Exposure Limits

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WHMIS: What You Need to Know

7. Exposure Limits

Exposure limits are established concentrations which, if not exceeded, will not generally cause adverse effects to the worker exposed. However, because of wide variation in individual susceptibility, a small percentage of workers may experience discomfort from some substances at concentrations at or below the established limit; a smaller percentage may be affected more seriously by aggravation of a pre-existing condition.

Exposure levels are intended for use as guidelines or recommendations in the control of potential health hazards and are not fine lines between safe and unsafe exposures, nor are they a relative index of toxicity.

The limits are based on industrial experience, and human and animal experimental evidence.

Exposure levels for many hazardous chemicals are included in the Regulation respecting the Control of Exposure to Biological or Chemical Agents - made under the Occupational Health & Safety Act of Ontario (O.Reg. 654/86). These are expressed as follows:-

TWAEV - Time-Weighted Average Exposure Value: The average airborne concentration of a biological or chemical agent to which a worker may be exposed in a work day or a work week.

STEV - Short Term Exposure Value: The maximum airborne concentration of a chemical or biological agent to which a worker may be exposed in any 15 minute period, provided the TWAEV is not exceeded.

CEV - Ceiling Exposure Value: The maximum airborne concentration of a biological or chemical agent to which a worker may be exposed at any time.

SKIN - This notation indicates that direct or airborne contact with the product may result in significant absorption of the product through the skin mucous membranes or eyes. Inclusion of this notation is intended to suggest that preventative action be taken against absorption of the agent through these routes of entry.

Except in certain circumstances these levels must be achieved without workers being required to wear and use personal protective equipment.

Note: The exposure limits listed on the MSDS may differ from the Ontario legal exposure limit.

Other notable occupational exposure limits that one may encounter on a Material Data Sheet (MSDS) are:-

  1. Permissible Exposure Levels (PELs) of the USA Occupational Safety and Health Administration (OSHA).
  2. Threshold Limit Values (TLVs) of the American Conference of Governmental Industrial Hygienists (ACGIH).

See "Glossary of Terms" for definitions.