<Table> <Tr> <Th> Color </Th> <Th> Condition </Th> </Tr> <Tr> <Td> Blue </Td> <Td> "Heavy pollution" in the next 24 hours </Td> </Tr> <Tr> <Td> Yellow </Td> <Td> "Hazardous" in the next 24 hours; or "heavy pollution" for three consecutive days </Td> </Tr> <Tr> <Td> Orange </Td> <Td> Alternate "heavy pollution" and "hazardous" days for three consecutive days </Td> </Tr> <Tr> <Td> Red </Td> <Td> Average of "heavy pollution" for four consecutive days, "hazardous" for two consecutive days or average AQI over 500 for one day </Td> </Tr> </Table> <Tr> <Th> Color </Th> <Th> Condition </Th> </Tr> <Tr> <Td> Blue </Td> <Td> "Heavy pollution" in the next 24 hours </Td> </Tr> <Tr> <Td> Yellow </Td> <Td> "Hazardous" in the next 24 hours; or "heavy pollution" for three consecutive days </Td> </Tr>

How long has air pollution been a problem in china