Analysis of Ambient Ethylene Oxide Mixing Ratios in the United States

Ethylene oxide (“EtO”) is a hazardous air pollutant with high cancer potency; chronic exposures as low as 0.02 μg m–3 or ∼11 ppt correspond to an estimated cancer risk of ∼100-in-1 million (or 10–4), based on U.S. EPA risk estimates. We analyzed a nationwide data set of EtO mixing ratios from 24 h integrated canister samples paired with gas chromatography–mass spectrometry, collected between mid-2018 and 2024. Annual averages declined from 185 ppt in 2019 to 65 ppt in 2024, though there was substantial site-level variability. Mean mixing ratios varied by land-use category: 190 (Near-Source (High Emitters)), 83 (Near-Source (Low Emitters)), 76 (Mid-Range), 70 (Urban), 66 (Suburban), and 61 ppt (Rural). Even the lowest mean suggests cancer risk greater than 500-in-1 million. Seasonal differences were pronounced, with summer (94 ppt) exceeding winter (53 ppt). EtO was positively correlated with temperature and with ozone, and mixing ratios were modestly higher on smoke-impacted days compared to nonsmoke days after controlling for temperature. Intersite comparisons showed heterogeneous agreement, including both highly correlated and uncorrelated site pairs across near-source and rural environments. These findings demonstrate persistent EtO presence, substantial spatial and temporal variability, and potential links to secondary production pathways and biomass-burning factors, emphasizing the need for continued monitoring and mechanistic studies.