Both ozone precursors and ozone itself are gases that travel through the atmosphere, sometimes over very long distances (several hundred kilometres). Thus it is not easy to distinguish between the pollution which we produce and that which is "imported" from neighbouring regions or countries... or further afield.
We therefore need action on an international scale! See International and European actions (HTML)
In the enlarged European Union, nearly 1800 measurement stations monitor changes in tropospheric ozone concentrations.
In Belgium, 37 stations all over the country, in both urban and rural areas, continuously measure the ozone concentration in the air. Measurement networks are managed by the three regions. An interregional cell called CELINE – IRCEL has been set up to coordinate all the networks and to provide the public with information. The results of the ozone concentration measurements can be consulted in real time in the form of maps on the CELINE website (WEB).
A few statistics
60 µg/m3 (micrograms per m3): this is the approximate background concentration of ozone in the ambient air in Belgium today, i.e. the annual average of all measurements taken in Belgium.
It is believed that this background concentration doubled during the twentieth century due to industrialisation and that if we do not reduce our emissions of ozone precursor pollutants, it will continue to rise by around 1% a year.
It is important to monitor the background concentration: it reflects the abundance of precursors in the air and the higher it is, the greater the maximum concentrations during ozone peaks are likely to be.
120 µg/m3: this is the guide value that the World Health Organisation recommends should not be exceeded, averaged over eight consecutive hours, and above which it is possible to begin observing effects on human health. In Belgium, this value was exceeded on 83 days at one measurement station at least in 2003. Since 1989 it has been exceeded on an average of 45 days per year.
180 µg/m3: this is the threshold for informing the public set by EU Directive 2002/3/CE (.PDF). A public information procedure is triggered when the ozone concentration exceeds this value for more than one hour at more than one measurement station.
An "ozone day" is when this value has been exceeded at more than one of the country's measurement stations.
During the summer of 2003, Belgium had 22 ozone days, including 12 consecutive days during August 2003, when highs were recorded at a many measurement stations.
240 µg/m3: this is the alert threshold set by European Directive 2002/3/CE (.PDF). If this ozone concentration level is exceeded for more than one hour, an alert procedure is triggered, entailing a public information procedure and implementation of specific measures.
In summer 2003, this ozone concentration was exceeded at several Belgian measurement stations over seven days, all during August 2003. During this same period, France, the Netherlands, Luxembourg, Germany and Austria all regularly recorded levels exceeding 240 µg/m3, on a wide scale, something which has rarely happened in recent years.