Pollutant

Description and main  sources

Potential effects on health/environment

Oxides of nitrogen

(NO_x)

All combustion processes in air produce oxides of nitrogen (NO_x). Nitrogen dioxide (NO₂) and nitric oxide (NO) are both oxides of nitrogen and together are referred to as NO_X. Road transport is the main source, followed by the electricity supply industry and other industrial and commercial sectors.

NO₂ is associated with adverse effects on human health. It can affect the liver, lung, spleen and blood. At high levels NO₂ causes inflammation of the airways. Long term exposure may affect lung function and respiratory symptoms. NO₂ also enhances the response to allergens in sensitive individuals. High levels of NO_x can have an adverse effect on vegetation, including leaf or needle damage and reduced growth. Deposition of pollutants derived from NO_x emissions contribute to acidification and/or eutrophication of sensitive habitats leading to loss of biodiversity, often at locations far removed from the original emissions. NO_x also contributes to the formation of secondary particles and ground level ozone, both of which are associated with ill-health effects. Ozone also damages vegetation.

Ozone (O₃)

Ground level (tropospheric) ozone is not emitted directly from any human-made source. Instead it forms in the atmosphere from a chain of photochemical reactions (with the help of sunlight) between various air pollutants, primarily NO_x and Volatile Organic Compounds (VOCs), but also CO and methane. Formation can take place over several hours or days and may have arisen from emissions many hundreds, or even thousands of kilometres away.

Exposure to high concentrations may cause irritation to eyes and nose. Very high levels can damage airways leading to inflammatory reactions. Ozone reduces lung function and increases incidence of respiratory symptoms, respiratory hospital admissions and mortality. Ground level ozone can also cause damage to many plant species leading to loss of yield and quality of crops, damage to forests and impacts on biodiversity. Ozone is also a greenhouse gas contributing to warming of the atmosphere.

Carbon monoxide (CO)

Formed from incomplete combustion of carbon containing fuels. The largest source is road transport, with residential and industrial combustion making significant contributions. CO reacts with other pollutants producing ground-level ozone

Substantially reduces capacity of the blood to carry oxygen to the body’s tissues and blocks important biochemical reactions in cells, leading to heart disease and damage to the nervous system. First symptoms include headache, dizziness and fatigue. People with existing diseases which affect delivery of oxygen to the heart or brain, such as angina, are at particular risk.