Indoor ozone and ozone reaction products: An emerging health threat

Ozone is a strong oxidizing air pollutant and a growing global health concern. Most human exposure occurs indoors, where it reacts with indoor chemicals to form ozone reaction products (ORPs). This paper reviews the health impacts of indoor ozone and ORPs, along with their associated sources. Indoor ozone primarily enters from outdoor environments via ventilation and infiltration and reacts with certain volatile organic compounds (VOCs) and human skin oils to produce ORPs. Both indoor ozone and ORPs have been linked to adverse health effects, including cardiorespiratory and skin issues, contributing to a significant disease burden. Nevertheless, considerable knowledge gaps remain concerning the health impacts of indoor ozone and ORPs. First, the association between indoor ozone and cardiorespiratory effects has shown inconsistency across various studies. Second, while ORPs consist of a complex mixture of numerous species in multiple phases, few epidemiological studies have examined the health impacts of the ORP mixture. Future research should focus on the mechanisms of pathophysiological changes induced by both short- and long-term exposure to indoor ozone and ORPs to resolve these inconsistencies, define ORPs' toxicity, and establish exposure-response relationships under real exposure conditions. These studies are crucial for generating the urgently needed knowledge to help develop policy and practical strategies to mitigate health risks from increasing ozone levels in the context of climate change.