The term "air pollution" refers to harmful particles, liquids or gasses in the atmosphere. Although such substances can occur naturally through events like fires and volcanic eruptions, most air pollution is caused by human activity. Sources of pollution are often classified as stationary (factories, furnaces, etc.) or mobile (vehicles, ships, aircraft, motorized devices, etc.). The burning of wood, coal and other fossil fuels is the single greatest contributor to air pollution, although many aspects of industry, agriculture, waste management and even everyday living also add to the problem. 

Air pollution can be either localized or global, depending on the type of pollution and on prevailing wind currents. Some types of pollution exist in the troposphere, which is the layer closest to earth, and affect mainly the region where the pollution first occurred. However, local particulate matter and gasses are also capable of traveling thousands of miles within the troposphere. Some of the air pollution on North America's west coast has been traced to China, proving that harmful material can travel all the way across the Pacific Ocean. Greenhouse gas pollution is truly a global problem; regardless of where the GHG pollution originates, it has the same effect on the overall atmosphere.

Air pollution is also usually described as either an indoor or an outdoor problem, although outdoor air pollution can certainly enter buildings and become an indoor air problem. In general, outdoor pollution is caused by industrial and vehicular emissions, while indoor pollution is caused by office equipment, office furnishings, and inadequate ventilation. Poor outdoor air quality can lead to or exacerbate asthma and other respiratory illnesses, while poor indoor air quality can lead to similar symptoms as well as a host of other problems, including Sick Building Syndrome (SBS) and Building Related Illnesses (BRI).