Citizen-science data reveal drivers of PFAS variability in backyard chicken eggs

Per- and polyfluoroalkyl substances (PFAS) are widespread synthetic contaminants that can accumulate in food, such as chicken eggs. Although European Union maximum levels for PFOS, PFOA, PFNA and PFHxS apply to commercialised eggs, backyard chicken egg concentrations frequently exceed these limits. Consequently, in the Netherlands, the national health authority advised against consuming backyard eggs. In response, many chicken owners tested their eggs and openly shared the results through a citizen-science initiative, resulting in the first large, citizen-led PFAS dataset worldwide. The dataset comprises 801 laboratory measurements and includes information on husbandry conditions, including uncovered outdoor space, flock size, feed type and water source. We conducted an exploratory analysis to identify variables associated with variation in the egg concentrations of the EFSA-4 PFAS: PFOS, PFOA, PFNA and PFHxS. Contamination was widespread, with only 36% of samples below EU maximum levels. PFOS dominated, accounting for 87% of the mean summed concentration. Repeated egg sampling revealed substantial temporal variation, indicating that concentrations are dynamic and that single measurements do not reliably reflect human exposure. Generalised additive models showed that surface area of uncovered outdoor space per hen was often the strongest predictor, with larger foraging areas being associated with higher PFAS levels. Larger flock size, higher egg production, and continuous access to food were associated with lower concentrations for three of the four PFAS. Diet and water source were not significantly associated with PFAS levels. Spatial patterns showed decreasing PFAS concentrations with distance from the coast, possibly caused by sea spray aerosol deposition, while PFOA levels were strongly related to distance from a fluorochemical plant, with the highest concentrations downwind. These results suggest that atmospheric deposition shapes PFAS spatial distribution, and uptake by free-ranging hens most likely occurs predominantly via ingestion of contaminated soil invertebrates. Future experimental studies could evaluate mitigation options targeting the soil compartment, including soil modification or limiting soil fauna access through chicken enclosure modifications.