Fetching the latest hour from CAMS…
Fetching the latest hour from CAMS…
Nothing here is a black box. Every index on this site is computed in the browser-visible source from published breakpoint tables. This page is the whole method.
You cannot score PM2.5 the way you score ozone. The EPA defines PM on a 24-hour average, ozone on 8 hours, and NO₂ and SO₂ on 1 hour, because that is the exposure duration each pollutant's health evidence is based on. We hold 48 hours of hourly model output to build these windows properly, and require 75 % data completeness before reporting an average.
The source data is in µg/m³ for everything. The EPA tables are in ppm and ppb, and India's CO table is in mg/m³. Gas conversion uses the ideal-gas molar volume of 24.45 L/mol at 25 °C and 1 atm, then the value is truncated to the decimal places the standard specifies — truncation, not rounding, because that is what the EPA method says.
Each averaged concentration lands in one row of the table and is linearly interpolated between that row's index endpoints. The overall AQI is the highest of the sub-indices, and the pollutant that produced it is the 'dominant' pollutant. Nothing is averaged across pollutants — an index is a worst-case statement, by design.
There is a real tension in asking for an instant AQI. The health thresholds behind the index are 24-hour averages, so a strict reading of the standard means you cannot know today's AQI until tomorrow. But if you just score the latest hour as though it were a full day, the number becomes wildly jumpy and overstates brief spikes.
The EPA's answer, which this app implements, is the NowCast: a weighted average of the last 12 hours where the weight given to older hours shrinks as conditions become less stable. The weight factor is w = 1 − (max − min) / max over the 12-hour window, floored at 0.5, and hour i back gets weight wⁱ.
In steady air, w approaches 1 and the NowCast behaves like a plain 12-hour mean. When smoke arrives and concentrations swing, w drops to its 0.5 floor and the most recent hour dominates — so the number moves within an hour instead of being diluted across a day. This is what AirNow publishes as the current AQI, and it is why our value can differ slightly from providers that simply score the latest hourly concentration.
United States Environmental Protection Agency · United States (used worldwide as a de facto standard)
A 0–500 scale where 100 marks the national air quality standard for each pollutant. Current-hour values use the EPA NowCast weighting for particulates, so the number reacts quickly when smoke or dust rolls in.
| POLLUTANT | AVERAGING | UNIT | CONCENTRATION → INDEX |
|---|---|---|---|
| PM2.5 | NowCast (12 h weighted) | µg/m³ | 0.00–9.00→0–509.10–35.4→51–10035.5–55.4→101–15055.5–125→151–200126–225→201–300226–325→301–500 |
| PM10 | NowCast (12 h weighted) | µg/m³ | 0.00–54.0→0–5055.0–154→51–100155–254→101–150255–354→151–200355–424→201–300425–604→301–500 |
| O₃ | 8-hour mean | ppm | 0.000–0.054→0–500.055–0.070→51–1000.071–0.085→101–1500.086–0.105→151–2000.106–0.200→201–300 |
| NO₂ | 1-hour | ppb | 0.0–53.0→0–5054.0–100→51–100101–360→101–150361–649→151–200650–1249→201–3001250–2049→301–500 |
| SO₂ | 1-hour | ppb | 0.0–35.0→0–5036.0–75.0→51–10076.0–185→101–150186–304→151–200305–604→201–300605–1004→301–500 |
| CO | 8-hour mean | ppm | 0.000–4.400→0–504.500–9.400→51–1009.500–12.400→101–15012.500–15.400→151–20015.500–30.400→201–30030.500–50.400→301–500 |
European Environment Agency · Europe
A band index from Good to Extremely Poor, anchored on WHO-aligned concentration thresholds. It is stricter than the US AQI at low concentrations: 25 µg/m³ of PM2.5 is already 'Moderate' here.
| POLLUTANT | AVERAGING | UNIT | CONCENTRATION → INDEX |
|---|---|---|---|
| PM2.5 | 24-hour mean | µg/m³ | 0.00–10.0→0–2010.0–20.0→20–4020.0–25.0→40–6025.0–50.0→60–8050.0–75.0→80–10075.0–800→100–120 |
| PM10 | 24-hour mean | µg/m³ | 0.00–20.0→0–2020.0–40.0→20–4040.0–50.0→40–6050.0–100→60–80100–150→80–100150–1200→100–120 |
| NO₂ | 1-hour | µg/m³ | 0.00–40.0→0–2040.0–90.0→20–4090.0–120→40–60120–230→60–80230–340→80–100340–1000→100–120 |
| O₃ | 1-hour | µg/m³ | 0.00–50.0→0–2050.0–100→20–40100–130→40–60130–240→60–80240–380→80–100380–800→100–120 |
| SO₂ | 1-hour | µg/m³ | 0.00–100→0–20100–200→20–40200–350→40–60350–500→60–80500–750→80–100750–1250→100–120 |
Central Pollution Control Board · India
A 0–500 scale on six categories from Good to Severe. It is the only major index that includes ammonia, and its PM2.5 bands are wider than the US EPA's above 100.
| POLLUTANT | AVERAGING | UNIT | CONCENTRATION → INDEX |
|---|---|---|---|
| PM2.5 | 24-hour mean | µg/m³ | 0.00–30.0→0–5030.0–60.0→50–10060.0–90.0→100–20090.0–120→200–300120–250→300–400250–500→400–500 |
| PM10 | 24-hour mean | µg/m³ | 0.00–50.0→0–5050.0–100→50–100100–250→100–200250–350→200–300350–430→300–400430–600→400–500 |
| NO₂ | 24-hour mean | µg/m³ | 0.00–40.0→0–5040.0–80.0→50–10080.0–180→100–200180–280→200–300280–400→300–400400–600→400–500 |
| O₃ | 8-hour mean | µg/m³ | 0.00–50.0→0–5050.0–100→50–100100–168→100–200168–208→200–300208–748→300–400748–1000→400–500 |
| CO | 8-hour mean | mg/m³ | 0.00–1.00→0–501.00–2.00→50–1002.00–10.00→100–20010.00–17.00→200–30017.00–34.00→300–40034.00–50.00→400–500 |
| SO₂ | 24-hour mean | µg/m³ | 0.00–40.0→0–5040.0–80.0→50–10080.0–380→100–200380–800→200–300800–1600→300–4001600–2000→400–500 |
| NH₃ | 24-hour mean | µg/m³ | 0.00–200→0–50200–400→50–100400–800→100–200800–1200→200–3001200–1800→300–4001800–2400→400–500 |
Guidance on this site is keyed to the six US EPA bands, the most widely recognised set, and drawn from EPA AirNow activity guidance and the WHO Air Quality Guidelines.
Air quality poses little or no risk.
Acceptable for most people, with a caveat for the unusually sensitive.
At-risk groups will start to feel this. Most others will not.
Everyone may begin to notice health effects.
A health alert. Risk is elevated for the entire population.
Emergency conditions. The entire population is at serious risk.