Airborne particle emissions from photocatalytic coatings and paints

Titanium dioxide (TiO₂)-based photocatalytic materials are increasingly proposed for indoor applications due to their ability to degrade volatile organic compounds (VOCs) and inactivate microorganisms. However, the same oxidation processes may lead to the formation of secondary airborne particles, potentially including SOA. In this study, we investigated whether transparent photocatalytic coatings and photocatalytic paints can act as indoor sources of ultrafine particles (UFPs) under controlled indoor lighting conditions. Chamber experiments were conducted by monitoring particle number concentration, particle size distribution, illuminance, and total VOCs under three lighting conditions (1200 lx, 300 lx, and darkness) using a lamp approximating the solar spectrum. Both materials generated particles under light exposure, while no emissions were observed under dark conditions. Mean emission rates at 1200 lx reached 2.2 × 10¹¹ part. min^- 1 m^- 2 for the coating and 2.8 × 10¹¹ part. min^- 1 m^- 2 for the paint, decreasing by approximately three orders of magnitude at 300 lx. Simultaneously, total VOC concentrations decreased from 250 ± 98 ppb to 210 ± 45 ppb, suggesting a link between photocatalytic VOC oxidation and particle formation. Particle size distributions were dominated by ultrafine particles with modal diameters around 10 nm, consistent with nucleation-driven processes. These findings indicate that photocatalytic coatings and paints can serve as significant indoor sources of UFPs under light exposure, highlighting a potential trade-off between the intended disinfection and pollutant-removal effects of photocatalytic materials and the resulting formation of secondary airborne particles. To the best of our knowledge, this is the first study to provide quantitative evidence of ultrafine particle emissions from photocatalytic coatings and paints under indoor lighting conditions, raising important considerations for the use of photocatalytic materials in indoor air quality management.