New Study Shows Correlation (But No Direct Causal Link) Between Firefighting and Cancer

It is axiomatic that firefighting is a hazardous occupation. But should the more insidious risk of developing cancer be added to list of more obvious hazards faced? Building on previous research that seems to point in this direction, a recent study, Mortality and cancer incidence in a pooled cohort of US Firefighters from San Francisco, Chicago and Philadelphia (1950-2009) undertaken by Robert D. Daniels, Travis L Kubale, James H Yin et al and published online October 14, 2013 [doi:10.1136/oemed-2013-101662] by Occup Environ Med, concludes that there may indeed be a correlation between a firefighter’s occupational exposure and an increased risk of certain cancers. In particular, the study suggests a link to malignant mesothelioma resulting from asbestos exposure.

Given the number of workers involved (the study states that there are over a million career and volunteer firefighters nationwide), the suggestion of an increased risk of cancer is concerning and warrants attention. Meta-reviews of previous studies (including a review by the International Agency for Research on Cancer (“IARC”)) reported risks for certain cancers. Nevertheless, the IARC took note of the limited available evidence, and concluded that firefighter exposures were only possibly carcinogenic. No direct causative link had therefore apparently been conclusively established.

Nor does this study provide one. What it does provide, however, is useful data on cancer incidents and mortalities amongst firefighters from different geographical locations over a long period of time. As such, it is another incremental step towards more fully understanding the risks involved.

Study Methodology

The study focused on the following cancer types: brain, stomach, oesophagus, intestines, rectum, kidney, bladder, prostrate, testes, leukemia, multiple myeloma, NHL, lung, COPD and breast cancer. The pertinent data was collected from records of a total of 29,993 firefighters serving in San Francisco, Chicago and Philadelphia between 1950-2009.

In developing the statistical methodology, the study calculated the person-years at risk (“PYAR”), and stratified them by gender, race, age and calendar year. A standardized mortality rate (“SMR”) was calculated using the ratio of the observed to the expected number of deaths. Cancer incidence was looked at in two different ways – the first used a multiple cancer approach which looked at all cancers occurring during the risk period, while the second looked just at the date of the first cancer diagnosis. The study also conducted several sensitivity analyses to scrutinize the results further (e.g. examining effects of age, length of employment and geographical location.)

Cancer Mortality Data

The study group was 97% male. With respect to these firefighters, the data indicated that their mortality rate was the same as the general population. Although heart disease was the leading cause of death, the results did indicate an increased rate of death from cancer. These were mainly from cancers of the respiratory, digestive and urinary systems. There was also a showing of excess malignant mesothelioma. There was reportedly “little evidence” of higher death rates from other cancers looked at in the study.

Female firefighter death rates were also the same as the general population. Unlike their male counterparts, the study reported little evidence of excess overall cancer mortality rates. Most cancer deaths were from breast cancer. Bladder cancer rates were also above the statistical norm.

Interestingly, non-Caucasian male firefighters had decreased all-cause mortality. Lung cancer deaths were below expectation. Only prostrate cancer seemed to be slightly elevated.

Cancer Incidence Data

The study results pointed to an overall cancer incidence increase “slightly above expectation” for males. The types of cancer incidence generally tracked the mortality data.

With respect to female firefighters, the study results indicated that overall cancer rates were elevated, but not significantly so. Bladder cancer rates were elevated, but based on very few cases. Half of all cases were breast cancer, and nearly all were diagnosed prior to age 55 (mainly between 50 and 54.)

While non-Caucasian firefighters did not show overall excess cancer incidence rates, there was a showing of excess prostrate cancer.

Sensitivity Analysis

The results for cancer mortality and cancer incidence rates by type were fairly constant across the three departments studied. Exceptions were for COPD and cancers of the lung, prostrate and brain. The differences in cancer mortalities were reportedly due to outlier decreased lung cancer and COPD in San Francisco firefighters and excess prostrate cancer and lung cancers in Chicago. As to cancer incidence, lung cancer incidence was lower in San Francisco but there was excess prostrate cancer incidence. Brain cancers incidence reportedly varied by location – higher in San Francisco and lower in Philadelphia.

