The study only looked at particulate matter with an aerodynamic diameter smaller than 2.5 micrometres. It based its on WHO norms that cite average annual particulate matter concentrations of no more than 10 micrograms per m³ as being desirable.
Just 7 of the 46 Belgian municipalities that took part in the study had particulate matter concentrations below the WHO norm. All 7 of these were in Wallonia.
However, at 25 micrograms per m³, the European particulate matter norm is less strict. All the Belgian municipalities surveyed were below this.
The average for Belgium as a whole (13.5 micrograms per m³) puts us about mid-table when compared to other European countries. Belgium came 16th out of the 32 European countries surveyed.
Of the 73 countries surveyed the world over, 53 had poorer average air quality than Belgium.