Air Quality Index Calculator
Estimate the dominant Air Quality Index (AQI) value from PM2.5, PM10, ozone, and NO2 concentrations using the EPA breakpoint method (AQI is the highest pollutant sub-index). Use it for indoor air quality monitoring or rough comparison against EPA outdoor AQI categories.
Last updated: September 2026
Formula below · 3 sources (epa.gov, iea.org, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
The Air Quality Index is a unitless 0-500 scale used by the US EPA to communicate air quality and health risk. For each pollutant the calculator finds the EPA breakpoint range containing your concentration and interpolates linearly: I = (I_hi − I_lo) / (C_hi − C_lo) × (C − C_lo) + I_lo; the AQI is the highest of the sub-indices, rounded to a whole number. Breakpoints (40 CFR Part 58, Appendix G, with the PM2.5 update of May 2024): PM2.5 24-hour µg/m³ 0–9.0 = 0–50, 9.1–35.4 = 51–100, 35.5–55.4 = 101–150, 55.5–125.4 = 151–200, 125.5–225.4 = 201–300, 225.5+ = 301–500; PM10 24-hour µg/m³ 0–54, 55–154, 155–254, 255–354, 355–424, 425–604; ozone 8-hour ppm 0–0.054, 0.055–0.070, 0.071–0.085, 0.086–0.105, 0.106–0.200 (1-hour ozone 0.125–0.164 = 101–150, 0.165–0.204 = 151–200, 0.205–0.404 = 201–300, 0.405–0.604 = 301–500; 8-hour values above 0.200 ppm use the 1-hour table); NO2 1-hour, entered in ppm and converted to ppb, 0–53, 54–100, 101–360, 361–649, 650–1249, 1250–2049 ppb. Concentrations are truncated as EPA specifies (PM2.5 to 0.1, PM10 to 1, ozone to 0.001 ppm, NO2 to 1 ppb). Choose the ozone averaging period that matches your reading; a 1-hour ozone value below 0.125 ppm has no AQI of its own. Readings above the top breakpoint are reported as 500. Standard EPA AQI categories: 0-50 Good, 51-100 Moderate, 101-150 Unhealthy for Sensitive Groups, 151-200 Unhealthy, 201-300 Very Unhealthy, 301+ Hazardous. PM2.5 is usually the limiting pollutant in most US locations; the WHO 2021 annual guideline is 5 µg/m³.
How to use
Example 1 — Moderate urban air. PM2.5 = 15 µg/m³, PM10 = 25, ozone = 0.060 ppm (8-hour), NO2 = 0.04 ppm (40 ppb). Step 1: PM2.5 sub-index = (100 − 51)/(35.4 − 9.1) × (15 − 9.1) + 51 = 62.0. Step 2: PM10 = 50/54 × 25 = 23.1. Step 3: ozone = (100 − 51)/(0.070 − 0.055) × (0.060 − 0.055) + 51 = 67.3. Step 4: NO2 = 50/53 × 40 = 37.7. Step 5: AQI = max = 67 (ozone), Moderate (51-100) — acceptable for most people, but unusually sensitive people may notice symptoms during outdoor exertion. Example 2 — Wildfire smoke event. PM2.5 = 60 µg/m³, PM10 = 80, ozone = 0.040, NO2 = 0.02. Step 1: PM2.5 = (200 − 151)/(125.4 − 55.5) × (60 − 55.5) + 151 = 154.2. Step 2: PM10 = (100 − 51)/(154 − 55) × (80 − 55) + 51 = 63.4. Step 3: ozone = 50/0.054 × 0.040 = 37.0. Step 4: NO2 = 50/53 × 20 = 18.9. Step 5: AQI = 154 (PM2.5), Unhealthy (151-200) — everyone may feel effects; limit outdoor activity and run air purifiers indoors.
Frequently asked questions
What does each AQI category mean for health?
EPA AQI categories with action recommendations: 0-50 Good (Green) — air is clean, no health effects expected; 51-100 Moderate (Yellow) — sensitive individuals may experience minor symptoms during outdoor exertion; 101-150 Unhealthy for Sensitive Groups (Orange) — children, elderly, people with heart/lung conditions should reduce prolonged outdoor exertion; 151-200 Unhealthy (Red) — everyone may experience symptoms, sensitive groups should avoid outdoor activity; 201-300 Very Unhealthy (Purple) — health alert for everyone, all outdoor exertion should be minimized; 301+ Hazardous (Maroon) — emergency conditions, everyone should stay indoors with windows closed. PM2.5 is the most commonly limiting pollutant globally; long-term exposure at even moderate levels (annual average 10-15 μg/m³) shortens life expectancy 1-3 years compared to clean air (annual average <5 μg/m³). The WHO 2021 air quality guidelines are much tighter than US standards — the WHO annual PM2.5 limit (5 μg/m³) is met by almost no major US city.
