Prevalence and Implications of Per- and Polyfluoroalkyl Substances (PFAS) in Settled Dust

Abstract

Purpose of Review

Per- and polyfluoroalkyl substances (PFAS) are a family of more than 7,000 fluorinated compounds. The carbon-fluorine bond of PFAS provides desirable hydrophobic and oleophobic properties and stability that has led to widespread usage in consumer products and industrial applications. The strength of the carbon-fluorine bond also prevents appreciable degradation once released into the environment. Consequently, various household products can release volatile and nonvolatile PFAS into the indoor environment that often concentrate in dust. We discuss the diversity of PFAS in settled dust, emission sources of these chemicals, changes in PFAS profiles in dust over the past century, and the implications for human health.

Recent Findings

Sources of PFAS found in dust include building materials and furnishings and consumer products used in typical indoor spaces. Daycares and workplaces are emphasized as locations with widespread exposure due to the presence of treated carpeting and industrial-strength cleaners. Comparison and interpretation of findings across studies are complicated by the different ways in which PFAS are screened across studies. We further discuss recent developments in non-targeted software for the comprehensive annotation of PFAS in indoor dust and emphasize the need for comprehensive and harmonized analytical workflows.

Summary

We highlight the detection and diversity of PFAS in settled dust collected from various indoor spaces, including locations with vulnerable subpopulations. There are opportunities for future research to leverage settled dust as a sentinel environmental matrix to evaluate the link between inhalation and ingestion routes of PFAS exposure to adverse health.

Introduction

The prevalence of per- and polyfluoroalkyl substances (PFAS) in the environment signifies a major public health concern given the emerging literature reporting their potential to cause or exacerbate disease. PFAS have been used in the USA since the 1940s, initially in non-stick cookware and waterproofing [1, 2]. PFAS use has since expanded to hundreds of consumer products, including food wrappers and disposable food containers, rain jackets, self-adhesive note paper, fabric softener, cosmetics, waterproof sunscreen, aqueous firefighting foam, and carpeting [3, 4]. Ingestion is considered a dominant exposure pathway directly through contaminated food [5–7] or drinking water [8–10] or indirectly through PFAS-laden packaging and cookware [11, 12]. PFAS-coated garments, personal care products, and stationery items can also lead to exposure through dermal absorption, although this is a less likely path of exposure [3, 13].

Consumer products can transfer both short- and long-chain PFAS to dust either through the air or directly from the product itself, although the actual mechanism of transfer is unknown [14, 15]. Given that individuals spend over 90% of their day inside on average, the indoor environment is a significant route of exposure to airborne PFAS and to PFAS-laden dust [11, 16]. Infants and young children are especially susceptible to chemical exposures through settled dust, given the extended periods that they spend on or near the floor coupled with their high hand-to-mouth activity [17, 18]. Toddlers (aged 7 months to 4 years) have been estimated to ingest 41 mg/day of dust in comparison to 2.6 mg/day by adults [19]. When accounting for body weight, that converts to roughly 3.3 mg dust ingested per kilogram of body weight every day for toddlers, whereas adults would only ingest 0.04 mg/kg every day, nearly two orders of magnitude less. In addition to ingestion, inhalation represents another exposure route. Adults have been estimated to be exposed to 3.5 pg/kg of PFOS in the air daily [13]. Inhalation also presents a concern for infants exposed to PFAS-contaminated dust given increased respiratory rates and proximity to resuspended dust [20, 21].

Legacy fluorinated hydrocarbons, including long-chain persistent perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are not easily biodegraded and bioaccumulate [22–25]. Studies have demonstrated the adverse health effects of these chemicals. Perfluorooctane sulfonic acid (PFOS) has been linked to delayed metacognition and behavioral problems in children [12, 26–28], as well as impaired kidney function and kidney cancer [12, 27, 29, 30]. Perfluorooctanoic acid (PFOA) has been associated with an increased risk of thyroid disease and thyroid cancers [31–33], testicular cancer [29], and ulcerative colitis [34]. PFOA has further been linked to obesity and lipid dysregulation, including hypercholesterolemia and hypertriglyceridemia [31, 35–38]. Furthermore, shorter chain replacement compounds have also been found to be likely toxic via various mechanisms [39]. For example, 6:2 FTOH has been shown to cause liver and kidney damage, developmental, and reproductive damage [40]. Short-chain PFCAs identified in dust have been associated with birth defects and neurodevelopmental conditions, as well as immune, respiratory, and cellular system effects in children [41–44].

