Indoor air carries more PFAS than the outdoor air around it

Most public discussion of PFAS exposure is about drinking water, and after that about food packaging. Two studies published three weeks apart looked at a route that gets far less attention: the air people breathe indoors, and the dust that settles in the rooms they spend most of their time in.

Eight schools in Porto

The first study collected 36 indoor and 8 outdoor air samples from eight primary schools in Porto, Portugal's second-largest city, and analysed them for 64 PFAS covering both perfluoroalkyl acids and their precursors [s1].

Two results stand out. The first is compositional. Precursors — compounds that are not themselves PFAAs but degrade into them — predominated over the terminal PFAAs, accounting for 66.5% of total PFAS indoors and 64.6% outdoors [s1]. Among the PFAAs themselves, perfluoroalkyl carboxylic acids were the dominant class in both indoor and outdoor air, at median concentrations of 3822.3 and 495.1 pg/m³ respectively [s1]. Among precursors, polyfluoroalkyl phosphate esters led at medians of 2811.6 pg/m³ indoors and 732.0 pg/m³ outdoors, followed by fluorotelomer alcohols at 1436.6 and 313.5 pg/m³ [s1]. The largest individual contributors indoors were trifluoroacetic acid, PFOS, nonafluoro-3, 6-dioxaheptanoic acid, di-sulfonamido polyfluoroalkyl phosphate and 8:2 fluorotelomer alcohol [s1].

The second is directional: all PFAS classes showed indoor enrichment relative to outdoor air [s1]. The building is not filtering an outdoor problem. It is contributing to it.

Because inhalation exposure is normalised to body weight, children came out with higher exposure than adults in the same rooms [s1]. Which compounds dominated that exposure depended on the scenario: fluorotelomer alcohols contributed most to estimated daily intake under the low-exposure scenario, while PFCAs predominated under the high-exposure scenario [s1].

Forty homes in the Faroe Islands

The second study takes advantage of an unusual natural experiment. The Faroe Islands have no local PFAS production and no manufacturing of consumer products, yet residents show elevated serum concentrations [s2]. Consumption of pilot whales, a traditional part of the Faroese diet, contributes to that exposure but does not fully account for observed trends in serum — which implies other pathways [s2].

Researchers measured air and dust across 40 homes, covering nonvolatile precursors, volatile neutral precursors and legacy ionic PFAS, and estimated daily intake for 15-year-old children [s2]. Indoor exposure came to 0.001, 0.012 and 0.129 ng/kg bw under low, intermediate and high scenarios [s2]. Set against the European Food Safety Authority's tolerable weekly intake, that is roughly 0.2%, 2% and 20% respectively, against about 60% from average pilot whale consumption [s2].

Indirect exposure to perfluoroalkyl carboxylic acids from air contributed the most to estimated intake, followed by indirect exposure to PFOS in dust under the high-exposure scenario [s2].

What the arithmetic implies

Diet still dominates in the Faroese case, by a wide margin at typical exposure. But the ratio is not fixed. The authors note that consumption of pilot whale is declining [s2] — and as the dietary term shrinks, the indoor term does not, which changes what a control strategy would have to address.

There is a second, subtler implication in both papers. The compounds dominating indoor air are largely not the ones regulations name. Precursors made up around two-thirds of total PFAS in the Porto schools [s1], and in the Faroese homes the largest single contribution came through indirect exposure — precursors that become PFCAs — rather than direct intake of the acids themselves [s2]. A monitoring programme built around PFOA and PFOS in water measures neither.

Limits worth keeping in view

Neither study measured anything in a person. Both estimate intake from environmental concentrations using exposure scenarios, which is a modelling exercise resting on assumptions about how much air a child breathes and how much dust a child ingests. Neither measured a health outcome.

The Porto study covers eight schools in one city, and the Faroese study 40 homes in a population whose exposure profile is genuinely unusual [s1] [s2]. Neither is a national picture, and the Faroese numbers in particular should not be transplanted to other countries — the pilot whale term has no equivalent elsewhere.

What is transferable is the structural finding, which both studies reach independently: indoor environments are enriched relative to outdoors, precursors dominate the mixture, and children take in more per kilogram of body weight than the adults beside them.

Sources

• [s1] Per- and polyfluoroalkyl substances (PFAS) in Portuguese primary schools: Occurrence in air and implications for children's inhalation exposure. Environmental Research, 21 August 2026. https://doi.org/10.1016/j.envres.2026.125533
• [s2] Contribution of Indoor Air and Dust to Children's Per- and Polyfluoroalkyl Substance Exposure in the Faroe Islands. Environmental Science & Technology, 30 July 2026. https://doi.org/10.1021/acs.est.6c02003