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Definitions Air pollutants Black carbon This is the part of particulate matter that most strongly absorbs light. It is created by the incomplete combustion of fuels and is a major part of soot. Carbon monoxide (CO) This gas is produced when carbon-containing fuel burns without an adequate supply of oxygen. Outdoor concentrations of carbon monoxide in the UK are generally low. Malfunctioning indoor gas heating appliances and other fuel-burning devices used indoors may generate high, toxic levels of CO. Diesel exhaust Diesel exhaust is composed of gases and particles. Diesel exhaust gases include compounds such as benzene, 1,3-butadiene, formaldehyde and polycyclic aromatic hydrocarbons (PAHs). Diesel exhaust particles (DEPs) have a range of sizes and compositions. Small DEPs often contain elemental carbon (soot), with many toxic compounds on their surface. These include organic compounds, sulphate, nitrate and metals. Metals and metalloids These include lead (Pb), mercury (Hg), arsenic (As) and nickel (Ni). Lead air pollution used to be dominated by emissions from road transport, but is now dominated mainly by activities in the iron and steel industry sectors. Mercury emissions are from a number of industrial production processes, but the most important is the manufacture of chlorine from Hg cells. Improved controls and new technologies have reduced emissions in the UK. Arsenic is a metalloid, ie its properties lie somewhere between those of the metals and the non-metals. The largest source of arsenic in the UK is the disposal of treated wood by burning. Nickel is mainly emitted as particles or associated with particles. UK emissions of nickel have declined significantly owing to reductions in the combustion of oil in power stations, industry and for domestic heating. Nitrogen dioxide (NO 2 ) This gas is a member of a family of compounds called oxides of nitrogen or NOx. Some NO 2 is produced directly by combustion. Another source of NO 2 is the oxidation of nitric oxide (NO) in the air; this can happen either slowly by reaction with oxygen or more rapidly by reaction with ozone. NO 2 levels are higher close to road traffic, or indoors where there is gas cooking. Oxides of nitrogen and particle concentrations are highly linked (correlated) in air samples from city roadside sites and it has been difficult to separate out their effects. © Royal College of Physicians 2016 xvii

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