Longer carbon chain PFAS (≥C6 for PFSA, ≥C8 for PFCA) in consumer products are increasingly being replaced with shorter chain fluorinated compounds [2, 25, 39, 45]. The enhanced mobility of shorter chain PFAS in soil, water, and air compared to longer chained compounds presents added risks of inhalation exposure and increased bioavailability [39, 46]. These more volatile compounds include polyfluoroalkyl phosphate esters (PAPs), fluorotelomer alcohols (FTOHs), and perfluoroalkyl sulfonic acids (PFSAs) [47, 48]. PAPs and FTOHs can also degrade forming shorter chained PFCAs and PFSAs, respectively [47, 48]. These shorter chained compounds have been detected in the air and further concentrate in dust indoors [17, 49].

We review the emerging literature evaluating the occurrence and distribution of short- and long-chain PFAS in settled dust. Dust from various indoor spaces is considered, including residential houses, daycares, and workplaces, and detected PFAS are discussed with respect to source (Figure 1). It is important to note that the major and minor sources of PFAS in household dust have not been studied in a controlled or thorough manner, and therefore relative levels from various sources cannot be discussed. Scientific gaps related to PFAS composition, contributing sources, and exposure assessment are identified, and utility of settled dust as an environmental matrix for PFAS exposure assessment is highlighted for use in future health studies.

Fig. 1.

Overview of PFAS detected in settled dust collected from various indoor spaces with tentative sources. Because many studies are targeted, potentially dominant PFAS in different emission sources may be missed, and hence this list is not comprehensive

Indoor Sources of PFAS in Settled Dust

Variance Across Space and Time

The diversity and magnitude of PFAS detected in indoor settled dust vary internationally, underscored by governmental regulations on use in consumer products, furnishings, and building materials. Comparison of levels across geographic regions revealed several notable trends (Table 1). Elevated concentrations of PFCAs, such as PFOA, have been reported for studies based in Canada [50] (552 ng/g) and the USA [51] (296 ng/g) while lower levels have been found for samples collected at sites in Egypt [52] (0.89 ng/g) and Sweden [53] (7.71 ng/g). Studies conducted in Sweden, Australia, and the Faroe Islands have consistently reported elevated levels of PAPs in settle dust [53, 54].

Table 1.

Studies of major PFAS compounds (PFCAs, PFSAs, PFES, PFSMs, PAPs, fluorotelomers) in settled dust from residential, childcare, and work environments. Reported concentrations in dust samples range from 0.01 to 1790 ng/g.

PFAS levels in settled dust could vary with season, since seasonal differences have been reported among airborne concentrations. Ambient temperatures can impact partitioning of semi-volatile compounds between settled dust and other indoor surface, such as building materials, furnishings, and consumer products [55, 56]. Our understanding of the dynamics of PFAS partitioning between these indoor compartment is still limited [57]. To better understand the seasonal variations in PFAS concentrations, one longitudinal study sampled air in living rooms in Tianjin, China, once from July to September and again in December. This study reported short-chain PFSAs and PFCAs to be elevated in the air during the winter, while longer chained PFCAs (i.e., perfluorononanoic acid (PFNA); perfluoroundecanoic acid (PFUnA)) have been reported to be higher in the summer likely due to ventilation, temperature, and sunlight [58, 59]. Most FTOHs have also been observed to attain maximum levels during the summer, whereas 6:2 FTOH has peak levels in winter, possibly due to seasonal attire [58]. Variation of PFAS across season and different indoor environments is ultimately dependent on the presence of PFAS sources, as well as changes in the environment including temperature and ventilation.

