The indicator shows the population-weighted concentration of ozone to which the urban population is potentially exposed. The principle metric for assessing the effects of ozone on human health is, according to the World Health Organisation’s recommendations (*), the daily maximum 8-hour mean. Ozone effects should be assessed over a full year. Current evidence is insufficient to derive a level below which ozone has no effect on mortality. However, for practical reason it is recommended to consider an exposure parameter which is the sum of excess of daily maximum 8-h means over the cut-off of 70 μg/m3 (35 ppb) calculated for all days in a year. This exposure parameter has been indicated as SOMO35 (sum of means over 35), and is extensively used in the health impact assessments, including the Clean Air for Europe (CAFE) Programme leading to the Commission Communication on the Thematic Strategy on Air Pollution. In 1996, the Environment Council adopted Framework Directive 96/62/EC on ambient air quality assessment and management. The third Daughter Directive (2002/3/EC) relating to ozone was adopted on 12 February 2002 with a long-term objective of 120 micrograms of ozone per cubic meter as a maximum daily 8-hour mean within a calendar year. The annual reporting must follow the Commission Decision 2004/224/CE of 20 February 2004 laying down arrangements for the submission of information under Council Directive 96/62/EC in relation to limit values for certain pollutants in ambient air. More recently, the Directive 2008/50/EC set a framework to define and establish objectives for ambient air quality and to harmonise methods and criteria among the Member States. (*) UN ECE (2004) Summary report prepared by the joint Task Force on the Health Aspects of Air Pollution of the World Health Organization/European Centre for Environment and Health and the Executive Body, EB.AIR/WG.1/2004/11.