Handbook of Solvents - George Wypych - ChemTech - Ventech!

15.2.1 Use of breath monitoring 1091
effect would be negligible the next morning. Breath samples collected the next morning
would reflect exposure on preceding days and the measurements would be independent of
level of physical exertion. Finally, individual differences in body build and metabolism
would have the greatest effect on breath levels, with the largest variations expected for metabolized
solvents.
Experimental data were used in a model, 59 which divided the body into four tissue
compartments: vessel rich group, muscle group, fat group and liver. Metabolism was assumed
to take place in the liver as a combination of a linear metabolic component and a Michaelis-Menten
component. Metabolic parameters and partition coefficients determined for
rats were scaled for body weight and were used in fitting results for humans. The model fit
very well the data reported from other controlled human exposure studies. 51,52,54
The BEI committee of the ACGIH recommends 2 three indices of PERC exposure:
1) PERC in end-exhaled air collected prior to the shift after at least 2 consecutive
workdays.
2) PERC in blood collected prior to the shift after at least 2 consecutive workdays.
3) Trichloroacetic acid (TCA) in urine voided at the end of the workweek. This index
is recommended only as a screening test because of the variability associated with urinary
excretion of TCA and, as such, no creatinine correction is necessary.
15.2.1.7.2 Styrene
The highest exposures to styrene take place in plants manufacturing glass-reinforced plastics,
particularly during lamination, where there is considerable evaporation of styrene.
Most of the inhaled styrene is absorbed and retained in the body. The first step in the major
metabolic pathway is the formation of styrene-7,8-oxide (phenyloxirane), a reaction catalyzed
by Cytochrome P-450 in the liver. The oxide is then hydrated to styrene glycol
(1-phenyl-1,2-ethanediol) by microsomal epoxide hydratase. The styrene glycol is conjugated
with beta-glucuronic acid, or is oxidized to mandelic acid and further to
phenylglyoxylic acid. In humans, less than 5% of absorbed styrene is eliminated via the
lung, whereas 90% is eliminated in the urine as mandelic and phenylglyoxylic acids.
A number of chamber and field studies have been reported, where blood, exhaled air
and urine have been studied as possible biological indicators. Although conditions and
study design differed among studies, certain consistent patterns emerged, which formed the
basis for subsequent recommendations.
In an early series of chamber exposures, 60 nine healthy males were exposed to styrene
concentrations of 50 to 370 ppm for periods ranging from 1 to 7 hours. Alveolar air samples
were collected during and for 8 hours following exposure. The authors showed that the
amounts of styrene present in the breath were directly related to the level and the duration of
exposure.
In another study, 61 fourteen healthy males were exposed to 50 and 150 ppm of styrene
in inspired air during rest and light (50 W) physical exercise. The duration of exposure for
each styrene concentration was 30 minutes, with total pulmonary ventilation, cardiac output,
and styrene concentration in alveolar air, arterial blood and venous blood measured
during and after exposure. During exposure at rest to either 50 or 150 ppm, the alveolar air
concentrations were about 20% of the inspired air concentrations. When alveolar ventilation
almost tripled with exercise at 50 W, the alveolar air concentrations increased slightly,
whereas arterial blood concentrations almost tripled. Alveolar air concentrations reached a
plateau within a minute or so, that persisted throughout the 30-minute period. Arterial blood
concentrations, however, rose rapidly during each such period and even more with increasing
exercise intensity (50, 100 and 150 W). Venous blood concentrations also rose sharply
during the 30-minute exposure periods without ever reaching a plateau. No linear relationship
was found between exposure duration and arterial and venous blood concentrations,