According to the World Health Organization, air pollution is “contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere. Household combustion devices, motor vehicles, industrial facilities and forest fires are common sources of air pollution. Pollutants of major public health concern include particulate matter, carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide. Outdoor and indoor air pollution cause respiratory and other diseases, which can be fatal.”

Components of Air Pollution
Air pollution is a mixture of pollutants and can have many different components. The following are the most common components of air pollution:

Particulate Matter (PM): A complex mixture of extremely small particles and liquid droplets. The major components of PM are sulfate, nitrates, ammonia, sodium chloride, black carbon, dust particles and water. PM comes from dust, dirt, soot, smoke, industry and vehicle exhaust as well as complex chemical reactions with other pollutants.

PM 10: Particulate matter of 10 micrometers in size. The most health-damaging particles are those with a diameter of 10 micrometers in size or less, (≤ PM10) (so both PM 10 and the smaller PM 2.5 size listed below) which can penetrate and lodge deep inside the lungs.

PM 2.5: Very small PM that are less than 2.5 micrometers are known as fine particulate matter or PM 2.5. These particles are less than 1/30 the width of a human hair. Once fine particles are in the lungs, they can affect the heart, blood vessels, and lungs.

Ozone (O3):  Ground-level ozone: Ground level ozone, one of the major components of photochemical smog, is formed by the reaction of pollutants such as nitrogen oxides (NOx) from vehicle and industry emissions and volatile organic compounds (VOCs) emitted by vehicles, solvents and industry with sunlight. As a result, the highest levels of ozone pollution occur during periods of sunny weather. Ground level ozone is known to be harmful to health.

Nitrogen dioxide (NO2):  A highly reactive gas. The major sources of NO2 are combustion processes (heating, power generation, and engines in vehicles and ships). NO2   is linked with a number of adverse effects on the respiratory system.

Sulfur dioxide (SO2): SO2 is a colourless gas with a sharp odour. It is produced from the burning of fossil fuels (coal and oil) and the smelting of mineral ores that contain sulfur. Linked with a number of adverse effects on the respiratory system.

CO: Carbon monoxide. A colorless, odorless gas emitted from combustion processes– from car exhaust, power stations and waste incinerators. CO can cause harmful health effects by reducing oxygen delivery to the body’s organs (like the heart and brain) and tissues.

Visit the CDC for more information on individual air pollutants:

Term Definitions:
Ambient air pollution is a broad term used to describe air pollution in outdoor environments.
WHO factsheet on ambient air pollution

Urban outdoor air pollution is a more specific term referring to the ambient air pollution experienced by populations living in urban areas, typically in or around cities.
WHO background information on urban outdoor air pollution

Indoor air pollution typically refers to the pollution caused by indoor cooking and heating practices used by much of the poor in developing countries. Use of inefficient fuels like coal and dung in leaky stoves and over open fires lead to high levels of health-damaging pollutants.
WHO Indoor air quality fact sheet, including WHO response

Indoor Air Quality is often used in places like the United States to talk about pollutants such as mold and radon that can build up in your house.  This term is also often used more generally, to discuss the overall air quality of a home.
CDC Frequently asked questions about indoor air quality