Toxic substances from coal burning include arsenic, fluorine, polycyclic aromatic hydrocarbons, and mercury. Coal tar (and substances containing polycyclic aromatic hydrocarbons, or PAHs) are true carcinogens. Gas-phase soot contains polycyclic aromatic hydrocarbons (PAHs). Coal tar is a complex mixture of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds. It is manufactured from aromatic hydrocarbons from petroleum. It is a complex mixture of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds. Anthracene, as many other polycyclic aromatic hydrocarbons, is generated during combustion processes. Cometary dust resembles interstellar grains which can include silicates, polycyclic aromatic hydrocarbons, and water ice. Naphthalene is the most abundant PAH among all the polycyclic aromatic hydrocarbons. Steam cracking is the process for producing ethylene and other alkenes from aliphatic hydrocarbons. It is not resistant to solvents, aromatics and some chlorinated hydrocarbons. Several chromium compounds are used as catalysts for processing hydrocarbons. The treaty is structured around several groups of halogenated hydrocarbons that deplete stratospheric ozone. Soot is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. Soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons.