A result that differs from your expectations can have various causes. CurieuzenAir confirms that air quality in Brussels has improved considerably in recent years. The exceedance level (i.e. the percentage of sites exceeding the EU limit value of 40 μg m-3) is only 1.4% in 2021, which is low when put into historical perspective.
An other important note is that the CurieuzenAir measurement of NO2 only provides information on air pollution related to traffic.
The effect of domestic heating and wood-burning on air quality is not properly reflected in our measurements. This is because these are primarily sources of fine particulates and less of NO2. Fine dust could not be measured in this project. A CurieuzenAir measurement in a low-traffic area where a lot of wood is burned will therefore probably give a low measurement result.