Risk assessment for combustion products of the gasoline additive MMT in Canada

The Scienceof the Total Environment 189/190(1996) 1I-20

Risk assessment for combustion products of the gasoline additive MMT in Canada M. Egyed”, G.C. Wood Monitoring and Criteria Division, Envirorimental Health Directorate, Health Canada, Ottawa, Ontario KlA OL2, Canada

Abstract

Methylcyclopentadienyl manganese tricarbonyl (MMT) has beenusedas an octane enhancerin Canadiangasoline since 1976.The main potential health concern is from manganeseoxidesproduced on combustion(mainly Mn,O,), given the known neurotoxicity of chronic inhalation of manganese(Mn) dust from mining and industrial use. Relevant epidemiologicalstudiesof occupational exposure to respirableMn are briefly reviewed; an ambient air referencevalue of 0.1 pg Mn/m3, and associatedinhalation tolerable daily intake (TDI) and tolerable daily uptake (TDU) of 0.035 and 0.021 pg/kg b.w./day are derived. Ambient levelsof PM,., (respirable)Mn in Canadiancities have remainedunchangedor have decreasedbetween1986and 1992,and do not reflect large changesin MMT usage during that time. Ambient levels of PM,,, Mn in Canadiancities in 1992were I 0.025 pg Mn/m3. Mean, 90th and 98th percentilesof PM,,, Mn inhalation uptake basedon ambient monitoring data from high traffic areasand from estimatesof personalexposureare below the inhalation uptake criterion. An assessment of exposurefrom air, food, water and soil revealedthat < 1% of total daily Mn uptake is derived from inhalation for all agegroups.Therefore, basedon current information, Mn derived from the combustionof MMT-containing gasolineis unlikely to represent a significant health risk to Canadians. Keywords:

Risk assessment; Gasoline; MMT; Manganese

1. Introduction An organic Mn-containing compound, methylcyclopentadienyl manganese tricarbonyl (MMT), has been used in Canada as an octane booster since 1976. An assessment conducted in 1978 by the Canadian Department of National Health and * Corresponding author.

Welfare [l] concluded that future predicted increases in ambient Mn concentrations with MMT in all gasoline would not be sufficient to constitute a hazard to human health. The U.S. Environmental Protection Agency (EPA) at that time rejected the use of MMT in unleaded fuel (although its use in leaded fuel continued) due to evidence of blockage of the catalytic convertors in service at the time by the combustion products of MMT,

0048-9697/96/$15.00 0 1996ElsevierScienceB.V. All rightsreserved PII SOO48-9697(96)05185-6

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and of elevated hydrocarbon emissions (a par-ticular concern in high pollution areas such as parts of California). Recent testing in the U.S.A. undertaken to support an application for a waiver of the prohibition on new gasoline additives under the Clean Air Act has concentrated on the possible blockage of catalytic convertors and increases in hydrocarbon levels, especially after long vehicle service (75 000 to 100000 miles). In a decision of July 14, 1994, these issues were judged to have been resolved satisfactorily, and unresolved health concerns with respect to airborne Mn were cited as the reason for rejection of the waiver application. The present risk assessment was undertaken in response to concern, heightened by this judgement, that Canadians were at some risk to health due to exposure to MMT-derived Mn. Historically, there has been little concern with MMT itself (although recently some concern has been expressed over possible genotoxicity and exposure through dermal absorption from accidental spills or deliberate use of gasoline as a solvent cleaner, and from misuse such as gasoline sniffing). MMT is added to gasoline as a catalyst, and in much smaller quantities than alkyl lead compounds (maximum 290 mg Pb/l in 1987 Canadian re~lations). The 1993 national average concentration of Mn in gasoline was 9 mg/l [a] while the maximum allowable concentration is 18 mg/l Mn, about l/16 the 1987 maximum for lead. MMT has not often been detected in air, as little appears to be emitted unburned into auto exhaust. It has a very short atmospheric half-life of about 15 s, and is converted to manganese oxides 131. The principal emission product of the combustion of MMT is a fine particulate matter of 0.1 to 0.4 pm diameter (similar in size to lead emissions) [3], composed of the insoluble Mn oxide Mn304, a mixed oxide in which two atoms are in the more oxidized 3 + valence state and one is in the reduced 2 + valence state. The health concerns with respect to using MMT in gasoline have focused on emissions of this oxide.

