US Air Quality Index from Nitrogen Dioxide, 1-Hour (EPA)

Nitrogen dioxide is the brown gas of traffic and combustion, and the US index for it is worked from the 1-hour average in parts per billion. As with the other pollutants, the EPA publishes breakpoints and a reading is placed by a straight line between the two around it. This page is the general straight-line step with the nitrogen dioxide breakpoints already in, so you enter the concentration $C$ and read the index $I$, or the reverse.

The breakpoints are 53, 100, 360, 649, 1249 and 2049 ppb, standing for index values of 50, 100, 150, 200, 300 and 500. A reading at a breakpoint gives that index exactly: 53 ppb gives 50, 100 ppb gives 100, 360 ppb gives 150 and 649 ppb gives 200. The upper bands are wide. The band from 100 to 360 ppb carries the index only from 100 to 150, so in that stretch a given change in concentration moves the index much less than it does lower down. The labels follow the index: 0 to 50 good, 51 to 100 moderate, 101 to 150 unhealthy for sensitive groups, 151 to 200 unhealthy, and above 200 worse.

In practice nitrogen dioxide seldom drives a city's overall index. Typical city air runs from 5 to 30 ppb, all of it below the 53 ppb that closes the good band. The overall US index takes the maximum over the pollutants: each one gets its own index, and the highest sets the headline number. Nitrogen dioxide is one of the pollutants in that comparison, but it is rarely the highest.

For a worked reading, take 80 ppb. It sits in the band from 54 to 100, which carries the index from 51 to 100. That band spreads 49 index points over 46 ppb, and the reading is 26 ppb above the bottom of it, so $49/46\times26+51=78.69565$. The EPA truncates the reading to a whole ppb and rounds the index; this page gives the continuous line. Canada's index takes nitrogen dioxide as a 3-hour mean instead. The breakpoints and equation are from Table 6 and equation 1 of the EPA's Technical Assistance Document (EPA-403/B-26-003, May 2026).