A LINEAR-MODEL RELATING BREATH CONCENTRATIONS TO ENVIRONMENTAL EXPOSURES - APPLICATION TO A CHAMBER STUDY OF 4 VOLUNTEERS EXPOSED TO VOLATILE ORGANIC-CHEMICALS

A linear model relating levels of volatile organic chemicals (VOCs) in exhaled breath to personal exposures at environmental (parts per bill ion) levels has been developed and evaluated in a chamber study of fou r human volunteers. The purpose of the model is to allow estimation of VOC concentrations in the body from measurements of exposure, or conv ersely to estimate previous exposure from a measurement of exhaled bre ath. The model differs from previous models in considering long-term i nhalation al low or moderate concentrations rather than instantaneous intake (as in drug administration) or intermittent exposure at high co ncentrations (as in occupational situations). The model is based on a mass balance approach using one or more compartments to represent dist ribution of the chemical in the body. The main observable parameters i n the model are the residence times tau(i) in the compartments, their ''capacities '' A(i), and the fraction f of the parent compound that i s exhaled under equilibrium conditions. The basic equations for the on e-, two-, three-, and n-compartment cases are derived. Solutions to th ese equations for the cases of a sudden constant high exposure, a sudd en constant low exposure, and a linearly increasing exposure are provi ded. These solutions can be readily applied to more complex exposure s cenarios.The chamber study suggests residence times on the order of a few minutes in the blood and 1-2 hr in the vessel-rich group of tissue s. It also provides an upper-limit estimate of about 6-8 hr in the ves sel-poor group of tissues. The design of the chamber study did not all ow an estimate of the model parameters for fat; a subsequent chamber s tudy has provided initial estimates of 50-100 hr. Field studies of per sonal exposures and breath concentrations of several hundred persons s uggest values of f ranging from 0.1 for xylenes and ethylbenzene to ab out 0.9 for tetrachloroethylene.