Shale oil contains polycyclic aromatic hydrocarbons which are carcinogenic. Refractory DOM largely comprises highly conjugated molecules like Polycyclic aromatic hydrocarbons or lignin. Toxic substances from coal burning include arsenic, fluorine, polycyclic aromatic hydrocarbons, and mercury. Tobacco smoke contains over fifty known carcinogens, including nitrosamines and polycyclic aromatic hydrocarbons. Certain rare polycyclic aromatic compounds are carcinogenic. Hydrocarbons are also abundant in nebulae forming polycyclic aromatic hydrocarbon (PAH) compounds. The purine nucleoside bases are notable polycyclic aromatic heterocycles. Processed, smoked and cooked meat may contain carcinogens such as polycyclic aromatic hydrocarbons. Coal tar (and substances containing polycyclic aromatic hydrocarbons, or PAHs) are true carcinogens. Type IV kerogen comprises mostly inert organic matter in the form of polycyclic aromatic hydrocarbons. Naphthalene is the most abundant PAH among all the polycyclic aromatic hydrocarbons. Polycyclic aromatic compounds characteristically reduce to the radical anions. Cometary dust resembles interstellar grains which can include silicates, polycyclic aromatic hydrocarbons, and water ice. Early examples were derivatives of polycyclic aromatic compounds. They belong to the chemical class of polycyclic aromatic hydrocarbons.