified procedure used in this paper assumes that all of the important distributions are log-normal, which is appropriate for natural systems in which negative values are impossible (Gilbert, 1987). It permits analytical calculation (through variance propagation) of the distributions of any exposure that is estimated as a product or quotient of parameters.

Academic Press

2. METHODS 1. INTRODUCTION

Methyl tertiary-butyl ether (MTBE) is used as an oxygenate in reformulated gasoline (RFG) and oxy-

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0273-2300/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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Human exposures via environmental media can be estimated by (EPA, 1989)

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D Å (C 1 CR 1 ET 1 EF 1 ED)/(AT 1 BW), (1) where D is the dose in mg/kg-day, C is the concentration in the medium of exposure (mg/m3 for air, mg/liter for water), CR is the contact rate with the medium (m3/hr for air, liters/day for drinking water), ET is the exposure time (hours per event for air, unity for drinking water), EF is the exposure frequency (events per year for air, days per year for drinking water), ED is the exposure duration (years of exposure), AT is the averaging time (days) appropriate for the toxicological endpoint of concern, and BW is the body weight in kilograms. Equation (1) historically has been evaluated as a ‘‘reasonable maximum exposure (RME)’’ for an individual who is among the most exposed persons but whose exposure is within the range of those reasonably anticipated. It is evaluated by using a combination of central or average point values for its parameters along with a few that are considered high-end (e.g., 90th or 95th percentile). The RME dose has been used to set cleanup criteria or other regulatory standards for environmental protection. Recently, some of the parameters have been represented by distributions instead of by point estimates and then propagated by Monte Carlo techniques to produce a distribution of doses; the 95th percentile or some other point on the distribution is used for developing standards. The distribution is usually accomplished for a defined ‘‘population at risk,’’ for example, the people living in a community adjacent to a hazardous waste site or an industrial source of emissions to air. For this paper, the distributional technique was extended to understand the distribution of MTBE exposures over the entire United States population for the purposes of characterizing risks, not for setting standards. Its results can be used to estimate the number of people exposed to a given toxicologically significant dose or higher or to estimate the incidence of cancer attributable to MTBE (if MTBE should prove to be carcinogenic in humans). As with the Monte Carlo technique for development of standards, some of the parameters were estimated as distributions, while others were defined by point estimates (often with a conservative bias tending to overestimate rather than underestimate dose). This study used the entire distribution, however, rather than focusing on one point of the distribution. Furthermore, the distributional propagation was accomplished in closed form by forcing all the distributions to be log-normal. To develop the national distributions of exposure via air and water, distributions were developed for subpopulations and then combined. The subpopulations studied are shown in Table 1. A two-parameter log-normal distribution of exposures for each of those populations was developed from data on concentrations in the media (air or water) and data or assumptions regarding

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the exposure characteristics and size of the populations. The methods were different for air and water because of differences in the sources of data and patterns of exposure. 2.1. Atmospheric Exposures

The data needed for the analysis of MTBE exposures via the atmosphere included the sources of MTBE emissions, the concentrations in air, the exposure characteristics for each exposed population, and the physiologic characteristics of exposed persons. 2.1.1. Sources of Atmospheric MTBE Because MTBE is used to varying degrees in different areas of the United States, only some people are regularly exposed to it. According to the National Reformulated Gasoline Hotline (NRGH, 1995), about 73 million people or 30% of the population lived in areas where MTBE was used in reformulated gasoline or oxyfuels in 1995. The remainder of the population experiences relatively low exposures and does not significantly influence the aggregate exposure picture. The major sources of atmospheric exposures to MTBE have been identified (Brown, 1995) as releases from manufacturing, blending, distribution, and storage; releases from refueling activities; emissions from vehicles (evaporative and tailpipe); and other sources such as small gasoline engines in lawnmowers. The emissions from the stationary sources were estimated from data in the Toxics Release Inventory (TRI) for 1993 (EPA, 1995). The emissions from gasoline stations were estimated from the release of gasoline per gallon dispensed (Mueller, 1989), the annual consumption of gasoline in cities where oxygenated fuels are in common use (NRGH 1995), and the average content of MTBE in fuel (10% by weight). The emissions from vehicles were estimated from emissions per vehicle or vehicle-mile (Stump, 1994) and census information on the number of vehicles and miles driven (Bureau of the Census, 1995). Data regarding emissions from the other sources were not found. 2.1.2. Concentrations in Air Concentrations of MTBE measured in workplace air, vehicle cabins, or ambient air were obtained from published and unpublished sources located through May 1995. When available, the original data sets were analyzed; otherwise, the authors’ statistics were used. Data were accepted at face value and used as presented unless there was sufficient reason to reject the entire data set (e.g., when neither original data nor the results of a log-normal analysis were available). Seventy-two sets of qualifying data were compiled from 14 sources as shown in Table 2. Data were entered into an Excel spreadsheet (available from the authors as hard copy

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TABLE 1 Populations Exposed to MTBE Category Workers with occupational exposures

Drivers exposed to MTBE in gasoline

People living near facilities involving MTBE The general public Potable water exposures

Description

Medium

Manufacturing Blending gasoline Transporting MTBE Distributing gasoline Dispensing gasoline Vehicle repair and related Professional drivers Commuters Other drivers Gasoline station customers Manufacturing and blending facilities Gasoline stations and storage facilities Ambient exposures Water affected by leaks and spills involving MTBE Water affected only by atmospheric deposition of MTBE People living in areas where MTBE is not regularly used

Air Air Air Air Air Air Air Air Air Air Air Air Air Water Water Water

or on diskette) and analyzed as log-normally distributed data for the geometric mean (GM) and geometric standard deviation (GSD). Where data were reported as less than the detection limit, the stated detection limit was used, unless all values in the data set were nondetects, in which case the detection limit was used only as a point of reference. For several of the environments in which MTBE occurs, more than one data set was available for estimating concentration distributions. In some cases, the different data sets reported measured concentrations that differed by as much as two orders of magnitude for exposure conditions that were described in similar words. Whether these differences are due to errors in measurement or interpretation or reflect truly different conditions is not known. After examination of the GMs for each data set and the number of measurements and conditions represented, a single value falling within the range was selected to represent the overall GM. If the individual GMs were within a factor of three, the GSD was taken from the upper half of the individual GSDs cited. If the GMs showed greater variability, the GSD was increased to account for either true variability or uncertainty about the true values. Details of the selection process can be found in the report by Brown (1995). The subjectivity of this procedure was unavoidable given the mix of sampling schemes and results represented. Using emissions estimates with appropriate atmospheric dispersion models, it is also possible to predict MTBE concentrations in various environments. Brown (1995) used simple box models to judge the credibility of the measured concentration information and found reasonable agreement for all but the air near service stations, where the measurements were taken at the periphery of the station but the box extended to a distance of 0.1 mile. Because the population exposed was

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defined as including everyone living within the box, Brown decreased the GM concentration but increased the GSD. 2.1.3. Exposure Characteristics To evaluate Eq. (1), values for time of exposure per day, days of exposure per year, and years of exposure per lifetime were needed for each exposed population. Values were based on accepted risk assessment assumptions and additional situation-specific data sources. Only the latter are discussed here. Data from the Bureau of the Census (1995) on commuting times and from Wixtrom and Brown (1992) on other drivers suggest that 1 hr per day is a good estimate for time spent in vehicles per day for the driving population. While customers refueling their vehicles may spend 5 min or more at the gasoline service station, the average time spent near the pump island is lower. Based on the estimated total refueling time over a year (Wixtrom and Brown, 1992), an average of 3 min per refueling event and 70 events per year was used. The duration variable was represented as a distribution rather than as a point estimate. Log normal parameters were estimated from data on mean and 95th percentile estimates for duration of employment in one job and duration of residence in one location (AIHC, 1994). Gasoline station attendants are much less likely to stay on the job for a long time than other workers, and the GM duration for them was reduced to 2.5 years from 4 years, but the same GSD was used (3.0, implying a 95th percentile duration of about 15 years). Based on the estimate that an average driving lifetime is 50–55 years (Wixtrom and Brown, 1992), the GM duration for drivers was taken to be 50 years and the GSD was chosen to allow a 95th percentile driving

