Perhaps of more concern, given our ability to have greater control over its release to the atmosphere, are the man-made air pollutants, which can have a detrimental impact on ambient air quality. 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. Depending on the nature of the fuel and the type of combustion process, pollutants released into the atmosphere from the burning of fossil fuels include nitrogen oxides, sulphur dioxide, carbon monoxide, particulate matter, lead and volatile organic compounds (VOCs). 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.