Cancer odds among Ohio firefighters: data from the Ohio Cancer Incidence Surveillance System (OCISS) 1996–2019

Introduction

Firefighting was recently determined by the International Agency for Research on Cancer (IARC) to be ‘carcinogenic to humans’ or a Group 1 carcinogen with sufficient evidence for mesothelioma and bladder cancer in humans.1 The agency also determined that there was limited evidence that occupation is a risk factor for several other cancers. Firefighters are exposed to various risk factors while performing their firefighting duties. Although, structural firefighters wear self-contained breathing apparatus (SCBA) for respiratory protection while in fires, they still experience the uptake of many smoke constituents into their bodies.2 Structural firefighters may remove their SCBAs during firefighting tasks with less apparent smoke or involving less intense smoke exposure such as during overhaul.2 3 Dermal absorption through various pathways is also a route of exposure and could substantially contribute to the overall exposure to some structural fire smoke constituents.3 Various Group 1 carcinogens are present within structural fire smoke including benzene, benzo(a)pyrene, which is a polycyclic aromatic hydrocarbon (PAH) and formaldehyde.2 3 Other cancer risk factors associated with structural firefighting include shift work, stress and behavioural factors such as dietary differences.1 4 5 Specifically, shift work is classified as being ‘probably carcinogenic to humans (Group 2A carcinogen) by IARC.6

Evidence of the carcinogenicity of structural firefighting has been presented in past studies. Compared with the general population, excess cancer incidence and mortality of 9% and 14%, respectively, were observed in the analysis of mortality and cancer registry-based data set of firefighters employed by the fire departments of three large cities in the USA, including Chicago, San Francisco and Philadelphia.7 Also, the risk of incidence of all cancers combined was increased among firefighters by 6% in a similar study that included data from the five Nordic countries.8

The conduct of previous studies of cancer risk among firefighters has been limited to a few metropolitan areas (eg, Boston, Chicago, Philadelphia and San Francisco) and states (eg, Florida and Massachusetts) in the USA and a few other countries and regions (eg, Australia, Canada and the Nordics). We conducted a study of cancer odds among firefighters in the state of Ohio in the USA. As of May 2022, there were 18 790 career firefighters employed in the state.9 This study represents the first documentation of cancer risk among firefighters in Ohio. Therefore, the study expands the geographical representation of available data. This is important as firefighting practice, exposures and associated risks of specific cancers may vary geographically.10 11

The analyses of cancer risk among Ohio firefighters in this population-based study relies on data from the state’s cancer registry, the Ohio Cancer Incidence Surveillance System (OCISS). The inclusion of information about occupation in the OCISS database facilitated the conduct of the study. Cancer registry data presents the benefit of a large sample size with representation from both rural and urban areas. Persons who did not report firefighting as an occupation in the OCISS database served as the comparison population for our primary analyses of cancer risks. Additionally, we compared cancer risks among firefighters to those people in the database who reported working as police to correct for the healthy worker effect. Similar to firefighters, police are occupationally exposed to several carcinogens including traffic-related air pollution and shift work.12 However, police are not actively engaged with fire and are less likely to experience the intense smoke emission that is associated with firefighting. While there is less documentation of cancer risk among police, increased risks of incidence of or mortality due to colon, kidney, oesophagus, digestive system, prostate, testis, skin and male breast cancers and/or non-Hodgkin’s lymphoma were observed among police compared with other individuals in some studies.5 12–15

Methods

Results

Among the 3397 firefighters, the majority of them were men (86.78%), white (92.73%), non-Hispanic (93.38%), married (67.68%) and never used tobacco (19.81%) (table 1).

The mean age of first cancer diagnosis among firefighters was 65.91 with 28.82% of them being diagnosed between 2010 and 2014. Similar distributions were observed among police, with the exception that the age at cancer diagnosis was lower at 62.94 years and 18.5% of them being current tobacco users. The distribution of gender among the general population was different with the majority being women (51.01%).

The number of different cancer types among firefighters, police and the general population aged 18–85 was calculated (online supplemental table 1). This descriptive analysis was used to collapse some of the cancer types (ie, oral cavity and pharynx) as well as determine which cancer types would be used in the logistic regression analysis, which was limited to those cancer types that had an n great than, or equal to 5.