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TABLE 2 Sources for Data on Atmospheric Concentrations of MTBE Source Anderson et al., 1995

Hinton, 1993 API, 1989 Buchta, 1993

White et al., 1993 Mannino et al., 1993 Moolenaar et al., 1993 AREAL, 1993

Schweiss, 1993 Lioy et al., 1994

Clayton, 1991 Hartle, 1993

Johnson, 1993

Hinton, 1993

Hinton, 1993

Aponte-Pons, 1995

Data set number(s)

Relevant population

Number of data points

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

General public Gas customers Mechanics, etc. General public General public Gas customers SS attendants SS attendants Refinery neighbors Mechanics, etc. Mechanics, etc. SS attendants Mechanics, etc. Mechanics, etc. Mechanics, etc. Mechanics, etc. SS attendants Mechanics, etc. Commuters Mechanics, etc. Mechanics, etc. Mechanics, etc. Mechanics, etc. SS attendants mechanics, etc. General public General public Gas customers Commuters Commuters Commuters Gas customers Commuters Gas customers SS attendants SS attendants SS attendants Gas customers SS attendants SS neighbors Gas customers SS attendants SS neighbors Gas customers SS attendants SS neighbors Manufacturing Manufacturing Manufacturing Manufacturing Blending Blending Blending Blending Blending Blending Blending Blending Transporting Transporting Transporting Distributing Distributing Distributing Distributing General population General population General population General population General population General population General population General population

8 3 3 2 2 4 * * * 7 3 3 7 5 5 4 4 7 * 10 8 * * 14 * * * * 6 3 2 * * 18 22 26 * * * * * * * * * * * * * * * * * * * * * * * * * * * * 6 7 11 12 12 14 13 13

* Unknown; authors’ statistical analysis used. 259

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lifetime of 67 years. The same distribution was used for gasoline customers. For commuters, the maximum duration is about 45 years; the GM of 20 years and GSD of 1.5 yields a 95th percentile of 39 years. 2.1.4. Physiologic Characteristics For exposures to MTBE by inhalation, the only physiologic characteristics pertinent to the evaluation of Eq. (1) are breathing rate and body weight. The breathing rate was represented by a point estimate for each of the exposed populations. All of the occupational exposure estimates use the breathing rates associated with moderate activity for males, averaging 2.5 m3/hr (AIHC, 1994). Commuting and refueling are considered to entail a light to moderate activity level for both genders, averaging 1.35 m3/hr. All other exposures are considered to be at a daily average rate of 0.75 m3/hr, appropriate for both adults and children. Although breathing rate theoretically should correlate with body weight, a reasonable correlation coefficient was not derivable from the AIHC data and the point estimate was used. Body weight varies significantly by gender and age, and the exposed populations have different proportions of males and females, and of children and adults. The log-normal distribution of body weight for adult males (AIHC, 1994) was used for all the worker populations, which are 90% or more male. A log-normal distribution of body weight for all adults was derived from the normal distribution presented by AIHC (1994) and used for all the drivers and gasoline customer populations. The residential and general population exposure groups contain both adults and children of both sexes, and the appropriate distribution of body weight depends on the averaging time for the health effect of concern. Because dose depends on the inverse of body weight, that inverse of body weight must be averaged over the period of exposure. For residential exposures near MTBE facilities, the averaging period is 30 years, assumed to span birth to age 30. For that period, the inverse-averaged body weight was calculated to be 30 kg. For general population exposures, the averaging period is 70 years and the inverse-averaged body weight was calculated to be 45 kg. 2.1.5. Propagation of Distributions For each exposed population, Eq. (1) was evaluated by substituting distributions for MTBE concentration, exposure duration, and body weight along with point estimates for contact (breathing) rate, exposure time per event, and event frequency. With all the distributions log-normal, the GM of the dose distribution was calculated by substituting the GM from each of the distributions. The GSD for the dose distribution was calculated by adding the squares of the logarithms of the GSDs for the component variables, taking the

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square root, and exponentiating. In most cases, the GSD for concentration is greater than the GSD for exposure duration and much greater than that for body weight; the variability in concentration therefore tends to dominate the overall estimate of GSD. The entire analysis was conducted both for an averaging time relevant to the evaluation of cancer risk (a 70-year lifetime) and an averaging time relevant to the evaluation of chronic noncancer health effects (1 year). Because acute health effects are often evaluated using the concentration averaged over a specified time period rather than the total dose over that period, concentration distributions were produced for time periods near 5 min, 30 min, and 8 hr, as appropriate. The first could be used to evaluate acute effects from refueling, for example, the second for short-term occupational tasks, and the last for daily occupational exposures. 2.1.6. Combining Distributions To derive an overall distribution for the entire U.S. population, the individual distributions were combined taking into account the fact that some people belong to more than one exposed subpopulation. For example, a person could be employed in an MTBE-related occupation, live near an MTBE facility, commute, and refuel. Of the 73 million people estimated to live in a heavily MTBE-using area, an estimated 83% 6.4% 9.6% 1.1%

do not live near an MTBE facility live near a gasoline service station live near a gasoline storage facility live near a manufacturing or blending facility 72% are drivers and potential gasoline station customers 41% are commuters 1.2% are professional drivers 0.4% work in auto shop or in other MTBE-exposed jobs 0.2% work in gasoline stations 0.01% distribute gasoline 0.002% transport MTBE 0.0025% work in refineries where MTBE is blended into gasoline 0.0012% work at facilities that manufacture MTBE. In any 1 year, the first four categories are mutually exclusive, as are the last seven. The commuters are a subset of the gasoline station customers. Although workers may be somewhat more likely to be commuters or at least drivers, that relationship was not estimated. No simple formula exists for combining the distributions of overlapping log-normal distributions, some of which are additive and some are not. As shown under Results, there is relatively little overlap between the occupational and nonoccupational exposures; within these two groups, membership is often mutually exclu-

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sive. This fact means that exposures at home or during driving are unimportant for the total exposures of the occupational groups, and the numbers of people with exposures greater than any specified dose can be simply added to obtain the total number of people with such exposures. Other simplifications based on the observed results are discussed later. 2.2. Potable Water Exposures

The classes of data needed for the analysis of MTBE exposures via potable water were essentially the same as for atmospheric exposures, but the data sources and analysis techniques differed. 2.2.1. Sources of MTBE in Potable Water Some MTBE is likely discharged with wastewater from the MTBE manufacturing and blending facilities described in Section 2.1.1. Leaks or spills of MTBE or of MTBE-containing gasoline can also occur at any point in the chain of use. Finally, the MTBE escaping to the atmosphere as described above can be deposited on surface water or on soil for percolation into groundwater. MTBE discharges to surface water during manufacturing of the pure product as well as during blending into gasoline have been estimated for industrial sources and compiled in the TRI (EPA, 1995). No specific estimates were found of the amount of MTBE released as spills from transfer operations or leaks from storage tanks and pipelines. Because MTBE evaporates rapidly if exposed to the atmosphere, spills are less likely to be a significant source of MTBE in potable water than leaks from tanks, especially slow, hard-todetect leaks from smaller gasoline storage tanks that have not been upgraded recently. These point source leaks can produce localized groundwater plumes with dissolved-phase MTBE concentrations in the range 100–100,000 mg/liter (Davidson, 1995). The groundwater can then be a source of MTBE in potable water. Although discharges to surface water can also occur, their influence on concentrations in potable water is likely small because MTBE is rapidly diluted and volatilized into the atmosphere (Rykowski, 1996). Another potential source of MTBE in potable water is dry or wet deposition of atmospheric MTBE. As most MTBE will be in vapor phase, little potential exists for it to deposit significantly through dry processes. Although local concentrations can be higher, general atmospheric concentrations of MTBE are unlikely to exceed 2 ppbv (Rykowski, 1996). Based on equilibrium partitioning of MTBE between water and air, concentrations of MTBE in cold wet precipitation should be less than 2 mg/liter, with lower concentrations at higher temperatures (Ryskowski, 1996). A portion of the precipitation will infiltrate into the soil and MTBE may

