Secondary pollutants occur when a primary pollutant reacts with another, often natural substance and creates a different pollutant. Diesel-run cars and some industrial processes release lead fumes, and at 0.5 micrometers in size, they are extremely harmful primary air pollutants. Natural gas burned in houses for heating, most types of fires as well as power plants that release carbon dioxide, carbon monoxide, and sulphur dioxide, are also primary air pollutants. Primary air pollutants are released from the reaction of burning fossil fuels, like when a motorized vehicle running on gasoline emits carbon monoxide. They can be observed in the form of soot, dust, smoke, fumes and mists, but some are invisible to the human eye. Primary air pollutants are small solids and particulates (liquid particles) that are suspended in the air and come at less than 10 micrometers in size.