We reviewed the literature for sources that contribute to PFAS in settled dust from residential, childcare, and occupational environments. These sources include rugs and carpets, food packaging, cosmetics, building materials and furnishings, paper products, vehicles, clothing, insecticides, and electronics. There is insufficient data to suggest which of these are dominant contributors to settled dust PFAS, which vary across study regions and households.

Rugs and Carpets

Floor coverings are a major source of PFAS. PFAS are widely applied to rugs and carpet to confer water, soil, and stain resistance [60]. These textile coatings (e.g., Scotchguard) have been reported to yield high levels of FTOHs, PFCAs, and PFSAs in settled dust, raising concerns about occupant exposure levels [50, 55, 61]. Decreased PFAS levels have been reported in spaces with handmade, non-treated carpets [52].

Higher levels of various PFAS in dust have been observed in homes, childcare facilities, and workplaces with carpeting as compared to indoor spaces with other flooring materials such as waxed wood, laminate, vinyl tiling, and linoleum [16, 49, 50, 62–65]. For example, findings from settled dust collected from childcare facilities and offices have demonstrated increased 6:2 FTOH, 8:2 FTOH, and 10:2 FTOH concentrations in spaces with carpeting compared to other flooring materials [49, 53, 66]. Furthermore, longer chained PFCAs, including PFNA and perfluorododecanoic acid (PFDoDA) have been detected at higher levels in childcare [44, 66], residential [49, 50, 63, 64], and office spaces [49] with carpeting when compared to similar spaces without. Interestingly, PFAS has been reported to be applied to floor polish, laminated floor covering, and resilient linoleum [67]. Therefore, it is important to note that flooring materials may also emit PFAS and increase PFAS exposure, albeit at a lesser degree then carpeting. Beyond higher potentially higher levels of PFAS in carpeting, a further reason for carpets leading to higher PFAS may be the ability for carpets to trap PFAS containing dust and emissions, leading to longer term exposure.

PFOS and other longer chain PFAS in water- and stain-repellent coatings have been replaced in many countries by a short-chain PFAS, for example, (C4): perfluorobutanesulfonic acid (PFBS) [68, 69]. This replacement compound has been detected in settled dust samples collected in homes from Canada, China, the Czech Republic, Egypt, Norway, Spain, and the USA [18, 49, 51, 52, 54, 58, 63, 64, 70–72]. This compound has also been found in daycares and preschools in the USA and Sweden [53, 66]. In childcare facilities across California, a positive correlation has been reported between PFBS levels in settled dust (8.37 to 386 ng/g) and paired carpet samples (5.13 to 884 ng/g) [44]. Elevated levels of PFBS have also been detected in offices with wall-to-wall carpeting, as reported for settled dust samples collected from office spaces in Greece [62].

While legacy fluorinated coatings are being replaced/phased out [73], older rugs and carpets continue to be a source of N-methylperfluorooctane sulfonamidoethanol (MeFOSE) and long-chain PFAS such as PFOS and PFOA [15], with MeFOSE being detected in various homes from Australia (51 ng/g) to Canada (742 ng/g), at elevated levels, as compared to Spain (0.51 ng/g) [54, 63].