Age-at-risk generally did not seem generally to make a difference with the exception of incidences of prostrate and bladder cancer. The excess rate for the former was found in the 45-59 year age bracket, while the age pattern of excess bladder cancer was “unclear.” Mortality rates for cancers of the breast, oesophagus and kidney were higher for firefighters less than 65.

Results Analysis

The results indicate that while firefighters do appear to enjoy better health than the general population (possibly due, the study posits, to health requirements for entering and remaining in the occupation) there exists a “modest excess” in overall cancer mortality and incidence rates. Additionally, a twofold increase in malignant mesothelioma incidence and mortality rates is indicated, a reportedly new finding not previously noted in connection with U.S. firefighters. The assumption is made that malignant mesothelioma is largely attributable to asbestos exposure “with sparse evidence of other causes.”

The study asserts that these findings may be applied broadly to all firefighters as “[t]he lack of significant differences in results between fire departments, end-points, and analytic techniques suggest that the pooled study findings are robust and generalizable to similar firefighter populations.”

Limitations of Study

The study cautions that its findings are to be tempered by the following limitations:

> As to the findings on excess digestive cancers, primarily oesophageal and colorectal, the study acknowledged that “information on occupational causes is sparse.” While noting that there is “limited evidence” that exposure to asbestos and diesel exhaust may be “weakly associated” with gastrointestinal cancers, the relationship appears de minimus when compared to factors such as diet, obesity, physical activity, tobacco and alcohol consumption.

> As to the findings of an increased risk of oral, pharyngeal and laryngeal cancers, the primary risk factors are tobacco and alcohol use, with such occupational exposures as wood dusts, smoke, asbestos, PAHs and acid mists occupying a much lower rung on the risk factor ladder.

> Lifestyle factors play a wildcard role. For example, smoking among firefighters is reportedly less frequent than among the general population. While this is consistent with lower incidence of COPD and stroke, it is inconsistent with increased digestive, oral and respiratory cancers. Similarly, other studies have suggested increased obesity among firefighters. This condition, often coupled with a diet high in fat and meat, can be associated with an increased gastric or colorectal cancer risk. However, other associated diseases such as heart disease, diabetes and stroke were not found at an increased level.

> Information on alcohol consumption was “sparse and inconsistent.” There was excess mortality from cirrhosis and other chronic liver diseases reported, but fewer than expected deaths from alcoholism. The study suggests that reasons for the excess liver diseases may be exposure to chemical toxins or infectious diseases.

> The study admits that the results involving female firefighters “merit cautious interpretation.” This is due to the small sample size (less than 4% were women) and the lack of “confirmatory results.”

> Healthcare availability, including medical screening, may impact the findings. Although excess bladder and prostrate cancers was found in firefighters aged between 45 and 59, and this early onset “suggests an association” with occupational exposure, it could also be a result of more frequent screening among firefighters and improved healthcare availability.

> Employment duration may not be directly proportional to exposure potential. Further analysis on this issue is apparently underway.

Conclusions

The study found a “small to moderate” increase in risk of certain cancers in firefighters, primarily in the respiratory, digestive and urinary systems. Rather than making definitive conclusions, the study stated that these results “strengthen evidence” of a correlation between firefighting and cancer. With respect to the malignant mesothelioma findings, the authors were willing to make a more definite conclusion, finding that there was likely a “causal association” with firefighting, based on the assumption that “asbestos is the only known causal agent for malignant mesothelioma.”

Certainly, more work remains to be done, particularly in the area of exposure analysis and the influence of lifestyle factors and it is to be expected that further studies will bring greater clarity to this still murky debate.

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