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thus enter the groundwater at concentrations of up to 2 mg/liter. 2.2.2. Concentrations in Water Concentrations of MTBE measured in potable water were obtained by Davidson (1995). Davidson found very little information in the published literature, but five state environmental agencies in the MTBE-using areas had sampled for MTBE in wells and, occasionally, in surface water. Davidson compiled all available data by type of water supply (surface or well, public or private), classified them into ranges of concentrations, and entered them into an Excel spreadsheet (available from the author as hard copy or on diskette). When multiple results existed for a single well, the highest MTBE concentration was used (Davidson, 1995). Detection limits varied from 0.2 to 5 mg/liter; the median detection limit was 0.5 mg/liter. The classifications supported by the available data were: not detected (less than detection limit), detection limit to 10 mg/liter, 10 to 20 mg/ liter, 20 to 200 mg/liter, and 200 mg/liter or more. The numbers of water systems in each range were counted and expressed as a fraction of all the systems in a category. From these distributions, the fractions of all wells in public or private systems, and the fractions of all wells in MTBE-using areas, an overall distribution of water supplies by concentration was developed. (The few available data from surface water sources confirmed the supposition that MTBE in potable water from these sources is not a significant contributor to national exposure.) The distribution was then fit by a log-normal function, which was used in all further calculations. 2.2.3. Exposure Characteristics Because the majority of the 73 million people living in the MTBE-using areas rely on regional public water supplies for their potable water, their exposures to MTBE in groundwater are not tied to the proximity of an MTBE facility. People with private wells are more likely to experience high or low extremes of exposure. To the extent that the data compiled for this paper are representative, they incorporate variations in water supply, both public and private. Therefore, the exposed populations were defined only in terms of the water supply they used, not by other characteristics. People are potentially exposed to MTBE in potable water through three principal pathways: direct ingestion of MTBE with drinking water, inhalation of MTBE that volatilizes from potable water during showering and other open uses of water, and dermal absorption from water during bathing or other water-contact activities such as dishwashing. Doses via ingestion, inhalation, and dermal absorption all depend on the MTBE concentration in water. Because that concentration is not expected to vary substantially over time periods of

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months, the principal concern is with chronic exposure leading to chronic toxicity or cancer. The daily dose was averaged over a year for use in assessments of chronic toxicity or a lifetime for use in assessments of potential carcinogenicity. 2.2.3.1. Ingestion. For drinking water, the amount of water ingested daily is the contact rate. (Drinking water includes beverages made from affected water; although some MTBE could be ingested with foods washed or cooked in water, that contribution is expected to be minimal.) The daily rate of water ingestion can vary from individual to individual and among different activities for the same person. According to the American Industrial Health Council (AIHC, 1994), tap water intake by people of all ages can be represented by a log-normal distribution with a GM of 0.963 liters/ day and a GSD of 1.7. The arithmetic mean of this distribution is 1.1 liters/day. Tap water excludes water in purchased beverages (milk, soft drinks). The distribution was used rather than the more familiar point estimate of 2 liters/day in the expectation that such secondary sources of water would be processed in a manner that allowed most of the MTBE to evaporate. Although the rate of water ingestion is expected to correlate to some extent with body weight, the details of the correlation are not known, and, for the purposes of this assessment, the two distributions were assumed to be uncorrelated. 2.2.3.2. Inhalation. The distribution of average daily doses of MTBE inhaled after volatilization into indoor air from use of tap water in the home was estimated by the screening methods of Schaum et al. (1992), who proposed different models for estimating concentrations during showering and for general household exposure. Using data on shower duration from James and Knuiman (1987), the general household model gave higher dose values. With that model, the concentration in air from all household sources of water is given by Ca Å Cw 1 f 1 WHF/(HV 1 ER 1 MC),

(2)

where Ca is the average concentration in residential air in mg/m3, Cw is the water concentration in mg/liter, f is the fraction of MTBE volatilized (dimensionless), WHF is the water flow rate for the whole house in liters/day, HV is the house volume in m3, ER is the air exchange rate for the house in air changes per day, and MC is the mixing coefficient (dimensionless). The fraction f was estimated to be 17%, based on a comparison of MTBE’s Henry’s Law constant (0.0177 in the dimensionless form) with those of other VOCs. The mixing coefficient adjusts for the fact that people may preferentially be active in areas of the house with higher than average concentrations. The values for

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WHF (890 liters/day), HV (400 m3), ER (30 changes per day), and MC (0.5) were selected from the ranges suggested by Schaum et al. (1992). The calculated value for Ca was used in Eq. (1) along with the same data and assumptions about breathing rate and time of exposure as were used as for people living near MTBE facilities (see Section 2.1.3). 2.2.3.3. Dermal absorption. The distribution of exposures from dermal absorption during bathing was estimated from Eq. (3) (Brown and Rossi, 1989), a variant of Eq. (1): D Å (Cw/1000) 1 (0.9 1 SA) 1 (ET 1 EF/365) 1 PC/(1000 1 BW), (3) where Cw is the water concentration in mg/liter, 1000 is the number of cm3 per liter, SA is the total skin area in cm2, 0.9 is the fraction of skin area exposed while bathing, ET is the time per bath in hr, EF is the frequency of baths per year, 365 is the number of days per year, 1000 is the number of mg per mg, and BW is the body weight in kg. The greatest skin exposures occur when people bathe in freshly drawn bath water before a significant fraction of the MTBE has volatilized. Approximately 90% of the total body skin area could be exposed for the duration of the bath. While dermal absorption would occur during showering as well, the total absorption was estimated to be substantially lower because only a thin layer of water contacts the skin. Data on the distribution of bathing durations and frequencies were not found in the sources surveyed, but a cumulative distribution of discrete values for shower duration is given by AIHC (1994). It is consistent with a log-normal distribution with GM of 6.8 min and GSD of about 1.5. Baths were estimated to last twice as long (GM Å 13.6 min) with the same GSD. No data on frequency of baths vs showers was found; for the purposes of this paper, daily bathing or showering was assumed, with showers four times as frequent as baths (i.e., 292 showers and 73 baths per year). 2.2.4. Physiologic Characteristics Although people move frequently, rarely staying in the same residence for more than 30 years, they may not move out of the local water supply area and are likely to remain somewhere in the MTBE-using areas. Therefore, an exposure duration of 70 years and the inverse-averaging of body weight over 70 years (see Section 2.1.4) were used for exposures via water. The total skin area can vary from individual to individual and increases from birth to adulthood for the same individual. Although AIHC (1994) provides an estimating equation that takes into account both height and weight in estimating body surface area, suf-

