agglomeration — a zone with a population concentration in excess of 250000 inhabitants or, where the population concentration is 250000 inhabitants or less, a population density per km2 which for the Member States justifies the need for ambient air quality to be assessed and managed 9.

alert threshold — a level beyond which there is a risk to human health from brief exposure and at which immediate steps shall be taken by the Member States as laid down in Directive 96/62/EC 6.

ambient air — outdoor air in the troposphere, excluding work places 1.

assessment — any method used to measure, calculate, predict or estimate the level of a pollutant in the ambient air 4.

fixed measurements — measurements taken in accordance with Article 6(5) of Directive 96/62/EC 16.

level — the concentration of a pollutant in ambient air or the deposition thereof on surfaces in a given time 3.

limit value — a level fixed on the basis of scientific knowledge, with the aim of avoiding, preventing or reducing harmful effects on human health and/or the environment as a whole, to be attained within a given period and not to be exceeded once attained 5.

lower assessment threshold — a level specified in Annex V, below which modelling or objective-estimation techniques alone may be used to assess ambient-air quality in accordance with Article 6(4) of Directive 96/62/EC 14.

margin of tolerance — the percentage of the limit value by which this value may be exceeded subject to the conditions laid down in Directive 96/62/EC 7.

natural events — volcanic eruptions, seismic activities, geothermal activities, wild-land fires, high-wind events or the atmospheric resuspension or transport of natural particles from dry regions 15.

oxides of nitrogen — the sum of nitric oxide and nitrogen dioxide added as parts per billion and expressed as nitrogen dioxide in micrograms per cubic meter 10.

PM10 — particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 10 μm aerodynamic diameter 11.

PM2,5 — particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 2,5μm aerodynamic diameter 12.

pollutant — any substance introduced directly or indirectly by man into the ambient air and likely to have harmful effects on human health and/or the environment as a whole 2.

upper assessment threshold — a level specified in Annex V, below which a combination of measurements and modelling techniques may be used to assess ambient-air quality, in accordance with Article 6(3) of Directive 96/62/EC 13.

zone — part of their territory delimited by the Member States 8.