Any compound composed of elemental carbon and hydrogen, hydrocarbons may also contain chlorine, oxygen, nitrogen, and other atoms. Hydrocarbons are classified according to the arrangement of carbon atoms and the types of chemical bonds. The major classes include aromatic or carbon ring compounds, alkanes (also called aliphatic or paraffin) compounds with straight or branched chains and single bonds, and alkenes and alkynes with double and triple bonds, respectively. Most hydrocarbon fuels are a mixture of many compounds. Gasoline, for example, includes several hundred hydrocarbon compounds, including paraffins, olefins, and aromatic compounds, and consequently exhibits a host of possible environmental effects. All of the fossil fuels, including crude oils and petroleum, as well as many other compounds important to industries, are hydrocarbons. Hydrocarbons are environmentally important for several reasons. First, hydrocarbons give off greenhouse gases, especially carbon dioxide, when burned and are important contributers to smog. In addition, many aromatic hydrocarbons and hydrocarbons containing halogens are toxic or carcinogenic.