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ficient precision can be obtained by using the approximation that skin area increases as the 2/3 power of body weight. All other parameters equal, therefore, the dose of MTBE absorbed into the body was taken to be proportional to skin area divided by body weight or body weight to the negative 1/3 power. The constant of proportionality was derived from the mean skin area and the mean body weight. For adults, the mean surface area used was 18,400 cm2 (Brorby and Finley, 1993). At a mean body weight of 72 kg, skin area is given by 18,400 1 (BW/72)2/3 Å 1064 1 (BW)2/3. This formulation makes it unnecessary to treat separately the variability in skin area, as it is incorporated in the variability of body weight. The actual amount of MTBE absorbed into the body depends on the amounts of MTBE in the source media and the fractions of those amounts that are absorbed into the body from the lungs, gastrointestinal tract, or skin. For inhalation and ingestion exposures, absorption is usually characterized by a single fraction applicable to the route. That fraction can differ between ingested and inhaled MTBE. Moreover, the fractional absorption may differ between humans and the experimental animals on which toxicology testing of MTBE was performed. If humans and animals do not differ in this respect, however, the assumption of 100% absorption may be used with a dose–response relationship derived from the animals’ applied dose rather than the actual absorbed dose for that route of exposure. The ratio of the absorption fractions for the two routes can then be applied to convert intake to dose for the second route. Currently, there is insufficient information available to determine if there are differences in the absorption rates of MTBE between experimental animals and humans (Dourson and Velazquez, 1996). For the purposes of this paper, it was assumed that animals and humans have similar absorption characteristics. Furthermore, it was assumed that absorption of ingested MTBE is complete (100%). Because animal studies suggest that absorption from the lungs is about half of the administered dose (Dourson and Velazquez, 1996), the dose must be corrected for that route of administration when combining it with those from ingestion or dermal absorption. In this paper, the exposure estimates were developed in terms of administered dose, not absorbed dose, when summarizing the distributions separately for the drinking water (ingestion) and household air (inhalation) routes. When combining doses across routes of exposure, the inhalation dose was discounted by 50%. No empirical data on the dermal permeation coefficient for MTBE were located. Therefore, the permeation coefficient, PC, was estimated by the methods of Brown and Rossi (1989) and McKone and Howd (1992). Brown and Rossi use only the octanol/water partition coefficient Kow to estimate PC. Mackay et al. (1993)

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report a range of Kows from 8.7 to 20 for MTBE; the calculated PCs do not differ much over this range. Using a Kow of 13.5, PC was estimated by the Brown and Rossi method to be 0.0055 cm/hr. McKone and Howd also used molecular weight (88.13 for MTBE) in their method; PC was estimated to be 0.0064 cm/hr. A value of 0.006 cm/hr was selected for use in this paper; it is consistent with the empirical value of 0.015–0.017 measured for diethyl ether (EPA, 1992), a smaller compound that would be expected to be somewhat more permeable. The concentration of MTBE in tissues depends not only on the rate at which it is absorbed into the body but also on its possible differential distribution to various tissues, the rates at which it is cleared from those tissues through metabolism of the MTBE to other compounds, and the rates at which it is excreted via urine, feces, exhaled air, etc. If the experimental animals used in developing the dose–response relationships do not differ from humans in these rates, then no adjustment need be made for toxicokinetics. Lacking data to indicate substantial differences between humans and animals in toxicokinetics, Dourson and Velazquez (1996) assumed no difference. Therefore, no adjustment was necessary in this paper. 2.2.5. Propagation of Distributions As with atmospheric exposures, the distribution of doses from exposure to MTBE via potable water was propagated from distributions of the component variables under the assumption that all the variables could be represented either by point estimates or by log-normal distributions. Concentration, ingestion rate, and body weight were all treated as distributions for the estimation of doses by ingestion with drinking water. For inhalation with indoor air, only concentration and body weight were treated as distributions. The breathing rate and time spent at home were both treated as point estimates. For dermal absorption from bath water, both concentration and skin area divided by body weight were treated as distributions. Time per bath, frequency of bathing, and permeation coefficient were all treated as point estimates because their variances are likely to be much less than that for concentration. 3. RESULTS 3.1. Atmospheric Exposures

3.1.1. Source Contributions The TRI (EPA, 1995) indicated that petroleum refineries formed the largest source category among the industrial sources at about 3 million pounds per year, followed by industrial organic chemicals (presumably, mostly MTBE manufacturing) at about 0.34 million pounds (Table 3). While emissions from service stations

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TABLE 3 Emissions of MTBE to the Atmosphere

a

Source type

Number reporting

Total emissions (million lb/year)

Refs.

Petroleum refineries Industrial organic chemicals Motor vehicles and car bodies Pharmaceutical preparations Internal combustion engines Petroleum bulk stations and terminals Other industries Gasoline service stations (refueling) Evaporative emissions Tailpipe emissions

81 17 9 5 3 2 24 — — —

3.0 0.34 0.013 0.068 0.058 0.028 0.162 1.3 (8)a 39 (49)a 110 (130)a

EPA, 1995 EPA, 1995 EPA, 1995 EPA, 1995 EPA, 1995 EPA, 1995 EPA, 1995 Brown, 1995 Brown, 1995 Brown, 1995

Values in parentheses estimated by Rykowski (1996).

during refueling were estimated to be of the same order as industrial sources (1.3 to 8 million pounds), emissions from about 192 million vehicles in the United States, of which 144,000,000 are automobiles (Bureau of the Census, 1995), were estimated to be dominant: 149–179 million pounds, with tailpipe emissions accounting for about 73% and the remainder due to evaporation, including about 12% from running losses and 15% from hot soak losses (Brown, 1995). 3.1.2. Concentration Distributions The available data were deemed sufficient to generate concentration distributions for 13 population groups as shown in Table 4. The arithmetic means shown were calculated from the distributions, not from the original data. The occupationally exposed populations experience far higher concentrations than do

other populations, but generally for shorter times. The available data rarely support confidence in the distributions; the most consistent data were probably for gasoline station customers, with a GSD of 2.1. Note that the concentrations estimated for that group are experienced only for very short periods, about 5 min once or twice per week. For some of the occupational groups, both true variation and uncertainty are high and contribute to the overall high GSDs. Hinton (1993) compiled some data for short-term worker exposures during maintenance, etc., which can reach hundreds of milligrams per cubic meter. The data were variable and situation-dependent, and they were not analyzed quantitatively for this paper. 3.1.3. Exposure and Physiologic Characteristics While the concentration information presented above can be used directly for assessment of acute health

TABLE 4 MTBE Atmospheric Concentration Distributions for Exposed Populations

Population ID

Number of data sets

Geometric mean concentration (mg/m3)

Geometric standard deviation

Arithmetic mean concentration (mg/m3)

Manufacturing workers Blending workers Transportation workers Distribution workers Gasoline station workers Mechanics, etc. Professional drivers Commuters Other drivers Gasoline station customers Manufacturing and blending neighbors Gasoline station and storage neighbors General public

2 2 1 1 7 11 [4]a 4 [4]a 7 4 3 4

300 1000 870 470 2000 300 40 40 20 150 3 30 1

6.0 6.0 10.6 6.3 4.0 3.5 2.5 2.5 2.5 4 2.1 3.5 4

1,500 5,000 14,000 2,600 5,200 660 61 61 30 390 4 66 2.6

a Same data sets used for all drivers; probably most applicable to commuters. GM reduced for other drivers because they drive on less congested streets.

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ATMOSPHERIC AND POTABLE WATER EXPOSURES TO MTBE

TABLE 5 Summary of Populations Exposed to MTBE via Air Population ID

Number of persons

Duration of event

Events per year

Years of exposure (GM, GSD)

Manufacturing workers Blending workers Transportation workers Distribution workers Gasoline station workers Mechanics, etc. Professional drivers Commuters Other drivers Full-service customers Self-service customers Manufacturing and blending neighbors Gasoline storage neighbors Gasoline station neighbors General public

883 1800 1489 7705 150,000 300,000 900,000 30,000,000 22,800,000 15,800,000 37,000,000 815,000 7,000,000 4,700,000 67,500,000

8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 1 hr 1 hr 3 min 3 min 18 hr 18 hr 18 hr 24 hr

