Definition of the air quality index

The index gathers as one representative number the concentrations of several polluants in ambient air. It is scaled from 1 (excellent air quality) to 10 (awful quality). The computation is performed by using data obtained from the telemetric networks that measure continuously the air quality in the 3  Regions. The index is thus a understandable representation of the measurement technical results for most of the public, allowing a global understanding of the ambient air quality. 

Which pollutants are used to compute the index?

The index is based on the concentrations of O3, NO2, SO2 and PM10 particles. A "characteristic value" is computed every day for these 4 pollutants and then compared to a concentration scale.
The concentration scales are based on the  European guidelines concerning the assessment and management of ambient air quality.
Based on the number of allowed exceedances of the new European limit values (the target value for ozone), an index value of 6, 7 or 8 is assigned to that limit value. The other scale divisions were defined by evaluating the range of concentrations measured in the telemetric networks of the three Belgian Regions.
The table below shows, for each pollutant, the relation between the measured concentrations mesured (in g/m), the index value and the corresponding scale.

Pollutant

g/m

SO2

24h average

0 - 15

16 - 30

31 - 45

46 - 60

61 - 80

81 - 100

101 - 125

126 - 165

166 - 250

> 250

NO2

hourly max

0 - 25

26 - 45

46 - 60

61 - 80

81 - 110

111 - 150

151 - 200

201 - 270

271 - 400

> 400

O3

max 8-hour average

0 -  30

31 - 45

46 - 60

61 - 80

81 - 100

101 - 120

121 - 150

151 - 200

201 - 270

> 270

PM10

24h average

0 - 10

11 - 20

21 - 30

31 - 40

41 - 50

51 - 70

71 - 100

101 - 150

151 - 200

> 200

index

1

2

3

4

5

6

7

8

9

10

rating

exellent

very good

good

fairly good

moderate

poor

very poor

bad

very bad

horrible

Which stations are used to compute the index ?

The "characteristic value"  for a given pollutant, for the whole Region or at some specific locations is obtained by computation of the average of the concentrations measured in the representative stations. The quality index is equal to the highest sub-index (i.e. the worst air quality) of the sub-indices of the 4 pollutants. The index is not computed whenever a sub-index is missing. The index is calculated in real time and for the last 15 days. The index is calculated for the country, the three Regions, the major conurbations and some specific districts.

Warning

The index is only a qualitative representation of the ambient air quality. It has little scientific meaning. Reports, studies and other scientific interpretations should only be performed by using measurement data.