Biological monitoring

The biological exposure indices (BEI) set by the ACGIH for methylene chloride are a methylene chloride blood concentration of 0.5 mg/l during shift and 0.2 mg/l methylene chloride in end of shift urine (ACGIH, 2000).

Methylene chloride can be detected in exhaled breath for several hours after exposure. The methylene chloride concentration in exhaled air generally reflects the amount inhaled at ambient concentrations of up to 500 ppm. Breathing ambient air concentrations of 200 ppm will result in about 80 ppm in expired air. Methylene chloride can also be measured in blood. Unfortunately, measurement of methylene chloride in expired air or blood is seldom practical in clinical situations and blood concentrations are difficult to interpret. Workers exposed to currently permissible levels usually have blood concentrations of only 1-2 mg/l. Few studies have been published with methylene chloride blood concentrations and these are limited to severe or fatal poisonings (ATSDR, 1993a,b).

Carboxyhaemoglobin can be detected in the blood of non-smokers about 30 minutes after exposure to methylene chloride. An ambient air concentration of 200 ppm methylene chloride corresponds to a carboxyhaemoglobin concentration of 4% to 9%. Exposure to methylene chloride concentrations of 500 ppm for several hours results in a carboxyhaemoglobin concentration as high as 15%. Ratney et al. (1974) reported that an 8 hour exposure to 180 ppm methylene chloride would correspond to a carboxyhaemoglobin concentration of 6-12% in non-smokers, and that at a constant exposure to 180 ppm the carboxyhaemoglobin concentration increases at approximately 0.5% per hour. However, other factors may influence the carboxyhaemoglobin concentration, including exercise, smoking and exogenous exposure to carbon monoxide. Therefore, the carboxyhaemoglobin concentration may not correspond directly to inhaled concentrations of methylene chloride.

Urinary concentrations of formic acid (an intermediate product in the metabolism of methylene chloride) have also been used to monitor exposed workers.

It is estimated that an 8 hour exposure to approximately 150 ppm methylene chloride is equivalent to an 8 hour exposure to 35 ppm carbon monoxide in so far as both exposures will result in a carboxyhaemoglobin concentration of approximately 5%. This study was carried out in sedentary volunteers (DiVincenzo and Kaplan, 1981a). Smokers or physically active workers are likely to have higher carboxyhaemoglobin concentrations than sedentary workers exposed to the same ambient concentration of methylene chloride (DiVincenzo and Kaplan, 1981b). Carboxyhaemoglobin concentrations may increase both during and after exposure (Astrand et al., 1975).

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