Mesothelioma is a cancer of the tissue lining the lungs and other organs caused by exposure to inhaled asbestos fibres. It is characterized by a long latency period: the onset of disease can be as much as 50 years after exposure to asbestos fibres. There is no cure and survival is extremely poor.
Risk factors
Asbestos is the commercial name for a group of naturally occurring mineral fibres. It was historically used in many commercial applications including insulation, roofing, cement pipe and sheets, gaskets, and friction materials. Use of asbestos was banned in Canada in 2018, although it is still found in many older products and buildings. The Burden of Occupational Cancer in Canada Study estimates that approximately 80% of mesothelioma cases are attributable to occupational asbestos exposure.
The greatest risks of mesothelioma were observed among workers employed in construction, asbestos mining, and some metal industries and occupations. Mesothelioma typically develops a minimum of 20 to 40 years after asbestos exposure [2], so diagnoses among ODSS cohort workers may be related to occupational asbestos exposure many years prior. However, although asbestos is now banned in Canada [3], workers may still be exposed particularly through work in construction and renovation of older asbestos-containing buildings.
Within the construction industry, insulators and pipefitters and plumbers have the highest risks of exposure to asbestos and mesothelioma as a result of the widespread use of asbestos in insulation and pipes in buildings built prior to 1980 [4, 5]. Many other trades are also at increased risks of mesothelioma through handling asbestos-containing materials or working in an environment with those material during renovation, maintenance or demolition of older buildings.
Unlike Quebec where asbestos mining was a large industry, Ontario’s asbestos mining was much smaller and only operated until 1978 [6, 7]. Nevertheless, ODSS results demonstrate that asbestos miners in Ontario are at a dramatically increased risk of mesothelioma. This an expected finding given the extraordinarily high levels of exposure likely to have occurred among asbestos miners.
Asbestos was commonly used as an insulating material in some hot metalworking processes [8]. Primary metal industry workers may operate furnaces and other heating apparatuses insulated with asbestos.
Brake and clutch components have contained asbestos for many decades [9], and workers involved in vehicle repair and maintenance were observed to have increased risk of mesothelioma in the ODSS. Many vehicles have asbestos-containing materials, and exposure from repair of these vehicles may continue to put these workers at risk for decades.
Figure 1. Risk of mesothelioma diagnosis among workers employed in each industry group relative to all others, Occupational Disease Surveillance System (ODSS), 1983-2016
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 mesothelioma diagnosis among workers employed in each occupation group relative to all others, Occupational Disease Surveillance System (ODSS), 1983-2016
Figure 1. Crude incidence rate of mesothelioma among males in the ODSS, 1985-2014 (n=2.18 million)
The crude incidence rate is the number of cases of disease diagnosed per person per year of follow-up in the ODSS cohort. The age-standardized incidence rate is the rate we would observe if we removed the influence of aging in the cohort over time. The general population of males in Ontario in 1995 was used as the standard. All rates are expressed per 100,000 persons per year.
Figure 2. Age-standardized incidence rate of mesothelioma among males in the ODSS, 1995-2014 (n=2.18 million)
