This is a descriptive study. Information on the PFAS variables was obtained from NHANES, 2005-2014. The NHANES has been designed as a nationally representative and complex cross-sectional survey conducted in 2-year cycles utilizing questionnaires, physiological examination, and laboratory tests. Detailed information on the questionnaires, methodologies, data, and documentation is available on the NHANES website https://wwwn.cdc.gov/nchs/nhanes/AnalyticGuidelines.aspx (accessed March 20, 2024).

In this study, we included participants aged 20 years and older who underwent blood tests for PFAS, reported their industry and occupation information, and were currently employed (N = 4476). Analyses for this study are based on data collected from the five NHANES survey cycles from 2005–2006 to 2013–2014. The selection of cycles was determined by the availability of industry and occupation data. The recent NHANES 2015–2016 and 2017–March 2020 (Pre-Pandemic) public-released data files do not include the industry and occupation codes. A flowchart outlining the inclusion and exclusion criteria of this study is presented in Supporting Information S1: Figure I. Detailed sample sizes by demographics and industry/occupation groups are provided in the first column of each table. The ethics approval for the NHANES was obtained from the Ethics Review Board of the National Center for Health Statistics (NCHS) [27], and all adult participants provided written informed consent.

Serum PFAS concentrations were obtained from blood samples collected during the health examination. PFAS concentrations were measured among a random subsample comprising approximately one-third of individuals aged 12 years and older in each NHANES cycle. We used the NHANES-provided sample weight that accounts for this subsampling to appropriately calculate nationally representative estimates.

We assessed the distribution of five major serum PFAS compounds, including PFOS, PFOA, PFHxS, PFNA, and PFDA. The five PFAS were measured in NHANES cycles from 2005–2006 to 2011–2012. In the 2013-2014 cycle, the concentrations of linear PFOA (n-PFOA) and the sum of branched isomers of PFOA (Sb-PFOA) for PFOA, and those of linear PFOS (n-PFOS) and the sum of perfluoromethylheptane sulfonate isomers (Sm-PFOS) for PFOS were summed to estimate the total concentrations of PFOA and PFOS for each participant [28].

Serum specimens are processed, stored, and shipped to the Division of Environmental Health Laboratory Sciences, National Center for Environmental Health, CDC, for analysis. PFAS quantitation in serum used solid phase extraction coupled with High Performance Liquid Chromatography-isotope dilution-tandem Mass Spectrometry [29]. The limits of detection for the serum PFAS varied from 0.08 to 0.20 ng/mL over the cycles as shown in Supporting Information S1: Table I. If the value of one isomer was less than the limit of detection (LOD), then the imputed value was used. The imputed value was calculated as the LOD divided by the square root of 2 [28]. The detailed specimen collection and laboratory methods are reported in the NHANES MEC Procedures Manual and Laboratory Procedure Manual [30, 31].

Demographic data were obtained from the NHANES household questionnaire. The data included sex (male, female), age (20–34, 35–49, 50–64, 65+ years), race and Hispanic ethnicity (Non-Hispanic White, Non-Hispanic Black, Non-Hispanic other races or multiple races, and Hispanic), and country of birth (United States, other countries including US territories). At the time of these analyses, the most recent data available from the NHANES occupational questionnaire was for 2005–2006 through 2013–2014. The data were coded into occupation and industry groups based on 4-digit US Census Bureau codes that are consistent with the Standard Occupational Classification (SOC) system and the North American Industry Classification System (NAICS). The public-use data files provided 22 broad industry groups and 23 broad occupational groups. We included all industry and occupation groups in this study with the exception of ‘Armed Forces’.

Descriptive statistics are provided for the participants’ demographic characteristics and for mean levels of serum PFAS by these characteristics. The geometric mean was used to assess the PFAS serum concentration. The geometric mean is commonly used to estimate central tendency for data that are distributed with a long tail. The geometric mean estimates and 95% confidence intervals were generated using NHANES survey subsample weights for US nationally representative estimates, considering the complex survey design and adjusted for age and survey cycle. Unreliable estimates, characterized by sample sizes less than 30 or relative standard errors exceeding 0.3, are marked with a symbol (ℙ) in the analysis tables [32]. All data analyses were performed using SAS-callable SUDAAN v12 software provided Research Triangle Institute, Research Triangle Park, North Carolina.

This study used pooled NHANES Lab data from 2005–2014 to estimate the serum PFAS concentrations of US adult workers across selected demographic characteristics, occupations, and industries. Our findings show that women, Hispanics, and non- US-born workers had lower mean serum concentrations of PFAS. These results are consistent with previous studies conducted on the US adult population [33, 34]. Additionally, we observed a dramatic decrease in PFAS concentrations among workers from 2005–2006 to 2013–2014 cycle, consistent with findings reported by Sonnenberg and colleagues [34]. This decline is likely influenced by the reduction in the use of PFAS chemicals in the United States since 2000 and may continue to decline as additional PFAS products are phased out through 2025 [35, 36]. Moreover, many manufacturers have developed non-fluorinated alternatives to PFAS-based finishing agents in response to increasing demand for more environmentally friendly options [37, 38]. Additionally, both federal and state governments in the US have implemented regulations on the sale and distribution of products containing PFAS [39-41].

The primary findings of this study show that there are wide variations in the mean serum PFAS concentrations across the 21 industries and 22 occupation groups. Among industry groups, the workers in Construction and Real Estate/Rental/Leasing sectors exhibited the highest mean serum PFAS concentration, followed by Utilities, while those in Private Households had the lowest mean serum PFAS concentration, followed by Health Care/Social Assistance and Accommodation/Food Services sectors.

