Air Quality Index Explained: How AQI Is Calculated
The EPA AQI is not a simple average. Here is how it actually works, what each pollutant does to your body, and how to use the data to make smarter daily decisions.
How AQI Is Calculated
The EPA converts raw pollutant concentration data into a single number using a piecewise linear formula. Each of the six pollutants has its own breakpoint concentration table. The formula converts concentration to a sub-index, and the highest sub-index becomes the reported AQI.
This means a day with high ozone but low PM2.5 can still show an AQI above 100, driven entirely by the ozone sub-index. When AQI is reported, the "responsible pollutant" is the one whose sub-index is highest.
PM2.5 Sub-Index Formula (Plain English)
The EPA formula for each pollutant sub-index is:
- Cp — the measured pollutant concentration (e.g., micrograms of PM2.5 per cubic meter)
- Bpl and Bph — the lower and upper breakpoint concentrations from the EPA table that bracket Cp
- Il and Ih — the AQI values corresponding to Bpl and Bph
- Ip — the resulting sub-index for that pollutant
In plain English: the formula asks where your measured concentration falls between the upper and lower breakpoints, then scales that position proportionally onto the corresponding AQI range. The result is the sub-index for that pollutant.
The Six Pollutants in Detail
PM2.5 — Fine Particles
Particles smaller than 2.5 microns in diameter. The most important pollutant for daily health decisions. PM2.5 can penetrate deep into lung tissue and cross into the bloodstream. Long-term exposure is linked to heart disease, stroke, lung cancer, and reduced lung development in children.
PM10 — Coarse Particles
Dust, pollen, mold spores, and other particles between 2.5 and 10 microns. These are filtered by the nose and upper airways for most people but still cause irritation. PM10 spikes are common in desert cities, agricultural areas, and during high wind events.
Ozone (O3) — Ground-Level
Ground-level ozone is not emitted directly. It forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) from cars and industry react in the presence of sunlight. Hot summer afternoons tend to produce the highest ozone levels. Ozone is a respiratory irritant that reduces lung function with repeated exposure.
Carbon Monoxide (CO)
Produced by incomplete combustion. Odorless and colorless. Outdoor CO is typically highest near heavy traffic or industrial facilities. Dangerous at high concentrations because it binds to hemoglobin and reduces oxygen delivery to organs.
Sulfur Dioxide (SO2)
Produced by burning coal, oil, and other fossil fuels, and by smelting operations. SO2 irritates the respiratory system, can trigger asthma attacks at high concentrations, and contributes to acid rain and fine particle formation.
Nitrogen Dioxide (NO2)
From traffic, power plants, and other combustion sources. NO2 contributes to ozone formation and fine particle pollution. High NO2 areas correlate with higher rates of respiratory illness, particularly in communities near major roads or industrial zones.
How Often Is AQI Updated?
- AirNow.gov — Updates hourly from official regulatory monitoring stations.
- PurpleAir sensors — Update every 2 minutes with real-time data from community-owned sensors. Require a correction factor (see FAQ below) to align with regulatory readings.
- IQAir app — Aggregates multiple sources, updates vary by source.
- State and local air quality agencies also publish hourly updates.
AQI Data Sources
- AirNow.gov — Official US government source. Most reliable for regulatory-grade data.
- State air quality agencies — Some states run their own networks with additional monitors.
- IQAir GlobalAirQuality — Aggregates data from multiple countries. Good for travel.
- PurpleAir — Community sensor network. Best for hyperlocal real-time data, especially in areas far from official monitors.
Wildfire Smoke and AQI
During active wildfires, AQI can spike from Good to Hazardous within hours. Wildfire smoke is primarily fine PM2.5 particles from incomplete combustion of wood, vegetation, and structures.
Wildfire PM2.5 is particularly concerning because the particles are very fine and the smoke can travel hundreds of miles from the fire source. Western US cities regularly see Hazardous AQI during fire season even when no fires are visible nearby.
Standard HEPA air purifiers help with wildfire smoke. Look for purifiers with both HEPA and activated carbon filtration to capture both particles and the gases present in smoke.
Limitations of AQI
- AQI is measured at outdoor monitoring stations. The nearest official monitor may be miles from your home, and local conditions can differ significantly.
- Indoor AQI is not measured by the EPA network. You need an indoor air quality monitor to know your actual indoor levels.
- AQI does not measure all pollutants. VOCs and radon, for example, are not included.
- PurpleAir sensors fill hyperlocal gaps but require correction factors to align with regulatory monitors.
Using AQI to Make Daily Decisions
| AQI | Outdoor Exercise | Windows Open | Purifier Setting | Mask |
|---|---|---|---|---|
| 0-50 | Yes, any activity | Yes — good ventilation day | Low or off | Not needed |
| 51-100 | Yes for most people | Yes, with caution if sensitive | Low | Not needed |
| 101-150 | Reduce intensity if sensitive | No — keep closed | Medium | Consider for sensitive groups |
| 151-200 | Limit for everyone | No — keep closed | High | Recommended outdoors |
| 201-300 | Stay indoors | No | Maximum | N95 if outdoors |
| 301-500 | Stay indoors | No — seal gaps if possible | Maximum | N95 required outdoors |
Frequently Asked Questions
Why does PurpleAir show higher AQI than AirNow?
PurpleAir uses optical particle counters that can read higher than the regulatory beta attenuation monitors used for AirNow. The EPA provides a correction factor algorithm (AQandU or LRAPA) that brings PurpleAir data closer to regulatory readings. PurpleAir's map now applies EPA correction by default, which reduces the gap significantly. During wildfire smoke events, even the corrected PurpleAir values may differ from official AirNow readings.
What AQI is safe to exercise outside?
AQI 0-100 is generally safe for outdoor exercise for healthy adults. At 101-150, sensitive individuals (asthma, heart conditions, elderly, children) should reduce intensity and duration. At 151 and above, outdoor exercise is not recommended for anyone. Check AirNow.gov before long outdoor workouts, especially in summer when ozone levels peak in the afternoon.
What causes high AQI days?
Several sources drive high AQI events: wildfires (PM2.5 spikes rapidly), summer ozone formation (hot days plus traffic emissions plus sunlight), industrial sources, and weather inversions that trap pollutants near ground level. Geographic factors like valleys and basins increase severity. Stagnant air with no wind prevents pollutants from dispersing.
Does AQI include indoor air quality?
No. AQI measures outdoor ambient air quality at official monitoring stations. Indoor air quality is a separate measurement. You need an indoor air quality monitor to know your actual indoor levels. Indoor AQI can be better or worse than outdoor, depending on your home's ventilation, HVAC filtration, air purifiers, and indoor pollution sources.