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

Sulphur dioxide is the gas of smelters, refineries, coal burning and volcanoes, and the US index for it is worked from the 1-hour average in parts per billion. This page is the general straight-line step with the sulphur dioxide breakpoints filled in: enter the concentration $C$ and read the index $I$, or enter an index and get the concentration behind it. Away from smelters and refineries readings are typically under 5 ppb, far below the first breakpoint, so ordinary air sits in the lowest band.

There are only four breakpoints: 35, 75, 185 and 304 ppb, standing for index values of 50, 100, 150 and 200. A reading at a breakpoint gives that index exactly: 35 ppb gives 50, 75 gives 100, 185 gives 150 and 304 gives 200. Between two neighbours the index follows a straight line, with its own slope in each band. 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 scale stops at 200 because the EPA's 1-hour table does. The 1-hour table ends at 304 ppb. For the upper half of the scale the EPA switches to the 24-hour average, so this page does not answer above 304 ppb. A high-sulphur episode beyond that has to be judged on the 24-hour average, and if you have the 24-hour breakpoints you can supply them to the general interpolation page.

For a worked reading, take 50 ppb. It sits in the band from 36 to 75, which carries the index from 51 to 100. That band spreads 49 index points over 39 ppb, and the reading is 14 ppb above the bottom of it, so $49/39\times14+51=68.58974$. The EPA truncates the reading to a whole ppb and rounds the index to a whole number; this page gives the continuous line. 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).