4
HUMAN EXPOSURE

To understand the risk associated with use of oxygenated fuels, it is necessary to understand the exposure pathways and characteristics of human contact with oxygenated fuels and the products of their combustion in motor vehicles. Multiple pathways exist for exposure to gasoline and its components, including occupational contact with fuel components through product distribution and use. The general public is exposed during refueling and use of gasoline products. Exposures can also occur from the environmental transport and transformation of gasoline constituents released as evaporative or tailpipe emissions from in-use vehicles.

The major pathway of exposure to oxygenated fuels and other gasoline products is assumed to be inhalation. However, ingestion and uptake through dermal contact might also be important in some cases. For example, occupational exposures of service-station attendants, automobile mechanics, and distribution workers may involve dermal as well as inhalation exposures. Similarly, the general public has the potential for dermal exposure during the handling and use of gasoline products, such as while cleaning and



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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels 4 HUMAN EXPOSURE To understand the risk associated with use of oxygenated fuels, it is necessary to understand the exposure pathways and characteristics of human contact with oxygenated fuels and the products of their combustion in motor vehicles. Multiple pathways exist for exposure to gasoline and its components, including occupational contact with fuel components through product distribution and use. The general public is exposed during refueling and use of gasoline products. Exposures can also occur from the environmental transport and transformation of gasoline constituents released as evaporative or tailpipe emissions from in-use vehicles. The major pathway of exposure to oxygenated fuels and other gasoline products is assumed to be inhalation. However, ingestion and uptake through dermal contact might also be important in some cases. For example, occupational exposures of service-station attendants, automobile mechanics, and distribution workers may involve dermal as well as inhalation exposures. Similarly, the general public has the potential for dermal exposure during the handling and use of gasoline products, such as while cleaning and

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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels degreasing. Ingestion may also be an important pathway of exposure for individuals using well or surface waters contaminated by oxygenated fuels. Well- and surface-water contamination has not been effectively characterized to date. The concentration, duration, and frequency of contact with oxygenated fuels are vital components of exposure; they influence uptake of a substance by the body, resulting doses to target organs, and health effects. Potentials for acute effects and symptoms can be influenced by the duration and frequency of contact with the contaminant. In general, higher concentrations encountered for shorter periods (e.g., during refueling) tend to be responsible for acute effects, and longer-term low-level concentrations are generally associated with chronic health effects. The use of oxygenated fuels is designed to reduce exposures to CO emitted from motor-vehicle tailpipes. Such fuels might increase or decrease the air concentrations of organic toxics associated with evaporative or tailpipe emissions (e.g., benzene and formaldehyde). Therefore, comprehensive risk evaluation would require a comparison of the risks resulting from shifting exposures from environmental contaminants of conventional gasoline to those of oxygenated fuels. DATA REVIEWED BY THE INTERAGENCY REPORT The interagency report contains two documents referred to as HEI and OSTP reports. Each of these reports has sections on exposure assessment but takes a different approach to the issue. The primary focus of both reports is on inhalation exposures; substantially less attention has been devoted to the potential for dermal exposures and direct ingestion of oxygenated-fuel components. As stated above, these routes of exposure might also be important in some cases.

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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels The HEI report reviews occupational- and non-occupational-exposure studies and presents a summary of the range of concentrations and exposures observed in the literature. To date, there have been no studies that are representative of a defined sample of the population. The reported studies indicate concentrations that span 5 orders of magnitude and generally decrease with increasing averaging periods. These results are presented in the HEI report, which concludes that the existing data provide a rough estimate of exposure ranges associated with various activities for the general population, [while] the frequency and distribution of these activities and the amount of exposure by dermal and oral routes are uncertain. Because of these limitations, using these data to calculate a cumulative exposure for use in risk assessment is not appropriate. The OSTP report does not heed this warning and uses the same set of exposure studies evaluated by HEI to estimate cumulative exposures for two hypothetical exposure scenarios that are created to evaluate lifetime cancer risk. The exposure section of the OSTP report makes it clear that the cumulative exposure estimates are based on hypothetical scenarios, with one of the two designed to represent a ''reasonable worst-case" scenario. However, the limitations can easily be missed by a reader in the subsequent risk characterization, in which the worst-case scenario is coupled with potency estimates to calculate lifetime cancer risks. The exposure characterizations in the HEI and OSTP reports are critiqued below. COMMITTEE CRITIQUE OSTP REPORT The OSTP report reviews several studies of MTBE exposure, including ambient studies, an in-vehicle study, and several occupational studies. These studies present results from samples of personal

