Health- and vegetative-based effect screening values for ethylene

Chemico-Biological Interactions 241 (2015) 87–93

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Health- and vegetative-based effect screening values for ethylene Neeraja K. Erraguntla ⇑, Roberta L. Grant Texas Commission on Environmental Quality, Toxicology Division, Austin, TX, USA

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Article history: Available online 26 February 2015 Keywords: Ethylene Ethylene oxide Health effects Vegetation effects Texas

a b s t r a c t Ethylene (ET) is ubiquitous in the environment and is produced both naturally and due to anthropogenic sources. Interestingly, the majority of ambient ET contribution is from natural sources and anthropogenic sources contribute only a minor portion. While microbes and plants naturally produce a large amount of ET, mammals are reported to produce only a small amount of ET endogenously. Anthropogenic sources of ET include the combustion of gas, fuel, coal and biomass. ET is also widely used as an intermediate to make other chemicals and products and is also used for controlled ripening of fruits and vegetables. Although, a review of human and laboratory animal studies indicate ET to be relatively non-toxic, there is concern about the potential toxicity of ET because ET is metabolically converted to ethylene oxide (EtO). EtO has been classified to be carcinogenic to human by the inhalation route by the International Agency for Research on Cancer (IARC) cancer. ET, however, has been classified as a Group 3 chemical which indicates it is not classified as a human carcinogen by IARC. Several studies have reported ET to cause adverse effects to plant species (vegetation effects) at concentrations that are not adverse to humans. Therefore, the Texas Commission of Environmental Quality (TCEQ) conducted detailed health and welfare (odor and vegetation) based assessments of ET to develop both health and vegetative based toxicity factors in 2008 in accordance with TCEQ guidelines. The health assessment based on wellconducted animal toxicity studies resulted in identification of higher points of departures and subsequently higher effect screening levels (ESLs) that were more than a magnitude higher than the threshold adverse effect level for vegetative effects for ET. Further, based on a weight-of-evidence evaluation of potential mutagenic and carcinogenic mode-of-actions for ET it appears the metabolic conversion of ET to EtO is of insufficient magnitude to cause concern of potential cancer risk. Therefore, the short-term ESL for air permit reviews and air monitoring evaluations is the vegetation-based ESL of 1200 ppb as it is more than a magnitude lower than the health-based acute ESL of 150,000 ppb. Similar to the acute derivation, the chronic evaluation resulted in the derivation of a chronic vegetation based ESL of 30 ppb that was much lower than the chronic ESL of 1600 ppb. In summary, the TCEQ’s acute and chronic ESLs for vegetation will protect the general public from short-term and long-term adverse health and welfare effects. The general public includes children, the elderly, pregnant women, and people with pre-existing health conditions. Ó 2015 Elsevier Ireland Ltd. All rights reserved.

1. Introduction ET (CAS No. 74-85-1) is produced both naturally and due to anthropogenic sources. The Organization for Economic Cooperation and Development (OECD) reports the majority of the global ET emissions (approximately 74%) to be from natural sources whereas the remainder (approximately 26%) of global ET Abbreviations: AMCV, Air Monitoring Comparison Value; DSD, Development Support Document; EPA, Environmental Protection Agency; ESL, effects screening level; ET, ethylene; EtO, ethylene oxide; ReV, reference value; TCEQ, Texas Commission on Environmental Quality. ⇑ Corresponding author at: Texas Commission in Environmental Quality, P.O. Box 13087 (MC-168), Austin, TX 78711-3087, USA. http://dx.doi.org/10.1016/j.cbi.2015.02.010 0009-2797/Ó 2015 Elsevier Ireland Ltd. All rights reserved.

emissions comes from anthropogenic sources [38]. All types of vegetation including plants, microbes and marine algae naturally produce ET. In addition decomposition of organic matter by soil microorganisms and burning of wood and biomass during forest fires can release ET. Volcanic emissions and natural gas leakage release a relatively small amount of ET [38]. A relatively small amount of ET is produced endogenously in mammals through lipid peroxidation of unsaturated fats, oxidation of free methionine, oxidation of hemin in hemoglobin, and metabolism of intestinal bacteria [26]. ET has been reported to be the largest volume organic chemical produced worldwide and is produced mainly by the steam-cracking of hydrocarbons. It is considered a basic building block in the

