US Air Quality Index from Ozone, 8-Hour (EPA)

This is the US Air Quality Index for ground-level ozone, worked from the 8-hour average that the EPA's rule requires. It is the general straight-line step with the ozone breakpoints filled in. Because it is built on an 8-hour average, a single hour's reading is not what the table asks for; give it the average. The same relation runs backwards, from an index to the concentration it stands for.

Units are the first thing to get right. The EPA's table is written in parts per million, to three decimals, while this page works in parts per billion: 0.070 ppm is 70 ppb, so the table's figures are multiplied by 1000. The breakpoints are 54, 70, 85, 105 and 200 ppb (0.054, 0.070, 0.085, 0.105 and 0.200 ppm), standing for index values of 50, 100, 150, 200 and 300. Between two neighbours the index follows a straight line, and each band has its own slope. 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.

The EPA's own example is 0.078 ppm, which is 78 ppb. The EPA truncates the ppm reading to three decimals, finds that it lies between the 70 and 85 breakpoints, in the band that carries the index from 101 to 150, and the straight line gives 125.5. The EPA reports 126 because it rounds to a whole number; this page leaves the continuous line. In the lower band, 60 ppb sits between 55 and 70 (index 51 to 100), so $49/15\times5+51=67.33333$. Readings at the breakpoints give their indexes exactly: 54 ppb gives 50, 70 gives 100 and 85 gives 150.

The limit is built into the table. Above 200 ppb the 8-hour table stops and the EPA's 1-hour table takes over, so this page does not answer there; for the 1-hour table, supply its breakpoints to the general interpolation page. Canada's index handles ozone differently: it takes ozone as a 3-hour mean along with two other pollutants. The breakpoints and equation are from Table 6 and equation 1 of the EPA's Technical Assistance Document for the Reporting of Daily Air Quality (EPA-403/B-26-003, May 2026).