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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels or microenvironmental exposures that were not selected as representative samples from the cities in which monitoring was conducted. They provide a general view of individual exposures and provide some information on the range of concentrations that might be observed in areas where MTBE is used. Point estimates of concentrations from the microenvironmental measurements are combined with estimates in the OSTP report to provide cumulative estimates of exposure. To estimate potential human exposures, two exposure scenarios of hypothetical exposure sequences are constructed. Scenario I is based on a person who visits a gasoline station 1.5 times/week, commutes 10 h/week, visits an auto-repair shop 4 times/yr, and spends 57 h/week in an office or public building. The home of the person in Scenario I is assumed to have a detached garage and not be near a gasoline station or highway. Scenario II is similar, with the exception that (1) the second hypothetical person is exposed to gasoline refueling concentrations that are 10 times higher, (2) a garage is attached to the home of this individual, and (3) the person is assumed to spend time outdoors near a gasoline station or heavily traveled highway. With these microenvironmental assumptions, Scenario II is assumed in the OSTP report to represent a reasonable worst-case exposure. Both scenarios are arbitrarily defined but may help to bound potential high-end lifetime exposures. Microenvironmental concentrations used in these scenarios are rounded up to the next highest half order of magnitude in order to provide a high estimate of exposure. However, the committee noted that lifetime exposures derived for both exposure scenarios exceed the range of 24-h exposures estimated on the basis of empirical data presented in the HEI report. The OSTP-report scenarios, therefore, are exceedingly conservative. While it is often useful in a data-limited situation like this to generate hypothetical scenarios as distributional exposure data become available, these data should be incorporated into the exposure distributions.

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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels The OSTP report states that there are limitations to estimating exposure, because of limitations in the models relating automobile emissions to ambient-air quality; and, for purposes of risk characterization, Scenario II is assumed for a lifetime. HEI REPORT The HEI report presents a useful summary of the studies reporting on occupational and nonoccupational measurements of personal and microenvironmental exposures to MTBE. These studies report on convenience (nonprobability) samples of exposure and were collected for different averaging times. Therefore, they are not suitable for direct use for construction of probabilistic exposure distributions in a risk assessment. However, the collective data, as presented in Figure 4-1, do provide information on the range of concentrations observed for different time averages. These exposure data, however, can be used to provide a "reality" check for quantitative risk assessments and to construct boundary conditions or exposure ranges for MTBE concentrations that might be encountered in a variety of locations and activities. Using this information, the committee concluded that HEI could have performed a quantitative risk assessment using the exposure data represented by the median (approximately 0.13 ppb) or maximum (approximately 0.01 ppm) daily MTBE exposures as estimates of exposure. Both of the hypothetical lifetime-exposure scenarios derived in the OSTP report lie above the range of 24-h concentrations presented in the HEI report. The lower of the two OSTP lifetime-exposure scenarios used an average daily MTBE exposure of 0.018 ppm during the oxygenated-fuel season. This hypothetical scenario was generated by accumulating time-weighted concentrations assumed to exist in multiple microenvironments (e.g., refueling, commuting, at work, and at home). Scenario II, constructed as a hypothetical worst case was even further outside the range of

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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels Figure 4.1. Time-weighted average exposures of individuals in the general public to MTBE. Only studies that provided ranges of exposure levels are included. The solid lines across the bars indicate median values. The numbers at the tops of the bars correspond to the numbers in parentheses in the "Sampling Site" column of Table III.3 (of HEI 1996)), where the same data are expressed. An asterisk (*) denotes the minimal detectable concentration. Source: HEI, 1996.

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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels observed 24-h concentrations presented in the HEI report. In a data-limited situation like this, bounding scenarios are appropriate for conducting risk analyses. The scenarios, however, should be realistic, and the limitations to generalizability need to be stressed wherever the scenarios are used. In addition to MTBE exposures, the HEI report provides a qualitative discussion of potential changes in concentrations of the atmospheric transformation products of MTBE, CO, air toxics, and O3 precursors. The general direction of change for these gases is discussed, but the report argues that data are not sufficient to present a quantitative evaluation of the shift in concentrations. The report concludes that the impact of oxygenates on ambient CO concentrations has not been fully evaluated. A comparative risk assessment requires data on the influence of MTBE on human exposure to these other constituents, and this committee believes that a quantitative framework should and could be established for this evaluation. Establishing a framework for consideration of all exposures will help identify important data gaps and provide for a direct comparison of the risk and benefits between conventional and oxygenated fuels. CONCLUSIONS The committee finds important deficiencies in the OSTP exposure analysis, which calculates cumulative-exposure estimates for two hypothetical scenarios. The lifetime exposures calculated for these scenarios are 10 times higher than the maximum daily exposures based on empirical data that are summarized in the HEI report. In addition to a reasonable worst-case scenario, OSTP should generate a more-realistic baseline exposure. Other emission products should also be considered. The HEI report provides a comprehensive and useful summary

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Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels of MTBE-exposure studies. Concentration ranges encountered in occupational and nonoccupational situations are adequately represented. HEI expressed the opinion that the data are too limited to calculate cumulative exposures for risk assessment. However, the committee's opinion is that the data are sufficient to bound a quantitative risk analysis and to develop a framework for conducting a comparative risk assessment of conventional and oxygenated fuels. A quantitative framework should also be established for evaluating changes in exposure from related emission products. RESEARCH NEEDS Routine ambient monitoring of MTBE and one of its major products of photo-oxidation, tertiary-butyl formate, should be initiated in communities where MTBE is used. (TBF has the potential to accumulate and persist in the atmosphere and its toxicity is unknown.) Representative personal-exposure monitoring of MTBE in an exposed population is needed in order to describe the distribution of exposures and for input into risk analyses. Such exposure monitoring should include the characterization of each individual's time-activity patterns, especially in the microenvironments where important exposures are likely to occur (NRC, 1991b). The most important factors affecting personal exposure should be determined in such a study. The relationship between fixed-site community monitoring and personal exposures to MTBE and other pollutants of concern resulting from the use of oxygenated fuels, should be evaluated in order to determine the information value of using community monitoring for assessing human exposure.