MRO Magazine

Plant Safety: Regulators change exposure limits of HazMat laws


June 14, 2001
By PEM Magazine

For the first time in 14 years, Ontario has significantly updated its occupational exposure limits for hazardous chemical substances in the workplace. Chemicals such as asbestos, benzene, lead and silica are some examples. Over time, the reduced exposure could decrease the incidence of occupational diseases among the province’s workers and reduce compensation costs for employers.

Up until September of 2000, exposure to some 587 chemical agents was governed by regulations under the Occupational Health and Safety Act (OHSA).

The majority of these exposure limits were only regulated in 1986 and were based on the then-current list of recommended limits of the American Conference of Governmental Industrial Hygienists (ACGIH). Since then, these limits have not been significantly updated by the Ministry of Labour, but the ACGIH reviews its occupational exposure limits as technological advances, new medical knowledge and more data become available.

The province has adopted the 1999 ACGIH recommended limits where they are more protective than the current Ontario limits.


Many foreign jurisdictions including several American states, the federal government and most Canadian provinces and territories have adopted ACGIH limits.

The new limits for 202 hazardous chemical substances took effect on Sept. 30, 2000. Limits for two more (benzene and 1,3-butadiene) will be subject to interim reductions while consultations take place prior to a further reduction.

Regulators will reduce the levels of two more occupational exposure limits (glutaraldehyde and petroleum coke [total dust]) within two years; consultations will be held to determine the fate of seven other substances in the futurte. These include: carbon monoxide, formaldehyde, manganese (dust and compounds), ozone, polyvinyl chloride dust (total dust), styrene (monomer), wood dust (not otherwise classified, total dust).

Many Ontario businesses have been in compliance with the updated limits even before September of 2000.

The limits are expressed in something called threshold limit value-time weighted average (TLV-TWA). This is the time-weighted average concentration for a conventional eight-hour workday and a 40-hour work week, to which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect.

For more information on Hazardous materials, visit the following sites:

Simon Fridland, P.Eng., is the president of S.A.F.E. Engineering, a company specializing in Per-Start Health and Safety Reviews and audits for fire code compliance. You can reach him at (416)447-9757.
The threshold limit value-short-term exposure limit (TLV- STEL) is the concentration to which it is believed that the workers can be exposed continuously for a short period of time without suffering from irritation, chronic or irreversible tissue damage or narcosis of sufficient degree to increase the likelihood of accidental injury, impair self-rescue or materially reduce work efficiency. It is not a separate independent exposure limit, rather, it supplements the TLV-TWA limit where there are recognized acute effects from a substance whose toxic effects are primarily of a chronic nature. STELs are recommended only where toxic effects have been reported from high short-term exposure to either humans or animals.

A STEL is defined as a 15-minute TWA exposure which should not be exceeded at any time during a workday even if the eight-hour TWA is within the TLV-TWA. Exposure above the TLV-TWA up to the STEL should be no longer than 15 minutes and should not occur more than four times per day. There should be at least one hour between successive exposures in this range.

There are established testing procedures to identify concentrations of dangerous substances. If they exceed the recommended TWA-TLV or STEL limits, proper engineering controls are required to correct the situation.