Over the past 60 years, many organizations in numerous countries have proposed occupational exposure limits (OELs) for airborne contaminants (1). The limits or guidelines that have been the most widely accepted both in the United States and in most other countries are those issued annually by the American Conference of Governmental Industrial Hygienists (ACGIH) and are termed Threshold Limit Values Ò (TLVs) (1-10).The usefulness of establishing OELs for potentially harmful agents in the working environment has been demonstrated repeatedly since their inception (3, 5, 6). It has been claimed that whenever these limits have been implemented in a particular industry, no worker has been shown to have sustained serious adverse effects on his health as a result of exposure to these concentrations of an industrial chemical (7). Although this statement is arguable with respect to the acceptability of OELs for those chemicals established before 1980, and later found to be carcinogenic, there is little doubt that millions of persons have avoided serious effects of workplace exposure due to their existence.