Since carpets serve as both sources and sinks of PFAS in dust [15], carpet material, the composition of water and stain repellants, the frequency of applying these repellants, and cleaning procedures are all key to understanding and predicting exposure levels and the PFAS species one is exposed to. Both of these predictions are important for understanding the health risks imposed on those exposed to carpet within indoor environments. PFAS levels from carpeting and flooring materials are dependent on cleaning and treatment procedures. Treated carpets are some of the highest emitters of certain PFAS in the indoor environment. A study in Canada reported settled dust from homes with treated carpets contained an order of magnitude higher levels of PFHxS (2,780 ng/g) and PFOS (1,090 ng/g) than homes with untreated carpets (PFHxS: 17 ng/g; PFOS: 132 ng/g) [50]. Additionally, FTOHs are present in carpet stain repellants and floor waxes [61]. The use of cleaning products can further elevate PFAS exposure: the frequency of surface cleaning has been positively correlated with perfluorotridecanoic acid (PFTrDA) in settled dust [18]. This compound has been detected in settled dust from carpeted indoor spaces, ranging from 6.93 ng/g in residential locations [18, 54, 63, 64] to 2.62 ng/g in childcare facilities [44, 66] and 21.6 ng/g in office spaces [49]. Other PFAS in cleaning products include PAPs/diPAPs, FTOHs, and fluorotelemer sulfonates (FTSs), where PAPs have been detected in settled household dust in Australia and the Faroe Islands [3, 61, 74]. Notably, FTSs are common in industrial cleaners, and thus, the predominance of FTSs in childcare facilities emphasizes the impact of PFAS in non-residential settings [3, 61, 66, 74].

Regional differences in the contribution of carpets and rugs to PFAS in dust have been observed for different countries (Table 1). For example, the treatment of flooring, carpet, and rugs with PFAS-containing formulas is the contributor to PFAS in these materials, and countries including Egypt, with less treated carpeting, have lower levels of PFAS in homes [52]. Elevated levels of PFSAs, notably perfluorohexane sulfonic acid (PFHxS) and PFOS, have been found for dust samples collected in North America (Canada [50], USA [51]) as compared to Europe (Sweden [53], Greece [62], Spain [63]); increased levels have been attributed to installation of wall-to-wall carpeting in North America. Furthermore, certain species of PFAS are only elevated in certain countries in carpeted spaces, suggesting industrial production and/or cleaning procedure may introduce a different profile of PFAS species depending on region. For example, FTOHs have also been detected in carpeted indoor spaces with elevated levels reported by studies based in the USA [44, 49, 51, 66], in contrast to Europe which had lower levels detected [52, 53, 63].

Consumer Products

Cosmetics, including foundation (powder, liquid, waterproof items), lip products, and nail polish, can contain PFCAs, diPAPs, and PFSAs [3]. These compounds commonly contain solubilizers as well as foaming and dispersing agents (i.e., Masurf FS-130, Masurf FS-2240), which are known to contain diPAPs, a PFAS which is also a precursor of PFOA and PFDA [75, 76]. Masurf-FS-130 is found in cosmetics, alcohol-based hand sanitizers, and sunscreen, while Masurf FS-2240 is found in floor polishes, water-based latex paints, and cleaning products [75]. Studies exploring cosmetic products have reported PFCAs and PAPs dominate total PFAS concentrations [77]. Partitioning of fluorinated compounds between cosmetic products, settled dust, and air remains unclear.

Food Packaging

PAPs, FTOHs, PFCAs, and PFSAs are commonly used in food packaging for their water- and oil-resistant properties [75, 76]. These compounds can migrate into food, especially in the presence of emulsifiers and heat [78, 79], raising concerns about elevated exposures and the associated impact on health. PFAS concentrations in disposable food packaging vary. Bread and dessert wrappers and food packaging have been reported to have higher levels of fluorinated chemicals, while items such as paper cups have been reported to have low PFAS concentrations [80]. Studies have shown level vary across geographic regions with 10:2 FTOH and other long-chain FTOHs more commonly detected in food wrappers assessed in China compared to the USA [79]. Interestingly, short-chain replacement compounds of PFOS and PFOA have been detected in food packaging from the USA [80]. These compounds have also been found in indoor settled dust collected in various indoor environments where packaged food is handled. PAPs and PFCAs in dust have been found in homes internationally (Canada, Faroe Islands, Spain, Norway, Sweden, Greece, Egypt, Nepal, Japan, and Australia) [52, 54, 57, 75, 81]. MonoPAPs and diPAPs accounted for the bulk of the total PFAS measured in settled dust collected at most of these study locations; Egypt was the exception with lower levels of PAPs and elevated PFOS [52]. PFCAs have also detected in settled dust from restaurants and coffee shops in the USA and Europe with PFBA reported to be the most common compound; these studies specifically attributed these fluorinated compounds to disposable food package [62, 80, 82]. In a few regions, PFAS has been banned in food contact materials (including food packaging) including in Denmark and certain cities or states in the USA (e.g., San Francisco) [83]. The regulations have had an impact, with tested food packaging in Denmark, for example, being detected to be free of PFAS treatment.