2. Health assessmentand derivatjon of exposure objectives

The chronic inhalation of Mn dust by Mn miners and ore processors has been known for many years to be capable of causing severe neurotoxicity, resulting in a condition known as ‘manganism’ or ‘manganic madness’. Less severe symptoms, sometimes seen in industrial workers in Mn-emitting smelters, are similar in many respects to symptoms of Parkinson’s Disease, and include slow clumsy limb movements, tremors, and facial rigidity, due to interference with the dopaminergic nerve transmission system of the brain and the eventual death of brain cells. However, Mn is an essential element required by the body for the proper functioning of several enzymes, including glutamine synthetase in the brain. Although Mn is naturally abundant in food and soil and is considered one of the least toxic of the metals, inhaled Mn is more hazardous than ingested Mn because of the bypassing of gastrointestinal mechanisms for homeostasis resulting in greatly increased absorption and distribution throughout the body. In addition, trivalent Mn, which is emitted due to fuel combustion, is the form linked to the destruction of dopamine in the brain [4]. In order to develop a reference concentration for airborne Mn against which exposures of Canadians could be compared and the risks to health assessed, four recent epidemiology studies [5-s] of workers occupationally exposed to airborne Mn dust were examined. The four studies were consistent in that subtle neurological disorders were associated with chronic exposure to Mn dust in each study. A well-designed and conducted study [5] on workers in a Belgian battery factory exposed to MnO, dust was selected as the critical study because of the availability of doseresponse info~ation based on individual exposure assessments and work histories for each worker, in contrast to the remaining studies [G-8] which could compare only the group averages of Mn exposure for controls and workers. In this study, 92 workers were compared to 101 controls from a nearby polymer plant; a number of potential confounding factors, including age, level of

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steadiness

L

CC$ll~Ol.

Quartile 1

Quartile 2

Quartile 3

Quartile 4

mg Mnlcubic me&e x years (N=~/qu~l~) Fig. 1. Dose-response for respirable Mn dust cumulatjve exposure and abnormal psychomotor performance. Sources: Roels et al. 1992, Dr H. Roels, letter to U.S. EPA, Refs. [9,10].

physical work, socioeconomic status, education, hobbies, personal habits (smoking and diet) and previous and present work histories including exposure to other neurotoxi~~ts (lead, mercury, organics) were taken into account. Abnormally poor performances, compared to controls, were found in tests on eye-hand coordination, visual reaction time, and hand steadiness, and were associated with a measure of cumulative exposure to respirable Mn dust (MnO,) that took into account the number of years worked and the job history. Division of the group into four equal quartiies of 23 workers revealed some evidence of dose-response for each endpoint (Fig. 1). No significant differences in the three outcomes were shown for the first quartile compared to controls, while significant increases were observed in the 2nd, 3rd and 4th quartiles (P = 0.04 - 0.001). Average cumulative respirable Mn dust exposure of 264 yg/m3 y per 8-h work shift for a 5 day week with average exposure duration of 2.6 years in the first quartile was converted to a continuous 24-h, 7 day/week equivalent of 32 pg/m3. 264pg/m3y

x 10m3/d x 5d = 31.53 pg/m” 2.6 y 23 m3/d x 7 d

A conservative ambient air value of 0.1 lug/m” was then derived, by application of an uncertainty factor of 300 to account for human variability (10 x -the study was conducted with young healthy males), less than lifetime exposure (10 x only 2.6 years average in this group and 5.3 years average in the entire cohort), and other weaknesses in the study and overall database (3 x statistical weakness from only 23 members/group, exposure to a different Mn oxide than Mn,O,, lack of reproductive data) [9]. This value is onetenth the 1987 WHO ambient air objective of 1 ~g~rn3, and is on the lower end of the U.S. EPA’s ‘most likely range’ of 0.09-0.2 @g/m3 [20]. It is equivalent to an inhalation tolerable daily intake (TDI) of 0.035 ,ug,/kg b.w./day, derived as follows: Reference air concentration/h x no. of hours/ day exposure + assumed body weight (of exptl. subjects in Roels et al. 1992) = TDI

0.10 - 0.11 ,ug,/m3/h x 24 h/d 70 kg b.w.

