Particulates - sometimes referred to as particulate matter or just particles - are tiny pieces of dust, soot and other materials suspended in the atmosphere. These can be produced naturally, by volcanoes or forest fires for example, but are also caused by incomplete combustion of fossil fuels, particularly from diesel engines. Because diesel is a denser fuel than petrol, it needs more oxygen for all its hydrocarbons to react completely, and the unburnt carbon - soot or black smoke - is emitted through the exhaust pipe.