Understanding Air Quality Index (AQI): Everything You Need to Know
Sources reviewed: 2026-07-16
Definitions, thresholds, and safety guidance are checked against the primary sources listed below.
What Is AQI? What Does AQI Mean?
What is AQI? AQI stands for Air Quality Index. It's a standardized measurement used worldwide to communicate how clean or polluted the air is and what health effects you might experience.
What does AQI mean for your daily life? Simply put, it's a number that tells you whether it's safe to breathe the outdoor air. The higher the AQI number, the more polluted the air, and the greater the health risk.
What Is a Normal AQI? Understanding AQI Levels
A "normal" or safe AQI is typically considered to be under 50. This is the "Good" range where air quality poses little or no risk. Let's look at the complete breakdown:
AQI Levels Explained
The AQI is divided into six categories, each corresponding to a different level of health concern:
| AQI Value | Level of Concern | Description |
|---|---|---|
| 0 - 50 | Good | Air quality is satisfactory, and air pollution poses little or no risk. |
| 51 - 100 | Moderate | Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution. |
| 101 - 150 | USG | Members of sensitive groups may experience health effects. The general public is not likely to be affected. (Unhealthy for Sensitive Groups) |
| 151 - 200 | Unhealthy | Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects. |
| 201 - 300 | Very Unhealthy | Health alert: everyone may experience more serious health effects. |
| 300+ | Hazardous | What does hazardous air quality mean? It means emergency conditions. The entire population is more likely to be affected. |
Major Pollutants
The AQI focuses on health effects you may experience within a few hours or days after breathing polluted air. EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter, including PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide.
How to protect yourself?
When the AQI is high, take simple steps to reduce your exposure:
• Check current air quality in your area.
• Limit outdoor exertion, especially if you fall into sensitive groups.
• Use air purifiers indoors.
• Wear N95 masks if you must go outside during high pollution levels.
Sources and further reading
These primary sources were used to check definitions, thresholds, safety guidance, and scientific context.
Sources reviewed: 2026-07-16
- U.S. EPA AirNow: Air Quality Index (AQI) Basics
AQI ranges, colors, pollutants, and health-concern categories.
- U.S. EPA AirNow: Using the Air Quality Index
How current AQI and AQI forecasts should be used for activity planning.
What AnyWeather adds
We turn the official U.S. AQI categories into a plain-language planning guide and explain where local indexes may use different scales.
This article is an independent synthesis of public sources; the listed institutions do not endorse AnyWeather. For weather or health risks, prioritize local official alerts and professional advice.
Frequently Asked Questions
What does AQI stand for?
AQI stands for Air Quality Index. It is a dimensionless number used to report daily air quality and how clean or polluted the air is, along with the associated health effects. The higher the AQI value, the greater the level of air pollution and health concern.
What is a good AQI number?
An AQI of 0–50 is considered 'Good' and 51–100 is 'Moderate' — both are generally acceptable. Values from 101–150 are unhealthy for sensitive groups, 151–200 are unhealthy for everyone, and anything above 300 is hazardous.
What pollutants does the AQI measure?
The AQI is based on five major pollutants: ground-level ozone (O₃), particle pollution (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). PM2.5 is often the most closely watched because it can reach deep into the lungs and bloodstream.
What should I do when the AQI is high?
When the AQI is above 100, sensitive groups (children, older adults, and people with respiratory conditions) should limit outdoor activity. Above 150, everyone should reduce time outdoors, run an indoor air purifier, keep windows closed, and wear an N95 mask if going outside is necessary.
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