Carboxyhemoglobin levels in methylene chloride-exposed employees

The Occupational Safety and Health Administration (OSHA) has proposed a reduction in the permissible exposure limit for methylene chloride from 500 parts per million (ppm) to 25 ppm (8-hour time-weighted average [TWA]). Part of the rationale for lowering the standard is a concern over potentially adverse cardiac effects secondary to elevated carboxyhemoglobin (COHb) levels as a by-product of methylene chloride metabolism. Employees exposed to methylene chloride as part of a triacetate fiber production process had average values of COHb ranging between 1.77% and 4.00% in the nonsmoking group and between 4.95% and 6.35% in a smoking group, with individually measured methylene chloride exposures averaging up to 99 ppm (8-hour TWA). A dose-response effect was seen only in the nonsmoking group. Additional daily cumulative exposure to methylene chloride did not produce increased levels of COHb. Data from this study support the fact that the COHb levels resulting from exposure to methylene chloride at or below the current American College of Government Industrial Hygienists limit of 50 ppm (8-hour TWA) are of a sufficiently low level that they are unlikely to produce adverse cardiac effects in humans.