The occupation of firefighting, including wildland firefighting, was recently classified as a Group 1 Known Human Carcinogen by the International Agency for Research on Cancer (IARC), based on sufficient evidence of mesothelioma and bladder cancer in firefighters (Demers et al. 2022). Wildland firefighters (WFFs) are routinely exposed to wildfire smoke, which contains combustion byproducts including polycyclic aromatic hydrocarbons (PAHs) (Adetona et al. 2017). Naphthalene is a PAH and has been classified by IARC as a Group 2B Possible Human Carcinogen (IARC 2002). PAHs have also been found to be associated with adverse health effects including kidney damage and lung function abnormalities (Abdel-Shafy and Mansour 2016; Patel et al. 2020).

Currently, there is no National Fire Protection Association(NFPA)-compliant, National Institute for Occupational Safety and Health (NIOSH)-approved respiratory protection available to WFFs (Domitrovich et al. 2017). As such, inhalation exposure to PAHs and other combustion byproducts is a concern for WFFs. Recent studies have also shown that dermal exposure may contribute to the WFFs’ PAHs body burden (Cherry et al. 2021; Cherry et al. 2023). Several factors relevant to WFFs can increase the potential for dermal absorption of PAHs, including elevated skin temperature and sweat (Luo et al. 2020; Probert et al., 2024). Biomonitoring is needed to evaluate PAH exposure for all routes including inhalation and dermal.

Indeed, previous studies have found that WFFs’ post-shift urinary mono-hydroxylated PAH metabolite (OH-PAH) concentrations increase significantly relative to their pre-shift concentrations (Adetona et al. 2017, Cherry et al. 2023). A recent study found that baseline (pre-fire) WFF OH-PAH concentrations were higher than general population median concentrations (Barros et al. 2024). Studies have also found that inhalation exposures for WFFs can vary by job position, (Reinhardt and Broyles 2019, Navarro et al. 2021, Navarro et al. 2023) and a biomonitoring evaluation may help to better understand WFFs’ potential exposure to PAHs based on job position. As such, we sought to evaluate exposure to PAHs, stratified by job position, for WFFs during a wildfire incident.

This assessment was conducted as part of the NIOSH Wildland Firefighter Exposure and Health Effects (WFFEHE) Study (Navarro et al. 2022). This analysis evaluated PAH exposure for an Interagency Hotshot Crew (IHC) of 19 WFFs at a wildfire incident near Salmon, Idaho in 2019. A detailed description of the entire study rationale, design, and methods were reported previously (Navarro et al. 2022, Navarro et al. 2023). Briefly, consenting WFFs on the IHC operated in three different roles: crew member (N = 9), saw team (N = 8), and overhead (N = 2). Crew members performed ground level fire management tasks such as using hand tools to dig firelines, carrying drip torches to ignite fuels, etc. WFFs on a saw team used chainsaws to fell trees and remove brush. Overhead positions directed and oversaw WFFs performing operational tasks. WFFs participating in this study were non-smokers. To account for small sample sizes when stratifying by job position, we grouped crew members and overhead because their job activities were similar. This study was reviewed and approved by the Centers for Disease Control and Prevention (CDC)/NIOSH Institutional Review Board (IRB)^§.

