Comprehensive Guide to the Air Quality Index (AQI)

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Comprehensive Guide to the Air Quality Index (AQI)

Understanding the Air Quality Index (AQI)

The Air Quality Index (AQI) serves as a crucial tool for assessing and communicating the quality of air to the general public. It provides a clear, understandable measure of how polluted the air currently is or how polluted it is forecast to become. The AQI focuses on health effects individuals may experience within a few hours or days after breathing polluted air.

The Basics of AQI

The AQI is a numerical scale ranging from 0 to 500, where higher values indicate higher levels of air pollution and greater potential health risks. The primary pollutants considered in calculating AQI include:

AQI Categories and Health Implications

The AQI is divided into six categories, each corresponding to a different level of health concern:

AQI Range Health Implications Color Code
0 - 50 Good: Air quality is considered satisfactory, and air pollution poses little or no risk. Green
51 - 100 Moderate: Air quality is acceptable; however, for some pollutants, there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution. Yellow
101 - 150 Unhealthy for Sensitive Groups: Members of sensitive groups may experience health effects. The general public is not likely to be affected. Orange
151 - 200 Unhealthy: Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects. Red
201 - 300 Very Unhealthy: Health alert; everyone may experience more serious health effects. Purple
301 - 500 Hazardous: Health warnings of emergency conditions; the entire population is more likely to be affected. Maroon

How AQI is Calculated

Calculating the AQI involves measuring concentrations of the aforementioned pollutants in the air, typically expressed in parts per million (ppm) or micrograms per cubic meter (µg/m3). These concentrations are then converted into a corresponding AQI value using standardized formulas developed by environmental health agencies.

For instance, the formula to determine the AQI for a specific pollutant, such as PM2.5, involves the following steps:

  1. Measure the pollutant concentration in µg/m3.
  2. Use the linear interpolation formula:
  3. AQI = ( ( Ihigh - Ilow ) / ( Chigh - Clow ) ) * ( C - Clow ) + Ilow


Real-Life Example

Imagine a bustling city with a high volume of traffic and industrial activity, leading to increased levels of PM2.5. On a particularly smoggy day, air quality monitors detect a PM2.5 concentration of 75 µg/m3. Using the AQI formula, this pollutant level converts into an AQI value indicating that the air quality is unhealthy for sensitive groups, thereby triggering health advisories for individuals with respiratory issues and other vulnerable populations.

Why AQI Matters

Understanding the AQI is critical for public health as it empowers individuals and communities to take precautionary measures when air quality levels deteriorate. For instance, during high AQI days, schools might limit outdoor activities, people with asthma might need to use their inhalers more frequently, and cities could issue health advisories.

FAQs about AQI

What does an AQI of 100 mean?
An AQI of 100 corresponds to the upper limit of the 'Moderate' air quality category, which means that air quality is generally acceptable, but some pollutants may pose a slight health risk to very sensitive individuals.
How can I check the AQI in my area?
You can check the AQI using various apps and websites provided by environmental agencies, such as the EPA's AirNow.
What can I do to protect myself on days with high AQI?
On days with a high AQI, limit outdoor activities, keep windows closed, and use air purifiers indoors. Individuals with respiratory conditions should follow their doctor's advice.


The AQI is an invaluable resource in understanding and responding to air quality issues. By staying informed about AQI levels, individuals can take steps to protect their health and contribute to a collective effort in addressing air pollution.

Tags: Environment, Pollution, Air Quality