31 145. Summary table of EU limit values and ceilings for certain polluants: long-term exposure short-term exposure national ceiling TSP 150μg/m3 300μg/m daily limit (not to be exceeded >18 days per year) n.a. PM10 40μg/m3 50μg/m3 daily limit (not to be exceeded >35 days per year) n.a. SO2 120μg/m3 350μg/m3 hourly limit (not to be exceeded >24 times a year) and 125μg/m3 daily limit (not to be exceeded >3 times a year) 320 ktons (For plants existing before 1 July 1987) NOX 40μg/m3 200μg/m3 hourly limit (not to be exceeded >18 times a year) 70 ktons (For plants existing before 1 July 1987) GHG n.a. n.a. 110,2 Mtons (to be reached at the latest in 2012) 3 146. The limit value for long-term exposure, expressed as the annual mean daily concentrations of a pollutant, measures the average level of exposure of individuals, without taking account of short-term periods of exposure or seasonal variations in pollutant concentrations. It describes ambient air quality and make it possible to assess the cumulative long-term effects of poor air quality. Limit values for short-term exposure are set at a higher level but may only be exceeded on a certain number of days or for a certain number of hours each year. This indicator measures the risks associated with brief periods of high pollution concentrations, which are considered to be responsible for short-term peaks in mortality and morbidity. National ceilings, expressed as total volume (tonnes) of a specific pollutant, indicate the amount of pollution that can be emitted by certain – or all – sectors of activity in a particular country. 147. The limit value for TSP, laid down in Directive 88/779/EEC on air quality limit values and guide values for sulphur dioxide and suspended particulates, has been suspended by the EU, because it is no longer considered to be a sufficiently precise indicator of air quality for the purposes of health protection. 148. Directive 2001/80/EC on the limitation of emissions of certain pollutants into the air from large combustion plants1 aims to bring about a step-by-step reduction in annual emissions of sulphur dioxide (SO2) and nitrogen oxides (NOx) from existing plants and set emission limit values for sulphur dioxide, nitrogen oxides and dusts for new plants. 149. Different rules apply to existing combustion plants and to permits for the construction of combustion plants or licences for the operation of new plants. As regards the former, states were required to draw up progressive programmes for the reduction of total annual emissions by 1 July 1990 and to elaborate a national emission reduction plan (NERP) for existing plants which they had to communicate to the Commission by 27 November 2003 (such plans set up objectives, provide for measures and timetables for attaining them and refer to a monitoring mechanism). 1 Official Journal L 309 of 27.11.2001, p. 0001 – 0021; deadline for transposition 27 November 2002