Home Air Quality Indexes (AQI)

Air Quality Indices


Indices for the evaluation of air quality


(see Atmospheric Pollution – Atmospheric Boundary Layer, Contemporary measurement Technique, KOZANI 2004)



Environmental indicator: A numeric or descriptive categorization of many environmental parameters. It aims to provide a capacity for extracting information that may become useful to those relevant to the assessment of air quality and taking appropriate decisions.

  • Worldwide, several environmental indicators have been introduced, the use of which makes it possible to assess the air quality of an area often in a simple and understandable way to the general public.

  • The use of indexes facilitates the comparison of air quality of different geographical areas.


PSI (Pollutants Standard Index)

Composite air quality index, calculated from the concentrations of ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide and suspended particles.

  • Converts the concentrations of air pollutants into simple numerical values ranging from 0 to 500, which in turn are assigned to categories of air quality.
  • The PSI values determine the air quality in accordance to the relevant valuation list.


Assessment of air quality scale based on the PSI index values
(Boubel et al., 1994)


Air Quality Type

Possible health implications



None for the total population



Some or none



Slight deterioration of symptoms in the most vulnerable categories


Very Unhealthy

Significant deterioration - extensive symptoms



Early emergence of certain diseases, increasing worsening of symptoms, reduction of resistance levels of healthy people



AQI (Air Quality Index):


The index AQI (Air Quality Index, EPA, 40 CFR Part 58) is a modification of the index PSI, introducing an additional category described as "unhealthy for sensitive groups" and specific indicators for concentrations of PM10 and PM2.5.

 Air Quality valuation scale according to AQI values (EPA, 40 CFR Part 58). 


Air Quality Index


Levels in relation to health

Color scale

When index (AQI) lies in  range: ...the air quality conditions are: ...marked by
0 to 50 Good Green
51 to 100 Moderate Yellow
101 to 150 Unhealthy for sensitive groups Orange
151 to 200 Unhealthy Red
201 to 300 Very Unhealthy Purple
301 to 500 Hazardous Crimson



Air Pollution Index


Adopted in United Kingdom by Committee on Medical Effects of Air Pollution Episodes (COMEAP). It makes use of 1 – 10 scale divided in four colored regions : 

  • 1-3 (Low)
  • 4-6 (Moderate)
  • 7-9 (High)
  • 10 (Very High)


The total air pollution index of a region is calculated using the maximum concentration of the following 5 pollutants: 

  • Nitrogen Dioxide
  • Sulfur Dioxide 
  • Ozone
  • Particulate Matter PM2.5 
  • Particulate Matter PM10 






Nitrogen Dioxide

Sulfur Dioxide

Carbon Monoxide


Running 8 hourly or hourly mean *

Hourly mean

15 minute mean

Running 8 hourly mean

Running 24 hour mean

* For ozone, the maximum of the 8 hourly and hourly mean is used to calculate the index value