<Tr> <Td> Very high </Td> <Td>> 100 </Td> <Td>> 400 </Td> <Td>> 180 </Td> <Td>> 240 </Td> <Td>> 110 </Td> </Tr> <P> Frequently updated CAQI values and maps are shown on the www.airqualitynow.eu and other websites . A separate Year Average Common Air Quality Index (YACAQI) is also defined, in which different pollutant sub-indices are separately normalised to a value typically near unity . For example, the yearly averages of NO, PM and PM are divided by 40 μg / m ^ 3, 40 μg / m ^ 3 and 20 μg / m ^ 3, respectively . The overall background or traffic YACAQI for a city is the arithmetic mean of a defined subset of these sub-indices . </P> <P> The United States Environmental Protection Agency (EPA) has developed an Air Quality Index that is used to report air quality . This AQI is divided into six categories indicating increasing levels of health concern . An AQI value over 300 represents hazardous air quality and below 50 the air quality is good . </P> <Table> <Tr> <Td> <Table> <Tr> <Td> Air Quality Index (AQI) Values </Td> <Td> Levels of Health Concern </Td> <Td> Colors </Td> </Tr> <Tr> <Td> 0 to 50 </Td> <Td> Good </Td> <Td> Green </Td> </Tr> <Tr> <Td> 51 to 100 </Td> <Td> Moderate </Td> <Td> Yellow </Td> </Tr> <Tr> <Td> 101 to 150 </Td> <Td> Unhealthy for Sensitive Groups </Td> <Td> Orange </Td> </Tr> <Tr> <Td> 151 to 200 </Td> <Td> Unhealthy </Td> <Td> Red </Td> </Tr> <Tr> <Td> 201 to 300 </Td> <Td> Very Unhealthy </Td> <Td> Purple </Td> </Tr> <Tr> <Td> 301 to 500 </Td> <Td> Hazardous </Td> <Td> Maroon </Td> </Tr> </Table> </Td> </Tr> </Table>

What level does the air quality index have to be above to be considered hazardous