=0.0343 - 0.0377 pg/kg b.w.i’ day (rounded to 0.035 pg/kg b.w./day)

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.

1981

1983

1985

1987

1989

1991

1993

year Fig. 2. MMT

sales in Canada. Sources: Ethyl Canada Inc., Mississauga Ont. L5B 351; Ref. [9].

An uptake from inhalation of 0.021 ,ug/kg b.w./ day was further derived from the TDI, (based on 60% deposition and 100% absorption of respirable particulate), for use in comparing the exposure of various pop~ation age groups.

3. Exposure assessment The exposure of Canadians to Mn was examined based on: (1) temporal trends in ambient monitoring data and MMT usage in Canadian cities; (2) a detailed inhalation exposure assessment for all age groups and including the upper percentiles of the population; and (3) a multi-media exposure assessment to examine the fractional contribution of inhalation to total Mn exposure. 3.1. T~rn~~ra~ trends in ambient Mi? and MMT usage The phase-down of leaded gasoline in Canada began in 1977, and the elimination of lead-based additives was complete in 1990. This process was associated with an increase in the use of MMT as an alternative octane enhancer throughout the 1980s (Fig. 2). Specifically, MMT sales by Ethyl Corp., the sole or primary supplier of MMT in

Canada throughout this time, increased 52% from 1986 to 1989, and subsequently decreased 38% from 1989 to 1992. As discussed above, Mn oxide emissions from the combustion of MMT are of diameter less than 1 pm and are thus expected to contribute especially to the fine fraction of particulate Mn in air. Ambient monitoring data of fine particulate PM2.5 Mn in four of the largest Canadian cities between 1986 and I992 [ll] are presented in Fig. 3. The anomalous nature of the Montreal data is due to the presence of a silica- and ferro-manganese facility located 25 km southwest of the city, which ceased activity in 1991. Since the closure of this point source, PM,, Mn levels have decreased to levels similar to those in other large cities. For cities not influenced by large point source emissions of Mn, ambient PM,., Mn levels have remained constant and do not reflect the concomitant substantial shifts in MMT sales, although no detailed source apportionment data are available for respirable Mn in Canada. 3.2. Inhalation

exposure to manganese

Mean ambient concentrations of PM,, Mn measured in 1992 for various Canadian locations [l l] are provided in Fig. 4. The lowest value

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Fig. 3. Trends in ambient PM, 5 manganese levels in four Canadian cities (arithmetic mean + ND.). Source: Ref. [l 11.

(0.002 pg/m3) was measured in Kejimkujic, a national park in Nova Scotia which can be assumed to represent a background rural level. Low levels, 0,Ol l-0.013 pg/m3, were measured in small cities including St. John, Ottawa and Halifax; moderate levels of 0.015-0.019 pggjm3 were reported for Winnipeg, Victoria and Calgary; the larger cities Vancouver, Toronto, and Montreal, reported higher levels of 0.020-0.025 fig/m’. These data suggest that in cities without major industriai sources of Mn, PM,, Mn is associated with urbanization/city size. The highest concentrations of Mn (0.030-0.158 pg/m3) were measured in Hamilton and Sault Ste. Marie, smaller cities with large Mn emitting industries (iron and steel manufacturing). Thus, in the largest Canadian urban centres without indust~al sources of Mn, ambient levels of respirable Mn are substantially below the reference value of 0.1 pg/m3. Although no comprehensive study of ‘personal’ exposure to Mn has been carried out in Canada, several limited studies [12-151 have assessed personal inhalation exposure of small samples of target groups, primarily with the intention of

elucidating the effect of MMT use (Fig. 5). In particular, taxi drivers and garage mechanics were selected as having potentially high exposure, while office workers were selected as members of the general working population. Results were assessed for 24 h exposures (as Mn in respirable particulates and/or total suspended particulate (TSP), the former being approximately 50680% of the latter, and TPS thus being an overestimate of exposure). In general, levels are higher for taxi drivers than for office workers, and are highest for garage mechanics (although exposure of the latter group was complicated by an inadequate use of ventilation techniques). For all groups studied, average inhalation exposure was less than the reference value of 0.1 pg/m3 (respirable Mn). In addition, personal exposure of office workers was always lower than concurrently collected and comparable ambient monitoring data, and personal exposure of taxi drivers was similar to or lower than ambient monitoring data [9]. This may be indicative of the time spent in indoor environments~ which have been shown to have lower levels of Mn than outdoors [15,16]. Other studies have also indi-

