Leukemia is a cancer that originates in the blood stem cells. There are many different types of leukemia, grouped by where and how they develop. In adults, the most common leukemias are acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) [1]. Occupational exposure to benzene is an established risk factor for AML and acute non-lymphocytic leukemia (ANLL), and there is some evidence it may increase the risk of other leukemias. Formaldehyde is an established risk factor for myeloid leukemia, and a suspected risk factor for other leukemias [2,3]. The International Agency for Research on Cancer (IARC) concluded that employment in rubber manufacturing was a leukemia risk factor, but suggested that the risk may be associated with exposure to solvents, particularly benzene [4].
Known occupational risk factors [2,5]
Possible occupational risk factors [2,5,6]
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.
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.
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].
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].
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.
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
