The European Commission has refered Belgium and Bulgaria to the EU Court of Justice over persistently high levels of the dust particles which pose a major risk to public health. Sweden was also given a final warning over poor air quality. Traffic is the main source of air pollution in most of Europe where air pollution is high.
Poor air quality is the reason why low emission zones are implemented, and often one of the reasons for other urban access regulations.
For more information, see the European Commission's press release.