It is well established that exposures to toxic chemical air contaminants can be harmful, especially when the levels exceed permissible exposure limits. Laboratory work typically involves potential exposures to a variety of toxic chemical air contaminants under conditions such that the level of exposure could be excessive. Employee chemical exposure monitoring is done for a variety of reasons. The primary reasons are to identify and to quantify specific chemical contaminants present in the work environment. In the chemical laboratory, employee chemical exposure monitoring is simplified by two factors. First, the laboratory worker usually knows which contaminants are present in the laboratory from the nature of the reaction, plus a knowledge of the raw chemicals, end products, and wastes. Therefore, general identification of laboratory contaminants is rarely necessary and, as a rule, only quantification is required. Second, usually only a single contaminant of importance is present in the laboratory atmosphere and the absence of obvious interfering substances often permits great simplification of procedures. Nonetheless, one must be on guard continually to detect the presence of subtle and unsuspected interferences.