Cancer risk among firefighters

Firefighters in our study experienced statistically significant elevated risk of many cancers, including a priori cancer sites identified by the IARC, which include prostate and testicular cancer and NHL.

We observed a higher risk of prostate cancer among firefighters than reported by the IARC, similar to existing literature.1 7 8 10 11 Although it is unclear what exposures are linked to prostate cancer in firefighters, few factors have been postulated. Night and rotating shiftwork are common in protective services occupations and have been suspected to be involved in cancer development through disruption of the circadian rhythm.1 Shiftwork has been classified by the IARC as probably carcinogenic to humans, based on limited evidence in humans.1 The night shiftwork hypothesis was initially suggested for breast cancer and was extended to prostate cancer.12 Firefighters are exposed to toxins through fire and smoke, which also include suspected prostate carcinogens such as arsenic and cadmium compounds.13 It is also possible that firefighters have increased screening recommendations due to the nature of their job14; however, it is unclear what their actual screening behaviours are like compared with other workers. A more detailed description of prostate cancer in the overall ODSS cohort, including firefighters, has been previously published.15

We also observed a higher risk of testicular cancer (156%) among firefighters compared with all other workers and roughly twice the risk compared with the police. There is limited evidence supporting firefighting as an occupational exposure for risk of testicular cancer.1 4 Although the literature is sparse on occupational causes of testicular cancer, few complex chemical compounds that may be present in fire smoke have been linked to testicular cancer, with limited evidence.4 13 16

For NHL, we observed a higher risk than reported by the IARC when comparing firefighters with all other workers in our cohort and a non-statistically significant elevated risk when compared with only the police. A recent meta-analysis observed an elevated risk (not statistically significant) of incidence of NHL and a statistically significant elevated risk of mortality from NHL.4 Firefighters are exposed to benzene through fire1 4; however, the risk of NHL related to benzene exposure is less clear.17 18 They may also be exposed to polychlorinated biphenyl (PCB) through fire smoke, which has been linked to NHL, with limited evidence.13

We observed an increased risk of leukaemia, specifically CLL, among firefighters compared with all other workers. A recent meta-analysis did not see an association for leukaemia4 and previous studies reported varying findings.19 20 Tsai et al 19 observed a statistically significant elevated risk of leukaemia overall and for AML, with a non-significant elevated risk of CLL.19 Demers et al 20 observed a higher elevated risk of leukaemia in workers with 30 years of employment.20 Although the IARC has reported sufficient evidence for benzene exposure causing AML, there is also some evidence linking benzene exposure to CLL13 and firefighters can be exposed to benzene through fire and DEE.

Our finding for bladder cancer among firefighters is similar to the 12% increased risk reported in a recent meta-analysis,4 although non-significant in our study. PAH and heavy metals (arsenic) are known bladder carcinogens,13 with limited evidence for other exposures of DEE and soot.13 Exposure to these agents could be through exposure to fire and smoke. Firefighters can also be exposed to DEE when working on or near fire trucks that are frequently running, whether at the fire halls or when responding to emergencies.

Our increased risk of kidney cancer is within the incidence range reported by other studies (OR/SIR 1.27–1.59)19 21 22; this relationship has been shown to increase with duration in firefighting.23 Arsenic and cadmium compounds and perfluorooctanoic acid (PFOA) are potential exposures through fire smoke and have been linked to kidney cancer, with limited evidence.13 When compared with the police the relationship remained elevated, but no longer significant. This may be due to exposures to fire smoke that are not similar for the police.

We observed elevated mesothelioma among firefighters, which became significant when restricting our comparison group to the police. Many workers in the ODSS may be exposed to asbestos (eg, construction workers, maintenance staff), whereas the police may be less likely to be exposed to asbestos, which explains the strengthening relationship when compared with the police. Our finding was similar to other studies (SIR=2.29–2.59), although these findings are based on firefighters compared with the general population.5 21 24 Firefighters are exposed to asbestos fibres from burning older buildings24 and may be more exposed during the overhaul stage when they are involved in tasks related to ripping up walls, floors and ceilings.

