Indoor ozone reaction products: Contributors to the respiratory health effects associated with low-level outdoor ozone

Low-level outdoor ozone (O₃) exposure has been associated with adverse respiratory health effects, whereas substantially higher O₃ concentrations have been required to exert measurable effects in controlled studies. This discrepancy remains poorly understood. After entering indoors, a substantial portion of O₃ reacts with indoor chemicals to generate ozone reaction products that are potentially more toxic than O₃ itself. We hypothesize that ozone reaction product exposures contribute to the adverse respiratory effects associated with low-level outdoor O₃ exposure. In a panel study of 70 healthy adults, each was measured four times during a low-ozone season (maximum 8-h average: 29 ± 13 ppb). We found that higher average outdoor O₃ concentrations, irrespective of whether participants were outdoors or indoors, were significantly associated with worsened spirometric lung function (i.e., FVC, FEV₁, FEF_25-75) and airway mechanics (i.e., R₅, R₂₀) indicators. Per interquartile range (IQR) increase in average outdoor O₃ exposure when participants were indoors with windows closed (exposure proxy for ozone reaction products + indoor O₃) was significantly associated with worsening of multiple respiratory function indicators including FVC, FEV₁, FEF_25-75, Z₅, R₅, and R₂₀ by 0.56–3.08%. In contrast, per IQR increase in average outdoor O₃ exposure when participants were outdoors or indoors with windows open (exposure proxy for O₃ without ozone reaction products) was only significantly and adversely associated with worsening of one respiratory function indicator X₅ by 1.4%. These findings support our hypothesis and suggest further evaluation of indoor ozone reaction products' contribution to adverse health effects induced by outdoor O₃ exposure.