The Air Quality Index Explained: Understanding Air Pollution Levels and Health Impacts

Every morning, millions of people check the weather before stepping outside. But how many check the air quality? The air we breathe is invisible, yet it carries a range of pollutants that can significantly affect our health – sometimes within hours of exposure. To make this invisible threat visible and understandable, scientists and governments developed the Air Quality Index (AQI) – a standardized, publicly accessible tool that translates complex pollution data into a single, actionable number. Whether you live in a megacity or a mid-sized town, understanding what the AQI tells you – and how it is calculated – is an essential part of making informed decisions about your health every day.

What is the Air Quality Index (AQI)? 🔗

The Air Quality Index is a numerical scale used by environmental agencies to communicate how polluted the outdoor air currently is, or is forecast to become. The U.S. Environmental Protection Agency (EPA) developed the AQI as a tool for reporting outdoor air quality and its effects on health. It runs on a scale from 0 to 500, and the higher the AQI value, the greater the level of air pollution and the greater the health concern. An AQI of 50 or below indicates good air quality posing little or no risk, while values above 300 signal hazardous conditions requiring emergency-level precautions.

The concept of a unified air quality index dates back to 1968. The U.S. National Air Pollution Control Administration first developed the index to draw public attention to air pollution and push local officials to take action on emission controls. Since then, countries around the world have adopted their own versions. In India, the National Air Quality Index (NAQI) was launched in September 2014 by the Ministry of Environment, Forest and Climate Change as part of the Swachh Bharat Abhiyan, with the Central Pollution Control Board (CPCB) and IIT Kanpur developing the technical framework.

At its core, the AQI exists to bridge the gap between technical environmental data and everyday public understanding. Newspapers, radio, television, and websites report AQI levels year-round so that people can take steps to protect themselves, children, and others from unhealthy levels of air pollution. It is not just a number – it is a public health tool.

AQI categories and health implications 🔗

The AQI is divided into color-coded categories that communicate not just the level of pollution, but the specific health risks associated with each level. The EPA has assigned six levels of health concern, each with a name, color, and associated public health meaning. Here is a breakdown of what each category means for the air you breathe:

Good (0-50): Green 🔗

At this level, air quality is considered satisfactory and air pollution poses little or no risk to the public. This is the ideal range – outdoor activities are safe for everyone, including children and those with respiratory conditions.

Moderate (51-100): Yellow 🔗

Air quality is acceptable at this level; however, some people who are unusually sensitive to ozone might experience breathing issues. For the general population, outdoor activities remain low-risk, but sensitive individuals should begin paying attention.

Unhealthy for sensitive groups (101-150): Orange 🔗

When air quality is in this range, people in sensitive groups – whether due to medical conditions, exposure conditions, or innate susceptibility – may experience health effects when engaged in outdoor activities. This includes individuals with asthma, heart disease, older adults, pregnant women, and children. The general public is largely unaffected at this stage, but sensitive groups should reduce prolonged outdoor exertion.

Unhealthy (151-200): Red 🔗

At this level, everyone may begin to experience health effects, while members of sensitive groups may experience more serious problems. Activities like jogging, cycling, or playing outdoors should be minimized, especially for at-risk groups.

Very unhealthy (201-300): Purple 🔗

When the AQI reaches this range, the risk of health effects is increased for everyone. People should avoid going outside as much as possible, and correctly fitted N95 masks are recommended when outdoor exposure is unavoidable.

Hazardous (301-500): Maroon 🔗

Air quality in this range triggers health warnings of emergency conditions. The entire population is more likely to be affected by serious health effects. At this level, governments may invoke emergency plans and order major pollution sources to curtail emissions.

India’s NAQI uses a slightly different labeling system. The six Indian AQI categories are Good, Satisfactory, Moderately Polluted, Poor, Very Poor, and Severe. The “Severe” category in India broadly corresponds to the Hazardous range internationally, and cities like Delhi have frequently breached this threshold. Delhi’s AQI regularly falls into the “severe” or “hazardous” category, with health impacts ranging from respiratory disorders like asthma and COPD to developmental problems in children, immune disorders, and cardiovascular disease.

How AQI is calculated 🔗

The AQI is not a simple average of pollution levels. It is a carefully structured index that converts the measured concentrations of specific air pollutants into a standardized scale, allowing different pollutants to be compared on the same terms. The calculation follows a defined process involving pollutant selection, concentration measurement, sub-index derivation, and final aggregation.

Step 1: Identifying the key pollutants 🔗

The U.S. EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. India’s NAQI goes further. The Indian AQI considers eight pollutants – PM10, PM2.5, NO₂, SO₂, CO, O₃, NH₃, and Pb – for which short-term national ambient air quality standards are prescribed. Among all these, ground-level ozone and fine particulate matter (PM2.5) are consistently identified as the most significant threats to human health.

