These results are based on all leukemia types combined but occupational risk factors vary by leukemia type. In the ODSS, the greatest incidence of leukemia was observed for workers with exposure to benzene, including mining and transportation industry employees and firefighters.

Mining and quarrying

Increased leukemia risk among miners has been previously observed [7], and several studies have explored radon exposure as a possible risk factor, with inconsistent findings [8,9]. Uranium miners, who might be expected to have the greatest exposure to radon, showed an increased risk of leukemia compared to other workers in the ODSS, but the relative risk was lower than for miners in other mine types, so it is unlikely that this exposure is the driving risk factor. Uranium miners are also exposed to gamma radiation, which may be contributing to the observed risk. A previous study of Ontario uranium miners found no increased leukemia risk [10]. Thus, benzene may be a more significant risk factor for mining industry workers who can be exposed to benzene used in degreasing and use of gasoline-powered equipment, and combustion of petrochemical products.

• •  Mining and quarrying including oil and gas field, occupations: 1.36 times the risk
    • Cutting, handling and loading occupations: 1.59 times the risk
    • Other rock and soil-drilling occupations: 1.13 times the risk
  • Mines (including milling), quarries and oil wells industries: 1.33 times the risk
    • Non-metal mines: 1.78 times the risk
    • Metal mines: 1.42 times the risk
      • Miscellaneous metal mines: 1.53 times the risk
      • Gold quartz mines: 1.35 times the risk
      • Uranium mines: 1.27 times the risk
    • Quarries and sand pits: 1.14 times the risk

Transportation

Increased risks were observed among transportation industry workers, likely due to engine exhaust, which accounts for the largest source of benzene exposure in the general environment [11].

• • Transportation industry: 1.10 times the risk
    • Taxi cab operations: 1.40 times the risk
    • Moving and storage, used goods, uncrated: 1.32 times the risk
    • Other transportation: 1.21 times the risk
    • Urban transit systems: 1.17 times the risk
    • Other truck transport: 1.15 times the risk
    • Bus transport, interurban and rural: 1.15 times the risk

Firefighters

Firefighters are exposed to benzene and formaldehyde which are common in fire smoke, known leukemia risk factors [12,13], as well as many other chemicals. Increased leukemia risk has been previously reported among firefighters [14]. In Ontario, primary acute myeloid leukemia, chronic lymphocytic leukemia, and primary acute lymphocytic leukemia are classified as ‘prescribed diseases’, meaning that under certain conditions, these cancers are assumed to be work-related and are eligible for compensation [15].

• • Firefighting occupations: 1.41 times the risk

Other groups

No excess risk of leukemia was observed among workers employed in rubber and plastics manufacturing industries, and too few cases were observed among workers in petroleum refineries to examine the association in that group. Excess risk observed among metal machining occupations and motor vehicle manufacturers may be linked to metalworking fluid exposure [16,17]. The risk among health workers could be linked to x-ray exposure, but too few cases were observed among radiological technicians to further explore this association. Acute myeloid leukemia has been previously reported among brick and stone masons [18]. Risk factors among other groups in the ODSS with excess risk remain unclear.

• • Other occupations in medicine and health, not elsewhere classified: 2.56 times the risk
  • Physiotherapists, occupational and other therapists: 2.52 times the risk
  • Forming occupations: clay, glass and stone: 2.50 times the risk
  • Brick and stone masons and tile setters: 1.45 times the risk
  • Machinist and machine tool setting-up occupations: 1.34 times the risk
  • Motor vehicle manufacturer industries: 1.32 times the risk
  • Industrial farm machinery mechanics and repairers: 1.20 times the risk

Figure 1. Risk of leukemia diagnosis among workers employed in each industry group relative to all others, Occupational Disease Surveillance System (ODSS), 2006-2016

The hazard ratio is an estimate of the average time to diagnosis among workers in each industry/occupation group divided by that in all others during the study period. Hazard ratios above 1.00 indicate a greater risk of disease in a given group compared to all others. Estimates are adjusted for birth year and sex. The width of the 95% Confidence Interval (CI) is based on the number of cases in each group (more cases narrows the interval).

Figure 2. Risk of leukemia diagnosis among workers employed in each occupation group relative to all others, Occupational Disease Surveillance System (ODSS), 2006-2016 

The hazard ratio is an estimate of the average time to diagnosis among workers in each industry/occupation group divided by that in all others during the study period. Hazard ratios above 1.00 indicate a greater risk of disease in a given group compared to all others. Estimates are adjusted for birth year and sex. The width of the 95% Confidence Interval (CI) is based on the number of cases in each group (more cases narrows the interval).

Table 1. Surveillance of Leukemia: Number of cases, workers employed, and hazard ratios in each industry (SIC)

Table 2. Surveillance of Leukemia: Number of cases, workers employed, and hazard ratios in each occupation (CCDO) group