Biomonitoring

Many studies have shown that the urine concentration of 2,5-hexanedione can be correlated to n-hexane exposure (Perbellini et al., 1981a; Iwata et al., 1983; Mutti et al., 1984; Saito et al., 1991; Cardona et al., 1993; Periago et al., 1993).

However, there are some problems with the methods used for 2,5-hexanedione determination (Saito et al., 1991; Soriano et al., 1996) and there have been various attempts to refine them (Perbellini et al., 1990). The most common method (Perbellini et al., 1981) is time consuming and there may be loss of 2,5-hexanedione from the urine sample during the concentration procedure. Another simpler method (Fedtke and Bolt, 1987) is less sensitive and in addition there may be conversion of other metabolites to 2,5-hexanedione. It has been shown that strong acid hydrolysis, converts 4,5-dihydroxy-2-hexanone to 2,5-hexanedione, whereas weak acid hydrolysis converts it to 2,5-dimethylfuran (Fedtke and Bolt, 1987). Simultaneous analysis of several different metabolites in urine, including 4,5-dihydroxy-2-hexanone, may be a better indicator of exposure than 2,5-hexanedione alone (Soriano et al., 1996). However, research on this subject using human subjects is lacking. In addition, a biological limit has been established for 2,5-hexanedione only.

The biological exposure limit (BEI) set by the ACGIH for n-hexane is 2,5-hexanedione 5 mg/g of creatinine in urine (ACGIH, 2000). However, since this involves acid hydrolysis this concentration of 2,5-hexanedione must in fact be a total of 2,5-hexanedione itself and 4,5-dihydroxy-2-hexanone which has been converted into 2,5-hexanedione (Fedtke and Bolt, 1987).

A post-shift urine concentration of 2,5-hexanedione greater than 7.5 mg/l is associated with significant electromyographic abnormalities. However, this same study found three workers with significant electromyographic changes who had 2,5-hexanedione urine concentrations of between 3 and 4.5 mg/l (Governa et al., 1987). In another study of exposed workers without signs or symptoms of polyneuropathy but with high urinary concentrations of 2,5-hexanedione (mean 11 mg/l; range 5.3-24.2 mg/l) there were no significant changes in sensory nerve conduction velocities (SNCV) or motor nerve conduction velocities (MNCV). However, the mean amplitude of the sensory nerve action potentials (SNAP) was significantly decreased below normal (Pastore et al., 1994).

In animal studies it was noted that there was a sudden decrease in the urine concentration of metabolites when dosing was interrupted at weekends (Soriano et al., 1996) and this has implications for the timing of samples taken for biological monitoring. Similarly, a study of workers exposed to n-hexane demonstrated that the urinary concentration of 2,5-hexanedione was higher at the end of the working week (Cardona et al., 1993).

Urine concentrations of 2,5-hexanedione are unchanged in samples stored at -20 °C or 4 °C for 30 days, but deteriorate rapidly when stored at 25 °C (Saito et al., 1991).

Screening for evidence of polyneuropathy has also been recommended for exposed workers (Chang et al., 1993). Nerve conduction studies have been shown to be reliable and sensitive; evoked potential studies may also be useful. The MNCVs and SNCVs were normal in asymptomatic workers exposed to n-hexane with urine 2,5-hexanedione concentrations greater than the recommended BEI. However, there were significant decreases in the mean amplitude of SNAPs and this has been suggested as a criterion for early diagnosis of neuropathy in patients without clinical signs (Pastore et al., 1994).

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