Issue link: https://hi.iaq.net/i/670243
an increased amount of PM produced by chemical reactions in the air including nitrates and sulphates. Another way of describing PM is by origin. Primary particles are those that are emitted directly to the air. These include particles emitted during fossil fuel combustion, brake and tyre wear, and road dust resuspended by vehicles. Secondary particles are particles formed by atmospheric processes. These include ammonium nitrate and sulphate. The origins (precursors) of secondary particles include nitrogen dioxide, sulphur dioxide, ammonia and volatile organic compounds. Polychlorinated biphenyls These are synthetic chemicals that do not occur naturally in the (PCBs) environment. The commercial production of PCBs is banned. Sources of PCBs in the air are landfill sites containing electrical equipment such as transformers and capacitors. Incineration of municipal waste may lead to PCB pollution. Polycyclic aromatic These are produced when the burning of fuels is incomplete. PAHs hydrocarbons (PAHs) can exist in over 100 different combinations. Examples of PAHs are benzo(a)anthracene and benzo(a)pyrene. Sulphur dioxide (SO 2 ) In the UK, this gas is largely emitted from industrial sources including power stations. The contribution from motor vehicle exhausts has been much reduced in recent years, owing to the use of low-sulphur fuels. Volatile organic compounds These are organic compounds with a low boiling point, and therefore (VOCs) a tendency to evaporate. Benzene and 1,3-butadiene are two important VOCs that are emitted from petrol car exhausts and directly from petrol. These emissions from traffic have been reduced through the introduction of three-way catalysts for petrol cars. Total volatile organic compounds are also known as TVOCs. Other terms Association An association is the statistical relationship between two measured quantities. Air pollution studies that report associations are examining the relationship between concentrations of an air pollutant and a health endpoint. Such an association (or correlation) is not necessarily causal and may be due to chance, bias or some other factor. COC Committee on Carcinogenicity COMEAP Committee on the Medical Effects of Air Pollutants COSHH Control of Substances Hazardous to Health EC European Commission © Royal College of Physicians 2016 xix Definitions