What are the main sources of each pollutant?
PM2.5 (fine particulate): wildfires, diesel engines, residential wood burning, coal-fired power plants, industrial processes, agricultural burning, and secondary formation from SO2 and NOx in the atmosphere. PM10 (coarse particulate): road dust, construction, agriculture, sea salt, and pollen — typically more transient and lower health impact per μg than PM2.5. Ozone: not directly emitted; formed by photochemical reaction of NOx (from cars) and VOCs (from industry, paint, evaporated gasoline) in sunlight. Ozone is worst on hot sunny afternoons, downwind of urban areas. NO2: vehicle exhaust (especially diesel), power plants, industrial combustion; high in dense urban areas and near busy roads. Each pollutant has distinct geographic and temporal patterns: PM2.5 spikes in winter (residential heating) and during wildfire season; ozone peaks in summer afternoons; NO2 highest along major roads during rush hour. Indoor sources are often forgotten but dominate personal exposure: gas stoves emit large amounts of NO2 (often exceeding outdoor levels), candles and cooking generate PM2.5, off-gassing furniture emits VOCs.
What are the most common mistakes when interpreting AQI?
The biggest is comparing instantaneous readings to AQI categories that are designed for averaging periods. EPA AQI for PM2.5 uses a 24-hour rolling average; a 1-hour smoke spike of 100 μg/m³ that subsides to 5 μg/m³ might give a 24-hour average of 15 (Moderate) even though the spike itself was Unhealthy. The second is conflating outdoor and indoor air quality — indoor air is typically 2-5x more polluted than outdoor air due to cooking, cleaning products, off-gassing furniture, and stagnation; outdoor AQI does not protect you indoors. The third is treating AQI as a single national standard — different countries use different scales and breakpoints; what is Moderate in the US may be Healthy in India or Unhealthy in the EU. The fourth is ignoring multi-day exposure — chronic exposure to even Moderate AQI causes meaningful long-term harm, while a single Unhealthy day matters less for long-term health. The fifth is over-relying on AQI maps without checking nearby monitor distance; in some regions monitor density is sparse and conditions 10 miles away may differ dramatically.
When should I NOT use AQI alone for health decisions?
Skip relying on outdoor AQI for indoor air quality decisions — install a PM2.5 monitor (PurpleAir, IQAir, AirThings) for your actual exposure. Avoid using single-monitor AQI for large regions with terrain or weather variation — wildfires, valleys, and inversions create dramatic local variation; a downtown monitor may read Good while a neighbourhood 5 miles away reads Hazardous due to a smoke plume. Do not use AQI for sensitive populations (severe asthma, COPD, infants, pregnant women) without consulting medical guidance specific to your condition; AQI thresholds are based on general population effects and individual sensitivities vary. Skip AQI as the sole metric for occupational exposures — workplaces with industrial pollutants need OSHA Permissible Exposure Limits and engineering controls, not just AQI. And do not use AQI for non-criteria pollutants (formaldehyde, benzene, lead, mercury, radon) — the AQI tracks only the six common air pollutants; many of the most harmful indoor and industrial pollutants are not included.
How can I reduce personal exposure to air pollution?
Top actions ranked by typical impact: (1) Switch from gas to induction cooking — gas stoves emit large NO2 and PM2.5 indoors, regularly exceeding outdoor WHO limits; (2) Run a high-efficiency air purifier with HEPA filter in your bedroom during sleep — you spend 30% of your life there, and improvements compound; (3) Keep windows closed and run recirculate-air on AC during wildfire season, high-pollution days, or after wildfire/forest-fire smoke arrives; (4) Avoid outdoor exercise during peak ozone (1-7 PM) and PM2.5 events (early morning during inversions); (5) Use N95 or P100 masks when outdoor PM2.5 > 100 μg/m³ — surgical and cloth masks do not filter fine particulate; (6) Choose homes/neighbourhoods at least 500 ft from major roadways (long-term exposure to road air causes 5-10% premature mortality risk); (7) For long-term planning, choose to live in cleaner-air regions; air pollution life-expectancy impact in dirty cities (Delhi, Lahore) is 5-10 years. Personal HEPA filters are remarkably cheap and effective — $200-400 for a Coway/Levoit running 24/7 in a bedroom can cut PM2.5 exposure 70-90%.