What is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a widely used system for communicating air pollution levels to the public. It simplifies complex air pollution data into a single value that indicates how clean or polluted the air is and what associated health effects might be a concern. The AQI scale ranges from 0 to 500, with higher numbers indicating worse air quality and greater potential health risks. These values are divided into six categories, from "Good" to "Hazardous," each represented by a specific color and health advisory.
How is the AQI Calculated?
The AQI is calculated based on measurements of key air pollutants, including particulate matter (PM2.5 and PM10), ground-level ozone (O₃), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Each pollutant has specific breakpoints, which translate its concentration into AQI values that fall into one of the six AQI categories.
The image below shows the breakpoints specifically for PM2.5, but similar breakpoints exist for all pollutants. For each pollutant, the AQI is calculated, and the highest value—called the "dominant" AQI—is reported. This means that even if multiple pollutants are being measured at a monitoring location, the AQI displayed is based on the pollutant with the highest concentration relative to its health impact.
Introducing AQI⁺
Note that different countries prescribe varying averaging periods for different pollutants when calculating AQI. For example, some countries calculate AQI using 24-hour averages, while others use shorter or longer timeframes. IQAir’s AQI⁺ provides a more immediate reflection of air quality by using hourly concentration averages for all pollutants. This approach aligns with official guidelines while offering more timely data than traditional 24- hour averages.
For example, U.S. AQI⁺ is based on the U.S. Environmental Protection Agency’s (EPA) AQI guidelines but enhances them with hourly updates to reflect current air quality conditions. Similarly, CN AQI⁺ follows China’s official reporting standards but also provides more real- time updates using hourly averages. This helps ensure that users are always receiving the most relevant information regarding air quality.
Understanding the Health Impacts of AQI
The Air Quality Index (AQI) provides guidance on health risks based on air pollution levels. Depending on the AQI level, sensitive groups and even the general population may need to take precautions. Below are the six AQI levels and their associated health recommendations. If the dominant pollutant is ozone or PM2.5, specific guidance may apply.
1. Good (0 - 50)
Air quality is excellent, and there are no expected health risks. It’s a great time to enjoy outdoor activities.
2. Moderate (51 - 100)
Air quality is acceptable for most people. However, sensitive individuals, such as those with pre-existing respiratory conditions, may experience mild symptoms like coughing or shortness of breath.
- Avoid ventilating indoor spaces with outdoor air, and close windows to prevent outdoor air pollution from entering your home. Learn more about how outdoor pollution can affect your indoor air here.
3. Unhealthy for Sensitive Groups (101 - 150)
Members of sensitive groups—such as children, the elderly, and people with heart or lung conditions—may experience health effects, while the general public is less likely to be affected.
- Sensitive individuals should avoid outdoor activities and consider wearing an air pollution mask outdoors if necessary. Ventilation is discouraged, and a high-performance air purifier should be turned on if indoor air quality is unhealthy.
4. Unhealthy (151 - 200)
Everyone may begin to experience adverse health effects, with sensitive groups at greater risk.
- Limit outdoor activities, especially if ozone is the dominant pollutant. Sensitive groups should avoid long or intense exertion outdoors, while others should take more breaks during outdoor activities.
- If PM2.5 is the dominant pollutant, it’s recommended to stay indoors as much as possible, and consider turning on an air purifier to maintain clean indoor air.
5. Very Unhealthy (201 - 300)
Health alerts apply to everyone, with a greater risk of serious health effects for the entire population.
- Everyone, particularly sensitive groups, should avoid all outdoor activities. Whether ozone or PM2.5 is dominant, move activities indoors and keep physical exertion to a minimum.
6. Hazardous (301 - 500)
AQI values in this range signal emergency health warnings for all individuals.
- Avoid all outdoor physical activity and stay indoors. Keep activity levels light and follow tips to maintain good indoor air quality, such as using high-performance air purifiers to keep particle levels low indoors.
These health recommendations are essential for protecting yourself and your family from the harmful effects of air pollution.
Comparing U.S. AQI and CN AQI
Both the U.S. and China use AQI systems to measure similar pollutants, but the U.S. AQI generally has stricter thresholds for classifying air quality as unhealthy. This can result in
different AQI values for the same pollutant concentrations in each country. However, IQAir’s U.S. AQI⁺ and CN AQI⁺ both provide real-time, hourly updates, allowing for a more responsive and consistent view of air quality globally.
Where Can You Access AQI Information?
You can access real-time AQI data, including U.S. AQI⁺ and CN AQI⁺ values, on the IQAir air quality map and in the IQAir AirVisual app. This helps you stay informed about current air quality conditions and make decisions to protect your health, whether you’re at home or traveling.