Compared with the general population, firefighters had increased odds of oesophageal cancer (OR=1.83, 95% CI: 1.43 to 2.33) within OCISS (table 2).

They were also at increased odds of cancers of the skin (OR=1.23, 95% CI: 1.06 to 1.42), brain (OR=1.37, 95% CI: 1.08 to 1.73), cranial nerves and other nervous system (OR=1.38, 95% CI: 1.00 to 1.91) and thyroid (OR=1.52, 95% CI: 1.18 to 1.96). In addition, cancer of the soft tissue including the heart had an increased OR among firefighters compared with the general population within the cancer registry, although it did not reach statistical significance (1.40 (95% CI: 0.99 to 1.99)). Finally, compared with the general population, firefighters had decreased odds of cancer of the pancreas (OR=0.78, 95% CI: 0.61 to 0.99), larynx (OR=0.69, 95% CI: 0.48 to 0.98), lung and bronchus (OR=0.81, 95% CI: 0.73 to 0.89) and bladder (OR=0.85, 95% CI: 0.72 to 0.99).

Compared with police, firefighters had an increased odds of cancer of the brain (OR=1.40, 95% CI: 0.99 to 1.99) and thyroid (OR=1.53, 95% CI: 1.05 to 2.23) in OCISS. Cancer of the soft tissue including the heart, eye and orbit and mesothelioma were increased among firefighters compared with police but did not reach statistical significance. Finally, they had a decreased odds of cancers of the pancreas (OR=0.61, 95% CI: 0.45 to 0.84), lung and bronchus (OR=0.84, 95% CI: 0.73 to 0.97) and bladder (OR=0.79, 95% CI: 0.64 to 0.98). They also had a decreased odds of cancer of the stomach although it did not reach statistical significance (0.69 (95% CI: 0.46 to 1.03)).

Odds of cancer was examined across occupations stratified by gender (tables 3 and 4).

Among the 2948 male firefighters, compared with the male general population in OCISS, firefighters had an increased odds of cancer of the soft tissue including heart (OR=1.57, 95% CI: 1.11 to 2.24), skin (OR=1.25, 95% CI: 1.08 to 1.46), prostate (OR=1.24, 95% CI: 1.14 to 1.35), brain (OR=1.35, 95% CI: 1.05 to 1.74), cranial nerves (OR=1.60, 95% CI: 1.14 to 2.25) and thyroid (OR=1.41, 95% CI: 1.03 to 1.94). Compared with the male general population, male firefighters had a decreased odds of cancer of the pancreas (OR=0.73, 95% CI: 0.56 to 0.96), lung and bronchus (OR=0.80, 95% CI: 0.72 to 0.89) and bladder (OR=0.84, 95% CI: 0.71 to 0.99). Cancer of the oral cavity and pharynx and the larynx were also decreased in male firefighters compared with the general population but they did not reach statistical significance (0.80 (95% CI: 0.62 to 1.04) and 0.72 (95% CI: 0.50 to 1.02), respectively). When compared with male police, male firefighters similarly had an increased OR for prostate cancer (OR=1.28, 95% CI: 1.13 to 1.44) within OCISS, but there was no increase in ORs for skin, brain, cranial nerves or thyroid. Cancer of the soft tissue including the heart was increased among male firefighters compared with male police, however, it did not reach statistical significance. Cancer of the eye and orbit also had an increased OR but did not reach statistical significance. Similar to the comparison to the male general population, male firefighters had decreased odds of the pancreas (OR=0.57, 95% CI: 0.41 to 0.79), lung and bronchus (OR=0.86, 95% CI: 0.74 to 1.00) and bladder (OR=0.80, 95% CI: 0.65 to 1.00) compared with male police. Stomach cancer had a decreased odds, however, it did not reach statistical significance.

The numbers of cancer among the 449 female firefighters were small which led to unstable ORs and 95% CIs with wide ranges (table 4).

