Abstract
Photocatalytic oxidation (PCO) is increasingly employed for the removal of volatile organic compounds (VOCs) from indoor air. A growing body of evidence indicates that PCO can generate toxic by-products, including formaldehyde, acetaldehyde, benzene, and other hazardous compounds. These by-products are recognized environmental contaminants with established carcinogenic and other adverse health effects. This review examines the key determinants of by-product formation—indoor VOC concentrations, relative humidity, light irradiance, and catalyst surface properties— with emphasis on indoor-relevant concentration ranges. The health risks associated with these by-products are assessed using metrics such as the hazard quotient (HQ), hazard index (HI), and cancer risk (CR). Calculations based on literature-derived datasets indicate that these indicators often exceed acceptable thresholds, suggesting potential health concerns associated with the PCO technology. Furthermore, evaluations of commercially available PCO-based air cleaners show that some devices may emit more by-products than they remove, resulting in a net degradation of indoor air quality. To support safer deployment, mitigation routes are outlined—operational optimization, catalyst engineering, and integrated adsorptive stages—complemented by predictive modeling to identify safe, energy-efficient operating windows. Based on current evidence, widespread residential deployment of PCO-based air cleaners warrants further evaluation; priorities include long-term exposure assessment under realistic loads, refinement and validation of predictive models, and field confirmation that engineered operating windows reliably control by-products without compromising indoor air quality.