🍃 Air Quality Index (AQI) & PM2.5 Checker
An air quality checker for any city: real-time US AQI with the PM2.5, PM10, ozone and nitrogen dioxide readings behind it, and what the index hides.
What Air Quality Index (AQI) & PM2.5 Checker Does
This tool reports the current air quality for a place, giving the US AQI alongside the individual pollutant concentrations behind it — PM2.5, PM10, ozone and nitrogen dioxide.
An air quality index is not a measurement. It is a national convention for converting pollutant concentrations into a single number on a scale, and different countries have chosen different conventions. The same air therefore scores differently depending on which scale you apply, sometimes dramatically.
The other thing worth knowing before reading any AQI is that it reports the single worst pollutant, not an average of them. A reading of 120 means one pollutant reached 120 on the scale; the rest could be near zero. That is why the individual concentrations shown underneath matter more than the headline number.
How to Use Air Quality Index (AQI) & PM2.5 Checker
- Type any world city name or click a popular city preset
- Click Check AQI
- View the real-time US AQI score, pollution category, health recommendations, and PM2.5/PM10 breakdown
Formula Used by Air Quality Index (AQI) & PM2.5 Checker
How a concentration becomes an index value
index = I_lo + (I_hi − I_lo) × (C − C_lo) ÷ (C_hi − C_lo)
- C
- Measured pollutant concentration
- C_lo, C_hi
- The concentration breakpoints of the band C falls in
- I_lo, I_hi
- The index values those breakpoints map to
Worked example
PM2.5 at 50 µg/m³, run through both scales' published breakpoints.
- US: 50 falls in the 35.5–55.5 band, which maps to index 100–150
- US: 100 + 50 × (50 − 35.5) ÷ (55.5 − 35.5) = 136
- Europe: 50 falls at the top of the 15–50 band, which maps to index 40–60
- Europe: 40 + 20 × (50 − 15) ÷ (50 − 15) = 60
Result: US AQI 136, "Unhealthy for Sensitive Groups". European AQI 60, "Moderate". Identical air, opposite advice, and both are correctly computed.
The index is a maximum, not a blend
AQI = max(index of PM2.5, index of PM10, index of O₃, index of NO₂, index of SO₂, index of CO)
- max
- Both the US and European indices take the worst single pollutant
Worked example
A live Delhi reading: PM2.5 30.7 µg/m³ but ozone 142 µg/m³.
- The particulate figure alone is mid-scale
- Ozone at 142 µg/m³ is high on both scales
- The reported index follows ozone, not the particulates
Result: US AQI 163. A visitor assuming the number describes dust or smoke would be looking at the wrong pollutant entirely — which is the argument for reading the breakdown rather than the headline.
The Same PM2.5 Concentration, Scored Both Ways
Computed from the published breakpoints — EPA for the US column, EEA 2024 revision for the European. Note that in practice the US applies its breakpoints to a 24-hour rolling average while Europe applies its own to an hourly value, which widens the gap further.
| PM2.5 (µg/m³) | US AQI | US verdict | European AQI | European verdict |
|---|---|---|---|---|
| 5 | 28 | Good | 20 | Good |
| 9 | 50 | Good | 28 | Fair |
| 15 | 61 | Moderate | 40 | Fair |
| 25 | 80 | Moderate | 46 | Moderate |
| 35 | 99 | Moderate | 51 | Moderate |
| 50 | 136 | Unhealthy for Sensitive Groups | 60 | Moderate |
| 75 | 164 | Unhealthy | 73 | Poor |
| 125 | 200 | Unhealthy | 94 | Very poor |
Measured Live: Five Cities, Two Scales
Queried 11 September 2026. The ratio between the two indices is not constant, because the scales diverge differently for each pollutant.
| City | US AQI | European AQI | PM2.5 (µg/m³) | Ozone (µg/m³) |
|---|---|---|---|---|
| London | 27 | 29 | 4.5 | 42 |
| New York | 56 | 30 | 9.9 | 66 |
| Los Angeles | 74 | 61 | 17.3 | 12 |
| Beijing | 123 | 67 | 46.9 | 52 |
| Delhi | 163 | 71 | 30.7 | 142 |
US AQI Categories
EPA scale. The index can exceed 500. This is the scale the tool displays.
