Some VOCs have carcinogenic and mutagenic effects on living organisms and human health. Moreover, the VOCs in urban air contribute significantly smog-photochemical reactions, leading to ozone formation, and tropospheric ozone is an undesirable constituent of air if present in appreciable concentrations. Aromatic hydrocarbons account for 20-40% of the total ambient VOCs in modern urban environments with benzene, toluene, ethylbenzene, and meta (m), para (p), and ortho- (o) xylene, collectively termed BTEX, being the most abundant among the aromatic hydrocarbons.