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ECSA statement on the OSHA’s methylene chloride Occupational Exposure Limit (1997)

198The US Agency for Occupational Safety and Health (OSHA) revised in 1997 its permissible occupational exposure limit for Methylene Chloride. OSHA moved from the old eight-hour average exposure limit value of 500 ppm to the following standards : 25 ppm as an 8 h time weighted average permissible exposure limit (TWA), and 125 ppm as a 15 minute permissible short term exposure limit.

ECSA understands that these OSHA standards for methylene chloride were based on the mouse carcinogenicity data. The mouse data were extrapolated to human by the use of mathematical models for risk calculation but ignoring the differences existing between men and mice regarding their respective sensitivity to methylene chloride. However, it was not clear how and why OSHA decided on the TWA value of 25 ppm.

Worldwide industry sponsored explanatory experiments have clearly demonstrated that the mechanism operating in mice for the methylene chloride tumour induction cannot take place in human due to fundamental differences between mice and men in the enzymatic systems which metabolise methylene chloride in the target cells. All the industry studies are now published in the peer reviewed literature. The conclusions from the mechanistic studies are that methylene chloride cannot be regarded as a potential human cancer causing agent on the basis of the mouse carcinogenicity data. Thus the mouse data should not be used to establish the occupational exposure limits.

ECSA recalls that the OSHA limit values have legal applications only in the USA. In Europe, each national limit value must b referred to. In most European countries the current eight-hour TWA exposure limits for methylene chloride range between 15 and 100 ppm. These values were generally not based on carcinogenicity data. The lowest limits were generally established on the basis of the methylene chloride transformation into carbon monoxide in the body, a metabolic transformation which occurs similarly in both animals and humans.

ECSA is confident that methylene chloride can be used safely under the current European working exposure limit conditions and following the handling recommendations provided by the producers in their Material Safety Data Sheets.