Sunny dayOzone is a secondary pollutant gas, formed by "photochemical" reactions – chemical reactions driven by sunlight – in the lower atmosphere (the troposphere)*. Oxides of nitrogen and volatile organic compounds·produced predominantly by traffic, are the main pollutants that act as "precursors" for ozone formation in these reactions.

These pollutants rise above urban areas and react with sunlight to create ozone (which forms part of photochemical smog seen over large cities). These reactions may take place for several days and the resulting ozone can be carried long distances, accumulate and reach high concentrations often far away from the sources of the original pollutants.

As a result of this, and the fact that ozone reacts with NO from traffic emissions in urban areas and near busy roads to form NO2, levels of ozone in the air are often higher in rural areas downwind of cities.

Ozone episodes

Ozone formation is more of a problem in summer as the sun is stronger and this leads to increased rates of photochemical breakdown of NO2 and thus generation of ozone. Ozone can reach high concentrations on sunny, windless days in particular, when precursor pollutants are poorly dispersed and accumulate over urban areas, forming ozone as they drift slowly to rural areas. These ozone “episodes” are important in terms of health, particularly for susceptible groups such as asthmatics (see below).

Ozone is also a “transboundary” pollutant, as precursor pollutants generated from industry and road traffic in one country may drift to and create ozone in another. For example, in the South of England, during summer, a significant proportion of ozone is formed from primary pollutants emitted in Europe.

Health effects

Ozone is an oxidising agent and acts as an irritant, producing inflammation of the respiratory tract. At high concentrations ozone irritates the eyes, nose, throat, causing coughing and discomfort whilst breathing and exposure over several hours can lead to damage of the lining of the airways. This is followed by inflammation, narrowing and increased sensitivity (to stimuli such as cold air and exercise) of the airways. This is called “airway hyperresponsiveness”. Ozone may also increase the sensitivity of the airways in susceptible people (e.g. hayfever sufferers) to allergens such as pollen.

People vary greatly in their response to ozone. Some people find their lung function (ability to breath) is not significantly affected whilst others find that it is. Interestingly, unlike other pollutants, asthmatics do not seem to be as susceptible to ozone as might be expected, compared to non-asthmatics. Narrowing of the airways (called “bronchoconstriction”) upon exposure to air pollutants is usually much greater in asthmatics but this does not seem to be the case with ozone. However, some studies have shown that the inflammatory response in the lungs following ozone exposure is greater and lasts for longer in asthmatics. Studies have also shown that there are more asthmatics admitted to hospital on days when ozone levels are higher, which suggests that exposure to ozone can make a person’s asthma worse.

More information

  • Department for Environment, Food and Rural Affairs (Defra)'s advisory Air Quality Expert Group (AQEG) Ozone in the UK (2009)
  • World Health Organization (WHO)  Health Risks of Ozone from Long-Range Transboundary Pollution (2008)
  • World Health Organization (WHO) Air Quality Guidelines - Global Update 2005. Particulate Matter, Ozone, Nitrogen Dioxide and Sulphur Dioxide (2006)

*Ozone is also produced in the upper atmosphere (the stratosphere) by the action of ultraviolet light on oxygen, forming the ozone “layer”.


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