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Indices definition
To present the air quality situation in European cities in a comparable and easily understandable
way, all detailed measurements are transformed into a single relative figure: the
Common Air Quality Index (or CAQI). Three different indices have been developed to enable
the comparison of three different time scale:
- An hourly index
which describes the air quality today, based on hourly values and updated every hours,
- A daily index
which stands for the general air quality situation of yesterday, based on daily values and updated once a day,
- An annual index
which represents the city's general air quality conditions throughout the year and compare to European air quality norms.
This index is based on the pollutants year average compare to annual limit values, and updated once a year.
However, the proposed indices and the supporting common web site www.airqualitynow.eu are designed to give a dynamic picture of the air quality situation in each city but not for compliance checking.
 THE HOURLY AND DAILY COMMON INDICES
These indices have 5 levels using a scale from 0 (very low) to > 100 (very high), it is a relative measure of the amount of air pollution. They are based on 3 pollutants of major concern in Europe: PM10, NO2, O3 and will be able to take into account to 3 additional pollutants (CO, PM2.5 and SO2) where data are also available.
The calculation of the index (automatically done by this website) is based on a review of a number of existing air quality indices, and it reflects EU alert threshold levels or daily limit values as much as possible. In order to make cities more comparable, independent of the nature of their monitoring network two situations are defined:
- Background, representing the general situation of the given agglomeration (based on urban background
monitoring sites),
- Roadside, being representative of city streets with a lot of traffic, (based on roadside monitoring
stations)
The indices values are updated hourly (for those cities that supply hourly data) and yesterdays
daily indices are presented.
Common air quality index calculation grid
Index
Class |
Grid |
 |
 |
Mandatory
pollutant |
Auxiliary
pollutant |
Mandatory
pollutant |
Auxiliary
pollutant |
| NO2 |
PM10 |
PM2.5 |
CO |
NO2 |
PM10 |
O3 |
PM2.5 |
CO |
SO2 |
| 1 hour |
24 hours |
1 hour |
24 hours |
1 hour |
24 hours |
1 hour |
24 hours |
| Very High |
>100 |
>400 |
>180 |
>100 |
>110 |
>60 |
>20000 |
>400 |
>180 |
>100 |
>240 |
>110 |
>60 |
>20000 |
>500 |
| High |
100 |
400 |
180 |
100 |
110 |
60 |
20000 |
400 |
180 |
100 |
240 |
110 |
60 |
20000 |
500 |
| 75 |
200 |
90 |
50 |
55 |
30 |
10000 |
200 |
90 |
50 |
180 |
55 |
30 |
10000 |
350 |
| Medium |
75 |
200 |
90 |
50 |
55 |
30 |
10000 |
200 |
90 |
50 |
180 |
55 |
30 |
10000 |
350 |
| 50 |
100 |
50 |
30 |
30 |
20 |
7500 |
100 |
50 |
30 |
120 |
30 |
20 |
7500 |
100 |
| Low |
50 |
100 |
50 |
30 |
30 |
20 |
7500 |
100 |
50 |
30 |
120 |
30 |
20 |
7500 |
100 |
| 25 |
50 |
25 |
15 |
15 |
10 |
5000 |
50 |
25 |
15 |
60 |
15 |
10 |
5000 |
50 |
| Very Low |
25 |
50 |
25 |
15 |
15 |
10 |
5000 |
50 |
25 |
15 |
60 |
15 |
10 |
5000 |
50 |
| 0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
- NO2, O3, SO2: hourly value / maximum hourly value in µg/m3
- PM10, PM2.5: hourly value / maximum hourly value or adjusted daily average in µg/m3
- CO: 8 hours moving average / maximum 8 hours moving average in µg/m3
THE COMMON ANNUAL AIR QUALITY INDEX
The common annual air quality index provides a general overview of the air quality situation in a given
city all the year through and regarding to the European norms.
It is also calculated both for background and traffic conditions but its principle of calculation
is different from the hourly and daily indices. It is presented as a distance to a target index,
this target being derived from the EU directives (annual air quality standards and objectives):
- If the index is higher than 1: for one or more pollutants the limit values are not met.
- If the index is below 1: on average the limit values are met.
The annual index is aimed at better taking into account long term exposure to air pollution based
on distance to the target set by the EU annual norms, those norms being linked most of the time
to recommendations and health protection set up by World Health Organisation.
Common annual air quality index calculation grid
| |
NO2 |
PM10 Year Average |
PM10 exceedences daily average |
Ozone # days 8-hour averages > 120µg/m3 |
SO2 |
Benzene |
Overall Index |
| Target value (µg/m3) |
40 |
40 |
31 |
25 |
20 |
5 |
|
| Target index |
1.0 |
1.0 |
1.0 |
1.0 |
1.0 |
1.0 |
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Full details on the index calculation are available in the report
Comparing Urban Air Quality across Borders (pdf in english).
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Indices definition 
