The most common source of man-made air pollution outdoors is the burning of fossil fuels , such as coal, oil and gas, in power stations, industries , homes and road vehicles . Other sources of these pollutants include forest burning, chemical, fertiliser and paper manufacture, and waste incineration. These pollutants are all called primary pollutants because they have direct sources. By contrast, there are no direct ground-level emissions of ozone to the atmosphere. Most ozone in the atmosphere near ground level is formed indirectly by the action of sunlight on VOCs in the presence of nitrogen dioxide, although a small amount comes from high up in the atmosphere where it forms naturally in the ozone layer.
Ozone is a secondary ambient air pollutant.
Both primary and secondary pollutants are, to a greater or lesser extent, detrimental to human health , depending on their concentration in the air, and the sensitivity of the individual. Consequently, national and international legislation exists to regulate and control the amount of pollution emitted to the atmosphere, and to ensure that objectives for improving ambient air quality are achieved.