Reactions of ozone with human skin lipids: sources of carbonyls, dicarbonyls, and hydroxycarbonyls in indoor air

Abstract

This study has used proton transfer reaction-mass spectrometry (PTR-MS) for direct air analyses of volatile products resulting from the reactions of ozone with human skin lipids. An initial series of small-scale in vitro and in vivo experiments were followed by experiments conducted with human subjects in a simulated office. The latter were conducted using realistic ozone mixing ratios (approximately 15 ppb with occupants present). Detected products included mono- and bifunctional compounds that contain carbonyl, carboxyl, or alpha-hydroxy ketone groups. Among these, three previously unreported dicarbonyls have been identified, and two previously unreported alpha-hydroxy ketones have been tentatively identified. The compounds detected in this study (excepting acetone) have been overlooked in surveys of indoor pollutants, reflecting the limitations of the analytical methods routinely used to monitor indoor air. The results are fully consistent with the Criegee mechanism for ozone reacting with squalene, the single most abundant unsaturated constituent of skin lipids, and several unsaturated fatty acid moieties in their free or esterified forms. Quantitative product analysis confirms that squalene is the major scavenger of ozone at the interface between room air and the human envelope. Reactions between ozone and human skin lipids reduce the mixing ratio of ozone in indoor air, but concomitantly increase the mixing ratios of volatile products and, presumably, skin surface concentrations of less volatile products. Some of the volatile products, especially the dicarbonyls, may be respiratory irritants. Some of the less volatile products may be skin irritants.

Fig. 1.

Plots of mixing ratio as a function of time; 0–11 min: clean glass wool, 100 ppb O₃; 12–57 min: glass wool soiled with skin oil, 100 ppb O₃; 58–75 min: glass wool soiled with skin oil, no O₃. (Upper) Mixing ratios of primary products. (Lower) Mixing ratios of secondary products.

Fig. 2.

Plots of mixing ratio as a function of time for acetone, 6-MHO, decanal, and 4-OPA in air that has passed over a subject’s forehead. (Upper) Air containing 50 ppb of O₃. (Lower) Air containing no O₃.

Fig. 3.

Mixing ratios of O₃ (values plotted are 1/10 measured values), 6-MHO, and 4-OPA in the simulated office. (Left) First scenario: two subjects entered at 10:00 and remained in the room until the end of the experiment. (Right) Second scenario: two subjects entered the simulated room at 09:00; at 12:00 the ozone generators were turned on; at 13:30 the subjects left the room, and the ozone generators remained on.

Fig. 4.

Schematic of ozone reacting with squalene on exposed skin. The initial reaction produces both gas phase and surface-bound primary products. Ozone further reacts with surface bound primary products (see Table 3) to produce additional gas-phase products.