240 240 240 240 240 240 240 240 365 70 70 350 350 350 365

4, 3.0 4, 3.0 4, 3.0 4, 3.0 2.5, 3.0 4, 3.0 4, 3.0 20, 1.5 50, 1.2 50, 1.2 50, 1.2 8, 2.5 8, 2.5 8, 2.5 70, 1.0

effects where the risk is related to the time-averaged concentration over a relatively short period, time-averaged dose information is needed for assessing the potential risks of cancer or noncancer chronic toxicity. Table 5 summarizes the population exposure and physiologic characteristics used in estimating doses from MTBE concentrations. Table 5 also shows the estimated sizes of the exposed populations. The numbers of people exposed to MTBE in manufacturing, blending, transportation, and distribution were estimated by Hinton (1993). All of the other population sizes were estimated from national statistics and then adjusted for the 30% of the U.S. population living in areas where MTBE is required in gasoline (NRGH, 1995). The number of service station workers was estimated from the number of ‘‘nonsupervisory’’ workers in dispensing facilities, the number of mechanics was estimated from the number of ‘‘nonsupervisory’’ workers in ‘‘Automobile Repair Services and Parking,’’ and the number of professional drivers was taken as those listed as ‘‘motor vehicle operator’’ (Bureau of the Census, 1994, 1995). The number of people with driver’s licenses in 1993 was used to estimate the number of people exposed to MTBE while driving (Bureau of the Census, 1995), and this total was also assumed to be exposed as gasoline station customers. Only some of these drivers are regular commuters (Bureau of the Census, 1995). The sizes of the populations residing near MTBE facilities were estimated from the assumed areas of influence (3.14 square miles for manufacturing plants and refineries, 0.283 square miles for bulk storage facilities, and 0.0314 square miles for gasoline stations), the number of facilities (52, 5000, and 30,000, respectively), and a typical suburban density of 5000 people per square mile (Bureau of the Census, 1995). The balance of the 73 million people residing in the MTBE-

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Body weight (GM, GSD) 76, 76, 76, 76, 76, 76, 76, 72, 72, 72, 72, 30, 30, 30, 45,

1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.22 1.22 1.22 1.22 1.3 1.3 1.3 1.3

using areas experience general ambient exposure to MTBE. 3.1.4. Distributions of Doses The distributions of doses for each exposed population were estimated using Eq. (1), the methods discussed in Section 2.1.5, the concentration distributions presented in Table 4, and the exposure and physiological characteristics presented in Table 5. The results are shown in Tables 6 and 7. The GM doses were obtained by inserting the GM values for all the variables into Eq. (1), while the GSDs were estimated by using the procedure discussed in Section 2.1.5. In general, the GSDs for concentration were dominant in calculating the overall GSDs. The arithmetic means (AMs) were calculated from the parameters of the log-normal distributions. Tables 5 and 6 differ with respect to the appropriate exposure and physiologic characteristics and averaging time (1 year for chronic toxicity and 70 years for carcinogenicity), but not with respect to concentration distributions. The parameters of these distributions are plotted in Figs. 1 and 2; in addition to the GM and AM, the 95% range (2.5–97.5 percentiles) is shown. Note that the distributions for the worker populations, except for the professional drivers, do not much overlap the distributions for the residential or driving populations. 3.1.5. Combined Distributions As explained in Section 2.1.6, people can belong to more than one of the above groups. However, the cluster of occupational groups and the cluster of residential groups is each mutually exclusive. Note from Fig. 1 that fewer than 2.5% of any of the bottom eight distributions overlap the GM of the top six occupational groups. This fact means that exposures at home or dur-

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TABLE 6 Estimated Chronic Doses for Populations Exposed to MTBE in Air Population ID

Number of persons

Geometric mean dose (mg/kg-day)

Geometric standard deviation

Arithmetic mean dose (mg/kg-day)

Manufacturing workers Blending workers Transporation workers Distribution workers Gasoline station workers Mechanics, etc. Professional drivers Commuters Other drivers Gasoline station customers Manufacturing and blending neighbors Gasoline station and storage neighbors General public

883 1800 1489 7705 150,000 300,000 900,000 30,000,000 22,800,000 52,800,000 815,000 11,700,000 60,500,000

5.19E-02 1.73E-01 1.51E-01 8.13E-02 3.46E-01 5.19E-02 3.74E-03 2.47E-04 3.75E-04 2.70E-04 1.30E-03 1.30E-03 4.00E-04

6.0 6.0 10.6 6.3 4.0 3.5 2.5 2.5 2.5 2.5 2.1 6.0 4.0

2.58E-01 8.62E-01 2.44E/00 4.42E-01 9.05E-01 1.14E-01 5.69E-03 3.75E-04 5.71E-04 4.10E-04 1.7E-03 6.47E-03 1.05E-03

ing driving are unimportant for the total exposures of these groups. As they are mutually exclusive, the number of people with exposures greater than any specified dose were simply added to obtain the total number of people with greater exposures. The results of this process are summarized in Table 8. The columns for 10 and especially 100 mg/kg-day are speculative and involve extrapolations well beyond the range of reliable data. In any case, the number of people exposed to those levels is small and could well be zero. The same exercise was conducted for the three mutually exclusive residential groups. The results are shown in Table 9. In this case, notice that numbers at 0.1 mg/kg-day, the lowest number from the worker summary, are also speculative through substantial extrapolation. Because both the highest end of the residential population distribution and the lowest end of the occupational population distribution involve small fractions of the total populations, the likelihood of them being

the same people is insignificant, and there is no need to add the doses. While there are about 240,000 workers with estimated doses below 0.1 mg/kg-day, that value is only about 10% of the number of residential exposures at 0.01 mg/kg-day and above. Within the precision of the original numbers (no better than one significant figure, although three have been shown for clarity), there is no need to adjust the totals for the lower dose levels. The various driver exposures do overlap substantially with the residential exposures. Both the commuters and the other drivers have very similar arithmetic mean daily doses (approximately 0.0005 mg/kg-day). This dose is only about half of that for the general public residential exposures and a smaller fraction of the facility neighbor exposures. Within the precision of the estimates, it is reasonable simply to use the combination of the residential and occupational distributions as adequately representing the distribution for all ex-

TABLE 7 Estimated Lifetime Doses for Populations Exposed to MTBE in Air Population ID

Number of persons

Geometric mean dose (mg/kg-day)

Geometric standard deviation

Arithmetic mean dose (mg/kg-day)

Manufacturing workers Blending workers Transporation workers Distribution workers Gasoline station workers Mechanics, etc. Professional drivers Commuters Other drivers Gasoline station customers Manufacturing and blending neighbors Gasoline station and storage neighbors General public

883 1800 1489 7705 150,000 300,000 900,000 30,000,000 22,800,000 52,800,000 815,000 11,700,000 60,500,000

2.97E-03 9.89E-03 8.60E-03 4.65E-03 1.24E-02 2.97E-03 2.14E-04 1.41E-04 2.68E-04 1.93E-04 1.23E-04 1.48E-04 4.00E-04

8.2 8.2 13.5 8.5 5.9 5.3 4.2 2.7 2.5 2.5 4.7 7.5 4.0

2.70E-02 9.00E-02 2.55E-01 4.62E-02 5.91E-02 1.19E-02 5.94E-04 2.33E-04 4.14E-04 2.98E-04 4.11E-04 1.12E-03 1.05E-03

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FIG. 1. Distributions of chronic average daily dose for population groups exposed via air.

FIG. 2. Distributions of lifetime average daily dose for population groups exposed via air.