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si I I I

4

+-i-t

s~unrr:Rsferenwiii

[Mnl

-+-i+

+--I

Guglm31

Fig. 4. Annual arithmetic mean PM,, manganese in ambient air (1992). Source: Ref. [I 11.

cated a link between air Mn concentrations and vehicle activity: Loranger et al [17] report significantly (P < 0.05) higher levels of TSP Mn in high traff?c areas compared to low traffic areas in Montreal, and TSP Mn levels of 0.028 ,ug/m3, 0.044 pug/m3 and 0.053 ,ug/m3 were reported for Toronto sites of low, medium and high traffic volume, respectively [ 181.

taxi drivers, Toronto # office workers, Toronto # taxi drivers, Toronto t offtce workers, Toronto t taxi drivers, Montreal t office workers, Montreal t garage mechanics, Montreal $ garage mechanics, Montreal * $ taxi drivers, Montreal $ outdoors, Windsorn indoors,Windsor fi 24”hour, Windsor g

PMlo was chosen as the relevant parameter for this exposure assessment as the reference value was derived from a study in which respirable Mn was collected as a fraction with an upper cutoff at 7 pm and MMAD equal to about 5 pm (that is, approximately equivalent to PM,) [S]. Since PM, is a smaller sbfraction of PMro, a reference value based on PM, will be conservative in comparisons

r ”””” ” - *’ ”-

IE2[respirable Mn] @g/m’)

1 ’ i i

F I I -i II p

/

/

j

0

0.02

0.04

I 0.06

I 0.08

-i- 0.1

0. 12

lMn1 ~~m3) Fig. 5. Estimates of 24-h personal exposure to manganese. Refs: # [12&t [13]; $ 1141;7 [15]; * under ventilated conditions.

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Table 1 Exposure to respirable PM,, Mn (fig/m’) based on ambient Mn data (AMB) and on extrapolation personal exposure (PE)

17

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from PTEAM

distributions of

Population exposure

AMB St. John, N.B. (pg/m’)

AMB Montreal, Que. bg/m7)

PE St. John, N.B. (fig/m’)

PE Montreal, Que. (fig/d

Mean 90th percentile 99th percentile

0.011 0.021 0.024

0.025 0.034 0.041

0.01 I 0.02 0.052

0.025 0.046 0.118

Sources: References [9,1 I, 16,191

involving larger PM,, measurements. Two sets of air data were assessed: (i) the most recent available ambient monitoring data (1992); and (ii) a translation of these data to population estimates of personal exposure based on PTEAM study results, a large study of population personal exposure to respirable Mn conducted in a suburb of Los Angeles, California [16]. For the latter, ratios of the arithmetic mean, 90th and 99th percentiles of personal exposure to the outdoor mean were calculated from the 24-h PTEAM data provided by Wallace [19]. These ratios were used to convert annual ambient PM,,, Mn means for Canadian cities to estimates of the mean, 90th and 99th

percentiles of personal exposure for populations in those Canadian cities (for more detail on this methodology see reference [9]). Estimates of inhalation exposure are presented for St. John, New Brunswick (a small city, < 100 000) and Montreal, Quebec (a large city, > 2 million population) with annual mean levels of ambient PM,, Mn of 0.011 pg/m3 and 0.025 The Montreal monitor p g/m3, respectively. recorded the highest mean PM,, Mn level for any city (uninfluenced by industrial Mn sources) in Canada in 1992, and is located at the intersection of two expressways, both with high traffic volume. It is thus assumed to represent a worst case

Imean __________________ algw19( __ 0.03 109th % q 4

b)

0.035

,5,

0.03

t

0.025

0.02

0.015

0.01

0.005

0

St. John C-11 yew

Monb-eal St John e-11 years) p2oyeam) (we 9fw)

