The EU's air quality directives (2008/50/EC Directive on Ambient Air Quality and Cleaner Air for Europe and 2004/107/EC Directive on heavy metals and polycyclic aromatic hydrocarbons in ambient air) set pollutant concentrations thresholds that shall not be exceeded in a given period of time. In case of exceedances, authorities must develop and implement air quality management plans. These plans should aim to bring concentrations of air pollutants to levels below the limit and target values. Selected EU standards and the World Health Organization (WHO) guidelines are summarised in the table below. These apply over differing periods of time because the observed health impacts associated with the various pollutants occur over different exposure times. The WHO guideline values are set for the protection of health, and are generally stricter than the comparable politically agreed EU standards.