Wildfire smoke drives most unhealthy air days during pregnancy

Clinical takeaway: Add wildfire exposure assessment and smoke-reduction counseling to prenatal care during fire seasons, with particular attention to patients in low-income, rural, American Indian and Alaska Native communities and areas with limited maternity care access.

Cleaner air on average can hide the days when exposure is highest. During wildfire season, pregnant patients may still encounter unhealthy fine-particle spikes even where year-round pollution has declined.

Decades of US pollution controls lowered ambient fine particulate matter (PM2.5), but those policies do not directly address wildfire smoke. Prenatal PM2.5 exposure has been linked to preterm birth and low birth weight, making the changing source mix clinically relevant. A national population-based modeling study assessed how wildfire smoke shaped prenatal fine-particle exposure trends across the contiguous US over nearly two decades.

By 2017 to 2019, 21.9% of infants had been exposed during pregnancy to at least one day when fine particulate matter reached 35 µg/m³ or higher. Among them, about 65% had an exceedance attributed to wildfire smoke. The overall rate had fallen sharply from 67.3% in 2003 to 2005, but the wildfire-attributable rate changed little, to 14.1% from 14.9%.

Average daily prenatal PM2.5 fell 34%, to 8.45 from 12.82 µg/m³. Wildfire’s share of total exposure nevertheless more than doubled, to 8.19% from 3.71%.

Wildfire’s share of air pollution rose most in low-income counties, to 11.51% from 5.31%. Among racial and ethnic groups, American Indian and Alaska Native infants had the smallest decline in total PM2.5 and the largest increase in wildfire’s share. In counties with maternity hospital shortages, wildfire’s share reached 10.57%, compared with 7.62% in counties with adequate access.

Investigators estimated prenatal fine-particle exposure for 64.5 million live births by linking birth records with modeled daily pollution in 2,842 US counties. Fire-attributable pollution was defined as the difference between simulations with and without biomass-burning emissions, including smaller contributions from prescribed and agricultural fires.

Wildfire smoke is increasingly an episodic prenatal exposure rather than only a regional environmental concern. Falling non-fire pollution does not eliminate short periods of high exposure, but this study modeled pollution rather than measuring birth outcomes or individual risk.

“The central contribution of this work is a shift in perspective. Prior research has mapped very well where wildfire smoke occurs, but public health resources are allocated to populations, not to acres,” Menglu Liang, an assistant clinical professor at the University of Maryland School of Public Health, said. “With these results, we can begin to answer who is exposed and where they live. We need this information to target interventions and allocate resources fairly.”

Source: Liang, et al (2026 Aug 7). Front Environ Health. Wildfire contributions to prenatal PM2.5 exposure in the contiguous United States, 2003-2019