US Air Quality Index from PM2.5 (EPA)

This is the number on the AirNow maps and the one most phone weather apps quote for smoke. It takes a PM2.5 concentration, meaning particles under 2.5 µm averaged over 24 hours, in µg/m³, and turns it into an index. It is the general straight-line step with the PM2.5 breakpoints already filled in, so you enter $C$ and read $I$. The relation also runs backwards: enter an index and it returns the concentration behind it.

The EPA fixes six breakpoints, 9.0, 35.4, 55.4, 125.4, 225.4 and 325.4 µg/m³ (the 2024 revision), that stand for index values of 50, 100, 150, 200, 300 and 500. Between two neighbours the index follows a straight line, and each band has its own slope, so equal steps in concentration do not give equal steps in index. The labels follow the index: 0 to 50 good, 51 to 100 moderate, 101 to 150 unhealthy for sensitive groups, 151 to 200 unhealthy, 201 to 300 very unhealthy, and above that hazardous. The line reaches 500 at 325.4, as the table's footnote 4 has it.

The page's first check is the edge of a band: 9.0 µg/m³ gives exactly 50. For a clear day in a city, 12 µg/m³ sits in the 9.1 to 35.4 band, which carries 51 to 100, so $49/26.3\times2.9+51=56.40304$, a moderate reading. A reading of 300 µg/m³ falls in the 225.5 to 325.4 band, which carries 301 to 500, and gives 449.40340. The EPA's own example is 35.9: the EPA truncates it to one decimal place, finds the 101 to 150 band and rounds 101.98 to 102. This page returns the continuous line, 101.98, and leaves the rounding to you.

Two cautions. The table is built for a 24-hour average, so feed it that rather than a single hour. And the breakpoints are the 2024 revision first published in EPA-454/B-24-002, carried in the EPA's Technical Assistance Document (EPA-403/B-26-003, May 2026, Table 6), so check which revision an older table uses before comparing. This page covers fine particles only; a city's overall US index takes the worst single pollutant, whereas Canada's index combines three. Run backwards, an index of 75 corresponds to 21.98163 µg/m³, and an index of 150 is the breakpoint 55.4.