A more than twofold statistically significant excess risk of melanoma was observed among firefighters. Our finding was higher than another study of Canadian firefighters where an HR of 1.58 (95% CI 1.11 to 2.25) was reported.7 Many studies have reported an increase in melanoma.19 24 25 A previous study found the risk of melanoma largely reduced when comparing firefighters and police, similar to our study.26 Firefighters may be exposed to solar radiation while working outside during calls, training or performing errands. They also have the potential to be exposed to PCB through inhalation or dermal absorption when exposed to fire and smoke.13 27

It is speculated that dermal exposure to soot resulting in exposure to other agents (eg, PCB, PAH) and smoke particulates may contribute to an elevated risk of melanoma.4 19 Light-coloured skin is a known genetic risk factor for melanoma.28 Although we do not have data on ethnicity or skin type in our cohort and there is a lack of reporting on ethnicity in Canadian firefighters, it is possible that firefighters have a higher proportion of Caucasians than other occupations. The relationship was attenuated when compared with the police and this may be because the police also spend time outdoors or because of ethnic similarities between the two groups.

We observed a statistically significant increased risk of colon cancer among firefighters, which supports the findings from a previous meta-analysis,4 although we only observed a non-statistically significant increased risk of rectal cancer. Findings may be related to shiftwork, ionising radiation and asbestos exposure based on limited evidence.12 13 29 30

We reported a higher pancreatic cancer rate than a Nordic study (SIR=1.17, 95% CI 0.94 to 1.45)24 and a recent meta-analysis on mortality (SMR=1.13, 95% CI 0.99 to 1.29).4 Another Canadian study did not report an association7; however, the association weakens when compared with the police.

Cancer risk among the police

The police had similar findings to firefighters for prostate and colon cancer and melanoma, among other associations. Findings in our study show increased cancer risk among the police, although results vary in the existing literature.

Evidence on the risk of prostate cancer among the police has been inconsistent,31–34 but a recent Canadian study reported a 28% excess risk.7 Similar to firefighters, findings may be linked to night and rotating shiftwork and increased screening behaviours, although it is unclear if police officers seek out prostate cancer screening more than other workers. Further details on prostate cancer among the police and the overall ODSS cohort have been previously published.15

A non-statistically significant increase in testicular cancer disappeared and became statistically protective when compared with firefighters. Previous studies showed mixed results.35 36 No clear relationship has been established regarding radar exposure during policing, which may explain this.37 38 Two studies examined if use of hand-held traffic radar units near the testicles would be a possible factor in the risk of testicular cancer.37 38

The increase in bladder cancer incidence among the police is similar (OR=1.40, 95% CI 1.00 to 1.90) to results found in Switzerland36 and is supported by a US study reporting a fourfold increase in mortality with 40 years or more of police experience.32 However, Wirth et al 6 reported a decreased incidence (SIR=0.64, 95% CI 0.39 o 0.99)6 and several mortality studies did not observe associations.34 39 It is unclear what exposures may be related to risk of bladder cancer among the police, but they may be exposed to diesel exhaust if working on roadways or near idling vehicles.

A non-statistically significant excess risk of NHL was observed among the police when compared with all other workers, and previous studies have primarily reported no association.6 31 36 However, Forastiere et al 34 did report an increased risk specifically in motorcycle police (OR 5.14).34 We observed a non-statistically significant elevated risk of myeloma among the police, similar to previous Canadian findings7; however, this was no longer observed when compared with firefighters. In a previous literature, the risk of myeloma among the police was not explicitly discussed and at times grouped into a broader cancer group of lymphomas. No association was observed for the different subtypes of leukaemia.

A more than twofold statistically significant excess risk of melanoma was observed among the police compared with all other workers. Our study finding was higher than two previous Ontario studies that reported increased risk of melanoma (HR=1.28, 95% CI 1.32 to 2.16; SIR=1.45, 95% CI 1.10 to 1.90).7 35 Mortality from melanoma was also found to be higher in a previous study.36 Findings may be related to solar radiation specifically for the police that work outdoors (eg, traffic police) or related to unknown carcinogen exposure. When compared with firefighters, the relationship attenuates and this may be a result of similar outdoor exposures or ethnic similarities between the two groups. Based on the Canadian census data from 2016, few Canadian police (∼8.4%) identify as visible minorities40 and this may bias our findings.

Although most research has not observed increase in thyroid cancer among the police,32 34–36 39 one study did report an increased risk (relative risk (RR)=2.21, 95% CI 1.23 to 3.66),41 higher than our study. Few known or suspected causes of thyroid cancer exist; however, exposure to X and gamma radiation for specialised tasks may exist among the police (eg, use of security/forensic devices).

