Air quality index

The information on air quality refers to the air quality index. The index colour refers to air quality in five classes shown in different colours and classified from good to very poor. The air quality index illustrates the relation of these classes to the health effects and norms.

If air quality is very bad, health effects are possible in sensitive individuals. If air quality is good or satisfactory, health effects, in light of research data, are very unlikely.

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

  • sulphur dioxide (SO2)
  • nitrogen dioxide (NO2)
  • thoracic particles (PM10)
  • fine particles (PM2.5)
  • ozone (O3) concentrations.

Table: Relation of the air quality index to effects.

​Air quality

​Health effects

​Other effects

​Very poor​Possible in sensitive population groups​Clear long-term impacts on vegetation and materials
​Poor​Possible in sensitive individuals​Clear long-term impacts on vegetation and materials
​Fair​Unlikely​Clear long-term impacts on vegetation and materials
​Satisfactory​Very unlikely​Clear long-term impacts on vegetation and materials
Good​Not found​Mild long-term impacts on the nature

 A low-level index is defined for each pollutant and the highest pollutant-specific index measured is the actual air quality index of that monitoring site. Therefore, the indexes of different monitoring sites aren’t completely comparable because the measured compounds may vary. For instance, if the index value rises to 51, the air quality class will change from good to satisfactory.

The reference,limit and threshold values of air quality, as well as the estimate, provided by THL's experts (National Institute of Health and Welfare), of the relation of concentrations to health risks underlie the index.

The air quality index used nowadays in Finland has been developed by YTV (i.e. the present HSY). The index was introduced in 1988. 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 indexes applied in other countries because it works on an hourly basis. Elsewhere the value is influenced by 24-hour, 8-hour and 1-hour averages.


Table: Definition of index classes according to the concentrations.






​​Very poor (>151)​>201​>351​>181​>201​>76
Poor (101-150)​151-200​251-350​141-180​101-200​51-75
​​Fair (76-100)​71-150​81-250​101-140​51-100​26-50
Good (<50)​<40​<20<60​​<20​<10

 The unit is a microgram per cubic metre (µg/m3).