Air quality · Hazardous
AQI 303.
An AQI of 303 sits in the Hazardous band (301–500) — emergency conditions. If fine particulate is driving it, that is roughly 226.5 µg/m³ of PM2.5. Here is where the number comes from, which pollutant produced it, and why that second question changes the answer to everything else.
Hazardous
Band 301–500 · Maroon · 1% through the band
the concentration behind the number
The AQI is not a measurement. It is a unit-less index produced by running an actual pollutant concentration through a piecewise linear transform, which means the transform inverts — every AQI value corresponds to a specific concentration, and that concentration is the number with physical meaning.
An AQI of 303 works out to each of the following, depending on which pollutant is responsible:
PM2.5
226.5 µg/m³
fine particulate, 24-hour average — the usual driver in smoke events
PM10
427 µg/m³
coarse particulate, 24-hour average — dust, pollen, mechanical sources
Ozone
n/d ppb
EPA does not define an 8-hour ozone AQI above 300
The transform
AQI = (Ihi − Ilo) / (BPhi − BPlo) × (C − BPlo) + Ilo
Where C is the measured concentration and BP/I are the breakpoints bracketing it. Solve for C instead of AQI and you get the figures above. Breakpoints here are the post-2024 EPA table, effective 6 May 2024, which lowered the PM2.5 ceiling for “Good” from 12.0 to 9.0 µg/m³ — a great many published tables still show the old numbers.
the number reports the worst pollutant, not the air
The AQI is computed separately for each monitored pollutant and then reported as the highest of those sub-indices. So an AQI of 303 does not describe the overall state of the atmosphere — it describes whichever single pollutant is currently furthest along its own scale, and says nothing about the others.
This matters more than it sounds, because the two pollutants that usually drive the US index behave in opposite ways across a day.
Ozone
Peaks mid-to-late afternoon
Ground-level ozone is not emitted; it is manufactured in place, by sunlight driving reactions between nitrogen oxides and volatile organic compounds. It therefore needs two things — precursor emissions and solar energy — and accumulates through the day as both build. Concentrations are typically lowest around dawn and highest between roughly two and six in the afternoon, on hot, stagnant, sunny days.
Wildfire smoke
Often peaks overnight and at dawn
Smoke particulate is transported rather than formed locally, and where it sits is governed by mixing. After sunset the surface cools, a stable layer forms, and the atmosphere stops mixing vertically — so smoke aloft settles toward the ground and concentrates in it. Daytime heating breaks that inversion and mixes the smoke through a deeper layer, which frequently makes the afternoon the cleaner part of a smoke day.
Which is why the standard advice inverts. “Exercise in the morning when the air is better” is correct on an ozone day and can be exactly wrong on a smoke day. An AQI of 303 on its own cannot tell you which one you are in — the dominant pollutant can, and most air quality apps will name it if you look.
the number on your screen is already behind
Particulate AQI is built on a 24-hour average, and an average is a lagging instrument by construction. During a fast-moving smoke event the reported figure can trail the air you are actually standing in by hours — improving while conditions worsen, or still elevated after a front has scoured the air clean.
The practical correction is to read the trend and the raw concentration rather than the index alone. Rising PM2.5 with a falling index means the average has not caught up yet.
what changes at 303
Everyone, seriously. Official guidance at this level, in the four decisions people actually make:
General information drawn from published EPA and AirNow category guidance, not medical advice. If you have a lung or heart condition, your own threshold may be lower than the category suggests.
what 303 does to the light
Daylight fails. Midday can read as dusk with a deep orange or brown cast, streetlights trip on, and the sun may be invisible rather than merely dimmed.
The uncomfortable symmetry of air quality is that the particulate degrading the air is the same particulate producing the colour — which is why the most spectacular skies of a smoke year arrive on its worst days. Why sunsets differ from place to place comes down largely to what the air column is carrying.
nearby readings
For the full explanation of the categories, how a plume finds you, and what actually changes at each threshold, read the air quality index explained. For the other number on your forecast that nobody translates, see the UV index.
Is an AQI of 303 good or bad?
An AQI of 303 falls in the "Hazardous" category (301–500), which is not good. Everyone, seriously. If fine particulate is the driver, 303 corresponds to roughly 226.5 µg/m³ of PM2.5 over 24 hours. The single most useful follow-up question is which pollutant is responsible, because ozone and wildfire smoke peak at opposite ends of the day.
Frequently Asked Questions
Is an AQI of 303 good or bad?
303 is not good. It sits in the "Hazardous" band, which runs 301 to 500 — note that "Good" is the specific name of the 0–50 band, so a Moderate reading is not the same thing as a good one. An emergency-conditions health warning. Everyone is likely to be affected. Readings here are rare and almost always indicate a major wildfire smoke event, a dust storm, or an industrial release close to the monitor. Everyone, seriously.
Can I exercise outside at AQI 303?
None outdoors. Minimise outdoor time entirely.
Should I open my windows at AQI 303?
Sealed. Filtered room, air cleaner running continuously, and no indoor combustion of any kind. If you do not have filtration, public guidance in smoke events commonly points to designated clean-air shelters.
What PM2.5 level does AQI 303 correspond to?
Roughly 226.5 µg/m³ as a 24-hour average, if PM2.5 is the pollutant driving the index. The same AQI of 303 corresponds to about 427 µg/m³ of PM10. The AQI reports whichever pollutant is worst at that moment, so the number alone does not tell you which one you are breathing.
Do I need a mask at AQI 303?
A fitted N95 or better for any outdoor exposure, however brief. Consider whether the trip is necessary at all.
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