Fig. 6. Inhalation uptake of PM,, Mn based on (a) exposure to ambient Mn and (b) extrapolation personal exposure. Sources: exposure, Ref. [I I]; PTEAM data. Refs. [16,19]; uptake. Ref. [9].

of PTEAM

Montreal p20 yesn)

distribution

of

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n EZSoil

IWater El Food

O-6 months

0.5-l

years

5-l 1 years

12-19 years

>20 years

Age Fig. 7. Uptake of manganese from air, food, water and soil. Inhalation uptake based on ambient PM,, Mn for Montreal Methodology as in Ref. [20]; data from Ref. [9] (background document).

scenario. Given the poor characte~zation of indoor Mn levels in Canada and the evidence that indoor levels are less than outdoor levels, the conservative assumption is made of 24-h exposure to outdoor ambient levels. Exposure levels of PM,, Mn based on ambient data and on estimates of population personal exposure are presented (Table 1). The results from the ambient monitoring data indicate that all estimates are well below the reference value of 0.1 figb37 and that Mn levels were higher in the larger city than in the smaher city. For estimates of personal exposure, all values were 54% or less of the criterion, except for the 99th percentile of the Montreal population which slightly exceeded the criterion. These inhalation exposure data were then used to estimate inhalation uptake for five age groups, two of which are presented in Fig. 6, including children aged 5 - 11 years who had the highest uptake on a body weight basis, and adults. Reference assumptions for each age group are based on Health Canada [20]. Based on ambient monito~ng data, inhalation uptake of Mn for all components of the population is less than 0.012 pg/kg b.w./day, or 57% of the reference

1992.

value of 0.021 pg/kg b.w./day. Inhalation uptake based on the estimates of personal exposure were well below 0.015 pg/kg b.w./day for the 90th percentile of the population in both cities and the 99th percentile in St. John. However, the 99th percentile of the population for both age categories examined in Montreal exceeded the reference value of 0.021 pg/kg b.w./day by loo/o to 50%. 3.3. ~~~t~-rn~diu exposure to rna~g~ne~e

A multi-media assessment of Mn uptake from air, food, water and soil was also undertaken. Assuming an uptake of ingested Mn of lOO%, 3% and 1.5% for children aged O-6 months, children aged 7 months to 4 years, and individuals greater than 5 years of age, respectively [9], estimates of uptake from all media are presented (Fig. 7). It is clear that for all age groups, dietary Mn (derived from the natural Mn content of foods such as grains) dominates total uptake and inhalation represents less than 1% of total daily uptake. The large contribution of soil to uptake by infants is due to the relatively high natural levels of Mn in

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Canadian soil, and to an assumption of high absorption of Mn after ingestion of dust by infants, compared to much lower absorption by toddlers. Assessment of intakes followed assumptions which are standard for Priority Substances List assessments by Health Canada [20].

4. Conclusion

In summary, constant or decreasing levels of ambient respirable Mn in Canadian cities do not reflect large changes in MMT usage that have occurred. However, ambient Mn levels have been associated with urbanization, traffic level and vehicle-related activities. Mean ambient PM,,, Mn levels in air are less than 2% of the reference value air value of 0.1 pg/m3 and limited personal exposure data, targeting groups that are more highly exposed to vehicle emissions, are also substantially below the criterion. Based on ambient monitoring data, the 99th percentile of exposure in highly trafficked areas in large urban centres is less than half of the exposure criterion. Estimates of population distribution of personal exposure suggest that exposure of 90% of the population is less than half the criterion, and the 98th percentile in a large city is 80% of the reference value, while the 99th percentile in a large city surpasses that value [9]. In addition, inhalation represents less than 1% of total daily Mn uptake for all age groups of the population. Thus, exposure to respirable Mn is considered low for 98-99X of the population, and the contribution from the combustion of MMT in gasoline is not likely to represent a substantial health threat to Canadians.

Acknowledgements

We would like to acknowledge the invaluable technical assistance of MS Veronique Morisset and Mr Barry Jessiman in producing this manuscript, and helpful comments provided by Dr R. Burnett and MS K. Hughes, as internal peer reviewers, Environmental Health Directorate, Health Canada.

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References VI

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