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TABLE 8 Estimated Number of Workers Exposed to Atmospheric MTBE at Chronic Dose Shown or Greater Dose (mg/kg-day) Population ID

0.1

1

10

100

Manufacturing workers Blending workers Transportation workers Distribution workers Gasoline station workers Mechanics, etc. Total

315 1,116 848 3507 122,000 90,000 218,000

44 295 315 665 33,300 2,730 37,300

16 21 56 34 1,143 4 [1,260]

— — 4 — 3 — [7]

Note. Brackets indicate speculative values.

posure environments combined. The resulting cumulative distribution is shown in Fig. 3. The curve is not uniformly convex, which reflects the influence of different populations’ distributions. The overall population arithmetic mean dose was estimated by weighting the individual population AMs by the corresponding population sizes. First, the mean exposure for motorists was estimated by adding the refueling dose (0.00041 mg/kg-day) to the mean of the driving dose (0.00046 mg/kg-day) to obtain 0.00087 mg/kg-day. Added to each residential dose, it yielded 0.00194, 0.00258, and 0.00734 mg/kg-day for the general public, manufacturing and blending neighbors, and gasoline station neighbors, respectively. The weighted mean for these exposures was 0.00281 mg/kg-day, which is the mean for all 73 million people, except for those employed in MTBE-related jobs. Finally, this (relatively small) dose was added to each of the worker groups’ arithmetic mean doses. Only for the professional drivers did it add more than 3% (to a total of 0.0085 mg/kgday). The overall weighted mean was then calculated to be 0.0053 mg/kg-day. Similar calculations were performed for the lifetime dose distributions. While the distribution for the general public stays the same, because they were assumed to have the same daily exposure for a lifetime, the other distributions all shift toward lower average daily dose.

(Fig. 2). Because the duration of exposure is lower for workers, their distributions shift most and overlap the other distributions more than in the chronic toxicity case. The combination of exposures for people belonging to more than one exposed group is similar to that for chronic exposure, but additional consideration was given to the changes in activity patterns that a person experiences with age. Children do not belong to the groups of gas station customers, commuters, or occupationally exposed persons, but the same person may do so later in life. Over a lifetime, a person may have several different jobs and eventually retire, quit commuting, and possibly quit driving. These relationships were too complex to analyze precisely for this paper. Instead, the analyses were conducted separately for each group, as in the chronic exposure analysis. Although the separation is less sharp, there is still relatively little overlap between the two distributions. There is substantial overlap between the occupational and residential distributions at 0.001, 0.01, and 0.1 mg/ kg-day. The numbers of people exposed to these doses was estimated to be 19 million, 1 million, and 35,000, respectively. The resulting overall distribution is plotted in Fig. 4. The population mean dose is calculated in the same way as for chronic dose. The driver dose was 0.000610 mg/kg-day. Added to the residential dose, it yielded 0.00166, 0.00102, and 0.00173 mg/kg-day for the general public, manufacturing and blending neighbors, and gasoline station neighbors, respectively. The population-weighted mean for these exposures was 0.00166 mg/kg-day. This dose was added to the means for the working populations and the overall mean was 0.00185 mg/kg-day. 3.2. Potable Water Exposures

3.2.1. Sources for MTBE in Water A total of almost 3.8 million pounds per year of MTBE was estimated to be released from 141 facilities reporting in the TRI (EPA, 1995). However, only 2.4% was reported as released directly to water. Petroleum

TABLE 9 Estimated Number of Residents Exposed to Atmospheric MTBE at Chronic Dose Shown or Greater Dose (mg/kg-day) Population ID

0.0001

0.001

0.01

0.1

Gasoline station and storage neighbors Manufacturing and blending neighbors General public Total

10,800,000 815,000 50,900,000 62,500,000

6,530,000 520,000 15,400,000 22,400,000

1,490,000 2,400 612,000 2,110,000

89,900 — 2,100 [92,000]

Note. Brackets indicate speculative values.

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269

FIG. 3. Cumulative distribution of people with chronic average daily atmospheric doses above those shown.

refineries (19) released 91,271 pounds, industrial organic chemicals plants (3) 129 pounds, and other facilities only 3.5 pounds in aggregate. The bulk of the releases were to the atmosphere. An additional 92,000 pounds was reported to be discharged to publicly operated water treatment plants, where any residual MTBE would likely be released to the atmosphere. Most of the reported emissions come from refineries where gasoline is blended and often stored as well, which are distributed across the United States. Some of the refineries manufacture the MTBE they use; the remainder is manufactured in plants classified as industrial organic chemicals. Many refineries also have petroleum bulk stations and terminals associated with them. Although manufacturing facilities are also widely distributed, over half of the production is concentrated in Texas and Louisiana. 3.2.2. Concentration Distribution in Water A detailed compilation of 1985 national water use information (Solley et al., 1988) estimated that 21,962 million gallons per day, or 55.2% of the potable water consumed (both public and private), was from surface water sources. Approximately 17,850 million gallons per day, or 44.8%, of the potable water consumed nationally was from groundwater sources. The latter includes 36.7% supplied by public wells and 8.16% from private wells (Solley et al., 1988).

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Very few data were found regarding MTBE concentrations in surface water. Two samples analyzed from surface water bodies used as public community water supplies in Wisconsin (an area using MTBE-oxygenated gasoline) showed no detectable MTBE to be present. State and federal agency personnel interviewed stated that MTBE is rarely analyzed for in surface water samples, partly because of the a priori knowledge that MTBE readily volatilizes and thus should not be expected to be found at detectable levels in surface water. Its Henry’s Law Constant is estimated to be 4–5 1 1004 atm-m3/g-mol at 207C (Robbins et al., 1993; API, 1991). In a water treatment experiment by Sorrel et al. (1985), 79% of the MTBE volatilized away in 24 hr, 96.4% of the MTBE volatilized away in 48 hr, and 99.7% of the MTBE volatilized away in 72 hr. The first value corresponds well with the 9-hr volatilization halflife calculated by Environment Canada (1992) from physical/chemical properties. Environment Canada (1993) also calculated that in a river one meter deep at temperature 207C and flowing at 1 m/sec with a 3 m/sec wind above, half the MTBE present would volatilize in 4.6 hr. At a water depth of 10 meters and a wind speed of only 1 m/sec, the theoretical half-life with respect to volatilization was estimated to be 267 hr. Further reductions of MTBE concentrations in surface waters used for public consumption would be expected as a result of dilution and volatilization during several

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STEPHEN L. BROWN

FIG. 4. Cumulative distribution of people with lifetime average daily atmospheric doses above those shown.

stages of water pumping, mixing, storage, transport, and treatment (Davidson and Parsons, 1996). Therefore, the occurrence of MTBE in water supplies from surface sources should not affect the overall distribution of exposures and was not estimated. More data were available on MTBE concentrations in groundwater than for surface water. Some of these measurements, however, were for water that is not tapped for potable use and the following discussion focuses on potable water. As with surface water, people in states with low MTBE use are expected to use groundwater with very low MTBE. Davidson (1995) located MTBE analyses from 1989 to 1995 on 2497 public groundwater systems and 1291 private groundwater wells in five states from the MTBE-using areas. Detection limits for the reported MTBE concentrations varied from 0.2 to 5 mg/liter, which posed difficulties for precise analysis of concentration distributions in this range. Furthermore, many of the detected concentrations were reported only in ranges, not as individual values. Therefore, a point-by-point statistical analysis of the data was not possible. However, a cumulative analysis of the percentages of concentrations below specified break points was possible and was used in this study. The results are shown in Table 10. At the higher concentrations of MTBE, especially those above 200 mg/liter, the water is less likely to be consumed because some people would smell or taste the

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MTBE at concentrations as low as 95 mg/liter (TRC, 1993) and might choose not to consume the water. Because it is easily detected at these levels, investigations are likely to be initiated to determine and eliminate the source of the MTBE in the groundwater. Therefore, even if such water is consumed for a time, the duration of exposure at such levels is likely to be short. By combining the concentration information with the consumption data described above, the distribution of the concentrations experienced by water users was extrapolated as shown in Table 11. The New Jersey Department of Environmental Protection (NJDWQI, 1993) reviewed MTBE occurrence and concentrations in public and private groundwater supplies from sev-

TABLE 10 Concentrations of MTBE in Sampled Water Wells Number of wells Concentration range (mg/liter) Less than detection limit Detection limit–10 10–20 20–200 200 or more

Public 2,430 63 1 4 1

Note. Source: Davidson (1995).

