US Air Quality Index from PM10 (EPA)

PM10 is the coarser end of airborne particles, anything under 10 µm: dust, road grit, pollen fragments and the larger part of smoke. The US index for it is worked from a 24-hour average in µg/m³. This page is the general straight-line step with the EPA's PM10 breakpoints filled in, so you enter the concentration $C$ and read the index $I$, or enter an index and get the concentration that produces it.

The EPA's breakpoints are 54, 154, 254, 354, 424 and 604 µg/m³, standing for index values of 50, 100, 150, 200, 300 and 500. A reading at a breakpoint gives that breakpoint's index exactly, which is the quickest way to see how the table is built: 54 gives 50, 154 gives 100, 254 gives 150 and 354 gives 200. Between any two neighbours the index follows a straight line, and the line reaches 500 at 604. 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. A typical city reading of 20 to 50 µg/m³ is below the 54 that closes the good band.

For a reading inside a band, take 100 µg/m³. It sits in the band from 55 to 154, which carries the index from 51 to 100. That band spreads 49 index points over 99 µg/m³, and the reading is 45 µg/m³ above the bottom of it, so $49/99\times45+51=73.27273$, a moderate reading. The only judgement is which band holds the concentration, and the breakpoints settle that.

Fine particles (PM2.5) have their own, much stricter table, so the same concentration reads higher there; do not use this page for them. The EPA truncates the concentration to a whole number and rounds the index, whereas this page gives the continuous line, so expect a difference in the last digit against a published figure. 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.