What is the air quality index?

The purpose of the different colour classes in the air quality index is to facilitate communication about air quality. The index categorises air quality into five different classes that describe the air quality using colours and words (see Table). The index is based on guideline, limit and threshold values for air quality, as well as on the Finnish Institute for Health and Welfare (THL) experts’ estimate of the relation of concentrations to health risks. When the air quality is poor, health effects are possible in sensitive individuals. When the air quality is good or satisfactory, health effects are very unlikely.

Relation of the air quality index to effects

Air quality Index colour Health effects Other effects
Very poor Violet

​Possible in sensitive population groups

​​Clear long-term impacts on vegetation and materials

Poor Red

​Possible in sensitive individuals

​​Clear long-term impacts on vegetation and materials

Fair Orange


​​Clear long-term impacts on vegetation and materials

Satisfactory Yellow

​Very unlikely

​​Mild long-term impacts on vegetation and materials

Good Green

​Not found

Mild long-term impacts on nature


The air quality index is calculated on an hourly basis for each monitoring site. The index covers all pollutants measured at the monitoring site. The calculation rules in the concentrations of different air pollutants:

  • sulphur dioxide (SO2)
  • nitrogen dioxide (NO2)
  • thoracic particles (PM10)
  • fine particles (PM2.5)
  • carbon monoxide (CO)
  • ozone (O3)
  • total reduced sulphur (TRS) compounds

A sub-index is calculated for each air pollutant measured at the monitoring site, and the highest pollutant-specific index measured is the actual air quality index of that monitoring site. The air quality indexes of different monitoring sites are not fully comparable because not all monitoring sites measure the same air pollutants.

The air quality index used in Finland was developed by YTV (now HSY) and THL. Our index was introduced in 1988, and the index calculated by the present method has been used for information communication since 1993. The calculation methods were revised in 2002 and in 2007. The index differs from the indexes used in other countries in that it works on an hourly basis. Elsewhere, the value is influenced by 24-hour, 8-hour and 1-hour averages.

Determination of index classes according to concentrations

In the table, the unit is micrograms per cubic metre (μg/m3), except for carbon monoxide CO (mg/m3).

  Index colour CO






Very poor (≥151) Violet








Poor (101–150) Red








Fair (76–100) Orange








Satisfactory (51–75) Yellow








Good (≤50) Green






​​ ≤10



Indicative air quality index for black carbon och LDSA

An indicative air quality index has been developed for the graphic display of concentrations of black carbon (BC), lung-deposited surface area (LDSA) of particles and particle number concentration (PNC).

There are so far no limit or guideline values for these air pollutants. Therefore, they are not included in the health-based air quality index developed by HSY and THL.

In the table the unit for black carbon is one microgram per cubic metre (μg/m3), for LDSA one square micrometer per cubic centimetre  (μm2/cm3) and for particle number concentration one thousand particles per cubic centimetre (103/cm3) .

  Index colour BC



Very poor (≥151) Violet




Poor (101–150) Red 7-12



Fair (76–100) Orange




Satisfactory (51–75) Yellow




Good (≤50) Green