Ambient air pollutant concentrations are measured by
air pollution monitoring networks in a number of countries
around the world. These measurements are combined with
mathematical models to forecast air pollutant levels over
24 to 48 hours. Both measured concentrations and predicted
levels are disseminated to the public in various ways. At
present, there is no accepted consensus standardization of
approaches or methods, but in general, most authorities
convert increasing concentrations of major air pollutants
(ozone, PM2.5, PM10, carbon monoxide, nitrogen dioxide, and
sulfur dioxide) into severity bands labeled with progressive
degrees of risk. For example, the US EPA’s Air Quality Index
(AQI) includes band ratings of ‘good, moderate, unhealthy
for sensitive individuals, unhealthy, very unhealthy, and
hazardous’. The Common Air Quality Index (CAQI), used
on the European Union’s Air Quality Now website, labels
band ratings as ‘very low, low, medium, high, and very high’.
The AQI, CAQI and other systems use different air
pollutant cut-off values to define bands. Therefore, the
severity bands are not directly comparable from country to
country, even though the severity terms may be the same,
because ratings are generally based on how a pollutant