Fact Sheet: Diesel Exhaust Exposure Limits

Occupational exposure limits are a measurement of concentration of a substance in air which is supposed to represent the maximum amount to which a person at work should be exposed over an eight hour shift. The exposure limits are supposed to be safe levels. If exposures are kept lower than the limit, then no-one should get sick.

Most toxic substances don''t have exposure limits which means that there are no restrictions on how much you can be exposed to. This includes many of the toxic substances which are produced by burning diesel fuels. Only about 650 chemicals of more than 100,000 in use in industry today have limits.

Most exposure limits are based on 8-hour averages. This makes it legal to expose workers above the limit for short periods of time, if the average exposure over eight hours is lower than the limit.

In Canada, the provinces base their regulations on the Threshold Limit Values (TLVs) set by the American Conference of Governmental Industrial Hygienists (ACGIH). The ACGIH changes these limits from time to time, so depending on when a province adopted the limits, the specific limits might be higher or lower.

In most of Canada, diesel particulate matter -- the cancer-causing soot particles in diesel emissions -- is not specifically regulated. Some provinces use an old limit recommended by the federal government''s mine research centre, CANMET, of 1.5 mg/m³ of respirable combustible dust. (Diesel particles make up about 2/3 of respirable combustible dust in mines which use diesel equipment.) CANMET has now recommended reducing the respirable combustible dust limit by half, to 0.75 mg/m³ , but no province has adopted this limit yet.

The ACGIH recently proposed an even lower limit of 0.15 mg/m³ for diesel particulate matter which may be adopted in 1997 or 1998. This is equivalent to a respirable combustible dust measurement of 0.225 mg/m³ . Newfoundland, Manitoba, the North West Territories and the federal government could be using this lower limit for diesel particulate as early as 1997 or 1998

The substances which make up diesel exhaust are found in differing amounts in the exhaust, depending on the fuel, engine type, maintenance procedures, load, and so on. Measuring one substance does not tell you that the others are under the exposure limit. Mines should be measuring and reporting exposures to ALL the components of diesel exhaust listed below.

The provincial 8-hour limits for some components of diesel exhaust are shown below:

ppm - parts per million , that is, number of parts of a particular gas in a million parts of air

mg/m³ - milligrams (thousandths of a gram) of a substance in a cubic metre of air

NR - Not Regulated

a - The Nova Scotia occupational health regulation adopted the ACGIH limits "as amended from time to time". Because the cabinet does not have the legal authority to amend the limits this way, Nova Scotia is currently stuck with enforcing these 1976 TLVs, although at least some inspectors write orders under the general duty clause which cite current, lower ACGIH limits.

b - These limits are slated to be reduced in the near future.

c - These jurisdictions use the ACGIH TLVs as they are annually updated, and may have a new limit for diesel particulate matter of 0.15 mg/m³ as soon as 1997.

d - These jurisdictions do not have a respirable dust limit, but only a total dust limit of 10 mg/m³ .

As you can see, not all provinces have adopted the lowest, most current TLVs for several toxic substances which make up diesel emissions. Our employers should be using the following:

Substance Lowest Recommended Limits

Carbon Monoxide (CO)

25 ppm

Sulphur Dioxide (SO₂ )

2 ppm

Diesel Particulate

0.225

mg/m³ measured as respirable combustible dust or
0.15 mg/m³ measured as elemental carbon

Total Respirable Dust

3 mg/m³