Defined as any unconfined part of the Earth's atmosphere, ambient air is the outdoor air in which humans and other organisms live and breathe. The content and quality of ambient air is directly affected by the day-to-day activities of humans. In turn, ambient air quality has a direct effect on both public health and the welfare of the Earth's ecosystems.

Air normally has no color, odor, or taste. It is a mixture of gases, primarily nitrogen, at about 78%, and oxygen, at about 21%, with the remaining 1% composed of carbon dioxide, methane, hydrogen, argon, and helium. Human activities, such as manufacturing and the burning of fossil fuels, cause changes in the chemical composition of ambient air through the release of chemical and industrial pollutants into the atmosphere.

Ambient air pollutants may include gases or particulate matter, which are small particles of dust, smoke, ash, pollen, or other substances. Many air pollutants have been found to be harmful to both the environment and human health. Pollutants known to have adverse health effects are called criteria pollutants. Criteria pollutants include ozone, lead, carbon monoxide, nitrogen dioxide, sulfur dioxide, and particulate matter.

Certain populations are most likely to be adversely affected by criteria pollutants in ambient air. These include infants, children, the elderly, and people with cardiopulmonary conditions. In children, criteria pollutants have been shown to increase the risk of respiratory tract infections and worsen the severity of asthma attacks. In adults, exposure to particulate matter is associated with an increased risk of hospitalization and death due to cardiovascular disease.

In the environment, changes in ambient air contribute to the development of acid rain and the greenhouse effect, the trapping of solar radiation in the Earth's atmosphere due to the presence of carbon dioxide, methane, and other gases. Ambient air pollution also contributes to depletion of the ozone layer, the region of the atmosphere that protects the Earth from harmful types of ultraviolet radiation.

To combat the detrimental effects of ambient air pollution, air quality standards have been established throughout the industrialized world. First enacted in the United States in 1970, the Clean Air Act required the Environmental Protection Agency to set National Ambient Air Quality Standards for criteria pollutants. These standards are designed to safeguard the public health, ensure the welfare of animals and crops, and protect the health of the Earth’s ecosystems.