Monocyclic aromatic hydrocarbons contain only one benzene ring, such as benzene, toluene, ethylbenzene, xylene, cumene, dodecylbenzene, etc. Polycyclic aromatic hydrocarbons are composed of two or more benzene rings (there are no carbon atoms shared by the two rings), which are connected by a single bond or through carbon atoms, such as biphenyl, triphenylmethane, etc. . The fused-ring aromatic hydrocarbon is a fused-ring hydrocarbon formed by fusing two or more benzene rings (so that the two benzene rings share a pair of carbon atoms), at least one of which is a benzene ring, such as naphthalene, anthracene, etc. The most important products in aromatics are benzene and xylene, followed by toluene, ethylbenzene, styrene, and cumene. Benzene and its smaller molecular weight homologues are flammable liquids, insoluble in water, and less dense than water; polycyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons are mostly crystalline solids. Aromatics are all toxic, of which benzene has the strongest effect on the central nervous system and blood. Condensed ring aromatic hydrocarbons have carcinogenic effects.