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chemical and petrochemical industries because it is an essential intermediate in the production of other industrial chemicals and polymers such as polyethylene, ethylene oxide, ethylene dichloride and ethylbenzene, styrene, and vinyl chloride [4]. Burning of hydrocarbons and biomass can also produce significant amounts of ET [38]. ET is a colorless gas (at ambient temperatures) with a faint sweet odor, is a liquid under pressure, and is slightly soluble in water. ET has a low blood-gas partition coefficient and does not accumulate in the body [38]. ET is also a highly flammable volatile gas that is considered to be a fire hazard at sufficiently high concentrations. In occupational settings, very high concentrations of ET can lower oxygen concentrations and has been reported to function as an asphyxiant [14]. ET is widely used as a fruit and vegetable ripening agent and as a pesticide. In the past, ET was widely used as an anesthetic agent but was discontinued because of its flammable properties and its ability to cause asphyxiation at high concentrations. Even though ET has been used as an anesthetic in the past and its continued and prevalent use as an artificial ripening agent indicate it to be non-toxic, there is concern about the potential toxicity of ET because ET is metabolically converted to ethylene oxide (EtO) (CAS No 75-21-8). EtO has been classified by the IARC to be carcinogenic to humans by the inhalation route (Group 1) [29,30]. EtO is also a genotoxicant and is a potent alkylating agent that can form adducts by interacting with cellular macromolecules such as DNA, RNA, and protein, although adduct detection does not necessarily translate to cancer [20,60,11,44,46,47]. ET is a unique chemical because it is both an air toxic and an important regulatory plant hormone. As an air toxic, ET can cause adverse vegetation effects in plants and crop species at concentrations not harmful to humans. As a plant hormone, it is produced naturally at many of the stages of plant growth, and has been reported to regulate both the morphological (e.g., leaf abscission and epinasty (leaf curling)) and physiological effects (e.g., bud formation, inhibition of flowering, photosynthesis, senescence, sprouting of buds, seed germination, and flower formation). In addition, ET can stimulate or inhibit the growth process. Interestingly, fruits (e.g., apples, oranges, and avocados) naturally release ET during maturation, which in turn promotes the ripening of the fruits. Interest in ET research spiked when it was identified to be phytotoxic to greenhouse plants [17]. Later, Abeles et al. [1] reported 10 ppb as the threshold concentration for physiological effects from studies in greenhouse experiments with ET. It was soon realized it was important to confirm if the extremely low threshold concentrations reported by Abeles and co-workers are relevant to field grown plants. Among the issues surrounding the applicability of the results reported by Abeles et al. [1] is the fact that greenhouse plants are normally exposed to very high concentrations of ET in a continuous manner, unlike field grown plants that generally experience lower concentrations of ET, and the exposure pattern is said to be discontinuous [51,52]. Greenhouse plants are also less hardy when compared to the field-grown plants and may experience more adverse effects [53]. Texas has a large petrochemical industry and there is potential for point sources of ET emissions to influence ambient ET levels, and subsequently, there is potential for community exposures from point sources of ET emissions. The TCEQ relies on its large air permitting program and extensive ambient monitoring network to both permit and monitor a suite of chemicals including ET [13]. The air permitting program and the air monitoring program use TCEQ’s science-based chemical specific values (Effect Screening Levels (ESLs) and Air Monitoring Comparison Values (AMCVs)) to permit and evaluate the potential for community exposures from point sources [50]. If the ambient monitoring reports exceedances,

then the exceedances are reviewed for their magnitude and frequency, prior to recommending if additional scrutiny is warranted. Systematic reviews of diverse streams of data (human chamber, animal, and mechanistic) are recognized to be integral to risk assessments by many entities [37,41]. ET is a well-studied chemical and there are well-conducted human chamber, animal toxicological, mechanistic, and vegetation studies. Because of the potential human exposure and adverse vegetation effects, the TCEQ in 2007 decided to conduct hazard assessments of both the health and vegetation end points to determine both health and vegetation based ESLs for ET. Specifically, the TCEQ’s assessments resulted in deriving up to four comparison levels: (1) acute health-based ESL, (2) acute vegetation-based ESL, (3) chronic health-based ESL and (4) chronic vegetation-based ESL. A detailed discussion of the hazard assessments is available in a Development Support Document (DSD) that also underwent an internal review and an external public comment period [49]. Since the DSD was prepared in 2008; a review of the current literature was conducted to determine whether the procedures to develop health and vegetation ESLs need to be updated. The purpose of this paper is to briefly present the procedures and ESLs determined for the chronic health and vegetation hazard assessments of ET and are based on TCEQ guidance developed in 2006 [48]. Due to space restriction, the results of the acute health and vegetation hazard assessments are only discussed briefly. 2. Methods Chronic and acute health-based and vegetation-based ESLs were derived after conducting hazard assessments following the TCEQ guidelines [48]. ESLs are intended to be comparison levels and are used in the TCEQ’s air permitting process to help ensure authorized emissions of air contaminants do not cause or contribute to a condition of air pollution. Specifically, chronic ESLs protect against chronic health effects and vegetation effects and acute ESLs protect against short-term health effects, nuisance odor conditions, and vegetation effects. Acute ESLs also consider that ambient exposure is dependent on meteorology and source emission patterns, and peak exposure could occur several times per day. Additional TCEQ guidance is available that describes how ESLs are used in the air permitting process [50,48]. 2.1. Development of health-based ESLs for ET For the health-based ESLs the TCEQ guidelines employed the four-step risk assessment process formalized by the National Research Council [34,35] and procedures recommended in numerous U.S. EPA risk assessment guidance documents and the scientific literature [55,56,57,39,36]. While, there are a few human exposure studies using ET these were mainly conducted to understand EtO burden in ET-exposed humans and hence did not provide adequate information to identify critical effects and consequently an appropriate point of departure (POD). The majority of ET exposure studies have been conducted using laboratory animals, and these studies were selected as key studies for both the chronic and acute health-based ESL determinations. The following analytical approach was used for hazard identification and dose-response assessments to derive health-based ESLs for ET: (1) conduct comprehensive literature review including physical/chemical properties and select key studies; (2) conduct human relevant mode of action (MOA) analysis; (3) choose the appropriate dose metric; (4) determine the POD for the key study(s); (5) conduct appropriate dosimetric modeling (i.e., duration adjustments) and determine the human equivalent POD (PODHEC); (6) select the critical effect and apply appropriate

