European Commission has recently proposed a particulate matter (PM) number limit for light-duty diesel vehicles under the Euro 5 and Euro 6 emission standards. To ensure that emissions of utlrafine particles are controlled, the commission has set up this new regulatory approach that tracks emissions in terms of mass and numbers. The proposed PM number emission limit is 5x10¹¹ particles per km. It will be applicable to all light-duty diesel vehicles at the Euro 5 (effective from 2009) and Euro 6 (effective from 2014) stage in Europe.