Biomonitoring

There are five methods to monitor exposure to toluene (Angerer and Krämer, 1997). These are measurement of:

• Toluene in expired air (Nomiyama and Nomiyama, 1974; Brugnone et al., 1976; Övrum et al., 1978; Apostoli et al., 1982; Monster et al., 1993)

• Toluene in blood (Brugnone et al., 1976; Brugnone et al., 1986; Campbell et al., 1987; Gill et al., 1988; Pekari et al., 1989; Kawai et al., 1994)

• Toluene in urine (Ghittori et al., 1987; Monster et al., 1993)

• Hippuric acid in urine (Caperos and Fernandez, 1977; Kira, 1977; Apostoli et al., 1982; De Rosa et al., 1987)

• Ortho-cresol (but not para- or meta-cresol) in urine (Truchon et al., 1996; Angerer and Krämer, 1997). The current ACGIH recommendations for biological exposure indices are listed in Table 21.1.

Table 21.1 Biological exposure indices for toluene (ACGIH, 2000)

Determinant

Sampling time

Biological exposure index (BEI)

o-cresol in urine

end of shift

0.5 mg/l

hippuric acid in urine

end of shift

1.6 g/g creatinine

toluene in blood

prior to last shift of workweek

0.05 g/l

Diet can affect the urinary concentration of hippuric acid. Benzoic acid and benzoic acid precursors (which are metabolised to hippuric acid) are present in food and drink (e.g., fish and egg products and soft drinks), particularly as preservatives. Benzoic acid is also formed from the amino acid phenylalanine. In the case of occupational exposure it may be impossible to eliminate the effect of diet when estimating the individual uptake of toluene using the urinary hippuric acid concentration (Campbell et al., 1987; Löf, et al., 1993; Angerer and Krämer, 1997). This is particularly a problem at low exposure concentrations (Campbell et al., 1987). In addition, the concentration of hippuric acid in the urine may be increased in individuals taking drugs which induce liver enzymes (e.g., phenobarbital) (Cohr and Stokholm, 1979).

Ortho-cresol is not a normal constituent of urine and normal concentrations are less than 0.9 |imol/l (Truchon et al., 1996). However, the concentration can vary and smokers have a 3-4-fold increase of ortho-cresol in urine (Nise, 1992).

Para-cresol is a normal constituent of urine, probably due to metabolism of the amino acid tyrosine (Angerer and Krämer, 1997). In 85 volunteers the urinary p-cresol concentration varied from 3.3-63 mg/g creatinine (Meulenbelt et al., 1990). Another study found that non-exposed individuals excreted up to 29 mg/l of p-cresol in urine (Woiwode et al., 1979). Consequently, p-cresol is not a suitable index of toluene exposure (Meulenbelt et al., 1990).

Although m-cresol is not a normal constituent of urine, it is usually only found following exposure to a high concentration of toluene (Meulenbelt et al., 1990). The concentration of m-cresol is usually much less than that of o- or p-cresol and it cannot be used as a biological indicator of low-level exposure. However, it was detected in 10 workers exposed to an average concentration of 280 ppm (range 100-600 ppm) of toluene, but did not correlate with toluene exposure (Woiwode et al., 1979).

There was also no correlation between air concentrations of toluene and o- or p-cresol in the urine (Woiwode et al., 1979). Urinary concentrations of hippuric acid and o-cresol have been correlated to air concentrations of toluene, but the correlation was weaker for o-cresol (De Rosa et al., 1987). Another study in volunteers exposed to a range of toluene concentrations found that urinary o-cresol concentrations were highly correlated with toluene exposure (Truchon et al., 1996).

Differences in methods of extraction and analysis (Angerer and Krämer, 1997) and co-exposure to other solvents may be a factor in the variation in results reported in different studies.

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