Building Materials

A diverse range of PFAS detected in settled dust has been linked to building materials, from flooring materials/finishes to insulation and wall coverings; unique PFAS profiles have been identified to each. Settled dust from rooms with plastic flooring have been reported to have elevated levels of PFOS compared to wooden flooring [17]. Wooden floors, however, can contribute to PFAS exposure if treated with sealants or adhesives, including PFPA, PFCAs (PFOA, PFHxA, PFHpA), and PFSAs (PFBS, PFHpS, and PFOS) [84]. Recent evidence also suggests PFAS may be emitted from coatings, oriented strand board, sealant, glass, façades and substitutes, awnings, and insulation [84, 85]. Paints have been found to have low PFAS levels in certain cases but further testing is needed to confirm findings [86]. While evidence exists that the PFAS content and diversity in indoor dust has changed across time, limited research currently exists on the contribution of building materials to this trend. This limited research does suggest building materials as a potential contributor to the shift in certain PFAS levels in household dust.

As new building materials emerge on the market and older building materials age, shifts have been observed in the PFAS detected in settled dust. Waterproof fabrics and architectural resins manufactured in the 1970s and 1980s have been associated with PFOS, PFNA, and polytetrafluoroethylene (PFTE) [87]. Despite the phase out of PFOS in 2003, other long-chain compounds and short-chain replacements for PFOS are still detected in renovated and newer homes. Specifically, studies have consistently found increased PFOA in homes built after 2003 in North America and Europe [18, 51, 64, 70]. As part of the 2010/2015 PFOA EPA Stewardship Program, PFOA synthesis has been phased out by eight companies in the USA and is no longer imported into the country as of 2012 [88]. Historical analysis of building materials for PFAS is needed to understand the contribution of newer and older building materials to PFAS loads in the indoor environment, while also considering that newer PFAS may degrade into PFOA and PFOS as products age. The construction style of older homes with greater natural ventilation and infiltration/exfiltration may result in decreased exposure to more volatile PFAS [18, 49, 89].

Furniture

Furniture treated with water- and stain-repellent coatings can contain PFAS, including various PFSAs and PFCAs [12]. Settled dust collected from couches in Norwegian homes and American offices has been reported to be dominated by PFOA, as well as PFHxA and PFBA [72, 90]. Comparison of residential spaces revealed elevated PFAS concentrations (approximately five times) in settled dust collected from rooms with older furniture items when compared to those rooms with newer furnishings [90]. Homes in Wisconsin constructed from 1968 to 1995 have been found to have high levels of PFOS and PFHxS, suggesting that furnishings, draperies, and floor covers manufactured pre-phase out of PFOS are likely sources [70]. Childcare facilities have also reported elevated levels of PFSMs and FTOHs in settled dust collected around nap mats; MeFOSE, EtFOSE, and 6:2 FTOH were most abundant in the mats and settled dust [66]. Exposure to PFAS derived from mats used for napping by children, as well as indirect exposure to fluorinated compounds in settled dust, presents an growing health concern given the prolonged contact children have with these items [66].

Clothing/Textiles

Clothing and other extiles have been identified as a source of PFAS. Garments with water-repellant coatings (i.e., Gore-tex) are a source of PFCAs (PFHxA) and FTOHs. These compounds have been detected in residential settled dust and specifically attributed to the presence of Gore-tex products [18, 45, 91, 92]. Firefighters’ protective turnout gear has been shown to be a source of PFSMs, notably EtFOSE, which has been detected together with its degradation product, EtFOSAA, in settled dust collected from gear storage locations [93, 94]. In line with these findings, a comparison of retail shops identified the highest levels of PFAS in settled dust collected from outdoor equipment stores; concentrations were dominated by FTOHs (6:2 FTOH and 8:2 FTOH), accounting for over 90% of total PFAS [58].

