Air pollution is the contamination of air, both indoor and outdoor, from the presence of harmful substances in the atmosphere1.
Currently, the pollutants causing the most significant health impacts in the UK are emissions of nitrogen dioxide and particulate matter.
Particulate matter is made up of tiny pieces of solids or liquids that we breathe daily. Once inhaled, particulate matter circulates around the human body and embeds itself into organs, causing health risks. Particulate matter may contain soot, smoke, dust, and dirt2 and is normally referred to as PM2.5 when smaller than 2.5 micrometres in diameter or PM10 when smaller than ten micrometres in diameter.
Primary particulate matter comes from human made sources such as smoke from fires, soot from vehicle exhausts, dust from tyres and brakes, as well as emissions from industry and natural sources such as pollen and desert dust. Secondary particulate matter is the combination of other chemicals, such as sulphur dioxide, nitrogen oxides and ammonia.
Nitrogen dioxide (NO2) is a gas that is mainly produced during the combustion of fossil fuels, along with nitric oxide (NO). The Air Quality Standards Regulations 2010 require that the annual mean concentration of NO2 must not exceed 40 µg/m3 and that there should be no more than 18 exceedances of the hourly mean limit value (concentrations above 200 µg/m3) in a single year. Most concentrations of nitrogen dioxide measured at the roadside come from local transport sources4.