Benzene

Benzene is one of the many cyclic hydrocarbons found in petroleum and coal. Exposure can occur from the refining and transport of benzene-containing liquids, emissions from burning coal and oil, gasoline service stations, and motor vehicle exhaust.

Benzene is a proven bone marrow toxin, reproductive hazard, and carcinogen. The allowable level of exposure to benzene has fallen markedly over the past four decades; in most provinces, the current 8-hour occupational exposure limit (OEL) is 0.5 ppm.

A medical surveillance program for benzene-exposed workers is required by regulation in some provinces, typically consisting of a health questionnaire, complete blood count, and physical examination. These regulations were established in the 1980’s when the OEL for benzene was about 20 times higher than today.  

Medical surveillance has limited preventive value in today’s workplace:

* a baseline health questionnaire and blood cell count is of value for comparison to future test results
and to assess individual health risk

* changes in blood cell counts are typically not seen with exposure below 5-10 ppm, and in most settings, repeat (periodic) blood cell counts is not warranted. Some occupations such as gasoline tanker drivers and railcar loaders can have intermittent exposure in this range, and require more rigorous exposure assessment.

* physical examination findings are non-specific and insensitive for benzene-related toxicity

* for acute, unanticipated, high-level exposure to benzene (e.g. spills, tank ruptures), monitoring of blood cell counts for three months may be worthwhile

* urine testing for S-phenylmercapturic acid (S-PMA) can estimate a worker’s 8-hour average exposure to benzene. This test can be useful following an unanticipated high-level exposure to benzene and at ‘end-of-shift’ to assess the effectiveness of workers’ respiratory protective equipment. For more information see Toxicological Monitoring.