Two NIOSH industrial hygienists accompanied the IHC into the field and documented environmental conditions and work activities, including the use of personal protective equipment (PPE). To assess the biological uptake of PAHs, WFFs were asked to provide pre- and post-shift urine samples on three consecutive days. Overall, 114 urine samples were collected. Urine samples were shipped to CDC laboratories to be analyzed for creatinine and seven OH-PAHs in four PAH biomarker categories: hydroxynaphthalenes (1-hydroxynaphthalene (1-NAP) + 2-hydroxynaphthalene (2-NAP)), hydroxyfluorenes (2-hydroxyfluorene (2-FLU) + 3-hydroxyfluorene (3-FLU)), hydroxyphenanthrenes (1-hydroxyphenanthrene (1-PHE) + 2-3-hydroxyphenanthrene (2-3PHE)), and 1-hydroxypyrene (1-PYR). Creatinine was measured using a Vitros Autoanalyzer (Ortho Clinical Diagnostic, Raritan, NJ). Most creatinine concentrations fell within 30-300 mg/dL (WHO 1996), except four samples that were excluded from all analyses. After enzymatic hydrolysis of conjugated OH-PAHs in 100 microliters of urine, the seven target OH-PAHs were quantified by online solid phase extraction coupled with high performance liquid chromatography-isotope dilution tandem mass spectrometry (Wang et al. 2017). The limit of detection (LOD) ranged from 0.008 to 0.09 micrograms per liter (μg/L), depending on the OH-PAH metabolite. Replicate samples were split from 5% of the samples and analyzed separately for quality assurance, and the replicates met method performance criteria for creatinine (<10%CV) and OH-PAHs (<15%CV).

Summary statistics are presented as geometric mean (GM), median, and range, stratified by metabolite, job position, and sampling time. Box and whisker plots with minimum, 25th percentile, median, 75th percentile, and maximum were generated by using differences of pre- and post-shift urinary concentrations, stratified by day and job position. For descriptive statistics, concentrations below the LOD were assigned values using the beta-substitution method (Ganser and Hewett 2010) conducted in R version 4.4.0 (R Core Team, 2024). Only two samples had 1-hydroxypyrene concentrations <LOD (Table 1). All concentrations reported were creatinine-corrected.

Wilcoxon rank-sum test was carried out to determine urinary concentration differences of OH-PAH biomarkers between job positions on each day. Wilcoxon signed-rank test was used to examine whether the change in OH-PAH biomarker concentrations between pre- and post-shift by job position and day significantly differed from zero. All statistical tests were performed in SAS version 9.4 (SAS Institute, Cary, NC) and were two-sided at the 0.05 significance level.

The detailed assessment of specific job positions, sampling strategy, and personal air sampling results has been published previously (Navarro et al. 2023). Briefly, our field observations included that more smoke was present on Day 2 relative to Day 1 or Day 3. The WFFs did wear gloves and did not wear respiratory protection during their work, and soot was observed on their skin during the sampling period.

Though there did appear to be marginal differences in PAH exposures by job position, no significant differences were found for any of the OH-PAH metabolites measured (all p-values > 0.05, results not shown). This was true for each individual OH-PAH metabolite (Table 1) and when grouped by their parent compound (Figure 1). Concentrations increased from pre- to post-shift for all WFF samples combined across the three days.

When stratified by job position and day, we observed significant increases from pre-to post-shift for most (88% or 37/42) comparisons (Table 1). Overall, OH-PAH metabolite creatinine-corrected concentrations were highest on Day 2. Specifically, the largest change from pre-to post-shift was observed for 2-NAP on Day 2 (median concentrations increased 425% from pre-to post-shift for crew member/overhead; median concentrations increased 146% from pre-to post-shift for saw team; Table 1).

Additionally, post-shift median creatinine-corrected concentrations of 1-NAP (once), 2-NAP (four times), and 1-PYR (twice) exceeded the 95^th percentile from the 2015-2016 National Health and Nutrition Examination Survey (NHANES) for non-smokers on at least one day (Table 1). Post-shift median creatinine-corrected concentrations for the other OH-PAH metabolites never exceeded the NHANES non-smokers 95^th percentiles. However, maximum post-shift concentrations (across all sample days) exceeded this percentile for every OH-PAH metabolite measured. 1-PYR also has a biological exposure index (BEI^®) of 2.5 μg/L (not corrected for creatinine; ACGIH 2024). No 1-PYR urine concentrations from this study were above 2.5 μg/L. ACGIH recommends the BEI for 1-PYR be adjusted for benzo(a)pyrene but we did not characterize benzo(a)pyrene exposure in this evaluation.

