Radical-induced autoxidation dominates squalene oxidative aging under environmental conditions.

Ozone has been viewed as the principal oxidant of skin oils and other unsaturated surface films, but their fate under realistic indoor conditions remains poorly constrained. Here, we use squalene (SQ), a major component of skin lipids, to investigate the dominant oxidation pathways in occupied indoor environments and laboratory flow reactors. With exposure to indoor air, SQ decay and reactive oxygen species (ROS) formation accelerated over time, consistent with autoxidation. Laboratory experiments with ozone alone drove first-order SQ loss and humidity-dependent emissions of volatile organic compounds (VOCs). In contrast, exposure to both ozone and •OH produced nonexponential SQ decays, higher ROS yields, a much stronger in-film •OH source, and condensed-phase products enriched in highly oxidized organic hydroperoxides that closely match those observed indoors. Unexpectedly, gas-phase •OH strongly suppressed VOC emissions. These results demonstrate that gas-phase radical-induced autoxidation governs SQ aging, challenging the prevailing ozone-centric view of indoor skin oil oxidation.