Step 2: Measuring pollutant concentrations 🔗

Pollutant concentrations are measured continuously at monitoring stations using calibrated instruments. The AQI is calculated by converting measured pollutant concentrations to a uniform index based on the health effects associated with each pollutant. The health benchmarks used are pollutant-specific and established by the EPA through the National Ambient Air Quality Standards, which the Clean Air Act requires the EPA to review every five years. In India, continuous ambient air quality monitoring (CAAQM) stations use instruments like beta-attenuation mass monitors to provide near-real-time pollution data.

Different pollutants are measured over different time periods. For example, particulate matter concentrations typically use 24-hour averages, while ozone readings may use 8-hour or 1-hour averages depending on the regulatory framework. Some AQI systems may use 24-hour averages for particulate matter, while others rely on shorter periods for certain pollutants like ozone.

Step 3: Calculating sub-indices using breakpoints 🔗

Once concentrations are measured, each pollutant’s raw value is converted into a sub-index using a set of predefined breakpoints. Each pollutant has a set of breakpoints – concentration ranges that correspond to different AQI values – allowing raw pollutant concentrations to be converted into AQI values that fall into one of the six health categories.

The mathematical formula used is a piecewise linear equation. As defined in U.S. federal regulations, the formula requires identifying the breakpoint concentrations above and below the measured value, along with their corresponding index values, and then interpolating to calculate the exact sub-index for that pollutant. This piecewise approach means the relationship between concentration and AQI is non-linear – an AQI of 300 does not mean twice the pollution of an AQI of 150, nor does it mean the air is twice as harmful.

Step 4: Selecting the dominant pollutant 🔗

After sub-indices are calculated for each pollutant monitored at a station, the final AQI value is determined not by averaging them, but by selecting the worst one. The highest AQI value – called the “dominant” AQI – is reported. This means that even if multiple pollutants are measured, the AQI shown to the public is based on the worst pollutant at that time, ensuring that the reported value reflects the most pressing health concern.

This is a deliberate design choice. The AQI is meant to represent the most urgent risk, not the average risk. If PM2.5 is dangerously high but ozone is moderate, the AQI will reflect the PM2.5 danger. Most cities forecast primarily for ozone and particle pollution because these pollutants are the major sources of unhealthy air quality around 99% of the time.

AQI as a forecasting tool 🔗

The AQI is not only used to report current conditions – it is also used to forecast future air quality. AQI forecasts heavily depend on temperatures, precipitation, wind, and cloud cover, since weather affects both pollution creation and the transport of pollutants from other areas. For instance, ozone levels tend to peak on hot, sunny afternoons, while PM2.5 can spike during winter temperature inversions when cool air traps pollutants close to the ground. Wildfire smoke is another increasingly important factor – higher AQIs in summer are increasingly attributed to smoke from wildfires in Canada and other regions, which can travel hundreds of miles and dramatically elevate local AQI readings.

Forecasts allow individuals, schools, and health agencies to plan ahead and reduce exposure before conditions worsen. When the AQI is expected to cross critical thresholds, authorities issue Air Quality Alerts – public advisories that recommend protective actions for sensitive populations and, in severe cases, for everyone.

Why understanding AQI matters 🔗

The AQI is one of the most effective public health communication tools available. Evidence indicates that air quality alerts are helpful, and advocacy from health care professionals is especially powerful – people are almost four times as likely to change outdoor activity levels when advised by a health care professional to do so during poor air quality conditions. This underscores that the AQI works best not just as a number on a screen, but as part of a broader system of health education and public awareness.

For vulnerable groups – including children and teens, people who are pregnant, those living with chronic illnesses, and people who spend a lot of time outdoors – tracking the AQI is especially important, as they are the first to feel the effects of ozone and particle pollution. Platforms like AirNow.gov in the US and the CPCB’s AQI portal in India provide real-time, publicly accessible AQI data that anyone can use to make daily decisions.

Understanding the AQI also supports broader environmental accountability. When communities consistently monitor and report air quality data, it creates pressure on policymakers and industries to address the sources of pollution – from vehicle emissions and industrial discharge to agricultural burning and construction dust. The index transforms what could be an abstract environmental concern into a concrete, daily measurement that anyone can understand and act on.

What do you think? If the AQI in your city regularly exceeds 150 during winter months, what changes – at the individual level and at the policy level – would you consider most urgent to address the problem? And given that the AQI reports only the single worst pollutant at any given time, do you think this approach gives the public a complete picture of air quality risks?

References

1. https://www.airnow.gov
2. https://airquality.cpcb.gov.in