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(97.32%) (2.52%) (0.04%) (0.16%) (0.04%)

Private 1,216 45 15 11 4

(94.19%) (3.49%) (1.16%) (0.85%) (0.31%)

ATMOSPHERIC AND POTABLE WATER EXPOSURES TO MTBE

TABLE 11 Extrapolated Percentage of Population Using Water by Concentration Range of MTBE Concentration range (mg/liter)

Percentage of the population exposed

Less than detection limit Detection limit–10 10–20 20–200 200 or more

98.5 1.21 0.11 0.13 0.04

Note. Source: Davidson (1995).

eral different state databases. They estimated that for this heavy MTBE-use state, approximately 0.5–1.0% of the wells may have been affected by MTBE. However, some of the agencies that collected the New Jersey data actively seek and sample contaminated wells. In addition, some duplicative reporting (i.e., one contaminated well sampled and reported in the database more than one time) may have occurred. Therefore, it seems likely that the New Jersey database overestimates MTBE occurrence for the state as a whole and would considerably overestimate MTBE occurrence in groundwater for states that have had less extensive MTBE use than has New Jersey or whose oxyfuels program began later. As indicated in Section 2.2.1, MTBE is quite unlikely to occur in groundwater at levels above 2 mg/liter solely as a result of atmospheric deposition. Therefore, almost all of the detections of MTBE were probably due to the local influence of leaking storage tanks. The analysis of the concentration data was therefore divided into two parts: one with no local influence by leaks and one of the higher reported concentrations that were due to leaks. Because concentrations due to atmospheric deposition can be detected only sporadically, only the distribution of MTBE from leaking tanks can be based on reliable data. That distribution can be restated as applying only to the estimated 1.49% of the population that experienced concentrations over the detection limit and may have been influenced by leaking storage tanks. Of those people, 85.2% consume water with less than 10 mg/liter MTBE, 90.3% consume water with less than 20 mg/liter, and 97.8% consume water with less than 200 mg/liter. The observed data points are consistent with a lognormal distribution. Figure 5 is a plot of the probits associated with the above percentiles versus the common logarithms of the concentration break points. A quantitative regression analysis showed an R2 of over 0.997 and estimates of approximately 0.36 for the GM and 23 for the GSD. While the meaning of extrapolation beyond the observable range is not clear, the formula gives reason-

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271

able approximations of the observed numbers. The arithmetic mean concentration estimated from this distribution is much higher than the geometric mean because of the high-value skew of the distribution; it is calculated to be 49 mg/liter. The 95th percentile of the distribution is estimated to be about 64 mg/liter, only slightly higher than the arithmetic mean. The remaining 98.5% of the population probably consumes water with concentrations influenced only by deposition of airborne MTBE. Because over half the total is from surface water sources and some of the remainder is from aquifers that are isolated from the shallow groundwater, most of this water is likely to have virtually no MTBE. Although no concentration in such waters can exceed 2 mg/l, the distribution of values below that ceiling is unknown. The assumption that all of the nondetects were at half the median detection limit (0.5 mg/liter) yields a typical concentration from atmospheric deposition of 0.25 mg/liter. This estimate was used for all of the people in the MTBE-using areas that are not exposed via leaks and spills. 3.2.3. Exposure and Physiologic Characteristics Of the approximately 73 million people who live in areas where MTBE is used in gasoline (NRGH, 1995), an estimated 98.5% or approximately 71.9 million consume water with very low levels of MTBE, while the remainder (about 1.09 million people) are estimated to consume water with a log-normal distribution having a GM of 0.36 mg/liter and a GSD of 23. The number of people using water at levels greater than any specified concentration can be calculated from the parameters of the distribution. The results are shown in Table 12. Note that over half of the people who are assumed to be exposed to MTBE from leaking storage tanks are estimated to consume water with less than 1 mg/liter of MTBE. The distribution thus may overlap the distribution of water with MTBE from atmospheric deposition only, which may cause exposures up to 2 mg/liter in some cases. Including the upper tail of the latter distribution would increase the number of people consuming water with more than 1 mg/liter by an unknown amount, but not the numbers consuming more than 10, 100, or 1000 mg/liter. The other exposure and physiologic parameters used in the calculation of the dose distributions were discussed in Section 2.2.3; they are summarized in Table 13. 3.2.4. Distribution of Doses 3.2.4.1. Ingestion of MTBE in tap water. The distribution of average daily doses of MTBE ingested in tap water was estimated from Eq. (1), with the averaging time and duration of exposure both taken to be a 70yr lifetime (25,550 days). Because the relationship is multiplicative and all of the distributions were repre-

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FIG. 5. Log-probit chart of cumulative concentration data for MTBE in water.

sented as log-normal, the geometric mean of the dose was estimated by substituting the GMs of each distribution. The estimated GM for the average daily dose is 7.7 1 1006 mg/kg-day for the population exposed to water that has been affected by leaks of MTBE. The variance in dose was estimated by combining the variances of the concentration, water consumption rate, and body weight. The logarithmic variance is the sum of the logarithmic variances for each variable if the distributions are all log-normal. Because the GSD of the concentration distribution is large in comparison to those for the consumption rate and body weight, the latter do not much affect the overall variance, and the GSD was calculated to be 24. The arithmetic mean dose is estimated to be 1.2 1 1003 mg/kg-day, almost as high as the estimated 95th percentile of 1.46 1 1003 mg/kg-day. In the population potentially affected by MTBE from leaking storage tanks, the corresponding distribution of doses is shown in Table 14(a). The last number in Table 14(a) has been extrapolated well beyond the re-

TABLE 12 Distribution of Concentrations of MTBE in Tap Water

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Concentration (mg/liter)

Population exposed to that concentration or more

1000 100 10 1

6,200 39,500 157,000 404,000

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gion of data and corresponds to a concentration of nearly 5 mg/liter, which is probably unrealistic as a long-term average in view of the objectionable taste and odor of MTBE at high concentrations and the fact that it would likely be associated with other gasoline components with even more objectionable properties. At the other extreme, the first line of the table corresponds to a concentration of 0.5 mg/liter, in the range where people using water affected only by atmospheric MTBE may be exposed. The total number of people exposed at that dose level therefore may be higher than shown. 3.2.4.2. Inhalation of MTBE evaporating from tap water. The inhalation dose was estimated from Eq. (1) using the concentration calculated from Eq. (2).

TABLE 13 Exposure and Physiologic Parameters for Estimating Doses of MTBE from Tap Water

Parameter

Units

Geometric mean

Bath duration Bath frequency Ingestion rate Inhalation rate Body weight Skin area exposed Permeation coefficient Exposure time (breathing)

min no/year liters/day liters/hr kg cm2 cm/hr hr/day

13.6 73 0.963 1.35 45 13,460 0.006 18

* Point estimate.

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Geometric standard deviation 1.5 * 1.7 * 1.3 * * *

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ATMOSPHERIC AND POTABLE WATER EXPOSURES TO MTBE

TABLE 14 Distribution of Doses from Exposure to MTBE in Tap Water Population exposed to that dose or more Average daily dose (mg/kg-day)

(a) Ingestion

(b) Inhalation

(c) Dermal

(d) Total

0.00001 0.0001 0.001 0.01 0.1

506,700 224,400 65,500 12,100 [1,400]

369,300 138,800 34,100 [5,300] [510]

35,000 5,600 [560] [35] [1]

540,000 260,000 83,000 17,000 [2,300]

Note. Estimates in ‘‘Total’’ not derived directly from other columns (see text). Brackets indicate speculative values.