We observed a statistically significant increased risk of colon cancer among the police and no association for rectal cancer. Bouchardy et al 36 observed a non-statistically significant elevated incidence of colon cancer among the police (OR=1.10, 95% CI 0.90 to 1.50),36 and another study did not observe any association (SIR=0.80, 95% CI 0.53 to 1.14).31 Findings for colon and rectum cancer could be related to shiftwork, among other possible exposures such as ionising radiation or asbestos.13

Our findings on female breast cancer add to the sparse literature on female police. One study reported no association for cancer mortality among female police32; however, other mortality and incidence studies reported an increase in male breast cancer among the police.7 34 Risk of breast cancer may be due to shiftwork exposure.13

Decreased risk among firefighters and police

We observed a statistically significant decreased risk of lung and laryngeal cancer among firefighters and police; these findings attenuated when comparing firefighters with the police and vice versa. Generally, research has shown no association of lung cancer among firefighters4 and police.6 However, in a large US cohort of firefighters an increase in lung cancer was observed,19 and in a Nordic cohort of firefighters an increase in lung adenocarcinoma was observed.24 It is possible that firefighters and police smoke less than other workers in the cohort. Firefighters in the USA have been shown to have lower smoking rates (<20%) than military personnel and other adult men.42 We could not account for tobacco smoking in our study, but it is unlikely that this has impacted our findings as we would have expected to see an elevated risk of lung and laryngeal cancers, which are associated with smoking.

Limitations and strengths

There are several limitations to this study. Occupation is only based on job title entered at the time of the worker’s compensation claim, which can lead to non-differential misclassification of the exposure group, biasing estimates towards the null. With job information at a single point in time, we cannot assess changes in jobs or duration of employment. Further, there are no exposure measurements or exposure surrogates used in this analysis, which limits our ability to evaluate exposure levels to specific cancer sites. Most WSIB claims are compensated for musculoskeletal-related injuries, which may introduce bias if exposures differ between compensated and non-compensated workers.

Lack of information on personal characteristics (eg, smoking habits, ethnicity, screening behaviours) limits our ability to control for possible confounders. Firefighters and police may have reduced smoking and alcohol consumption habits than other workers due to the physically demanding nature of their work and assessments prior to working as firefighters or police.43 A recent report on trends in cigarette smoking and smokeless tobacco among firefighters and law enforcement personnel showed a declining trend in cigarette smoking, but observed use of smokeless tobacco to be higher than in other workers.44 Body mass index has been thought to be a poor measure of obesity in individuals with high muscle mass and may not be appropriate for this study. Race and ethnicity may play a role in findings specific to melanoma as many firefighters and police are likely to be Caucasian.

Firefighters and police may have better access to screening recommendations compared with other workers. In Canada, firefighters are provided with health recommendations regarding cancer screening, but it is up to each worker to request screening from the primary physician. The healthy worker effect could still influence findings in this study as firefighters and police have stringent physical fitness requirements on entry into the profession and may be healthier than other occupations in the ODSS cohort, leading to healthy worker effect.

Comparison of firefighters with the police and vice versa may help to reduce potential confounding by these factors and the healthy worker effect. Our study makes multiple comparisons, which can lead to chance findings; however, many of our study findings are supported by previous literature and the interpretation is based on existing evidence.

This study also has several important strengths. Through linking administrative health data, this cohort provides a large sample of workers to identify many cancer sites, even relatively rare, based on cancer registry information, rather than mortality, which previous studies have focused on. The side-by-side analysis of firefighters and police, two groups that have similar entry requirements related to physical fitness and tend to have a similar socioeconomic status, provides a lens through which we can consider the unique occupational exposures of each group. Contribution to the body of literature on cancer incidence among the police, with the inclusion of female police, is also a strength of this study as there are limited studies on cancer among the police. The evidence from this study can provide insight on WSIB policies regarding prescribed cancers in Ontario for firefighters and be useful for workers, employers and policy makers. The ODSS cohort will be updated on an ongoing basis, providing up-to-date administrative health data, including cancer diagnoses. Our findings are comparable with previous studies and provide evidence for shared cancer risk among firefighters and police.

Our study had the ability to follow a substantial number of workers to examine cancer incidence. Our findings align with a priori cancer sites of interest among firefighters and for colon cancer risk among the police, among other cancers identified. Based on our findings, further investigation on exposures involved in these occupations is needed. Many of our findings attenuated when comparing firefighters with the police and vice versa, which may imply that there are common exposures involved, compared with other workers in the cohort. For example, shiftwork should be further investigated given the elevated risk of breast, prostate and colon cancer in these occupations. This can lead to the protection of workers and improvement of prevention strategies.