1. Introduction
Per- and polyfluoroalkyl substances (PFAS), also known as ‘forever chemicals’, are a global environmental and health concern due to their ubiquitous presence in the environment, tendency to bioaccumulate, and the growing evidence of adverse human health effects at very low levels of exposure [
1]. They are synthetic, thermally stable compounds with unique non-stick surfactant properties [
2]. As such, PFAS are widely used in consumer products including food packaging, cookware coatings, water-resistant products such as cookware, dental floss, and in-home furniture and carpeting [
1]. PFAS are also used for many industrial processes. Human PFAS exposure can occur through ingestion of contaminated food or water, inhalation, and there is evidence from rodent models of dermal adsorption [
3]. Contamination of ground water has been commonly reported around airports and military bases, because PFAS (primarily perfluorooctanesulfonic acid (PFOS) and perfluorooctanoicacid (PFOA)) have historically constituted one to five percent of Class B aqueous film-forming foams (AFFF) that are used for fire suppression in those settings [
4]. Other industrial point sources of community ground water have been reported [
5,
6,
7].
In humans, the serum half-lives perfluorohexanesulfonic acid (PFHxS), PFOS, and PFOA were reposted by one study as 5.3, 3.4, and 2.7 years, respectively [
8]. They are readily transported and have been detected in ecosystems from the Arctic to the Antarctic [
9,
10]. Almost all U.S. residents have detectable levels of one or more of the most studied long-chain PFAS: PFOA, PFOS, PFHxS, and perfluorononanoic acid (PFNA). PFAS exposure has been associated with multiple adverse human health outcomes including dyslipidemia, cardiovascular disease, immune suppression, kidney disease, and endocrine disruption [
1]. There is evidence that several long-carbon chain PFAS may be carcinogenic [
11]. In 2015 the International Agency for Research on Cancer classified PFOA as a group 2B (possible) carcinogen for kidney and testicular cancers [
12].
Firefighters can be exposed to PFAS through multiple pathways. Firefighters’ protective clothing (aka, gear or turnout gear) was historically treated with PFAS to provide water and stain resistance properties; evidence of PFAS in all layers of gear has been reported [
13]. Residential, commercial, and industrial building structure and vehicle fires may burn products that contain PFAS including electronics, furniture, carpeting, and insulation and release particles that can be inhaled or settle on gear and skin [
14]. Firefighters who use AFFF have been shown to have increased serum concentrations of PFOS and PFHxS that was positively associated with years of firefighting [
15].
Higher mean serum levels of some PFAS, including PFOA, PFOS, PFHxS, PFNA, and perfluorodecanoic acid (PFDA) have been observed among firefighters than those of demographically similar subsets of the general population [
16]. Most research assessing PFAS exposure in firefighters has been conducted among career (paid) firefighters. However, two-thirds (67%) of firefighters in the US serve as volunteers [
17]. Volunteer firefighters perform the same tasks as their career counterparts, but often with less protection and risk reduction.
There is a significant gap in our understanding of PFAS exposure in US firefighters and specifically, among volunteer firefighters. To begin to address this gap, we conducted a biomonitoring study in a large suburban volunteer fire department with the primary goal of assessing the distribution and levels of PFAS compounds detected in serum compared to the general US population as represented by the National Health and Nutrition Examination Survey (NHANES). NHANES is a population-based survey that publishes data in two-year cycles. When this study begun, the most recent cycle for which NHANES PFAS serum levels were publicly available was 2015–2016, and we completed our comparisons using those data. However, before the study was concluded PFAS data from the 2017–2018 NHANES cycle became publicly available. Serum concentration of some long chain PFAS are declining in the general population, including PFOS, PFOA and PFNA, [
18] (which have previously been reported as elevated in biomonitoring studies of firefighters). The publishing of the 2017–2018 presented an opportunity to add a secondary aim to the study: To assess whether a comparison with the NHANES data collected closer in time to our study data (2019) would alter our findings or interpretation. As such, we compared the distribution and levels of PFAS compounds among volunteer firefighter study participants to those of the two most recent NHANES cycles. Within the volunteer firefighters, we also explored associations between firefighting exposures and PFAS serum levels.
4. Discussion
We found that the prevalence and level of PFAS chemicals in serum differed between members of a large suburban volunteer fire department and the general US population, including that PFDoA was detected in 80% of study subjects but in none of the NHANES participants in the 2017–2018 cycle. As well, we observed significantly higher serum levels of PFNA, PFDA, and PFDoA, but lower levels of PFOS and MeFOSAA, than NHANES participants of the same age, gender, and race-ethnicity (adult, non-Hispanic white males). These findings are generally consistent with prior research in that firefighters have elevated serum levels of some long-chain PFAS when compared to the general population, including PFHxS, PFDA and PFNA. As well, some firefighters with history of using AFFF have elevated serum levels of PFOA and PFOS. However, much of the prior research on PFAS exposure among firefighters has been conducted among career firefighters. To our knowledge this is the first study of its size to evaluate PFAS exposure among volunteer firefighters.
In our side by side comparison of our study participants enrolled in 2019 to the two most recent NHANES cycles, 2015–2016 and 2017–2018, the overall interpretation of our findings would have been similar had we used the earlier NHANES cycle (2015–2016). However, because concentrations of PFAS are dynamic in the general population, with some average levels declining and some increasing, reliance on the 2015–2016 rather than the 2017–2018 data which is more temporally proximal to the 2019 CAPS study, would have had introduced errors in estimating differences between the prevalence and serum levels of some PFAS among our two study populations.