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uncertainty factors (UFs) to determine the reference value (ReV); (7) Calculate the appropriate health-based ESL by multiplying the appropriate ReV with 0.3 to account for cumulative effects during the air permit review process. The procedure to use a factor of 0.3 for health-based ESLs is to account for cumulative effects. This is a TCEQ science policy decision and will result in ESLs used for air permit reviews to be 70% lower than ReVs, used as comparison values for air monitoring [48,50]. These procedures were used for both the acute ESLs (using short-term toxicity studies) and chronic ESLs (using long-term toxicity studies). 2.2. Development of vegetation-based ESLs According to the World Health Organization, vegetation critical levels are defined as the concentration of pollutants in the atmosphere above which direct adverse effects on plants may occur according to present knowledge [61]. The TCEQ recommends identification of any available vegetation toxicity data during the data gathering phase for each chemical’s ESL development followed by the identification of adverse threshold effects or LowestObserved-Effect-Level (LOEL) as part of the hazard identification process. The magnitude of effects at various exposure concentrations and durations are then reviewed for the dose-response assessment. [48,50]. The LOELs or critical levels have replaced the previous terminology of adverse threshold vegetation effects as described in the TCEQ [48] guidance. Because adverse vegetation effects have been described for ET, the TCEQ first conducted a comprehensive literature review and identified key studies that represented both the acute and chronic duration exposure scenarios to determine LOELs or critical levels on vegetation. Hazard identification was based on a few criteria and included: (1) plant species native to Texas or known to be grown in the state; and (2) relatively moderate adverse effects such as defoliation, abscission of flower buds, epinasty, failure of seed filling and disproportionate leaf growth, rather than milder effects such as slight dry sepal injury (observed after the sepals are dried). The guidance further recommends mild effects if data exists or if it becomes available to suggest that mild effects progress to moderate effects under conditions of chronic exposure. 3. Results 3.1. Development of chronic health-based and chronic vegetation ESLs 3.1.1. Development of a health-based chronic ESL 3.1.1.1. Key study. Due to the unavailability of chronic inhalation exposure studies in humans, the TCEQ selected a well-conducted rat 2-year inhalation study conducted by the Chemical Industry Institute of Toxicology [25] as the key study to determine the chronic ReV and ESL. In the study, Hamm et al. [25] randomly divided 960 Fischer-344 rats into four groups of one hundred and twenty animals for each sex and exposed them to 0, 300, 1000, or 3000 ppm of ET for 6 h per day (h/d), 5 days per week (d/w) for 106 weeks. There were no reports of any chronic toxicity or oncogenicity at any of the concentrations tested. Comprehensive analysis of various tissues (e.g., kidney and nasal turbinates) indicated no signs of carcinogenic effects. While a variety of proliferative, degenerative, and inflammatory lesions were observed in both the control and treatment groups, the authors reported these types of lesions are typical of the animal and were not treatment related. It should be noted 3000 ppm is the highest concentration that could be safely studied for long-term chronic studies due to fire hazard issues [15]. Based on the absence of ET treatment related toxicity at the highest concentration tested, Hamm et al. [25] reported 3000 ppm as the No-Observed-Adverse-Effect-Level

