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INHALATION EXPOSURE TO METHYL TERT-BUTYL ETHER (MTBE) AND DIBROMOCHLOROMETHANE (DBCM) USING CONTINUOUS BREATH ANALYSIS
Citation:
GORDON, S. M. INHALATION EXPOSURE TO METHYL TERT-BUTYL ETHER (MTBE) AND DIBROMOCHLOROMETHANE (DBCM) USING CONTINUOUS BREATH ANALYSIS. U.S. Environmental Protection Agency, Washington, DC, 2005.
Impact/Purpose:
The primary study objectives are:
1.To quantify personal exposures and indoor air concentrations for PM/gases for potentially sensitive individuals (cross sectional, inter- and intrapersonal).
2.To describe (magnitude and variability) the relationships between personal exposure, and indoor, outdoor and ambient air concentrations for PM/gases for different sensitive cohorts. These cohorts represent subjects of opportunity and relationships established will not be used to extrapolate to the general population.
3.To examine the inter- and intrapersonal variability in the relationship between personal exposures, and indoor, outdoor, and ambient air concentrations for PM/gases for sensitive individuals.
4.To identify and model the factors that contribute to the inter- and intrapersonal variability in the relationships between personal exposures and indoor, outdoor, and ambient air concentrations for PM/gases.
5.To determine the contribution of ambient concentrations to indoor air/personal exposures for PM/gases.
6.To examine the effects of air shed (location, season), population demographics, and residential setting (apartment vs stand-alone homes) on the relationship between personal exposure and indoor, outdoor, and ambient air concentrations for PM/gases.
Description:
The oxygenate methyl tert-butyl ether (MTBE) has been added to gasoline to help meet national ambient air quality standards in those parts of the U.S. that are non-compliant for carbon monoxide. Although MTBE has provided important health benefits in terms of reduced hazardous air pollutants, the increasing occurrence and detection of MTBE in drinking water sources in California, New Jersey, and elsewhere has raised concerns about potential exposures from water usage and resulting health effects. In addition to MTBE, disinfection byproducts can be present in the water people use for showering, bathing, or drinking, as a result of the reaction of disinfection agents with organic material already present in water. Chlorine reacts with humic acids to form the trihalomethanes, which are the most common and abundant byproducts in chlorinated water. Besides chloroform, which has been widely studied, the byproduct dibromochloromethane (DBCM) occurs as a result of the chlorination process in those areas that naturally have bromide in their ground water. Relatively little information on exposure to this chemical is available.
This study was designed to determine the uptake by humans of MTBE and DBCM as a result of controlled, short-term inhalation exposures. Our approach made use of continuous real-time breath analysis to generate exhaled-breath profiles, and evaluate MTBE and DBCM kinetics in the body. Seven subjects were exposed continuously via face mask to 2,217 µg/m3 (542 ppbv) MTBE-d12 and 728 µg/m3 (85.6 ppbv) DBCM, except for several brief (~2-min) intervals during which breath measurements were taken. Total exposure time was ~30 min, followed by exposure to clean air for a further 30-60 min. Exhaled breath was sampled and analyzed with the real-time breath technology; blood samples were simultaneously collected from the subjects (3-4 samples during exposure; 2-5 samples post-exposure). The real-time technology was specially modified with a biofeedback exposure control system to allow us to make uptake measurements during the exposure period; breath measurements were taken continuously throughout the post-exposure period.
The exposures resulted in an increase in the measured breath concentration of MTBE-d12 from levels that were less than 10 - 20 µg/m3 (2 - 5 ppbv), the method detection limit, to 200 - 450 µg/m3 (50 - 110 ppbv) following exposure. MTBE-d12 blood concentrations increased from the limit of detection, 0.30 µg/L, to ~0.9 - 2.5 µg/L at the end of the ~30-min exposure period.
The time-course measurements of both exhaled breath and venous blood are well-described by the linear compartmental uptake and elimination models, the interpretation of which provides important information on the residence times of the compound in the body, the relative capacity of each compartment, and the fraction of the chemical exhaled unchanged at equilibrium. The breath uptake data were consistent with a one-compartment model. The mean value for the one-compartment uptake residence times T1uptake was 5.7 ± 2.4 (SD) min (range 3.3 - 9.8 min). In contrast, the breath decay phase data gave satisfactory two-compartment fits. The mean value for the first compartment decay residence times T1decay was 3.8 ± 1.9 (SD) min (range 2.4 - 7.8 min); for the second compartment, the mean decay residence time T2decay was 61 ± 11 (SD) min (range 46 - 73 min). The blood uptake data were also consistent with a one-compartment model and were convergent in almost all cases. The average blood uptake residence time was essentially the same as that for the breath. The quality of the blood decay data were such that we were only able to extract meaningful information from 2 or 3 data sets.
The mean MTBE-d12 total absorbed ("internal") dose was 149 ± 34 µg for the average 30-min exposure and a mean total ("applied") dose of 209 µg. The mean fraction of MTBE-d12 absorbed, or relative uptake, was 0.73 ± 0.04. The mean value for ƒ, the fraction of the MTBE-d12 exposure concentration exhaled unchanged was 0.29 ± 0.04. This value is in good agreement with the value recently reported by Lee et al. Using linear regression analysis, the mean blood/breath ratio for MTBE-d12 was found to be 6.7 ± 3.4. This value is significantly lower than values obtained in previous studies. The reason for this discrepancy is not clear.
By and large, background levels for DBCM in the exhaled breath were below the limit of detection, and the signal measured for this compound at m/z 129, the most abundant ion in the glow discharge mass spectrum, was exceptionally "noisy". The average signals during the uptake phase provided initial (pre-exposure) breath concentration values that ranged from 70 to 160 µg/m3 and rose to between 130 and 250 µg/m3 after 30 minutes. The high initial breath concentrations suggest that the measured signal at m/z 129 was probably elevated due to an unknown contaminant with fragment ions at the same mass. For tert-butyl alcohol (TBA), all of the blood measurements were below the detection limit.