ICAIR, the cumulative air index

A new index for air quality

ICAIR (Cumulative Air Index) is an index that cumulates four regulated pollutants (NO₂, O₃, PM2.5, PM10) at the spatial resolution of 25 meters over the entire region (HD modeling with measurement assimilation).

There is an hourly version with a 24h forecast so people can take the level of pollution into account in their daily activities: ICAIRh
It also exists in an annual version: ICAIR365

This new index takes into account the cumulative effects of different pollutants, making it possible to highlight areas of multiple exposure. In its hourly version, it’s based on the European Air Quality Index thresholds. In its annual version, it uses the WHO Air quality guidelines (AQG).

Pollution, an accumulation of exposure

The ICAIR index takes into account the cumulation of 4 pollutants (PM10, PM2.5, O₃ and NO₂), to calculate a unique air quality assessment value.
The choice to consider the sum of those 4 pollutants makes it possible to better assess the exposure of each person to pollution.

An example: on this diagram, ozone (O₃) (3), nitrogen dioxide (NO₂) (1) and fine particles (PM10 and PM2.5) (2) concentration curves represent a fictional summer day. Ozone concentration is generally the highest level, with a lower level in the city, compared to the suburbs and the countryside. If we considered the only major pollutant, the city could appear to be less polluted than peri-urban areas.

With ICAIRH, each pollutant concentration will be taken into account in the final air quality level assessment. And by making a curve combining the three pollutants, we realize that urban areas are actually more polluted than peri-urban areas.

ICAIRh represents the “multi-exposure” of populations.

An index to adapt our daily activity to air quality

ICAIRh is an hourly index. It translates, hour by hour, pollution throughout the region. This hourly index uses the European Index threshold, adds to it the principle of cumulation, and allows to schedule activities depending on the level of pollution.

ICAIRh

The applicable colour scale is a colour gradient as shown here. Depending on the level of pollution, some activities will be more recommended than others in certain areas (peri-urban rather than urban for example).

By clicking on the hourly map on the homepage, you will have access to:

How is the ICAIR index calculated?

The weighting of the pollutants is based on:
- the European Index thresholds, for the hourly index
- the WHO Air quality guidelines (AQG v2021), for the annual index
The indicator varies on an open scale with values generally between 0 and 10, without necessarily being stuck between those thresholds
ICAIR provides data rounded to the minimum precision of one digit after the decimal point, allowing the representation of:
- fine spatial variations
- pollution without a threshold effect
Because of the accumulation of pollutants, there is no exact match between the exceedance of a regulatory threshold and a value of this indicator.

How is ICAIRh calculated?

A sub-index Ip is calculated fo each pollutant "p"

For a representation of hourly variation, the hourly average is used for all pollutants. For NO₂ and O₃, we use the European index thresholds which are already on an hourly basis.

For PM10 and PM2.5 the European thresholds are defined on a daily basis.At the national level, the correlation between daily averages and hourly maximums per day gives a factor of about 2 for both fractions of particles. For the hourly index, we therefore refer to the European index thresholds doubled.

For a concentration [P] of the pollutant "p" and x between 1 and 5, if Sₓ<[P]<=Sₓ₊₁ (if [P] > S₅, we consider that x = 5) :

ICAIRh is then calculated as follows:

ICAIRh = max ( IPM10 ; IPM2.5 ) + INO₂ + IO₃

Note: we only take into account the maximum value between IPM10 and IPM2.5 to avoid double counts between PM10 and PM2.5.

How is ICAIR365 calculated?

ICAIR365

ICAIR365 is an annual index. It is based on annual averages of the four pollutants taken into account (PM10, PM2.5, O₃ and NO₂). These are aggregated together to give an overall picture of air quality over the past year. It then shows exposure to the sum of the four pollutants.

The applicable colour scale is a gradient using the colours of the European Index.