Non-Hodgkin lymphoma (NHL) is a group of cancers of the white blood cells that perform immunological functions in the lymphatic system. NHL is the sixth most common cancer in Canada with 10,400 Canadians projected to be diagnosed in 2020 [1].
There are few established risk factors for NHL and its causes are not completely understood. Exposure to ionizing radiation through medical imaging and infection with certain viral pathogens are non-occupational risk factors for NHL, though they may also be encountered in workplaces [2,3]. Many pesticides, both organochlorine (lindane, pentachlorophenol) and organophosphorus (glyphosate, diazinon, malathion), are known or suspected causes [4–7]. Other suspected occupational risk factors for NHL include trichloroethylene, used in dry-cleaning and various industrial manufacturing processes, benzene, chlorinated solvents, and PCBs [8–10].
Known occupational risk factors
Possible occupational risk factors
Increased risks of NHL were found among some groups in agriculture, mining, transportation, and electric utility workers. In Canada, DDT was phased out in the mid 1970s [11]. The use of PCBs has been banned since 1997 and lindane was deregistered for agricultural pest control in 2005 so excess risk observed in the ODSS cohort may reflect past exposures prior to their ban [12,13].
Among farming occupations, an increased risk was observed only among farmers, based on a small number of observed cases. No excess risk was observed for the more than 40 thousand farm and nursery labourers tracked in the ODSS. An excess risk was observed for workers in fertilizer manufacturing, but this was based on only a small number of NHL cases.
Increased risk of NHL was observed for many mining-related occupations however, the risk factors remain unclear.
Workers in transportation-related occupations are regularly exposed to engine exhaust fumes, which contain benzene [9].
Increased risk of NHL has been previously observed among electric utility workers in Ontario [14]. While PCBs have been banned in Canada, they may remain in existing electrical equipment [13].
Increased risks were observed for several additional groups, but were inconsistent, making it difficult to draw conclusions regarding potential NHL-related exposures.
Increased risk of NHL among firefighters may be linked to exposure to benzene, polychlorinated biphenyls (PCBs) and other chemicals during firefighting [3,10].
An increased risk was observed for foremen mechanics and repairmen (except electrical), but no excess risk was observed among mechanics more broadly. Similarly, foremen in metal machining occupations and electrical equipment assembly and repair had increased risk, but no excess was observed for other workers in these occupations. Longer durations of employment and exposure to NHL carcinogens among foremen compared to other workers may explain some of this difference in risk.
An excess risk was observed among radiological technologists and technicians, but only a small number of cases were observed for this group. Exposure to ionizing radiation through medical imagining is a risk factor for NHL, but this has not been established as a risk factor through work-related exposure [15].
Figure 1. Risk of NHL diagnosis among workers employed in each industry group relative to all others, Occupational Disease Surveillance System (ODSS), 1999-2016
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 NHL diagnosis among workers employed in each occupation group relative to all others, Occupational Disease Surveillance System (ODSS), 1999-2016