| AQI | Category | What it means |
|---|---|---|
| 0–50 | Good | Little or no risk |
| 51–100 | Moderate | Acceptable; unusually sensitive people may be affected |
| 101–150 | Unhealthy for Sensitive Groups | Sensitive groups may experience effects; most people will not |
| 151–200 | Unhealthy | Everyone may begin to experience effects |
| 201–300 | Very Unhealthy | Health alert; risk is increased for everyone |
| 301+ | Hazardous | Emergency conditions |
European AQI Categories and Thresholds
EEA bands as revised in 2024, applied to hourly concentrations. The index is the highest of the five pollutant sub-indices and can exceed 100.
| Band | AQI | PM2.5 | PM10 | NO₂ | O₃ |
|---|---|---|---|---|---|
| Good | 0–20 | 0–5 | 0–15 | 0–10 | 0–60 |
| Fair | 20–40 | 5–15 | 15–45 | 10–25 | 60–100 |
| Moderate | 40–60 | 15–50 | 45–120 | 25–60 | 100–120 |
| Poor | 60–80 | 50–90 | 120–195 | 60–100 | 120–160 |
| Very poor | 80–100 | 90–140 | 195–270 | 100–150 | 160–180 |
| Extremely poor | >100 | >140 | >270 | >150 | >180 |
How to Read Your Result
Averaging window is half the disagreement
Even before the breakpoints differ, the two systems are measuring over different periods. The US applies its particulate breakpoints to a 24-hour rolling average and its carbon monoxide breakpoints to an 8-hour one; Europe uses hourly concentrations throughout. During a short pollution spike — rush hour, a nearby fire — the European index responds immediately while the US index is still averaging the spike against the previous quiet twenty hours. During a long, steady episode the two converge. A tool reporting "current" AQI is therefore reporting two quite different senses of "current" depending on the scale.
What the WHO guideline looks like on either scale
The World Health Organization's 2021 guideline for annual PM2.5 exposure is 5 µg/m³. Air at exactly that level scores 28 on the US index and 20 on the European one — comfortably "Good" on both. The WHO 24-hour guideline of 15 µg/m³ scores 61 on the US scale and 40 on the European. Air can therefore sit at the top of a national "acceptable" band while exceeding the health guideline, which is a reminder that an index band is a communication device, not a safety threshold.
A forecast model, not a sensor on your street
These figures come from a forecast model — CAMS, at roughly 11 km resolution over Europe and 45 km globally — not from a monitoring station at your location. That is why a reading can differ from a nearby government monitor or a neighbor's low-cost sensor. The model is good at regional air masses and cannot see a specific road, a construction site or a bonfire. For enforcement-grade numbers, the reference monitors run by national agencies are the authority.
Sensitive groups is a specific list
The 101–150 band is named for the people it applies to: those with asthma or other lung disease, heart disease, older adults, children, and people exercising outdoors, who take in considerably more air per hour. For everyone else that band genuinely is unlikely to cause symptoms. It is worth knowing which side of that line you are on before deciding what a given number means for your day.
Limitations & Accuracy Notes
- Displays the US AQI only, even though the underlying request also returns the European index. The comparison above has to be made from the tables rather than in the tool.
- Model output, not a measurement. CAMS forecast data at roughly 11 km over Europe and 45 km globally cannot resolve street-level pollution, and will differ from a nearby reference monitor.
- Four pollutants shown — PM2.5, PM10, ozone and nitrogen dioxide. Sulfur dioxide and carbon monoxide contribute to the index but are not displayed here.
- A location with no US AQI in the model returns no index rather than a number. That is deliberate: the tool used to report a missing value as 0, which renders as "Good".
- No forecast and no history — current conditions only, so it cannot show whether the air is improving or deteriorating.
- No pollen, no wildfire smoke layer and no indoor air guidance.
- Place-name search resolves to a single coordinate, so a large city is represented by one point in a model grid cell.
- Not health advice. Band descriptions are the published category language, not a judgement about any individual's risk.
Frequently Asked Questions
What is the Air Quality Index (AQI)?
Why is PM2.5 particulate matter hazardous?
Why do you show two different index values?
What is PM2.5 and why does it matter most?
What should I do when the index is high?
Is this data from a station near me?
Why does the reading differ from another app?
What if no reading is available?
References & Further Reading
- Open-Meteo — Air Quality API documentation — Source of both breakpoint tables above, citing EPA thresholds for the US index and the EEA 2024 revision for the European one, plus the averaging periods each scale uses and the CAMS model resolutions
- US EPA AirNow — AQI Basics — The official US categories, the color scheme and the definition of the sensitive groups the 101–150 band is named for