The Air Quality Index (AQI) is a measurement of air pollutant concentrations in ambient air pollution and their associated health risks.

Air pollutants measured in AQI

There are six air pollutants measured in the index formula, including:

• PM2.5
• PM10
• carbon monoxide
• sulfur dioxide
• nitrogen dioxide
• ground-level ozone

An AQI number is assigned based on the air pollutant with the highest AQI number at the moment the air quality is measured. Only pollutants available from a given air quality monitoring station are measured, and many do not include all six pollutants equally. Because air quality changes throughout the day, a monitored location’s AQI changes with the level of measured air pollutant concentrations.

The index represents air pollutant concentrations with a number falling within a range of air quality categories. Within each category and number range, elevated health risks associated with rising air pollutant concentrations are identified.

The air quality index ranges from 0 to 500, though air quality can be indexed beyond 500 when there are higher levels of hazardous air pollution. Good air quality ranges from 0 to 50, while measurements over 300 are considered hazardous.

IQAir AirVisual platform AQI readings are based on the U.S. Environmental Protection Agency (EPA) National Ambient Air Quality Standards (NAAQS) to calculate AQI and to attribute code color.^4,5 AirVisual Series air quality monitors measure PM2.5, PM1, PM10, and carbon dioxide levels and use PM2.5, or fine particulate matter, to determine the AQI.

The AirVisual Series monitors AQI using PM2.5 measurements as the determinant for AQI readings because PM2.5 is widely available and considered the most hazardous air pollutant impacting human health.^6,7,8

PM2.5 is measured by micrograms per cubic meter (μg/m³). According to the U.S. EPA NAAQS, any measurement greater than 9.0 μg/m³ (US AQI 50) can be dangerous to human health.

Pictured: Air quality index chart with corresponding PM2.5 μg/m3.

AQI categories

Here’s how each of the AQI categories are represented, how they can impact human health, and some recommendations for what to do when the AQI has reached a higher category.

The categories are listed from least harmful to human health (“good”) to the most harmful (“hazardous”).

Good

• AQI: 0-50
• PM2.5 (μg/m³): 0-9.0
• Green

When categorised as good, air quality poses little to no health risk at an AQI of 0 up to an AQI of 50. You can enjoy your usual outdoor activities. You may choose to open your windows and ventilate your home to bring in outdoor air.

Moderate

• AQI: 51-100
• PM2.5 (μg/m³): 9.1-35.4
• Yellow

Due to the risk of respiratory illness symptoms, sensitive groups should greatly reduce outdoor exercise when air quality is moderate (US AQI 51-100). Avoid ventilating indoor spaces with outdoor air, and close windows to avoid letting outdoor air pollution indoors.

Note that sensitive groups for all categories include children, the elderly, pregnant people, and people with cardiac and pulmonary diseases.

Unhealthy for sensitive groups

• AQI: 101-150
• PM2.5 (μg/m³): 35.5-55.4
• Orange

When air quality is unhealthy for sensitive groups, everyone is at risk for eye, skin, and throat irritation as well as respiratory problems. The public should greatly reduce outdoor exertion.

Sensitive groups are at greater health risk, should avoid all outdoor activity, and should consider wearing an air pollution mask outdoors. Ventilation is discouraged. A high-performance air purifier should be turned on if indoor air quality is unhealthy.

Unhealthy

• AQI: 151-200
• PM2.5 (μg/m³): 55.5-125.4
• Red

Unhealthy AQI measurements mean that there is an increased likelihood of heart and lung aggravation as well as health impacts among the public, particularly for sensitive groups.

Everyone should avoid and wear a pollution mask outdoors. Ventilation is discouraged. Air purifiers should be turned on.

Very unhealthy

• AQI: 201-300
• PM2.5 (μg/m³): 125.5-225.4
• Purple

When air quality is very unhealthy, the public will be noticeably affected. Sensitive groups will experience reduced endurance in activities. These individuals should remain indoors and limit activities.

Everyone should avoid outdoor exercise and wear a pollution mask outdoors. Ventilation is discouraged. Air purifiers should be turned on.

Hazardous

• AQI: 301-500 or greater
• PM2.5 (μg/m³): 225.5 or greater
• Maroon

Everyone is at high risk of experiencing strong irritation and negative health effects that could trigger cardiovascular and respiratory illnesses.

Avoid exercise and remain indoors. Avoid outdoor exercise and wear a pollution mask outdoors. Ventilation is discouraged. Air purifiers should be turned on.

Differences between AQI systems

There are different systems used around the world for measuring air quality. The United States and China AQI systems are the most widely used systems – both measure the same group of air pollutants. However, each system rates AQI scores from 200 and lower differently.²

The U.S. AQI system generally yields higher scores for AQI under 200 because lower levels of concentrations of PM2.5 result in a higher AQI value. The U.S. revised its AQI standard in 2013 when it was found that lower levels of PM2.5 were associated with greater serious health consequences than was previously understood.³ Consequently, the U.S. AQI system is considered the more stringent standard and is the world standard.

Pictured: Comparison chart of China and U.S. air quality index differences.

The takeaway

The air quality index reinterprets potentially complex air quality data into a meaningful number with corresponding categories of pollutant concentrations and associated health effects.

Please note, though, that there is no safe level of air pollution, in particular as the AQI reaches closer to 50 and beyond. By knowing whether the air we breathe is safe and how it can impact our health, it’s possible to take action to help control the harmful health effects of air pollution.