The most described NP in this category is titanium dioxide (TiO2), which shows photocatalytic properties when illuminated under UV light, is considered inert, has low toxicity, and is not susceptible to photocorrosion.191 TiO2 is currently used in paints and lacquers,155 in self-cleaning windows, and for water purification.192 TiO2-containing paints have also been envisaged for purifying ambient air because photocatalytic TiO2 successfully removes volatile organic compounds (VOCs) when exposed to UV light.