<P> As of 2012, the EU - supported project CiteairII argued that the CAQI had been evaluated on a "large set" of data, and described the CAQI's motivation and definition . CiteairII stated that having an air quality index that would be easy to present to the general public was a major motivation, leaving aside the more complex question of a health - based index, which would require, for example, effects of combined levels of different pollutants . The main aim of the CAQI was to have an index that would encourage wide comparison across the EU, without replacing local indices . CiteairII stated that the "main goal of the CAQI is not to warn people for possible adverse health effects of poor air quality but to attract their attention to urban air pollution and its main source (traffic) and help them decrease their exposure ." </P> <P> The CAQI is a number on a scale from 1 to 100, where a low value means good air quality and a high value means bad air quality . The index is defined in both hourly and daily versions, and separately near roads (a "roadside" or "traffic" index) or away from roads (a "background" index). As of 2012, the CAQI had two mandatory components for the roadside index, NO and PM, and three mandatory components for the background index, NO, PM and O. It also included optional pollutants PM, CO and SO . A "sub-index" is calculated for each of the mandatory (and optional if available) components . The CAQI is defined as the sub-index that represents the worst quality among those components . </P> <P> Some of the key pollutant densities in μg / m for the hourly background index, the corresponding sub-indices, and five CAQI ranges and verbal descriptions are as follows . </P> <Table> <Tr> <Th> Qualitative name </Th> <Th> Index or sub-index </Th> <Th_colspan="4"> Pollutant (hourly) density in μg / m </Th> </Tr> <Tr> <Th_colspan="2"> </Th> <Th> NO </Th> <Th> PM </Th> <Th> O </Th> <Th> PM (optional) </Th> </Tr> <Tr> <Td> Very low </Td> <Td> 0--25 </Td> <Td> 0--50 </Td> <Td> 0--25 </Td> <Td> 0--60 </Td> <Td> 0--15 </Td> </Tr> <Tr> <Td> Low </Td> <Td> 25--50 </Td> <Td> 50--100 </Td> <Td> 25--50 </Td> <Td> 60--120 </Td> <Td> 15--30 </Td> </Tr> <Tr> <Td> Medium </Td> <Td> 50--75 </Td> <Td> 100--200 </Td> <Td> 50--90 </Td> <Td> 120--180 </Td> <Td> 30--55 </Td> </Tr> <Tr> <Td> High </Td> <Td> 75--100 </Td> <Td> 200--400 </Td> <Td> 90--180 </Td> <Td> 180--240 </Td> <Td> 55--110 </Td> </Tr> <Tr> <Td> Very high </Td> <Td>> 100 </Td> <Td>> 400 </Td> <Td>> 180 </Td> <Td>> 240 </Td> <Td>> 110 </Td> </Tr> </Table>

What is the unit of measurement for air pollution
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