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(NOAEL) for long-term chronic studies. Because of the absence of a Lowest-Observed-Adverse-Effect-Level (LOAEL) or any other adverse effects, the TCEQ considered 3000 ppm as a free-standing NOAEL. The TCEQ defines a free-standing NOAEL as a NOAEL not associated with any biological or statistical effect identified from a study with several dose levels or with only one dose level [50]. 3.1.1.2. Consideration of reproductive/developmental/effects. The TCEQ selected the study conducted by Aveyard and Collins [10] to determine the potential for ET to adversely affect fertility, pregnancy, maternal, and suckling behavior and growth and development of offspring on exposure to ET. The study was conducted in accordance with the OECD test guideline 421 and is a screening study often used to evaluate developmental and reproductive effects. In the study, groups of rats were exposed head-only to 5000 ppm of ET for 6 h/d for 2 weeks prior to mating and during the mating period. Female rats were also exposed to ET until gestational day 20. ET did not appear to cause any adverse effects and all the female rats became pregnant in the study. Additionally, there were no reported effects on body weight gain, food intake, fertility, fecundity, sex ratio, and pup weight or pup growth. No reproductive/developmental toxicity effects attributed to ET were determined from this study. 3.1.1.3. Mode-of-action (MOA) analysis, genotoxicity, and carcinogenic potential. Even though there is strong evidence ET is relatively non-toxic, there is concern that ET is metabolized to EtO via the cytochrome P-450 pathway and EtO has been classified to be carcinogenic to humans by the inhalation route (Group1) and is a genotoxicant. [28,21,32,22]. EtO is also potent alkylating agent and can form adducts by interacting with cellular macromolecules such as DNA, RNA, and protein (e.g., hemoglobin) [27,60,5]. Several investigators have reported detecting hemoglobin adducts in workers occupationally exposed to ET. For example, Tornqvist et al. [54] reported hemoglobin adducts (i.e., hydroxyethylvaline adducts) in serum of fruit storage workers exposed to approximately 0.3 ppm ET and Granath et al. [23] reported adducts in plastic industry workers exposed to ET. DNA adducts are considered as biomarkers of exposure and detection of adducts is not necessarily indicative of mutagenic potential [60,44,46,47,31]. ET by itself was not reported to be mutagenic in in vitro studies [59] and was not mutagenic or genotoxic in in vivo studies in rodents [58,60]. More recently, sub-chronic exposure of rats to ET up to 10,000 ppm did not result in significant increases in micronucleated erythrocytes in bone marrow [19]. (REACH Dossier summary IUCLID file for ethylene). In a 2-year study, Hamm et al. [25] did not find any signs of carcinogenicity in rats exposed to 3000 ppm, the highest practicable exposure concentration for chronic exposure scenarios due to concerns of fire hazard. Walker et al. [60] reported no significant increases in the Hprt gene of ET exposed animals. IARC has labeled ET as a Group 3 chemical which indicates it is not classified as a human carcinogen [28]. Nonetheless there appears to be continued interest in evaluating the rate of conversion of ET to EtO to better understand the estimated EtO body burden in humans exposed to ET [20,21,32,22]. Filser and his colleagues in 1992 and 2013 measured ET uptake and estimated body burden by exposing human volunteers to concentrations of ET ranging from 5 to 50 ppm up to 2– 4 h respectively [20,22]. In the 1992 study, Filser et al reported that a majority (98%) of ET was exhaled unchanged and only a small amount (2%) of the ET was absorbed and metabolized to EtO in humans. Toxicokinetic information from human chamber, animal exposure, and in vitro studies adds to the weight-of-evidence (WOE) that the metabolic conversion of ET to EtO is a rate-limiting step

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that will result in an insignificant amount of EtO being produced [21,18,44,32,22]. The TCEQ considered all the evidence from the different data streams (human chamber, animal exposure, and mechanistic studies) to conclude ET does not have a mutagenic MOA and there is no concern of potential cancer risk from ethylene exposure. Because the main purpose of this paper is to present procedures for an updated health and vegetation-based ESLs of ET and not to definitively assess/discuss the carcinogenic MOA of ET, only a brief summary is included in this manuscript on the proposed MOA that supports a non-mutagenic and non- carcinogenic MOA for ET. 3.1.1.4. POD and default exposure duration adjustments. The TCEQ selected the free-standing NOAEL (3000 ppm) from the Hamm et al. [25] study as the POD and conducted an adjustment from the discontinuous animal exposure regimen to a continuous exposure regimen with the following equation to determine the adjusted POD (PODADJ).

