The major sources of energy in the United States are coal, petroleum, and natural gas, all of which are known as fossil fuels. Fossil fuels were formed millions of years ago by the decomposition of animals and plants and thus are in limited supply. We are quickly depleting the available fossil fuels.

Coal is solid and is composed of large hydrocarbons and other compounds that contain sulfur, oxygen, and nitrogen. When it’s combusted, the sulfur it contains is converted to SO₂, which is an air pollutant. Petroleum is a liquid made up of hundreds of different components, but mostly hydrocarbons. It also contains some compounds that have functional groups containing sulfur, nitrogen, or oxygen. The first step in refining (processing) petroleum is to separate it into fractions based on the different boiling points of its components. Natural gas consists of hydrocarbons in the gas phase, primarily methane (CH₄).

Air pollution is the contamination of air by a variety of substances, causing health problems and damaging our environment. It has thinned the ozone layer above the earth, exposing us to harmful UV radiation from the sun. Some of the major pollutant gases are listed below.

Carbon monoxide: CO is produced from incomplete combustion of all types of natural and synthetic products, including cigarette smoke. When it builds up in high concentrations, it can be very toxic. Cities with heavy traffic problems are known for dangerous CO levels.

Carbon dioxide: CO₂ is the principal greenhouse gas and is primarily responsible for the greenhouse effect. It can be formed from all types of common human activity, such as burning fuels and even breathing.

Chlorofluorocarbons: Chlorofluorocarbons, or CFCs, are used in great quantities in industry, for refrigeration and air-conditioning, and in consumer products. When released into the air, they rise into the stratosphere, where they readily react with the ozone that constitutes the ozone layer, effectively degrading it.

Ozone: O₃ gas occurs naturally in the upper atmosphere, where it shields the earth from the sun’s dangerous ultraviolet rays. When found at ground level, however, it’s a pollutant. It can cause damage to humans (especially our respiratory system), the environment, and a wide range of natural and artificial materials. Vehicle exhaust and industry waste are major sources of ground-level ozone.

Nitrogen oxide and sulfur dioxide: NO_x and SO_x are major contributors to smog and acid rain. These gases both react with volatile organic compounds to form smog, which can cause respiratory problems in humans. Acid rain can harm vegetation, change the chemistry of river and lake water by lowering the pH so that it’s harmful to animal life, and react with the marble of statues and buildings and decompose them.