degree of acute toxicity should be treated as a "particularly hazardous substance" in the context of a specific planned use. This determination not only will involve consideration of the total amount of the substance to be used, but also will require a review of the physical properties of the substance (e.g., is it volatile? does it tend to form dusts?), its potential routes of exposure (e.g., is it readily absorbed through the skin?), and the circumstances of its use in the proposed experiment (e.g., will the substance be heated? is there likelihood that aerosols may be generated?). Depending on the worker's level of experience and the degree of potential hazard, this determination may require consultation with supervisors and safety professionals.

Because the greatest risk of exposure to many laboratory chemicals is by inhalation, it is essential that laboratory workers understand the use of exposure limits that have been established by agencies such as ACGIH

and OSHA. The threshold limit value (TLV), assigned by the ACGIH, defines the concentration of a chemical in air to which nearly all individuals can be exposed without adverse effects. The TLV-TWA (threshold limit value-time weighted average) refers to the concentration safe for exposure during an entire 8-h workday, while the TLV-STEL (threshold limit value-short term exposure limit) is a higher concentration to which workers may be exposed safely for a 15-min period. OSHA defines the permissible exposure limit (PEL) analogously to the ACGIH values, with corresponding TWA and STEL limits. TLV and PEL values allow the laboratory worker to quickly determine the relative inhalation hazards of chemicals. In general, substances with PELs or TLVs of less than 50 ppm should be handled in a fume hood. Comparison of these values to the odor threshold for a given substance will often indicate whether the odor of the chemical provides sufficient warning of possible hazard. However, individual differences in ability to detect some odors as well as anosmia, or "olfactory fatigue," can limit the usefulness of odors as warning signs of overexposure. LCSSs contain information on odor threshold ranges and whether a substance is known to cause olfactory fatigue. Finally, a variety of devices are available for measuring the concentration of chemicals in laboratory air, so that the degree of hazard associated with the use of a chemical can be assessed directly. The industrial hygiene offices of many institutions can assist labora-