Insecticides

Insecticides have been reported to be a source of PFAS. Products used for controlling ants contain EtFOSA. EtFOSA (pesticide commonly named Sulfluramid or Mirex-S) is the active ingredient often at around 0.3% (3000 ppm) and has been banned in certain countries (including the USA) but is still used commonly in certain regions of Latin America. Application of these insecticides has raised concern given the potential for biodegradation of EtFOSA to FOSAA, FOSA, PFOS, and PFOA in crops where the pesticide is often applied [95]. Elevated levels of EtFOSA have been reported in settle dust collected from Australian homes (2000 ng/g) prior to the products phase out in 2004 [65, 96]. EtFOSA has been detected in more recent studies in settled dust from homes in South America where this PFAS-containing insecticide is still in use [97].

Paper Products

Elevated concentrations of PFCAs (PFOA, PFHxA) and PFSAs (PFOS) have been detected in settled dust collected from offices in Belgium and Sweden [16, 98]. High-volume paper product use or paper manufacturing processes (i.e., newspaper and book publishers) have been suggested to contribute to increased exposure to PFOS and PFOA [16]; these compounds are known to be in inks, applied paper and cardboard, and found in the transfer belts of copiers and printers [16, 99, 100].

Vehicle-Related

Settled dust collected from vehicles has been found to contain elevated PFAS levels, attributable to water- and stain-repellent coatings on textiles. PFSMs (MeFOSE), PFCAs (PFOA), PFSAs (PFOS, PFBS), and FTOHs (6:2 FTOH, 8:2 FTOH, 10:2 FTOH) have been detected by studies in Egypt, the UK, and the USA [49, 52, 65]. A reduction of PFAS in cars was observed following regulations mandating replacement of PFOS with short-chain compounds, such as PFBS, in vehicles [2, 70]: vehicles manufactured between 1995 and 2001 have been reported to have 4.4 times higher concentrations of PFOA and PFOS, as compared to collected dust in vehicles from 2004 to 2007 in the USA [49]. A comparison of in-car PFAS levels was also reported to vary internationally; dust from cars in Egypt had lower PFAS concentrations compared to the USA (20 times) and the UK (67 times); elevated ambient temperatures in Egypt were thought to contribute to enhanced volatilization of these compounds and decreased levels in settled dust [49, 52, 65]. Dust from vehicles also poses a potential health risk to occupants, with levels and types of PFAS exposure varying based on the vehicle age and surface treatments.

Electronics

Indoor spaces containing large amounts of electronics, such as stores, offices, libraries, and internet cafés, have been reported to have elevated levels of PFCAs and PFSAs in settled dust, dominated by PFOS and PFOA [62]. These fluorinated compounds are derived from electronic wire coatings [101]. PFOS replacements for metal plating and electronics, such as 6:2 chlorinated polyfluorinated ether sulfonate (Cl-PFESA) and 8:2 Cl-PFESA, have also been identified in settled dust collected from residential sites in China [102–104]. Additional research on the impact of electronics to PFAS in dust exposures is needed, particularly in public and residential microenvironments.

Targeted and Non-targeted Analysis of Indoor Dust Samples and Harmonization of Analyses

