In a 2005 WHO report:
PM2.5:
10 µg/m3 annual mean
25 µg/m3 24-hour mean
PM10:
20 µg/m3 annual mean
50 µg/m3 24-hour mean
Note that pollution limits for particulate matter (PM) are only expressed in terms of particle size and concentration - strangely there is no consideration of what the particles are, i.e. there chemical composition. This anomaly probably arises because of the technical difficulty associated with measuring such chemical compositions. So in the interests of furthering our scientific understanding (and our health) there is probably a need for better monitoring technologies that can detect the chemical composition.
@akb Thank you so much for your comment and absolutely agree with you. Our team has been deep in research to particularly better understand this issue of composition.