Sulphur Dioxide (SO₂): a corrosive, acidic gas primarily emitted by burning fossil fuels, such as coal or crude oil.

• Sources: Industrial sources such as coal and oil-burning power stations, as well as domestic sources such as boilers and stoves.
• Health effects: SO₂ has an irritant effect on the lining of the nose, throat and airways, and the effects are often felt very quickly. At high concentrations can cause coughing, tightness in the chest and narrowing of the airways of the lungs.

Ammonia (NH3): a colourless, reactive gas released into the atmosphere only for a few hours. Once emitted into the atmosphere, NH3 can be rapidly deposited over areas near the emission source. However, when ammonia mixes and reacts with other gases in the atmosphere, it can form particulate matter (PM), which can exist for several days, and be transported large distances.

• Sources: The vast majority of ammonia emissions come from agricultural sources, in particular livestock manure, urine and the spreading of inorganic fertilisers.
• Health effects: The main health impacts of ammonia arise through its role in forming particulate matter (PM), which is known to be associated with short term and long-term health impacts (see section on particulate matter).

Carbon Monoxide (CO): a colourless, odourless, tasteless non-irritating gas produced as a by-product of burning fuels (gas, oil, coal and wood) without enough oxygen present.

• Sources: Household appliances – boilers, central heating systems, gas fires, water heaters, cookers, and open fires). Burning charcoal, running cars, and smoke from cigarettes.
• Health effects: Exposure to high indoor levels can be fatal, while exposure to lower levels can result in symptoms such as headache, nausea, vomiting, dizziness, feeling weak, confusion.

Ozone (O₃): a gas which occurs both in the earth’s upper atmosphere and at ground level.  Ground level O3, is not emitted directly into the air but is created by chemical reactions involving the oxides of nitrogen (NOx) and volatile organic compounds (VOCs). This happens when pollutants emitted by cars, industrial sites, and other sources chemically react in the presence of sunlight.

• Sources: In contrast to many air pollutants measured in the UK, there are no major emission sources of ozone itself. Ozone is formed and removed from the atmosphere through a wide range of chemical reactions and physical processes, the timescales of which range from minutes to years.
• Health effects: Irritation of the respiratory tract, eyes, nose, and throat causing inflammation, as well as aggravating lung diseases such as asthma (causing asthma attacks), emphysema and bronchitis. Ozone can also react with other chemicals in the air to form particulate matter (PM), or smog, which is also harmful to health.

Non-methane volatile organic compounds (NMVOCs): A large variety of chemically different composed organic compounds such as benzene, toluene, and ethylbenzene, which display similar behaviour in the atmosphere.

• Sources: Industrial processes, agriculture, petrol vapour, combustion processes, solvents, air fresheners, cleaning products and perfumes
• Health effects: In the outside atmosphere NMVOCs react with NOx in the presence of sunlight to produce ground level ozone (O₃) which can trigger inflammation and asthma. NMVOCs also pose a threat to health indoors, as reactions between NMVOCs and chemicals from combustion processes such as smoking, heating, cooking, or candle burning can produce chemicals such as formaldehyde – which is a human carcinogen and is known to cause irritation of eyes and upper airways at low concentrations. NMVOCs can also contribute to the concentrations of particulate matter (PM) which is known to be associated with short term and long-term health impacts (see section on particulate matter).
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