PODADJ ¼ POD  ðD=24 hÞ  ðF=7 dÞ where: PODADJ = POD from animal studies adjusted to a continuous exposure scenario. POD = POD from animal studies based on discontinuous exposure scenario. D = exposure duration, h per day. F = exposure frequency, days per week. The PODADJ = 3000 ppm  (6/24)  (5/7) = 535.71 ppm. 3.1.1.5. Default dosimetry adjustments. ET is relatively non-toxic even at high concentrations (up to 3000 ppm) with no reported point of entry (POE) respiratory effects or adverse effects in any tissues examined [25]. It meets the criteria for a Category 3 gas (relatively water-insoluble and unreactive) [55]. According to USEPA [55], if the animal blood: gas partition coefficient is greater than the human blood: gas partition coefficient, a default value of 1 is used for the regional gas dose ratio [(Hb/g)A/(Hb/g)H], (RGDR). Csanády et al. [18] reported the tissue:air partition coefficients for rat and humans to be very similar and the blood:gas partition coefficients for rats to be double that of human values. The TCEQ will therefore conservatively consider a default blood: gas partition coefficient of 1 and will calculate the human equivalent POD (PODHEC) using the following equation: PODHEC = PODADJ  (Hb/ g)A/(Hb/g)H. Where, PODADJ = adjusted point of departure. Hb/g = ratio of blood:gas partition coefficient. A = animal. H = human. The PODHEC based on Hamm et al. [25] study is 535.71 ppm  1 = 535.71 ppm. 3.1.1.6. Critical effect and adjustments of the PODHEC. Although no adverse health effects were reported, hepatic damage is conservatively presumed to be the potential critical effect, based on the findings that hepatic damage was the only adverse effect noted in acute rat studies, albeit the rats were pretreated with polychlorinated biphenyl (PCB) mixture that potentially resulted in cytochrome p450 enzyme induction and oxidative stress, prior to ET exposure [16,24]. The TCEQ took this conservative approach because there are subpopulations of humans whose p450 enzymes

may be induced, which potentially may make them more sensitive to ET exposure (e.g., humans receiving therapeutic phenobarbital). The TCEQ applied appropriate uncertainty factors (UFs) to derive a chronic ReV in accordance with the ESL guidelines [48]. An UF of 3 was applied to account for extrapolation from animals to humans (inter-species variability) (UFA) because default dosimetric adjustments from animal-to-human exposure were conducted which account for toxicokinetic differences but not toxicodynamic differences. The total UF of 100 (UA  UFH  UFD) was applied to the PODHEC of 535.71 ppm from the 2-year study conducted by Hamm et al. [25]. The chronic ReV is therefore = PODHEC/ (UFA  UFH  UFD) = 535.71 ppm/100 or 5.3571 ppm. The chronic ReV of 5.3571 ppm was rounded down to two significant figures and converted to ppb concentrations and is 5300 ppb. 3.1.1.7. Health-based chronic ReV and chronic ESL. The chronic ReV of 5300 ppb was then used to calculate the chronic ESL by using the hazard quotient (HQ) of 0.3 to account for cumulative effects during the air permit review. The chronic ESL = chronic ReV  HQ or chronic ESL = 5300 ppb  0.3 = 1600 ppb (Table 1). The TCEQ acknowledges the health-based ReV and ESL are conservative since they are based on a free-standing NOAEL and have a conservative UFD. The chronic ESL is significantly higher than the vegetationbased ESL (Section 3.1.2.2), the critical ESL used during the air permit review process. 3.1.2. Development of a vegetation-based chronic ESL 3.1.2.1. Key study. The Alberta Ethylene Research Project (The Alberta Canada Study) was selected as the key study among several studies because it contained detailed information on several plant species (e.g., barley, field peas, and canola) that were exposed to various concentrations of ET for different durations. The Alberta Canada Study was a multi-stake holder initiative that was jointly sponsored by the Provisional Government and petrochemical industries in Alberta, Canada [3,2]. The project was initiated to determine the concentration threshold at which short-term exposures to ET would cause significant effects on vegetative and reproductive parameters in selected cultivars of agricultural crops of interest to Alberta, Canada. Archambault and Li, the investigators of the Alberta Canada study, conducted extensive preliminary screening experiments with Ethephon ((2-Chloroethyl) phosphonic acid) to determine which plant species in each of 3 plant categories (i.e., cereal,

Table 1 Derivation of the chronic ReV and chronic ESL. Parameter

Summary

Study

Chronic toxicity and oncogenicity bioassay of inhaled ethylene in Fischer-344 rats Fischer-344 rats Medium Inhalation Potential hepatotoxicity 3000 ppm

Study population Study quality Exposure method Critical effects POD (free-standing NOAEL) Exposure duration Extrapolation to continuous exposure (PODADJ) PODHEC Total UFs Interspecies UFA Intraspecies UFH Incomplete database UFD (database quality) Chronic ReV (HQ = 1) Chronic ESL (HQ = 0.3)