Among WFFs during a wildfire incident, we observed significant increases in OH-PAH metabolite urinary concentrations from pre-to post-shift across three days. Results from this study suggest that WFFs can experience higher exposure to PAHs during their work shifts than the general population, regardless of their job position.

Navarro et al. (2023) reported that on Day 2, the IHC conducted a firing operation, which included crewmembers igniting fuels (unburned vegetation) to create a break in fuel between the main fire and the control lines. Day 2 was also when the most smoke was observed where WFFs were working. Navarro et al. (2023) also characterized personal air concentrations of naphthalene and found they were not elevated on Day 2 relative to Day 1 or Day 3 (Navarro et al. 2023). By contrast, our urinary results suggest that exposure to PAHs (in particular 2-NAP) was highest on Day 2. Considering the relatively low air concentrations of naphthalene and the fact that soot was observed on WFFs’ skin (including on their hands and arms), it is likely that dermal absorption contributed to the WFFs’ exposures, possibly during activities like mop up (removing partially burnt materials/extinguishing smoldering vegetation by covering with dirt). Previous studies show that naphthalene is the most prevalent PAH detected on skin wipes collected from WFFs (Cherry et al., 2023; Broznitsky et al., 2024).

Adetona et al. (2017) characterized urinary concentrations of several OH-PAH metabolites before and after conducting wildland firefighting activity on prescribed burns (intentionally set fires for resource management). Our study found higher post-shift GM concentrations compared to Adetona et al., (2017) for 2-NAP (Adetona GM= 12.1 μg/g creatinine) and lower GMs for the other OH-PAH metabolites including 1-NAP (Adetona GM=8.82 μg/g creatinine), 2-FLU (Adetona GM= 1.49 μg/g creatinine), 3-FLU (Adetona GM= 0.426 μg/g creatinine), 1-PHE (Adetona GM= 0.557 μg/g creatinine), and 1-PYR (Adetona GM= 0.576 μg/g creatinine).

Post-shift urinary concentrations of hydroxynaphthalenes in this study also exceeded median concentrations when compared to structural firefighters after they performed firefighting activities in a live-fire training environment (Mayer et al. 2023). Fent et al. (2020) also evaluated other OH-PAH metabolites in structural firefighters and reported that they had post-fire median concentrations of hydroxyfluorenes, hydroxyphenanthrenes, and 1-PYR above those found in the current study. This could be an artifact of the difference in fuels for wildland fires and structural fires. It is important to note that structural firefighters in all of these other studies wore self-contained breathing apparatus; the WFFs in the present study did not wear any respiratory protection. Dermal exposures were not evaluated in the WFFs from the current study, but soot was observed on their skin on all three days. Overall these findings, along with the results from Adetona et al. (2017), suggest that 2-NAP levels in the current study were elevated relative to what has previously been characterized in the literature for other USA WFFs and even structural firefighters.

A potential limitation of this study is that we did not characterize the biological absorption (i.e., urinary concentrations) of volatile organic compounds (VOCs), including toluene, ethylbenzene, and xylenes, that were measured in the air during this wildfire event (Navarro et al. 2023). We began sampling in the middle of the wildland season, so pre-shift samples on Day 1 could have been impacted by previous exposures from the season or from dietary intake (e.g., chargrilled foods). We grouped crew and overhead firefighters together even though their exposures may vary. We also did not evaluate dermal exposure to PAHs. Future studies could characterize VOC exposures for WFFs, evaluate dermal exposure to PAHs, and further assess how job position affects exposure to better understand WFFs’ overall exposure burden.

WFFs come into contact with and absorb PAHs while performing their duties resulting in PAH biomarker concentrations that can exceed those in the general population. No significant differences were observed by job position. The current study reveals a significant increase in urinary concentrations of seven PAH metabolites among WFFs from pre- to post-shift. Together, these findings suggest that, in addition to inhalation, dermal absorption may contribute to WFFs’ exposures to PAHs. WFFs could prioritize skin decontamination efforts (e.g., decontamination wipes after mop up) to reduce exposures.