Ambient formaldehyde (HCHO) is a leading contributor to cancer risk among the 187 hazardous air pollutants (HAPs) regulated in the United States (U.S.). Fires emit HCHO and its precursors and enhance secondary HCHO formation downwind, raising concerns about health impacts. However, the contribution of fire to ambient HCHO remains poorly quantified because ground-based monitoring is sparse and largely urban. Here, we present the first satellite-based quantification of fire-attributable HCHO enhancement and associated health risks across the contiguous U.S. from 2020–2023. We improve TROPOMI retrievals of HCHO columns to account for smoke aerosol effects, and evaluate the changes in column and surface HCHO under fire smoke using NOAA’s HMS smoke product. Annual mean HCHO shows a distinct increase under fire smoke, with vertical column density increased by 6%–16% and surface HCHO by 6%–11%. The spatial distribution of this enhancement aligns with major wildfire activities across California in 2020, the Pacific Northwest in 2021 and the Southern U.S. in 2022, and is most extensive in 2023, likely reflecting the combined influence of Canadian fire smoke intrusions, U.S. wildfires and elevated temperatures. Exposure at 2020–2023 levels translates to an estimated 275–482 excess lifetime cancer cases, indicating a non-negligible health burden from fire-related HCHO even in years of moderate fire activity.

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