6 h/d, 5 days/week, 2 years 535.71 ppm

535.71 ppm (gas with systemic effects based on default RGDR = 1) 100 3 10 3 (medium) 5300 ppb 1600 ppb

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legumes, and oilseeds) and 2 tree species were most sensitive to ET [9,8,6,7]. In their studies with field crops, sensitivity of seed yield to ET was the critical effect used to determine the relative sensitivity of the crop species. However, for the tree species, sensitivity to ET was based on the vegetative characteristics because vegetative characteristics are important parameters that determine the marketability of seedlings. In all the experiments, the order of exposures was randomly selected. In addition, the investigators also considered a background concentration of 10 ppb based on the findings by Reid and Watson [42] who reported that complete removal of ET from atmosphere would lead to detrimental effects on plant growth. This project included two exposure scenarios briefly summarized here. For detailed descriptions of the treatments and exposure regimens please see the Alberta’s Ethylene Crop Research Project Report titled ‘‘Response of Barley, Field Peas, Canola, and Tree Seedlings to Ethylene Exposure’’ [2]. For information on the other supporting studies the TCEQ considered please see the ET DSD that is available at the TCEQ website [49]. 3.1.2.1.1. Exposure scenario 1. In this scenario, Archambault and Li exposed barley plants to 50 ppb of ET for 0, 3, 6, 12, 18, and 24 days, and field peas to 50 ppb of ET for 0, 12, 16, 20, 24, and 28 days in growth chambers according to standard laboratory protocols. In the case of barley, the seed yield decreased by 41% when the plants were exposed to 50 ppb for 3 days and by 89% when the plants were exposed for 24 days. However, there were no effects on the above ground and root biomass, plant height, or tiller number after exposure to 50 ppb for 24 days. Field peas on the other hand were found to be relatively less sensitive to long-term exposures to ET. Exposure of field peas to 50 ppb of ET did not result in significant effects in plant height, number of pods, weight of pods, number of seeds, or seed yield. A LOEL of 50 ppb for long-term exposures was therefore determined from the studies on barley. 3.1.2.1.2. Exposure scenario 2. In the second exposure protocol barley, field peas, and canola were exposed to a range of ET concentrations (10–250 ppb) for 14 days The investigators reported a 63% reduction in seed yield of barley when barley plants were exposed to 30 ppb for 14 days. Based on these results, the TCEQ determined a LOEL of 30 ppb for long-term exposures.

3.1.2.2. Determination of chronic vegetation-based ESL. Vegetationbased ESLs are set at the LOELs for adverse effects and are determined in accordance with ESL guidelines [48,50]. Among all the key studies identified by the TCEQ, the barley exposure studies from the Alberta Ethylene Study (Section 3.1.2.1.2) reported the lowest threshold concentration of 30 ppb. The TCEQ therefore determined the chronic vegetation-based ESL to be 30 ppb (Table 2).

Table 2 Derivation of the chronic vegetation-based ESL. Parameter

Summary

Study

Alberta’s Ethylene/Crop Research Project Report III, 2001 (Section 3.1.2.1.2) Barley Growth chambers 63% reduction in seed yield for barley 30 ppb

Study population Exposure method Critical effects POD (threshold concentration) Exposure duration Chronic vegetationbased ESL

