Hot summer weather exacerbating ozone pollution

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News Published 14 Aug 2013 Last modified 23 Nov 2020
3 min read
Photo: © Orlando Sørensen
Unusually high temperatures this summer may be contributing to poor air quality in many European cities. Thresholds to protect health from ground-level ozone have been exceeded across Europe in recent weeks, according to preliminary data reported to the European Environment Agency (EEA).

Ozone formation increases during warm sunny weather depending on the level of ‘precursor pollutants’ present. Europe must therefore work hard to reduce the emissions of pollutants that cause ozone to protect human health.

Paul McAleavey, Head of EEA Air and Climate Change Programme

Ozone pollution has serious effects on health, especially for older people and children, or those with asthma and other respiratory problems. Reducing ozone pollution in the air depends on cutting the ‘precursor pollutants’ which lead to ozone formation.

When ozone levels in the air exceed certain thresholds – either the ‘Information Threshold’ or the higher ‘Alert Threshold’ – Member States must report such ‘exceedances’ to the European Commission, via the EEA, and inform their citizens.

Paul McAleavey, Head of EEA Air and Climate Change Programme, said: ‘Ozone formation increases during warm sunny weather depending on the level of ‘precursor pollutants’ present. Europe must therefore work hard to reduce the emissions of pollutants that cause ozone to protect human health’.

Ozone exceeding pollutant thresholds

July this year has been hotter than usual with warmer than average temperatures across most of Central and Western Europe. In July 2013, the average temperatures in Rome, Prague, Paris and Copenhagen were among the highest since 1996. These temperatures may have contributed to increased ozone levels.

Although April, May and June this year had fewer exceedances compared to the same period in 2012, the number of exceedances in July 2013 seems to be much higher than last year, according to preliminary data reported to EEA, showing that the Information Threshold for ozone was exceeded at approximately a quarter of all measurement sites in Europe.

In the first half of July concentrations exceeding the Information Threshold occurred mainly in northern Italy, Spain and southern France, but by the second half of the month similarly high pollutant concentrations were also found in parts of northern Europe. Ozone exceeded these limits in the Paris area (17 July) and in the Netherlands, Belgium and western Germany (22 to 23 July). At the end of the month most of the exceedances were registered in northern Italy, with high values occasionally occurring also in the Central European region.

Last summer levels of ozone were particularly low - the alert threshold was exceeded the fewest times since monitoring began in 1997. This is partly due to efforts to reduce air pollution in Europe. However, ozone is still a problem, with more than 98 % of the total EU urban population potentially exposed to ozone levels above World Health Organization guidelines to protect health.

What causes ozone?

Ground-level ozone is not directly emitted into the atmosphere but is formed from chemical reactions following the release of various ‘precursor pollutants’ from a wide variety of sources,  for example: fossil fuel combustion, road transport, refineries, solvents, vegetation, landfills, wastewater, livestock and forest fires.

The reactions that create ozone are catalysed by heat and sunlight – so it is a particular problem in the summer months, and southern Europe typically has much higher levels of ozone than the north.

Excessive ground-level ozone can cause respiratory problems, trigger asthma, reduce lung function and cause lung diseases. The mortality rate rises with increases in ozone exposure, according to several European studies. Unlike ozone in the stratosphere, which protects us from ultraviolet radiation, high levels of ground-level or ‘tropospheric’ ozone can also damage plants, reducing crop yields and forest growth, and also damage buildings and monuments.

Staying safe from poor air quality

Children, the elderly, asthma sufferers and others with respiratory illnesses are most vulnerable. If ozone levels are very high, it is advisable for these groups to avoid spending a lot of time outdoors. Ozone levels are usually higher in the afternoon, so vulnerable people may avoid the higher levels of pollution by working or exercising outdoors in the mornings or evenings.

You can find up-to-date information on ground level ozone concentrations across the pan-European region at the EEA’s real-time air quality maps. The site gives hourly ground level ozone concentrations for the current situation as well as recent episodes, based on up-to-date air quality data measurements. The website provides data from around 2 000 monitoring sites, allowing anyone to check air quality in a specific region or across Europe. Concentration data for other pollutants (PM10, NO2 and SO2) can also be found in this viewer.

The EEA website also provides information on exceedances of the different ozone thresholds measured this summer (both calculated from the real-time data and from the data officially reported by countries).

Historical validated data from previous years is stored in Airbase. The EEA also publishes reports overviewing the ozone situation in previous summer periods.


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