What Is AQI? Air Quality Index Explained (2026)
AQI stands for Air Quality Index. It is a numerical scale from 0 to 500 created by the US Environmental Protection Agency. The higher the AQI number, the worse the air quality.
What Is the AQI?
The Air Quality Index tells you how clean or polluted the outdoor air is in your area right now. The EPA designed it so that anyone can understand air quality at a glance without needing to interpret raw pollution data.
A score of 0 means clean air. A score of 500 means hazardous conditions. The index is color-coded and sorted into six categories so you can make quick decisions about outdoor activity and ventilation.
AQI Levels and Categories
| AQI Range | Category | Color | Who Is Affected |
|---|---|---|---|
| 0-50 | Good | Green | Air quality is satisfactory for everyone |
| 51-100 | Moderate | Yellow | Sensitive groups may notice effects from some pollutants |
| 101-150 | Unhealthy for Sensitive Groups | Orange | Children, elderly, and those with heart or lung conditions |
| 151-200 | Unhealthy | Red | Everyone may begin to feel health effects |
| 201-300 | Very Unhealthy | Purple | Serious health effects possible for all people |
| 301-500 | Hazardous | Maroon | Emergency conditions — everyone is at risk |
What Pollutants Does AQI Measure?
The AQI covers six major air pollutants. Each one is measured separately and converted to its own sub-index. The AQI you see reported is the highest of the six sub-indexes.
- PM2.5 — Fine particles smaller than 2.5 microns. These penetrate deep into lung tissue and can enter the bloodstream. The most important pollutant for daily health decisions.
- PM10 — Coarser particles up to 10 microns: dust, pollen, mold spores.
- Ozone (O3) — Ground-level ozone formed by sunlight reacting with vehicle and industrial emissions. A respiratory irritant.
- Carbon monoxide (CO) — Produced by combustion. Odorless and colorless.
- Sulfur dioxide (SO2) — From burning fossil fuels. Irritates airways and contributes to acid rain.
- Nitrogen dioxide (NO2) — From traffic and power plants. Linked to respiratory illness.
Why PM2.5 Matters Most
PM2.5 particles are fine enough to bypass the nose and throat and travel deep into the lungs. The smallest particles can cross into the bloodstream. Long-term exposure to PM2.5 is linked to heart disease, stroke, lung cancer, and reduced lung function. For daily indoor air quality decisions, PM2.5 is the number to watch.
How to Check Your AQI
- AirNow.gov — The official US government source. Enter your zip code for current AQI.
- IQAir app — Aggregates multiple data sources. Shows both outdoor and indoor AQI.
- PurpleAir sensor network — Hyperlocal real-time data from community sensors.
- Google search — Search "AQI [your city]" or "air quality near me" for a quick result.
- Local TV and radio weather reports often include AQI in their forecasts.
When to Take Action
- AQI 101 and above: Run your air purifier. Sensitive individuals should limit outdoor exertion.
- AQI 151 and above: Keep windows closed. Everyone should reduce outdoor activity.
- AQI 201 and above: Consider N95 masks if you must be outdoors. Air purifiers running at higher settings indoors.
Indoor vs Outdoor AQI
The AQI you see on AirNow.gov measures outdoor ambient air at official monitoring stations. Your indoor air quality can be better or worse depending on:
- How well your HVAC system filters air
- Whether you run an air purifier
- Indoor pollution sources like cooking, candles, and cleaning products
- How well sealed your home is
On high outdoor AQI days, keep windows closed and run your air purifier to keep indoor PM2.5 below the Moderate level. On low outdoor AQI days (below 50), ventilating by opening windows is a free way to improve indoor air quality.
How Air Purifiers Help
HEPA air purifiers capture PM2.5 and PM10 particles as air passes through the filter. True HEPA filters capture 99.97% of particles 0.3 microns or larger.
Activated carbon filters in air purifiers capture gases and VOCs (volatile organic compounds) that do not show up on PM sensors.
The CADR (Clean Air Delivery Rate) rating tells you how fast a purifier cleans a given room size. A higher CADR number means the purifier cleans air faster. Match the CADR to your room square footage when choosing a purifier.
Frequently Asked Questions
What is a good AQI number?
AQI 0-50 is Good. 51-100 is Moderate, which is acceptable for most people, but sensitive individuals may notice effects. AQI 101 and above indicates reduced air quality where outdoor activity should be limited for vulnerable groups including children, elderly, and people with asthma or heart conditions.
What does AQI measure?
AQI combines measurements of six major air pollutants: PM2.5, PM10, ground-level ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant is measured separately and converted to a sub-index. The reported AQI uses the highest sub-index of the six.
Is 100 AQI bad?
100 AQI falls in the Moderate category. It is not immediately dangerous for most healthy adults. People with asthma, heart disease, or lung conditions should reduce prolonged outdoor exertion. Children and elderly are also considered sensitive groups at this level. At 101 and above, the Unhealthy for Sensitive Groups category begins.
How do I improve indoor air quality?
Run a HEPA air purifier sized to your room — check the CADR rating to match it to your square footage. Ventilate by opening windows when outdoor AQI is below 50. Replace HVAC filters every 3 months. Reduce indoor sources such as cooking smoke, candles, and VOC-emitting cleaning products.