As has been discussed, settled dust contains a diverse array of PFAS, representing the majority of sources of PFAS from the indoor environment, ranging from consumer products, such as hair spray, rain jackets, food containers, and fabric softeners, to furnishings including couches, carpets, and blinds [80, 105–109]. Every product that contains PFAS is comprised of a diverse suite chemical moieties and carbon chain lengths to best match the chemical and physical properties required for the application. Furthermore, the mode of manufacturing, which has changed over time, has led to changes in PFAS structural diversity, with historical synthesis leading to numerous branched-chain PFAS, whereas current methods mainly lead to specific linear-chain PFAS [110]. The range of PFAS structures is enormous, due to the constant introduction of new compounds and both the stability of legacy compounds and possible byproducts when parent PFAS do breakdown in the environment; a non-exhaustive list of over 7000 unique chemical structures can be found in the EPA PFAS master list [111]. In addition, the manufacturing process may unintentionally introduce PFAS into products, further affecting exposure risk and detectable levels of compounds. Because of this enormous diversity of PFAS in products, the number of products that contain PFAS in indoor environments, and the assumption that settled dust is a sink for a wide range of these PFAS, it is important that methodologies are developed to be as comprehensive as possible (e.g., cover the wide range of PFAS which may exist). Dust collection methods (e.g., vacuum versus air filter), sample preparation, storage, extraction methods, and acquisition methods and instrumentation used can all impact PFAS coverage. Furthermore, data-analysis methods used for non-targeted approaches, which use high-resolution mass spectrometry (the most comprehensive profiling technique available), can significantly impact the resultant PFAS coverage. Future work applying non-targeted and comprehensive data-processing algorithms are needed to better characterize PFAS indoor exposure via dust as a sentinel matrix.

Most current methods for characterizing dust use targeted methodologies (often using triple-quadrupole mass spectrometers) and have the advantage of providing accurate concentrations and confident identifications but are generally limited to a short list of PFAS compounds (30–40, generally). Comprehensive non-targeted analysis that provides putative annotations is therefore needed to ensure that abundant and/or highly toxic PFAS, which are previously uncharacterized, are not missed. For example, using non-targeted methodologies, 74 PFAS were characterized in settled dust, compared to the 10–30 PFAS that are generally monitored in targeted analyses [112]. These included previously uncharacterized abundant species in settled dust, such as 2,3,4,5-tetrachloro-6-[[[3-[[(1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctyl)sulfonyl]oxy]phenyl]amino]carbonyl]-benzoic acid (TCBA-BA-PFOS) [112].

These non-targeted methodologies can be time-consuming and, hence, costly, limiting their adoption. Furthermore, each laboratory performs non-targeted analysis differently (both mass spectrometric analysis and data workup), resulting in challenges when assessing the confidence of the annotation results, and therefore, streamlined harmonization of results is not possible at the present time.

Proper annotation is one of the most challenging and time-consuming aspects of non-targeted analysis. Multiple steps must be considered by researchers, narrowing application to experts in the field, including MS/MS coverage using data-dependent, data-independent, and intelligent acquisition approaches, increasing library coverage using in silico approaches based on standard MS/MS and finding homologous series based on exact mass patterns within and across PFAS classes [113]. The recently released FluoroMatch software fully automates the non-targeted PFAS workflow [114] and provides comparable results to less automated approaches independently of vendor or analytical platform [115]. Future applications of automated tools for non-targeted PFAS annotation in dust include implementing FluoroMatch, CFM-ID, MetFrag, Compound Discoverer (ThermoFisher Scientific), and other software, each with the potential to provide insights into novel PFAS and previously undiscovered PFAS of concern from various household products and other sources [114, 116, 117]. Automation using open-source software not only increases coverage and throughput but, in addition, if accepted and adopted by numerous laboratories, increases harmonization and combines effort into a single user-friendly platform.

Conclusion and Future Directions

The present review highlights numerous sources of PFAS emission indoors, both suspected dominant and minor contributors, which accumulate in settled dust in the household, workplace, and schools, as well as the most prevalent species. Certain sources of PFAS have been found to be directly linked with PFAS levels in settled dust and serve as sources including carpets and rugs, cleaning products (and the frequency of cleaning), floor materials and treatments, and furniture. PFAS levels and types in settled dust have changed across the decades as PFAS in construction materials and consumer products shift in concentrations and types and as materials age. While mechanisms of transfer between PFAS in the air and dust are unknown, associations between compounds and their degradative products emphasize the need to reduce exposure, especially for infants and children who are more susceptible to breathing significantly more resuspended dust. As noted, the bioaccumulative nature of PFAS is associated with systemic health conditions among all ages. Therefore, the importance of categorization and identification of these exposure sources is paramount to phase-out efforts of these toxic PFAS, which can have detrimental effects on health.

