30 Legislation on air quality 137. Directive 96/62/EC1 on ambient air quality assessment and management establishes limit values and alert thresholds for a number of pollutants (see section below on the first daughter directive). 138. Ambient air quality must be monitored throughout the member states. Different methods may be used for this: measuring, mathematical modelling, a combination of the two, or estimates. Assessment of this type is mandatory in builtup areas with more than 250 000 inhabitants, or in areas where concentrations are close to limit values. Where limit values are exceeded, member states must devise programmes to reach them within a set deadline. The programme, which must be available to the public, must contain at least the following information: the location where the pollution is excessive; the nature and an assessment of the degree of the pollution; the origin of the pollution. 139. States must draw up a list of areas and conurbations where pollution levels exceed limit values. 140. Where the alert thresholds are exceeded, member states must inform the inhabitants and send the European Commission any relevant information (recorded pollution level, length of the alert, etc.). 141. Where certain geographical areas and conurbations have pollution levels below limit values, member states must keep below the said values. 142. The Directive contains provisions on the forwarding of information and on reports on pollution levels and the areas concerned. 143. The first daughter directive of Directive 96/62/EC, Directive 99/30/EC2, sets the limit values for sulphur dioxide (SO2), sulphur oxide (SOx), nitrogen dioxide (NO2), particulate matter (PM10) and lead in ambient air. 144. Directive 2001/81/EC set national emission ceilings regardless of pollution sources for certain atmospheric pollutants that cause acidification, eutrophication and tropospheric ozone formation – i.e. sulphur dioxide (SO2), nitrogen oxide (NOx), volatile organic compounds (VOC) and ammonia (NH3). States were required to introduce them by 2010 at the latest. 1 2 Official Journal L 296, 21/11/1996 p. 0055 – 0063; deadline for transposition 21 June 1998 Official Journal L 163, 29/06/1999 p. 0041 – 0060; deadline for transposition 19 July 2001