Photochemical oxidant (ozone) is a secondary contaminant generated by the photochemical reaction through solar exposure. The primary contaminant, which contains NOx and VOC as a main component, is emitted from factories and vehicles into air. It has been proven that photochemical oxidants are harmful to human health.

*VOC (volatile organic compounds) is a collective term for organic compounds such as toluene, xylene, ethyl acetate and many other substances. These VOC vapors travel through the air.