Wildland firefighter smoke exposure and risk of lung cancer and cardiovascular disease mortality
Introduction
Wildland firefighters suppress thousands of wildfires each year that burn across millions of acres in the U.S. (NIFC, 2017a). During 2017, more than 8.8 million acres burned and more than 26,000 wildland firefighters worked to suppress fire during the height of the summer wildfire season (NIFC, 2017a; NIFC, 2017b). Large forest fires in the western U.S. are nearly five times as frequent annually as they were in 1990 (NIFC, 2017a). These fires burn more land area and last much longer than in the past. The wildfire season is also much longer, as exemplified by the Thomas Fire near Santa Barbara, which became (on December 22, 2017) California's largest wildfire in modern history. The wildfire season in California typically ends in October, when the autumn rains begin (InciWeb the Incident Info, 2018).
Wildland firefighters are exposed to inhalation health hazards including hazardous air pollutants from the combustion of vegetative live and dead biomass (smoke) and the breathing of soil dust, while working long work shifts with no respiratory protection (Broyles, 2013; Naeher et al., 2007). Along with exposure to smoke on the fire line, wildland firefighters may be exposed to smoke at incident command posts (ICP) situated near the wildfire to support suppression operations (McNamara et al., 2012). Wildland firefighters conducting prescribed fires (intentionally ignited, low-intensity fires used for land management) are also exposed to smoke (Ryan et al., 2013). Wildfire smoke is a complex mixture of gas and particle-phase air contaminants, including acrolein, benzene, carbon dioxide, carbon monoxide, formaldehyde, polycyclic aromatic hydrocarbons, and fine and respirable particulate matter (PM with aerodynamic diameters ≤2.5 μm or ≤4 μm, respectively), which can contain amorphous carbon or soot (Naeher et al., 2007; Adetona et al., 2016). Soil disturbance from several work activities including fire line construction, mop-up, and open vehicle transportation, also exposes firefighters to mineral contaminants such as crystalline silica (Broyles, 2013).
Previous health studies of wildland firefighters examined acute health effects of smoke exposure across individual shifts and entire fire seasons. Liu et al., 1992, found that 63 wildland firefighters in California had significant declines of individual lung function (FVC, FEV1, and FEF25-75) and an increase in airway responsiveness post-season when compared with their pre-season baseline values (Liu et al., 1992). When examining cross-shift changes in lung function, Gaughan et al., 2014a, b, reported that wildland firefighters had a significant decline in lung function associated with high exposure to levoglucosan (a tracer for smoke from wood or vegetation combustion) (Gaughan et al., 2014a). Additionally, Adetona et al. (2017), Hejl et al. (2013), and Swiston et al. (2008), reported increased levels of biomarkers of systemic inflammation in firefighters after wildland fires and prescribed burns (Adetona et al., 2017; Hejl et al., 2013; Swiston et al., 2008). Booze et al. (2004), conducted a health risk assessment to characterize the risk of cancer and non-cancer health effects in wildland firefighters (Booze et al., 2004). The study concluded that there were elevated risks of developing cancer, primarily from exposure to benzene and formaldehyde, as well as non-cancer health effects from exposure to PM3.5 and acrolein. Recently, Semmens et al. (2016), conducted the first long-term health survey of wildland firefighters that examined the association between the duration of wildland firefighters’ careers and self-reported health outcomes (Semmens et al., 2016). The survey reported significant associations between the number of years worked as a wildland firefighter and history of ever diagnosis of two cardiovascular measures - hypertension and/or heart arrhythmia, as well as the need for knee surgery.
The U.S. Environmental Protection Agency regulates fine particulate matter (PM2.5) because there is robust epidemiologic evidence of associations between short-term exposures to PM2.5 and cardiopulmonary mortality, as well as increased risk of acute cardiovascular outcomes, including myocardial infarction, stroke, and arrhythmias (Atkinson et al., 2015). Along with the epidemiological evidence, experimental evidence from both animal and human studies supports the associations between exposure to PM2.5 and cardiovascular outcomes (Brook et al., 2017). Risk of lung cancer is also associated with exposure to ambient PM2.5 (Hamra et al., 2014). The recently reported results of a large U.S. cohort study of older individuals (nearly 19 million Medicare beneficiaries) showed increased risks of both cardiovascular mortality and lung cancer with increased PM2.5 exposure (Pun et al., 2017).
In our study, we conducted an analysis to examine long-term health impacts for wildland firefighters. Our objective was to estimate relative risk of lung cancer and cardiovascular disease mortality from existing PM exposure-response relationships using a measured PM concentration from smoke and breathing rates from wildland firefighter field studies across different exposure scenarios.
Section snippets
Methods
Wildfire Smoke Exposure-Response Relationship. To estimate the relative risk of lung cancer (LC) and cardiovascular disease (CVD) mortality from exposure to PM2.5 from smoke, we used the exposure-response (ER) relationships developed by Pope III et al. (2011); Pope et al., 2011. Briefly, they conducted a cohort study analysis from the American Cancer Society (ACS) Cancer Prevention Study II, which included 1.2 million adults, to examine the shape of the exposure-response relationships of PM2.5
Results
Table 1 presents the parameters we used to calculate daily dose of PM4 for the two exposure scenarios: (1) firefighter long season and (2) firefighter short season. Based on field study observations, firefighters worked an average of 13.6 h per shift. The mean concentrations of PM4 and crystalline silica measured on wildland firefighters was 0.53 mg m−3 and 0.026 mg m−3, respectively. After adjusting the measured shift average concentration of PM4 for dust exposure (crystalline silica),
Discussion
Our study objectives were to estimate lifetime risk of lung cancer and cardiovascular mortality from exposure to PM4 from smoke. Our analysis used measured PM4 concentration from smoke and estimated breathing rates that we collected as part of extensive field studies of wildland firefighters in the U.S. Using the PM2.5 exposure-response relationships developed by Pope III et al. (2011), we estimated that wildland firefighters had an increased risk of LC and CVD mortality, with RRs greater than
Declaration of competing financial interests (CFI)
All other authors declare they have no actual or potential competing financial interests.
Acknowledgements
This work was funded by the Joint Fire Science Program (Project ID: 13-1-02-14).
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