In addition to governmental regulation for the phase-out of PFOS and PFOA with short-chain replacement compounds, several green initiatives are focusing on sustainable solutions to restrict use of long-chain PFAS. These programs include NSF/ANSI 140–2015: Sustainability Assessment for Carpet and Oeko-Tex 100, both of which provide standards and certifications of sustainability for carpet and textile materials [15, 118, 119]. These regulations are limited, likely due to the regulatory and analytical challenges of including numerous PFAS, to only a few species. For example, Oeko-Tex 100 as of Jan 2021 only regulate PFOA and “PFOA like” substances as well as PFOS, 2-Propenoic acid, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl ester, PFHxA (C6), and PFBA (C4) [120, 121]. PFAS in outdoor, water-resistant clothing, represent another area of potential sustainability, as a study found that non-fluorinated clothing granted similar water-repellency when compared to short- and long-chain PFAS [106].

Because of the pervasiveness of PFAS in numerous consumer goods and construction materials, it is challenging to determine all the sources of PFAS in dust and their relative contributions. All studies reviewed here were performed in uncontrolled environments (people’s homes, office building, etc.), controlled studies where certain activities, consumer goods, etc., are introduced or removed with numerous replicates, would be helpful to quantify the influence of various sources for prioritization. These studies would also benefit from the creation of models to examine the fate and transport of PFAS and to quantify and relate PFAS exposure based on levels in settled dust. Furthermore, certain potential emission sources of PFAS in the indoor environment are understudied (e.g., most construction materials) and, hence, it is important to understand whether these are major unknown sources of PFAS in residential homes, childcare facilities, and office spaces. Future studies should be conducted to investigate the nature of compounds in newer construction materials, as PFAS levels and species contained in these materials and their corresponding emissions under different living conditions are presently unknown.

One challenge in understanding the diversity of PFAS in settled dust and how this diversity changes within different indoor environments across time is the difference in the types and number of PFAS assessed in different study locations. As Table 1 exemplifies, no study assesses all PFAS frequently observed and, hence, even commonly occurring PFAS measured in one study are often not screened for in another study by a different research group. Since targeted measurements of PFAS require an assumption on the dominant types of PFAS within a particular setting to measure and are not comprehensive, hundreds of PFAS that exist in the environment can be missed, including potential dominant species. An area of future work includes employing non-targeted studies of PFAS in settled dust, thus allowing for a more exhaustive meta-analyses on the types of PFAS, their sources, and their dominance in different microenvironments. Due to the limited number of non-targeted analyses of PFAS in settled dust, likely due to the expertise, time, and money needed to do these studies, it is impossible to do a meta-analysis on all studies to understand which types of indoor environments, what products, and which locations have the highest levels of PFAS other than the most common species (e.g., PFOS and PFOA). Furthermore, the extraction methods and instrumentation used can also influence the types of PFAS observed.

Harmonization of methodologies and automation of nontargeted workflows could help to make comparisons of PFAS levels in dust samples across different studies possible, which, in turn, would allow the prioritization of regulation, behavior, and materials and consumer products chosen to reduce indoor PFAS exposure. Automated software, including Compound Discoverer and FluoroMatch, are reducing the expertise and time needed for non-targeted analysis [114, 115]. Furthermore, reference materials, such as SRM 2585 (Organic Contaminants in House Dust), which contain reference levels of PFAS in house dust, can be used to validate quantitative approaches and ensure that the concentrations can be compared across studies and laboratories. As methodologies improve and PFAS coverage increases, along with more studies characterizing the indoor sources and environmental dynamics affecting levels of PFAS in dust, we will begin to understand the link between PFAS levels in settled dust, emission sources, and exposure levels. This may allow settled dust to become an important surrogate for determining the risk of PFAS exposure in different indoor environments and its subsequent impact on human health.