Enter the concentrations of PM2.5, PM10, ozone, NO2 and SO2, and this tool computes the US EPA Air Quality Index by piecewise-linear interpolation in real time, showing the overall AQI, the health category and the governing pollutant.
Parameters
PM2.5 concentration
µg/m³
Fine particulate matter, diameter ≤ 2.5 µm (24-h average)
Tropospheric ozone (8-h average). Main component of photochemical smog
NO₂ (nitrogen dioxide) concentration
ppb
From vehicle exhaust and combustion (1-h average)
SO₂ (sulphur dioxide) concentration
ppb
From coal and heavy-oil combustion (1-h average)
Results
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PM2.5 AQI
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PM10 AQI
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O₃ AQI
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Overall AQI
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Category
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Governing pollutant
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Sub-AQI bar chart — category colour bands
Horizontal bars for the five pollutants. The background is shaded into the six AQI categories (green → yellow → orange → red → purple → maroon); the longest bar governs the overall AQI.
Sub-AQI per pollutant (Chart.js)
PM2.5 concentration vs AQI — non-linear breakpoints
Piecewise-linear interpolation. For a concentration C in the band [C_lo, C_hi], the sub-AQI is interpolated into the band [I_lo, I_hi]. Slopes change at each band, so the overall mapping is a non-linear staircase function.
The overall AQI is the maximum of the five sub-AQIs. The pollutant reaching that maximum is the "governing pollutant" and is reported as the dominant health driver. Adding CO gives the full six-pollutant EPA formulation.
What is the Air Quality Index (AQI)?
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I hear on the weather forecast "today's AQI is 150" all the time. But what does that number actually mean? It has no units and I can't get a feel for it.
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Good question. AQI (Air Quality Index) is a dimensionless index that converts a mix of pollutants into a single "how bad is it for your health" scale. The big six are PM2.5, PM10, ozone, NO2, SO2 and CO, each measured in different units (µg/m³ or ppb). AQI normalises them into a common 0-500 range so you can directly compare "is the PM2.5 worse, or is the ozone?". US EPA designed it so that AQI = 100 corresponds exactly to the daily health standard for each pollutant — that's the magic value to remember.
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OK, a common scale. But how do you actually convert concentration into AQI? It's not just "concentration × something", is it?
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That's the interesting bit — it isn't linear. EPA publishes a "breakpoint table". For PM2.5, for instance: 0 to 12 µg/m³ maps to AQI 0-50; 12.1 to 35.4 maps to 51-100; 35.5 to 55.4 maps to 101-150, and so on. Inside each band the conversion is linear, but globally it's a staircase function with kinks at every breakpoint. So PM2.5 going from 12 to 12.1 jumps the AQI from 50 to 51 — a small concentration change, a discrete category shift. The bands are set by epidemiological evidence that health effects are non-linear in concentration.
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And after computing all five sub-AQIs, how do you get the final "overall AQI"? Average? Sum?
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Take the maximum — the worst-of approach. Imagine PM2.5 at AQI 200 and ozone at 30: the air is obviously bad because of PM2.5, and averaging would dilute that danger. Taking the max always reports the worst case, which is the conservative public-health choice. And the pollutant that produced that maximum becomes the "governing pollutant", which is what you need to target with mitigation. Reporting both the number and the dominant species is much more actionable than a single index alone.
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Does the governing pollutant change with season or location? Like Tokyo in summer vs winter?
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Absolutely — it flips dramatically with season and geography. Winter cities have low temperatures and stable stratification, so PM2.5 gets trapped near the ground; heating, traffic and trans-boundary transport combine and PM2.5 dominates. In summer cities and suburbs, strong sunlight drives the NOx + VOC photochemistry that produces ozone, so ozone wins. SO2 dominates near coal-fired power regions, NO2 along major highways in megacities — the patterns track human activity. That's why looking at just "the overall AQI" misses the story; you need to know "which pollutant is bad" to act on it.
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Last thing — is there a practical rule like "wear a mask above AQI 100"?
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EPA's category guide says: 101-150 "Unhealthy for sensitive groups" means asthmatics, cardiac patients, children and the elderly should cut outdoor activity. 151-200 "Unhealthy" means everyone should avoid prolonged outdoor exercise. 201-300 "Very Unhealthy" recommends N95 masks and staying indoors for everyone, and 301+ "Hazardous" is essentially "don't go outside". Practically, many people use AQI 100 as the cutoff for jogging or letting kids play outside, and AQI 50 as the cue to keep a HEPA purifier running. Note the WHO 2021 guidelines are far stricter — EPA's AQI 50 (12 µg/m³ PM2.5) is already "unhealthy" under WHO. Always check the standard used for your region.
Frequently Asked Questions
The US EPA AQI is computed per pollutant by piecewise-linear interpolation: I = (I_hi - I_lo)/(C_hi - C_lo) * (C - C_lo) + I_lo, where each concentration C maps into a 0-500 sub-AQI. The overall AQI is the maximum across all pollutants. For example a PM2.5 of 35 µg/m³ falls in the [12.1, 35.4] -> [51, 100] band, giving I = 49/23.3 * 22.9 + 51 ≈ 99. The breakpoint table is non-linear: it is set by EPA from epidemiological evidence, not by a simple ratio.
