The indicator shows the population-weighted concentration of PM10 and PM2.5 to which the urban population is potentially exposed. Fine and coarse particulates (PM10) are those whose diameter is less than 10 micrometres, whilst fine particulates (PM2.5) are those whose diameters are less than 2.5 micrometers. Particulates can be carried deep into the lungs where they can cause inflammation and a worsening of the condition of people with heart and lung diseases. The smaller the particles the deeper they travel into the lungs, with more potential for harm. According to the recommendations of the World Health Organisation (WHO) the annual mean concentration is the best indicator for PM-related health effects. In 1996, the Environment Council adopted Framework Directive 96/62/EC on ambient air quality assessment and management. The first Daughter Directive (1999/30/EC) relating to limit values for PM10 and other pollutants in ambient air fixed an annual limit value of 40 micrograms of PM10 per cubic meter (40 µg/m3). Note that the WHO guideline value is 20 µg/m3 (annual mean). 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. This does have limits for PM2.5. The limit value that was due to be met on 1 January 2015 is 25 µg/m3, which falls to 20 µg/m3 by 2020. Note that the WHO guideline value is 10 µg/m3 (annual mean). The directive 2008/50/EC also places a requirement on Member States to assess and reduce population exposure to concentrations of PM2.5 by 2020. The magnitude of the required reduction depends on national average concentrations between 2009 and 2011. Where concentrations for those years were greater than 22 µg/m3, all appropriate measures should be used to reduce below 18 µg/m3 by 2020.