This study found that those who were employed in Construction sector had one of the highest serum PFAS concentrations. PFAS chemicals are widely used in various building materials due to their resistance to heat, stains, oils, and water. These materials include roofing shingles (such as metal and asphalt shingles), weatherproofing membranes for flat and low-slope roofs, gutters, concrete, glass panels, ceramic fixtures, paints, carpets, caulking sealants, electric wires and cables, tapes, and composite wood products like plywood and wood fiber insulation [10, 42]. An Australian study has demonstrated that effective management practices (including enforcement and policing) and new treatment methods can address PFAS contamination in the construction industry [43].

We also found elevated serum PFAS concentrations among workers in the Utility sector. Within the energy industry, including renewable or alternative energy sectors, PFAS chemicals are extensively used in various materials or equipment, including sheathing for power cables, coatings for electrical wires, wind turbines, hydrogen fuel cells, high-capacity batteries, and solar panels [44]. It is possible that workers in energy-related fields may be exposed to PFAS [45]. This issue could warrant further study. Workers in Real Estate/Rental/Leasing sector had highest exposure to PFOS and PFOA. There are no reports as to how those in Real Estate may be exposed to PFAS. However, workers in ski rental and leasing, especially professional ski waxers, had up 25 times higher PFOS serum concentration than the general population due to workroom aerosols and ski waxes in a Norwegian study [46]. These workers may represent those with high PFAS concentration in the "Leasing" area of the Real Estate/Rental/Leasing sector.

Despite the widespread use of PFAS chemicals in various industries, we found that workers in the Agriculture/Fishing/Forestry sectors had high serum PFOS concentrations. PFAS have also found application in the agriculture industry. Certain pesticides contain PFAS to enhance wetting and penetration of insecticides. Lasee and colleagues found PFOS in 6 out of 10 tested insecticide formulae. Residual PFAS from these applications may remain in soil and be absorbed by crops in areas where insecticides are sprayed [47]. Given the broad range of activities within Agriculture, Fishing, and Forestry, future studies could analyze these industry sectors separately to better understand occupational exposure patterns.

For occupational groups, Installation/Maintenance/Repair occupations showed the highest mean serum PFAS concentration, followed by Construction/Extraction occupations. Food Preparation/Serving occupations had the lowest mean serum PFAS concentration, followed by Health Care Support/Service occupations and Personal Care/Service occupations. These findings underscore the importance of considering industry- and occupation-specific exposures when assessing serum PFAS concentrations in workers.

This study found that workers in Construction/Extraction occupations had the highest serum PFAS concentrations. Construction materials use PFAS for weatherproofing, corrosion prevention, and friction reduction, leading to higher exposure among workers in construction, maintenance, and manufacturing plants involved in producing these materials [11]. Additionally, due to their flame-retardant properties, chemical inertness, hydrophobicity, and dielectric strength, PFAS are intentionally incorporated into production in the electronics industry, including electronic devices, manufacturing processes, and semiconductor production [48].

Additionally, our study identified workers in Installation/Maintenance/Repair Occupations as having elevated serum PFAS concentrations. Fluoropolymers, a subset of PFAS, are extensively employed in the automotive industry for various applications. Fluoropolymers, a subset of PFAS, are used for various applications which include fuel lines, hoses (such as those for turbochargers and hydraulic systems), ABS brake lines, fuel containment and injection systems, lubricants, cylinder head gaskets, emissions reduction systems, and fuel cells and batteries in electric vehicles [49, 50]. Individuals working in auto repair shops, including workers and owners, are probably also exposed to occupational hazards associated with PFAS.

Limitations of this study should be noted alongside its strengths. While we compared serum PFAS concentrations among workers by industry and occupation groups, we are aware that individual serum PFAS concentrations would not result solely from occupational activities. Factors like individual lifestyles, including dietary habits and drinking habits at home, and exposure to materials in the living environment can also influence serum concentrations [2].

Second, the classification of 21 industries and 22 occupation groups used in this study is very broad, which may limit our ability to explore specific areas where high PFAS are used. For instance, a review article reported that workers in fluorochemical production plants had the highest serum concentrations (ranging from 120 to 11,000 ng/mL) [15]. As we mentioned in the Methods section, many PFAS have been phased out and replaced since 2000, and the current findings do not take into account these replacements. Data on occupation and industry from 2015–2016 and 2017–2018 have not been coded into Census industry and occupation groups, and industry and occupation were not collected in subsequent cycles. Additionally, classification codes were based on current work status, which may lead to misclassification, especially for workers who have changed jobs before or during the study period. Another limitation concerns the duration (i.e., half-life) of PFAS in the body. We do not have specific information on the excretion rates of each PFAS. However, research has shown that the estimated half-lives of PFAS vary widely due to several environmental and biological factors [2, 26, 33, 51], and thus readers could interpret our results keeping this limitation in mind.

Despite these limitations, a significant strength of this study lies in its utilization of a nationally representative database to obtain blood sample measurements and its adherence to a standardized classification system for occupational and industry groups.

In conclusion, our study reveals that workers in the construction, utilities, and installation/repair industry sectors exhibit elevated serum PFAS concentrations. These findings may contribute to the identification of worker groups with higher PFAS exposure for future studies investigating potential health impacts of occupational PFAS exposure. Future studies on PFAS exposure in the working population could consider more detailed occupational and industry groups, as PFAS levels can vary significantly within specific industry settings. By delving deeper into these variations, we can better understand the effects of PFAS on worker health and develop targeted interventions to mitigate potential risks.