Our observation that mean PFDA serum concentrations were significantly elevated in these volunteer firefighters compared to NHANES participants is notable. It is consistent with a 2015 biomonitoring study of 101 California career firefighters that reported their PFDA levels were three times higher than those in NHANES participants. Of note, the serum concentrations of PFDA levels observed in the California firefighters (and the 2011–2012 NHANES sample 0.90 μg/L; 95% CI, 0.78 to 1.03), were much higher than those we observed in this study [
24]. This may reflect that serum levels of some long-chain PFAS are falling as they are phased out of industrial use and consumer products due to health and environmental concerns. These career firefighters in California did not have detectable serum levels of PFDoA; in contrast the volunteer firefighters from New Jersey had a high prevalence (80%) and significantly elevated serum levels of PFDoA, with the geometric mean twice as high as in the NHANES participants. The source of PFDoA exposure in these volunteer firefighters is an important direction for future investigation. While these firefighters differed from those in the California study by both working structure (volunteer vs. career) and geography, it is noteworthy that in our study PFDA and PFDoA serum levels were positively correlated with both years of being a firefighter and never having worked as career firefighter. However, community exposure may also be contributing to PFAS exposures in the study participants as the area in New Jersey had a history of industrial pollution from dye manufactures and other industries which may use PFAS in their manufacturing processes [
25]. In a 2020 study of PFAS in surface water and fish conducted by sampling 11 waterways around New Jersey based on proximity to potential sources of PFAS in recreational areas. Four of the sampling areas were adjacent to the township where the current study was conducted because they are within the drainage basin of the Joint Base Maguire-Dix-Lakehurst complex. Of the water samples adjacent to the study town, all contained detectable levels of PFOA and PFOS but not of PFDoA or PFNA; PFNA was detected in one location [
26]. These results do not lend support to an environmental source of PFDoA in these firefighters.
We observed elevated serum concentrations of PFNA in our study participants, which was not associated with years of firefighting after controlling for age, occupation, and educational level. Other biomonitoring studies have observed elevated levels of PFNA, as well as PFHxS and PFOS among firefighters who have worked with AFFF [
15]. However, the volunteer fire department whose members participated in this study rarely used AFFF but some of the participants may have done so in previous departments or in the armed services, however we did not see an association between being a career firefighter an PFAS levels serum levels or with previous military service (data not shown). There are some areas of New Jersey with uniquely high PFNA ground water contamination, [
5] as well some studies in the general population that found diet can account for over half of PFNA exposure [
27,
28]. As such the lack of association with firefighting experience may point to an environmental rather than firefighting-related source. Including community members with a similar socio-demographic profile but no firefighting experience in future studies of PFAS exposure could offer insights into the important question of PFAS exposure sources.
A biomonitoring study of New Jersey residents conducted using remnants of laboratory specimen and blood donations acquired between 2016 and 2018 and used post-sample stratified weights to estimate population parameters reported a somewhat different distribution of PFAS in adult New Jersey males than was seen among thee volunteer firefighters. They observed lower geometric levels of PFNA (0.88 ng/mL, 95% CI: 0.77, 1.01] and PFDA (0.23 ng/mL, 95% CI: 0.18, 0.29], although in both cases the confidence intervals overlapped [
21].
The strong and consistent positive association observed between having any college education and serum PFDA and PFDoA concentrations was unexpected. We were unable to evaluate variation in household income in this study, but education is typically strongly correlated with socioeconomic status and associated lifestyle factors. Studies of dietary contribution to PFAS body burden find that the major contributors are fish, meat, fruit and eggs [
29]. Higher intake of fish and fruit are associated with higher income in the US and so this may explain part of the observed association. As well, PFAS are used on furnishings and in carpeting to make them stain resistant, so higher serum levels may reflect consumer product patterns associated with income. This is an important area for future research.
Our study had a number of notable strengths including being among the first study of its size to investigate PFAS exposure among volunteer firefighters. In New Jersey where this study was conducted, more than 80 percent of the approximately 37,000 firefighters are volunteers. In the US, the percentage of US firefighters who are volunteers is increasing while the number of career freighters is decreasing [
17]. Volunteer firefighters train for and perform the same tasks as career firefighters, but often with less protection and risk reduction. They are always on-call, and so could potentially accumulate more years of firefighting -related exposures than their career counterparts [
30]. Compared with career firefighters, significantly more US volunteer firefighters are females: 4 vs. 11%. We were unable to find reliable statistics on the proportion of US volunteer firefighters who are non-Hispanic white. In the career service, approximately 8% are African American, 8% Hispanic and 1 percent Asian or other. Future studies of volunteer firefighters should attempt to include more females and people of color as their occupational and exposure profile may differ.
Another strength of this study was that it was a scientific collaboration with the national FFCCS [
31]. We used their Annual Cancer Survey, an instrument that had already been used extensively within firefighter populations. However, a limitation inherent in capturing firefighting history by survey is obtaining an accurate exposure assessment for firefighters given the diversity in the types of calls they respond to, their variable call volume, and often their long duration of firefighting experience. The inability to precisely capture firefighters’ exposures may result in exposure misclassification, however such misclassification should likely be nondifferential.