14 days 30 ppb

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4. Discussion ET has been reported to be the largest volume organic chemical produced worldwide. It is largely used as an intermediate in the production of other chemicals and is considered to be the building block of the chemical and petrochemical industries. It is unique because it is an air pollutant that can cause adverse vegetation effects at concentrations not harmful to humans and is also an essential regulatory plant hormone. Even though the majority of ET in ambient air is produced from natural sources there is concern that fugitive emissions from stacks, flares, and leaks from pipe fittings from refineries can contribute to ET levels in ambient air and potentially expose communities living close to refineries (Health [26,38]. The Texas Clean Air Act mandates the State of Texas to prevent air pollution from causing adverse health and vegetation effects [48,50]. The TCEQ relies on its extensive air permitting and air monitoring programs for permitting, surveillance and compliance initiatives to promote safe concentrations of chemicals in ambient air [13]. Further, the TCEQ also conducts chemical specific hazard and/or risk assessments to determine chemical-specific ESLs and AMCVs to use in the TCEQ’s air permit and air monitoring programs, respectively. This assessment includes a discussion of the human relevant MOA and the derivation of chronic health-based and vegetationbased ESLs and/or AMCVs for ET. While there is a general consensus that ET is not genotoxic, mutagenic, or carcinogenic, there is concern that ET exposures can result in significant EtO body burdens because ET is metabolized to EtO and EtO has been classified by IARC to be carcinogenic to humans by the inhalation route (Group 1) and because EtO is a genotoxicant. Additionally, EtO is a potent alkylating agent that can form adducts by interacting with cellular macromolecules such as DNA, RNA, and protein [60,44]. To date only hemoglobin adducts have been reported in workers exposed to ET in occupational settings [54,23]. While DNA adducts are biomarkers of exposure, their use as critical endpoints in risk assessment is controversial because they occur naturally in very large numbers, and are not necessarily indicative of mutagenic potential [60,44,46,47,31]. Additionally, the types of genetic damage responsible for causing mutations and cancer in EtO treated animals are not known. The major DNA adducts caused by EtO, N7-(2-hydroxyethyl) guanine (7HEG) arise from multiple sources in vivo and are not thought to be promutagenic [47]. Toxicokinetic information from human chamber, animal exposure and mechanistic studies indicates a majority of ET to be exhaled unchanged, resulting in only a small amount (2%) for absorption and subsequent metabolism to EtO [20,22,32,45]. All these studies add to WOE that the rate of conversion of ET to EtO is a rate-limiting step and the EtO burden in humans exposed to ET to be insignificant. In addition to evaluating the MOA for ET, the TCEQ also considered animal toxicity studies and vegetation studies because there is concern that ET can cause adverse vegetation effects at concentrations not harmful to humans [49]. For these reasons, the TCEQ conducted updated human health and vegetation-based assessments to determine appropriate screening values. The TCEQ relied on well-conducted chronic and acute animal toxicity studies and extrapolated the results to humans following the appropriate duration and/or dosimetric adjustments. Results from the chronic, sub chronic and acute exposures add to the WOE that ET is relatively non-toxic even at highest practicable concentrations. Specifically, chronic inhalation exposures up to 3000 ppm did not produce any adverse effects including carcinogenicity in rats [25]. The results from the chronic study are supported by the results from a sub-chronic exposure study that reported no adverse effects up to 10,000 ppm [40]; data provided in [49]. Despite the relatively

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high concentrations and long exposure durations, no LOAELs were reported and only free-standing NOAELs were identified as the PODs. To calculate the health-based chronic ReV and ESL, the TCEQ used default UFs that are conservative in nature. A default UFH of 10 was applied to account for potentially sensitive populations. Although, there are no known sensitive subpopulations for ET exposure, it is known there are individual differences in the biotransformation of xenobiotics that can be attributed to induction of cytochrome p450 enzymes or polymorphisms in the glutathione S- transferase gene. These differences can play a critical role in cellular protection against oxidative stress and other xenobiotics [43,12]. An UFA of 3 was applied to account for extrapolation of animal data to humans because there was information on the toxicokinetics but there was uncertainty regarding pharmacodynamics. An UFD of 3 to account for database completeness was also conservatively applied. ET is relatively non-toxic and there are well-conducted sub-chronic and chronic animal toxicological studies. It could be argued that an UFD of 1 would suffice because of the absence of any adverse effects including developmental and reproductive effects in the animal studies. Using an UFD of 1 instead of 3 would result in slightly larger ReV and ESL values that would still be health-protective. In the case of ET, the TCEQ’s health-based ReVs and ESLs for ET are not the ESLs TCEQ would ultimately use in their air monitoring and air permitting programs. The TCEQ’s vegetation-based ESLs are much lower than the healthbased ReVs and ESLs (i.e., are more conservative), and will be the final lowest comparison value used in both the air monitoring and air permitting programs. A chronic health-based ESL of 1600 ppb was determined from the Hamm et al. [25] study. The TCEQ’s vegetation assessments resulted in identifying LOELs much lower than the health-based PODs. Specifically, the chronic vegetation assessment resulted in the identification of a LOEL (30 ppb) that was 53 times lower than the health-based ESL (1600 ppb). Similar to the chronic evaluation, the acute assessments resulted in the derivation of the acute vegetation-based ESL of 1200 ppb that was considerably lower than the health-based ESL of 150,000 ppb (data provided in [49]. Additionally, the MOA information adds to the WOE that the metabolic conversion of ET to EtO is a low concern for cancer. The vegetation-based ESLs were chosen as the final ESLs because these were lower and therefore more conservative than their respective health-based ESLs. The TCEQ also conducted a reality check by comparing the ambient ET levels reported from the TCEQ’s ambient air monitoring program to the derived ET ESLs. The TCEQ’s extensive ambient monitoring data reported relatively low ambient ET levels across the state [33]. For example, the highest annual average concentration of ET for the year 2012 was reported at the Dona Park site in Corpus Christi, Texas and was 5.77 ppbv which is five times below the chronic vegetation-based ESL (30 ppb) and 277 times lower than the health-based ESL (1600 ppb). Similar to the 2012 annual average, the 11-year annual average at the same site was only slightly less than the 2012 average and was 4.19 ppbv. The reported ET levels are from the TCEQ’s community air monitoring network that includes monitoring sites strategically placed to be representative of community exposures and meet the US Environmental Protection Agency (US EPA)’s criteria for placement of air monitors. The relatively low reported levels from the monitoring sites across the state across multiple years provide assurance that the general population including children and other sensitive population are exposed to low concentrations of ET. Therefore, the TCEQ’s acute and chronic ESLs for vegetation will protect the general public from both short-term and long-term adverse health and vegetation effects.