Compared with the female general population, an increased odds of cancer was only observed in the oral cavity and pharynx (OR=2.51, 95% CI: 1.38 to 4.57) in OCISS. Thyroid cancer was increased among female firefighters compared with the general population but it did not reach statistical significance (p=0.062). There were no cancers that female firefighters had a decreased odds of compared with the female general population. Compared with female police, female firefighters had increased odds of cancer of the corpus and uterus (OR=2.13, 95% CI: 1.03 to 4.37) and thyroid (OR=2.11, 95% CI: 1.04 to 4.29).

Discussion

Compared with the general population, firefighters had an increased odds of oesophageal, skin (excluding basal and squamous), brain, cranial nerves and other nervous system and thyroid cancer in the Ohio cancer registry used for this study. Male firefighters also had increased odds of cancer of the soft tissue including the heart, skin, prostate, brain, thyroid, cranial nerves and other nervous system, while female firefighters had an increased odds of cancer of the oral cavity and pharynx compared with the general population. Other similar event-only registry-based cancer studies among firefighters have observed increased odds of some of the same types of cancer, although only one of these studies was stratified by gender.17 19 21 22 In addition, meta-analyses have also observed increased risks of bladder, kidney, prostate, thyroid and melanoma among firefighters.25 26 Similarly, the IARC Working Group that recently assessed the carcinogenicity of firefighting reported sufficient evidence in its meta-analysis for increased risk of bladder cancer (16%, 95% CI: 8-26%) and limited evidence for prostate, testicular and melanoma.1 Finally, cohort studies conducted in North America, Australia or the Nordic countries detected increased risk of some of the same cancers, most notably skin and prostate.7 8 13 27–29 Consistent with previous studies, the odds of lung and bronchus cancer among firefighters was significantly decreased compared with both police and the general population.17 19 21 22

Previous studies and the IARC Working Group have suggested that a higher likelihood of screening among firefighters and the healthy worker effect could bias the results for some cancers.1 Therefore, police were included in the current study as an alternative referent group. Further research examining the incidence versus mortality ratio in a cohort study design is essential to assess evidence of the healthy worker effect or screening bias in firefighter studies.

Firefighters are occupationally exposed to many carcinogens including smoke constituents and shift work. A recent review summarised risk factors for some of the cancer types that had an increase in risk among firefighters in the current study.26 Although firefighters may be exposed to radiation, which is the only established causal factor for thyroid cancer, and potential risk factors that can affect thyroid function including pesticides and some heavy metals, their exposures to these hazards are not expected to be elevated.19 26 30 31 Also, there is some research demonstrating that exposure to brominated flame retardants can affect thyroid function.32 Radiation may also increase the risk for brain, soft tissue and bone cancer, while ultraviolet radiation can increase the risk for skin cancer.33 34 Exposure of firefighters to combustion emissions from structure and wildland fires could contribute to increased risk of some of the cancers with positive findings in this study, as tobacco smoking and cooking with solid biomass fuel including wood are associated with cancers of the oral cavity and/or the oesophagus.35 36 Components of emissions from these fires or chemicals that firefighters may interact with during firefighting including heavy metals, PAHs, halogenated organics and endocrine disruptors are associated with prostate cancer.26 In addition, night shift work has been shown to increase the risk of testicular cancer.26 Finally, certain behaviours that are common in the fire service such as alcohol consumption and poor dietary habits can increase the risk of cancer of the oral cavity, oesophagus and brain.37–39

Few studies have investigated potential biological pathways underlying increased cancer risk among firefighters, however, the IARC Working Group concluded there was strong mechanistic evidence that firefighting induces oxidative stress, epigenetic alterations and chronic inflammation.1 The genotoxic effect of firefighting smoke exposure has been indicated by its association with elevation of urinary mutagenicity and DNA damage/chromosomal aberration in blood and buccal cells.40–43 Finally, firefighting is associated with epigenetic modifications including DNA methylation and changes in microRNA expression.44 45

The current study observed that police in Ohio had an increased odds of cancer of the pancreas compared with the general population within the state’s cancer registry. Similar to firefighters, police are exposed to combustion emissions (vehicle exhaust), stress, shift work and sleep disruption.12 However, police had an elevated odds of cancer of the pancreas, lung and bronchus and bladder compared with firefighters. Studies on cancer risk in the police force are limited, but a few studies have observed an increased risk of melanoma, kidney cancer and leukaemia among police.13 15 46 These studies have also observed increased risk of brain, thyroid, male breast, colon, soft tissue, prostate and Hodgkin’s lymphoma among police, however, this was not seen in the current study.