With the parameter values discussed in Section 2.2.3.2, the GM concentration in air was calculated to be 0.0091 mg/m3. The duration of exposure was taken to be equal to the averaging time. The GM average daily dose was estimated to be 2.7 1 1006 mg/kg-day. The GSD contains only the variances for concentration and body weight and therefore was estimated to be 24. The calculated arithmetic mean dose is 4.2 1 1004 mg/kg-day and the corresponding 95th percentile is 5.1 1 1004 mg/ kg-day. For the population potentially affected by MTBE from leaking storage tanks, the dose distribution for inhalation is shown in Table 14(b). The last two rows in Table 14(b) were extrapolated well beyond the range of data and are therefore not reliable. The dose from the first row corresponds to a concentration below 2 mg/ liter and the number of people exposed might need to be increased to account for exposures from water containing MTBE from atmospheric deposition. 3.2.4.3. Dermal exposure to MTBE during bathing. The average daily dose from bathing in water containing MTBE was estimated from Eq. (3) using the parameter values shown in Table 13. The resulting GM dose was 2.6 1 1008 mg/kg-day for the population exposed to water that has been affected by leaks. This value is less than 1% of the corresponding estimated dose for direct ingestion. Although the variances in the parameters are generally not known, they are small

in comparison with the variance in concentration. The overall GSD is taken to be approximately 25, which yields an arithmetic mean dose of 4.6 1 1006 mg/kgday and a 95th percentile of 5.3 1 1006 mg/kg-day. In the population potentially affected by MTBE from leaking storage tanks, the distribution of doses is shown in Table 14(c). The last three rows in Table 14(c) were extrapolated well beyond the range of data and are therefore not reliable. All of the data in this table correspond to concentrations above 2 mg/liter and would not be influenced by consideration of exposures from water containing MTBE only from atmospheric deposition. The dermal absorption from showering was not estimated because of uncertainties regarding the applicability of immersion models to absorption from a thin film of water. Because dermal absorption from immersion was estimated to be small in comparison to ingestion of tap water and inhalation of volatilized MTBE, the dermal absorption from showering was ignored here. 3.2.4.4. Combined exposures. Because the exposures from drinking water, volatilization into residential air, and dermal exposure from bathing are all linearly related to the concentration in the source water, they are fully correlated. Therefore, it is appropriate to add the arithmetic means to estimate the arithmetic mean of the overall distribution. Before adding in the

TABLE 15 Average Daily Doses from Water Affected by Leaks and Spills of MTBE Average daily dose (mg/kg-day) Exposure scenario Ingestion of drinking water Inhalation in showera Inhalation in whole housea Dermal absorption in bath Combined exposures a

Geometric mean

Arithmetic mean

06

7.7 1 10 0.6 1 1006 1.4 1 1006 2.6 1 1008 Ç1 1 1005

1.2 1.1 2.1 4.6 1.4

1 1 1 1 1

10 1004 1004 1006 1003

Administered doses reduced by 50% to account for lower absorption via inhalation.

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95th percentile 1.5 1 1003 1.2 1 1004 2.6 1 1004 5.3 1 1006 Ç2 1 1003

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inhalation exposure, however, it was decreased by 50% to take into account relative absorption via inhalation in comparison to ingestion (see Section 2.2.4). The combined exposure is therefore estimated to have a arithmetic mean of 1.4 1 1003 mg/kg-day. Because doses were added, the overall distribution is not expected to be strictly log-normal, but it still should be highly skewed toward the higher concentrations. Typical values of exposure will be much lower, in the vicinity of the value that would be obtained by adding the GMs, namely somewhat less than 1 1 1005 mg/kg-day. Because the drinking water exposures make up about 85% of the total dose, the distribution of doses above given levels would resemble that in Table 14(d). When the distribution was approximated as log-normal with a GM of 1 1 1005 mg/kg-day and a GSD of 25, the values in Table 15 were derived. 4. DISCUSSION AND CONCLUSIONS

The results reported above show that individual exposures to MTBE can vary widely, both via the atmosphere and via potable water. A benchmark for significant exposures is provided by the EPA Reference Concentration (RfC) of 3 mg/m3 (EPA, 1996). With standard assumptions about breathing rate and duration of exposure, and with the application of the 50% discount factor for inhalation doses, that RfC corresponds to a Reference Dose (RfD) of 0.42 mg/kg-day. Tables 8, 9, and 14 show that most people receive chronic doses well below that RfD via either air or water. Although Table 8 suggests that chronic doses could be higher for people with occupational exposures, the highest exposure levels are statistical extrapolations rather than direct measurements. Moreover, any one person is unlikely to be exposed continually to potable water affected by leaks and spills or to remain in the worst occupational exposure situation for a significant fraction of a working lifetime. Therefore, the exposure levels in the upper percentiles of the distributions are probably lower than extrapolated above. Although the reported results were based on a substantial number of individual measurements of MTBE in air and water, the scope and representativeness of the sampling was limited. For both air and water, some bias toward sampling in areas of suspected problems may exist. For air, some of the exposed population group’s doses were estimated from only a few individual samples. For water, the sample set extended over only five states. If concerns about the health impacts of MTBE persist, additional and more systematic sampling should be conducted to improve the data. The results are also uncertain because of the many assumptions and calculational manipulations needed to convert concentration measurement data into exposure distributions expressed in terms of average daily dose. Although the distributional analysis was de-

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signed to answer questions about the variability of exposure in the population, no quantitative uncertainty analysis was performed. Future attempts to characterize MTBE exposure could benefit from such an analysis. Below are some qualitative observations about the reliability of the results. • Time frame of sampling. The data were collected only during a limited period in which MTBE was coming into widespread use. As MTBE use increases, so may the frequency of detections. • Statistical analysis of concentration data. This paper made the assumption that MTBE concentrations are distributed log-normally. While the analysis fits the observed distributions rather well, extrapolation beyond the range of observed values is quite uncertain. The upper tail of the distribution is unlimited, when in reality concentrations cannot exceed physical limits. This bias probably overstates the arithmetic mean but should not substantially affect the prediction of statistics within the usual range of interest (median or 50th percentile to the 99th percentile). • Population exposed. This analysis used a population in MTBE-using areas of 73 million. If areas were included where MTBE is used at lower levels than in RFG, the population estimate would rise, but the concentration distribution would fall. The U.S. Geological Survey (USGS, 1995) has stated that 109 million people live in counties where MTBE is believed to be used, probably including oxyfuel regions as well as RFG areas (although such is not stated in the USGS report). While oxyfuels contain 35% more oxygenates such as MTBE than does RFG, they are used for only a few months per year in winter. • Physiological parameters. Most of the physiological parameters are not only less variable than concentrations but also more certain. However, substantial uncertainty remains with some. In particular, the absorption of MTBE by humans at various levels of applied dose is uncertain for all routes, and especially for dermal absorption. • Other exposure parameters. The contact rates (water consumed, air breathed, skin area exposed) are all reasonably well characterized. The contact duration and frequency are less well known and the parameters of the distributions, if used at all, are moderately uncertain. While the impact on average doses is probably not great, the influence on estimates of unusually high doses may be substantial. The directions of these influences are not known with any certainty. • Exposure scenarios. Emphasis was placed on residential and occupational exposures of people with reasonably normal habits. Very unusual exposure patterns were not captured. Again, these should be sufficiently rare that they would not much influence the distributions in the usual range of interest. More important is the uncertainty in choosing a moderately

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ATMOSPHERIC AND POTABLE WATER EXPOSURES TO MTBE

long averaging period for the exposures. It is known that MTBE clears rapidly from the body, so tissue concentrations could be higher immediately after an exposure event than would be assumed from the time-averaged dose. According to Clary (1996), tissue concentrations may drop by a factor of two within hours after an acute dose. If biological effects are caused by shortterm peak concentrations rather than by total dose over time (e.g., area under the curve), then the averaging procedure used here may not be appropriate. ACKNOWLEDGMENTS This paper would not have been possible without financial support from ARCO Chemical Company. Special thanks go to Susan Youngren and Richard Hubner of EA Engineering, Science, and Technology for locating information vital to conducting these analyses and for other scientific and logistical support. James M. Davidson cooperated in developing the analysis of exposures via water and Michael E. Ginevan provided advice on improving the statistical analyses.

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