5. Conclusions Due to concerns of potential human and vegetation effects from exposure to ET from point sources, the TCEQ developed chronic and acute health- and vegetation-based ESLs for ET. The chronic evaluation resulted in the derivation of a chronic vegetation-based ESL (30 ppb) that was much lower than the chronic health-based ESL (1600 ppb). Similar to the chronic evaluation, the acute assessment resulted in the derivation of the acute vegetation-based ESL (1200 ppb) that was considerably lower, and therefore, more conservative than the health-based ESL (150,000 ppb) [49]. In summary, TCEQ conducted a WOE analysis of the MOA and concluded that the metabolic conversion of ET to EtO is a low concern for cancer and determined that TCEQ’s acute and chronic ESLs for vegetation will protect the general public from both short-term and long-term adverse health and welfare effects. The general public includes children, the elderly, pregnant women, and people with pre-existing health conditions. Conflict of Interest The authors declare that there are no conflicts of interest. Transparency Document The Transparency document associated with this article can be found in the online version. References [1] F.B. Abeles, P.W. Morgan, M.E. Saltveit, Ethylene in plant biology, second ed., Academic Press, Inc., San Diego, CA, 1992. 414 pp. [2] Alberta Environment. (2003). Assessment report on ethylene for developing ambient air quality objectives. ISBN No. 0-7785-2498-1 (online edition). Available at: . Accessed September 05, 2013. [3] Alberta Research Council. (2001). Response of barley, field pea, canola and tree seedlings to ethylene exposure. Alberta’s Ethylene Crop Research Project, Report III. [4] American Chemistry Council (ACC). 2004. Ethylene Product Stewardship Manual. Available at (accessed November 3, 2014). [5] American Conference of Governmental Industrial Hygienists (ACGIH). (2005). Ethylene: TLVÒ (2005). Chemical Substances seventh ed. Documentation ACGIHÒ Publication #7DOC-264. [6] Archambault D, Li X. (1999a). Report I. Design and Performance of ARC’s ethylene exposure system. Alberta Research Council. [7] Archambault D, Li X. (1999b). Report II. The effects of ethephon on barley, wheat, oats, field peas, and canola: A screening test for the determination of ethylene sensitivity. Alberta Research Council. [8] Archambault D, Li X. (2001). Report III. Response of barley, field pea, canola and tree seedlings to ethylene exposure. Alberta Research Council. [9] D. Archambault, X. Li, K. Foster, et al., A screening test for the determination of ethylene sensitivity, Environ. Monit. Assess. 115 (1–3) (2006) 509–530. [10] L. Aveyard, C.J. Collins, OECD 421 reproduction/developmental toxicity screening study by head-only exposure: experience with ethylene, Teratology 55 (1997) 47. [11] H.M. Bolt, J.G. Filser, Kinetics and disposition in toxicology. Example: carcinogenic risk estimate for ethylene, Arch. Toxicol. 60 (1–3) (1987) 73–76. [12] H.M. Bolt, R. Thier, Relevance of the deletion polymorphisms of the glutathione S-transferases GSTT1 and GSTM1 in pharmacology and toxicology, Curr. Drug Metab. 7 (6) (2006) 613–628. [13] T. Capobianco, S.M. Hildebrand, M. Honeycutt, J.S. Lee, D. McCant, R.L. Grant, Impact of three interactive Texas state regulatory programs to decrease ambient air toxic levels, J. Air Waste Manag. Assoc. 63 (5) (2013) 507–520. [14] F. Cavender, Aliphatic hydrocarbons, in: G.D. Clayton, F.E. Clayton (Eds.), Patty’s Industrial Hygiene and Toxicology, vol. 2B, John Wiley & Sons, New York, 1994, pp. 1221–1266. fourth ed. (Chapter 10). [15] Chemical Industry Institute of Toxicology. (CIIT). (1980). A twenty-four month inhalation toxicology study in Fischer-344 rats exposed to atmospheric ethylene. Final report. [Report prepared for the Chemical Industry Institute of Toxicology by Industrial Biotest Laboratories Inc, CIIT Docket No. 12000]. Chemical Industry Institute of Toxicology Research Triangle Park, NC.

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