Air pollutants of concern to health come from a variety of sources and can be divided up in several ways; gaseous and particulate; primary and secondary; indoor and outdoor. Information on individual pollutants can be found on the Air pollutants of relevance to human health page.
Gases and Particles
Pollutants are present in air as either gases or particles (or associated with particles). Important gaseous pollutants include sulphur dioxide (SO2), oxides of nitrogen (nitrogen dioxide and nitric oxide are collectively referred to as NOx), ozone, and carbon monoxide (CO). Particles (or “particulate matter”) come from numerous different sources and vary greatly in size, composition and in what is attached to them. They are complex mixtures of different substances, both organic and inorganic, which include carbon, sulphate, nitrate, ammonium, minerals and metals. They are generally divided up on the basis of their size (particle diameter) with the most common two groups being PM2.5 and PM10 (particles generally <2.5µm or <10µm in diameter respectively).
Other pollutants can straddle the divide between the gases and the particles, including polycyclic aromatic hydrocarbons (PAHs) which occur in gaseous form or bound to particles; metals such as lead, arsenic and nickel which usually occur in air as particles or bound to particles and mercury, which exists as a vapour; and volatile organic compounds (VOCs) such as benzene, 1,3-butadiene and formaldehyde, which occur as gases.
Primary and Secondary
Pollutants are also often classified as either primary or secondary pollutants. Primary pollutants are those released directly into the air from the pollutant source and secondary pollutants are those formed by chemical changes to primary pollutants. For example, SO2 is a primary pollutant produced by coal burning power stations. Nitrogen dioxide (NO2) is mainly a secondary air pollutant, formed by the oxidation of nitric oxide (NO, a primary pollutant produced by traffic and fossil fueled power stations). Ozone is also a secondary pollutant, produced when sunlight breaks down pollutants such as NO2 and VOCs.
Particles can also be primary or secondary. Primary particles, emitted directly into the air, include those from coal fired power stations and car exhausts. Primary particles also include naturally occurring materials such as pollen grains and their fragments. Secondary particles are those formed from other pollutants e.g. sulphates and nitrates formed from SO2 and NO2. A common secondary particle constituent is ammonium sulphate formed from ammonia and sulphuric acid. For more information see Particles
Indoor and outdoor
Concentrations and types of pollutants differ between indoor and outdoor air. Pollutants in indoor air relevant to health include CO, NO2, VOCs and Environmental tobacco smoke, whereas ozone, sulphur dioxide and particles are more relevant in outdoor air. Concentrations of pollutants can vary significantly between outdoor and indoor air – for example, carbon monoxide, although present in outdoor air, can reach dangerously high levels indoors if rooms are poorly ventilated. By contrast, levels of ozone, particularly during summer ozone episodes, are much lower indoors than outdoors (due to interaction with indoor surfaces).