Per- and polyfluoroalkyl substances in paired serum and breastmilk samples among pregnant farmworkers in Thailand.

Per- and polyfluoroalkyl substances (PFAS) are widely detected in pregnant persons and can be transferred to the developing fetus in utero. Breastfeeding may represent an important source of PFAS exposure for infants. However, studies quantifying levels of PFAS in breastmilk samples remain scarce, particularly in low- and middle-income countries. We examined breastmilk as a postnatal PFAS exposure source among mother-infant pairs in Thailand. Pregnant farmworkers were enrolled in the Study of Asian Women and their Offspring's Development and Environmental Exposures (SAWASDEE), a prospective birth cohort in Northern Thailand, between 2017 and 2019. We quantified levels of eight PFAS in maternal serum samples obtained during the second trimester, as well as in breastmilk samples obtained at 9.5 months and 11.5 months in infancy (N = 46 matched pairs). For each PFAS, we calculated lactational (serum to milk) transfer efficiencies and lactational estimated daily intake. PFOA, PFOS PFHxS, PFNA, PFDA, and PFUnDA were detected in >90% of serum samples. PFOS was detected in >60% of breastmilk samples obtained at 9.5 and 11.5 months, while PFNA was detected in >50% of 9.5-month breastmilk samples only. All remaining PFAS were detected in <50% of breastmilk samples. The lactational transfer efficiency for PFOS was 7.03% (SD = 5.78) and 5.83% (SD = 5.21) at 9.5 and 11.5 months, respectively. The lactational estimated daily intake for PFOS was 12.1 ng/kg bodyweight/day (SD = 5.49) and 10 ng/kg bodyweight/day (SD = 6.22) at 9.5 and 11.5 months, respectively. For PFNA, the lactational transfer efficiency and estimated daily intake at 9.5 months was 14.7% (SD = 14.3) and 6.14% (SD = 3.40), respectively. Within one of the first PFAS biomonitoring studies conducted in Thailand, we found that legacy PFAS were widely detected in serum, and some compounds were also detected in breastmilk of farmworkers. This study provides new evidence enhancing our understanding of postnatal exposure to PFAS.