Possible Effects of Ozone Chemistry on the Phase Behavior of Skin Oil and Cooking Oil Films and Particles Indoors

Heterogeneous reactions occurring between ozone and films or particles containing skin oil or cooking oil can be a sink of ozone and a source of organic compounds indoors with negative health effects. To predict this heterogeneous chemistry, information on the phase behavior (number and types of phases within these films and particles, and its evolution during ozone exposure, is needed. In this work, we used optical microscopy to directly observe the phase behavior of proxies for skin oil and cooking oil particles (diameters of 110-220 mu m) during exposure to O-3. As a proxy for skin oil, we used a multicomponent mixture of unsaturated lipids. For cooking oil, we used a commercially available canola oil. We also studied single-component unsaturated lipids (squalene, triolein, and oleic acid) and jojoba oil (mostly unsaturated monoesters) as simpler proxies for skin oil and cooking oil. Prior to reactions with ozone, the particles were all liquid. Exposure to ozone led to the formation of a new phase within the particles. For oleic acid, the new phase was most likely a high-molecular-weight liquid formed by liquid-liquid phase separation. For all other systems, the new phase was likely solid or semi-solid. Our results, together with the resistor model and the reaction extent, were then used to predict the time it takes for a new phase to form in skin oil and cooking oil films indoors. Based on our results, we predict that a new phase will form in skin oil films with a thickness of 0.1-1 mu m after a timescale of 3 h to 7 days under typical indoor ozone concentrations (6 ppb) and relative humidity (RH) conditions ranging from 5 to 65%. In cooking oil films, we predict that a new phase will form after 1.5-16 h for films of 0.1-1 mu m thickness under ozone concentrations of 6 ppb and RH conditions ranging from 5 to 65%. A new phase will likely impact diffusion rates and partitioning coefficients within the films and hence ozone loss rates indoors, production rates of respiratory irritants, and partitioning of these irritants between films, particles, and the gas phase.

Automated summary

This study observed the phase behavior of skin oil and cooking oil particles during ozone exposure using optical microscopy. It was found that a new phase was formed after exposure to ozone. Based on the results and models, the time required for the formation of a new phase under indoor conditions was predicted.