The overall AQI takes the maximum of the 5-6 sub-AQIs (worst-of approach). The pollutant that produces the maximum is called the governing pollutant and is reported as the main health driver. For example, if PM2.5 has a sub-AQI of 99 and the others are 70 or less, the overall AQI is 99 and the governing pollutant is PM2.5. In winter cities PM2.5 usually dominates; in summer suburbs ozone typically wins. The governing pollutant swaps with season and location.
AQI is binned into six categories: 0-50 Good (green), 51-100 Moderate (yellow), 101-150 Unhealthy for Sensitive Groups (orange), 151-200 Unhealthy (red), 201-300 Very Unhealthy (purple), 301+ Hazardous (maroon). Above 100, sensitive groups such as asthmatics, cardiac patients, children and the elderly start to feel effects. Above 150 the general population is affected, and above 200 outdoor activity should be limited. Japan's PM2.5 standard (35 µg/m³ daily) corresponds roughly to AQI 100.
The WHO 2021 guideline of 15 µg/m³ daily PM2.5 is about 2.3× stricter than the US EPA value. Japan's standard (35 µg/m³ daily) is closer to EPA, while China's GB 3095-2012 sits between them. This tool uses the official EPA AQI breakpoints, so if you want to evaluate against WHO, the displayed values will be roughly half of what WHO would consider unhealthy. China's AQI (HJ 633-2012) uses the same framework as EPA with slightly different breakpoints.
Real-World Applications
Environmental monitoring and public air-quality reports: The US AirNow service, Japan's Soramame, China's national air quality publication platform and many other government portals convert station data to AQI in real time and publish it on maps. Citizens read the colour (green to maroon) at a glance and decide whether to exercise outdoors or restrict children's activities. This tool helps engineers, researchers and public-health officers reproduce the EPA breakpoint computation themselves from raw concentrations.
Urban planning and emission-control impact assessments: Environmental Impact Assessments (EIAs) for new factories or traffic infrastructure use dispersion models (AERMOD, CALPUFF) to forecast concentrations, then translate them into AQI so the spatial reach can be mapped. For example "the new power plant raises SO2 by 30 ppb within 5 km, pushing the worst-case AQI from 80 to 125". Playing with this simulator helps you build intuition for which pollutants matter most when designing regulations.
Building ventilation and air-cleaner control: Smart HVAC in offices, schools and hospitals modulates fresh-air intake based on outdoor AQI. A typical rule is "above AQI 100, throttle outdoor air and switch to HEPA recirculation; above 200, seal completely". Designing that logic requires understanding AQI breakpoints. Consumer IoT air purifiers follow the same pattern, switching modes from outdoor sensor readings.
Epidemiology and health-impact communication: Long-term PM2.5 cohort studies (Pope et al., Beelen et al.) report relationships in µg/m³, but the message to the public is usually translated to AQI: "an AQI rise of 50 corresponds to X% extra mortality". Practising with the concentration-to-AQI mapping in this tool makes it easier to take published exposure-response curves and reinterpret them for your own neighbourhood.
Common Misconceptions and Pitfalls
The biggest misconception is "AQI is proportional to concentration". The chart in this tool shows that AQI vs concentration is a staircase. For PM2.5, the slope in the low band (0-12 µg/m³) is roughly 4.2 (= 50/12), in the middle band (35.5-55.4) about 2.5 (= 49/19.9), and in the high band (150.5-250.4) only about 1.0 (= 99/99.9). Doubling the concentration does not double the AQI. Low concentrations push AQI up steeply, high concentrations less so — this encodes the idea that once air is already "very unhealthy", further worsening only saturates the index. When you need ratios, always work in concentration.
Next, mixing averaging times. EPA breakpoints assume specific averaging windows: PM2.5 and PM10 use 24-h averages, O3 uses an 8-h average, NO2 and SO2 use 1-h averages, CO uses 8 h. This tool uses whatever number you type, but with real data you must respect the official averaging window for each species. Feeding a 1-h PM2.5 spike into the 24-h breakpoints will inflate the AQI. The real-time "NowCast AQI" is a different formula again — a weighted moving average that estimates a 24-h equivalent from sub-daily values — and is not the simple band-by-band interpolation done here.
Finally, "same AQI means same health impact" is not true. AQI = 100 driven by PM2.5, by ozone or by SO2 affects different vulnerable groups and demands different responses. If PM2.5 dominates, an N95 mask plus staying indoors is effective; if ozone dominates, you may want to reduce ventilation (indoor ozone is usually lower than outdoor); if SO2 dominates, asthmatic patients need particular care. Never act on the overall AQI alone — always also read the governing pollutant and the individual sub-AQIs. That's why this tool surfaces them separately.
How to Use
Enter PM2.5 concentration in µg/m³ using pm25Num field and select the averaging period (1-hour or 24-hour) via pm25Range dropdown
Input PM10, O₃, NO₂, and SO₂ concentrations in ppb or µg/m³ as specified, selecting appropriate averaging periods for each pollutant
The simulator applies EPA breakpoint equations to calculate individual pollutant AQI values, identifies the governing pollutant (highest AQI), and displays the overall Air Quality Index with category classification