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

Carbon monoxide is the odourless gas of incomplete combustion, and the US index for it is worked from the 8-hour average in parts per million. Outdoors it has fallen so far with cleaner engines that it rarely sets a city's overall index; 0.2 to 1 ppm is typical outdoors in a city. Indoors, near a faulty heater or a running engine, the same numbers matter a great deal, so the page is useful even though the outdoor index seldom turns on it.

The breakpoints are 4.4, 9.4, 12.4, 15.4, 30.4 and 50.4 ppm, standing for index values of 50, 100, 150, 200, 300 and 500. Between two neighbours the index follows a straight line, with its own slope in each band. The lowest band is the simplest because it starts at nothing: 0.0 ppm is index 0, so there is no offset to add. A reading of 2.0 ppm sits in the band from 0.0 to 4.4, which carries 0 to 50, so the index is $50/4.4\times2.0=22.72727$. Readings at the breakpoints give their index exactly: 4.4 ppm gives 50, 9.4 gives 100 and 12.4 gives 150.

The EPA's own example is 8.4 ppm. It falls in the band from 4.5 to 9.4 ppm, which carries the index from 51 to 100, and the straight line gives exactly 90. The EPA truncates the reading to one decimal and rounds the index, so a published figure can differ from this page's continuous line; here the two agree because the example happens to land on a whole number.

Mind the averaging time: the table asks for an 8-hour average, not a spot reading. The relation also runs backwards, returning the concentration in ppm for a given index, and a ppm reading can be turned into mass per cubic metre of air with the conversion page. 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), a US federal work in the public domain. The line reaches 500 at 50.4 ppm.