This is the first study to use OCISS data to examine the odds of different types of cancer among Ohio firefighters. A major strength is the size of the study. The OCISS data is a robust cancer registry since anyone providing diagnostic or treatment services to patients with cancer in Ohio is required to report their data, however, it is prone to errors as any cancer registry is.47 However, there is a limitation in using this data set to examine odds of cancer among firefighters. The entry of occupational information in OCISS was incomplete. Approximately 69% of people in the database reported this information either in the occupation or industry fields. People who provided fire or police departments as their employer but did not specify their occupation were not included in the analysis as firefighters or police because they could have been employed in other jobs (ie, administrative office positions). In addition, people who reported being a ‘fireman’ or ‘fire man’ as their occupation without providing a fire department as an employer were also not included in the analysis as firefighters since these terms are also used in other industries such as foundries.22 Finally, occupation was self-reported at the time of cancer diagnosis, therefore, it was possible to miss individuals who were retired from firefighting. It is also possible that the occupation reported was recently started before the time of diagnosis. The absence of occupation information has been reported to be non-random and could have biased the results in the current study either overestimating or underestimating the ORs depending on the cancer site.19 48 49 In addition, the analysis of the association between odds of cancer and severity of exposure (ie, number of fires worked and career length) was impossible as this was not included in the OCISS.

The results could also be biased if firefighting is involved in the aetiology of some of the control cancers since the controls consisted of people in the registry diagnosed with cancers other than the one being analysed. Based on previous studies examining cancer risk among firefighters, there are increased risks of several different types of cancer among firefighters. Therefore, by using people with other types of cancer as controls, there is the possibility that the ORs we observed were biased towards the null.17 20–22 While a previous event-only registry-based study of California firefighters restricted controls to cancers thought to not be associated with firefighting, there is no clear evidence in the literature to rule out the involvement of firefighting in the aetiology of major cancers and a recent similar study of Florida firefighters did not use these same restrictions.17 19 Also, there is a potential for bias in the ORs due to the fact that this is event-only data and the rate of control events are likely dissimilar among firefighters versus the reference population.31 Differentially distributed non-occupational cancer risk factors between firefighters and the control groups could have also caused bias.19 Therefore, the current results should be interpreted cautiously when generalising to the general population that is cancer-free.

While OCISS asked about tobacco use, almost half of the people included in the study had an unknown tobacco history. We examined the demographic variables among only those with unknown tobacco history and the distributions were similar to those among all people in the database, however because there is such a large number of individuals lacking this data, it is impossible to definitively conclude that smoking is not a confounder in this study. Furthermore, results were similar when we conducted logistic regression analysis with the inclusion of tobacco use as a covariate. In addition, a sensitivity analysis among only those individuals with information on tobacco use and controlling for it, resulted overall in similar ORs and similar trends. Finally, there was insufficient data to account for the impact of other potential confounders such as socioeconomic factors and certain health behaviours such as diet and physical activity.

Conclusion

This was the first study to indicate an increased odds of cancer of the oesophagus, skin, brain, cranial and other nervous system and thyroid among firefighters in Ohio. While this study has some limitations, these are important findings that suggest a need for further investigation. Firefighters were not only compared with the general population but also to police within the registry, a similar occupation group to minimise biases that are recognised when comparing to the general population. Finally, the current results showing increased odds of certain types of cancers were mostly consistent with the findings of previous studies conducted in other locations, at other time points and using different data sources and study designs. In conclusion, among Ohio firefighters there was an increased odds for cancer of the oesophagus, skin, brain, cranial nerves and thyroid compared with the general population, while odds for cancer of the pancreas, larynx, lung and bladder were decreased. Further hypothesis-driven studies are needed in Ohio to further determine the association of different cancer types with firefighting.