PM2.5 and the AQI, Explained

A weather app shows a single number — "AQI 89" — colours it yellow, and somehow that is supposed to tell you whether to go for a run. Underneath it is a small, exact piece of arithmetic: a concentration of fine particles in micrograms per cubic metre, pushed through a lookup table and a straight-line formula until it lands on a 0-to-500 scale. This guide takes the US EPA Air Quality Index apart from first principles for its most-reported pollutant, PM2.5: what the index really is, the breakpoint method, the full PM2.5 table, one concentration converted step by step, the six category bands and what the EPA honestly says each means, how PM2.5 differs from PM10 and ozone, and the one limit every AQI number shares — it only knows the concentration you fed it.

What the AQI actually is

The Air Quality Index is a unitless 0-to-500 scale, not a measurement in its own right. Different pollutants use different units — particles in micrograms per cubic metre (µg/m³), gases like ozone and carbon monoxide in parts per million (ppm) or billion (ppb) — and those numbers are not comparable on their face. The AQI translates each onto one common ruler so a single value can say "this air is roughly this bad" regardless of which pollutant is driving it.

Six pollutants have official EPA breakpoints: PM2.5, PM10, ground-level ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide. Each is converted to its own sub-index independently, and the reported AQI is the maximum of those sub-indices; the pollutant behind that maximum is the driving pollutant. That "take the worst one" rule is why a headline AQI is a floor on how bad the air is, never a full inventory of it.

The EPA breakpoint method

AQI is not a smooth curve; it is a piecewise-linear lookup. For each pollutant the EPA publishes a table of concentration breakpoints paired with AQI breakpoints, and each table row is a straight line segment. To convert a concentration Cp, you find the row whose concentration range straddles it, then interpolate between that row's AQI endpoints with the standard formula:

Ip = ((IHi − ILo) / (BPHi − BPLo)) × (Cp − BPLo) + ILo

Here BPLo and BPHi are the low and high concentration breakpoints of the row, ILo and IHi are the matching low and high AQI values, and Ip is the sub-index for that pollutant. Two details matter before you plug in numbers. First, the concentration is truncated, not rounded, to the table's precision — one decimal place for PM2.5 — so 30.09 becomes 30.0, and a value's extra digits are simply dropped. Second, the resulting Ip is rounded to a whole number only at the end.

The PM2.5 breakpoint table

PM2.5 is reported as a 24-hour average in µg/m³, truncated to one decimal. These are the exact concentration and AQI breakpoints this site's calculator uses, each row written as [concentration low – high → AQI low – high]:

The top two rows both fall inside the single Hazardous colour band (AQI 301–500): the category is one colour, but the concentration scale keeps splitting it so each segment stays linear. Each row also starts exactly one precision-step above where the last ended — 12.0 then 12.1, 35.4 then 35.5 — so no concentration qualifies for two rows at once. Anything outside 0.0–500.4 has no defined PM2.5 sub-index and is left out rather than extrapolated.

Worked example: 30.09 µg/m³ step by step

Suppose a monitor reports a 24-hour PM2.5 average of 30.09 µg/m³. Convert it by hand:

So 30.09 µg/m³ of fine particles reads as AQI 89, Moderate. Every input to that calculation came from the PM2.5 table above: the row boundaries 12.1 and 35.4, and the AQI endpoints 51 and 100. The same five steps handle any reading. Push it to exactly 35.5 and step 2 lands in the next row, where Cp − BPLo is zero, so the formula returns exactly 101 — one point into Unhealthy for Sensitive Groups, not 100. That single-point jump is the deliberate one-unit gap between rows doing its job.

The six categories and honest health guidance

The 0-to-500 scale is cut into six named bands, each with an official colour used on maps and forecasts. The EPA's guidance for each is deliberately about who should take care rather than precise medical claims:

Two honest caveats. The bands are thresholds on a continuous risk, not cliffs — AQI 99 and 101 describe almost the same air even though one is yellow and the next orange. And these are population-level messages: someone with asthma may react below the "Moderate" line, while a healthy adult may notice nothing at "Unhealthy." The category is a starting point for a decision, not a personal diagnosis.

PM2.5 vs PM10 vs ozone

PM2.5 gets most of the attention, but it is one of three pollutants that most often drive a daily AQI, and they behave differently. PM2.5 is fine particulate matter 2.5 micrometres across or smaller — small enough to reach deep into the lungs — reported as a 24-hour average in µg/m³. PM10 is coarser particles up to 10 micrometres (dust, pollen, smoke), also a 24-hour µg/m³ average but with its own, wider breakpoint table truncated to whole numbers rather than one decimal, so identical-looking concentrations do not produce identical AQIs across the two.

Ozone is different again: a gas measured in ppm as an 8-hour average, on a table whose defined 8-hour range tops out around 0.200 ppm before the EPA switches to a separate 1-hour method for extreme readings. Because the units and averaging windows differ, the commonest mistake in AQI arithmetic is a unit mix-up — ozone in ppb instead of ppm, or an hourly particle spike fed into a table that expects a daily average — either of which throws the result off by a wide margin.

The honest limit of any AQI

Every AQI number, including the one from this site's calculator, shares a ceiling on its meaning: it reflects only the concentration it was given. There is no live sensor and no network call behind a converter — it turns the value you enter into an index, so the answer is exactly as good as your input. If the reading is stale, mis-measured, or from a monitor across town, the AQI inherits every flaw while still looking authoritative. It also implements the US EPA definition; other systems (the European EAQI, UK DAQI, India's NAQI, China's AQI) use different breakpoints, so a given concentration reads as different numbers in different countries. Treat the AQI as a faithful translation of a concentration into a health-oriented scale — nothing more, nothing less.

Putting it together

The AQI is a 0-to-500 translation layer: each concentration is truncated to the table's precision, matched to the breakpoint row that straddles it, interpolated with Ip = ((IHi − ILo) / (BPHi − BPLo)) × (Cp − BPLo) + ILo, and the reported figure is the worst pollutant's sub-index. For PM2.5, 30.09 µg/m³ works out to AQI 89 (Moderate) using nothing but the table's own numbers. The six colour bands map to population-level EPA guidance, not personal medicine, and the number means only as much as the concentration behind it. Learn those few mechanics and the yellow "89" in your weather app stops being a mystery and becomes a calculation you could check by hand.

Open the AQI Calculator → to convert your own PM2.5, PM10 or ozone readings, or browse all Polyatic tools.