Understanding AQI — What Those Numbers Really Mean
A practical guide to the Air Quality Index — what each band means for your health, and what to do about it.
What the AQI number is designed to tell you
If you have ever opened an air quality app and seen a number such as 87, 152, or 301, you were looking at an Air Quality Index value. AQI is not a raw pollution measurement. It is a public communication scale that converts pollutant concentrations into a single health-oriented number. That number helps people quickly understand whether outdoor air is generally safe, whether sensitive groups should take precautions, or whether everyone should reduce exposure.
The value is usually calculated from measured levels of major pollutants, including fine particulate matter (PM2.5), coarse particulate matter (PM10), ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. Each pollutant has its own health breakpoint scale. The pollutant with the highest health concern at that moment typically drives the overall AQI category shown for a city or station.
How the health categories work
AQI values from 0 to 50 are commonly labeled Good. In this range, air pollution is low and most people can continue normal outdoor activity. Values from 51 to 100 are Moderate. Air is acceptable for the general population, but people who are unusually sensitive to particle pollution or ozone may notice symptoms during long or intense outdoor activity.
From 101 to 150, the category becomes Unhealthy for Sensitive Groups. Children, older adults, pregnant people, outdoor workers, athletes, and people with asthma, COPD, heart disease, or other chronic conditions should start reducing prolonged exertion outside. Between 151 and 200, the air is Unhealthy for everyone. At this point, even healthy adults may experience throat irritation, coughing, chest tightness, or reduced exercise tolerance.
Values from 201 to 300 are Very Unhealthy and should be treated as a serious health warning. Outdoor exercise should be postponed, windows should stay closed where possible, and high-efficiency filtration becomes more important indoors. AQI above 300 is Hazardous. That level indicates emergency conditions where everyone should minimize outdoor exposure and follow local public health instructions.
Why PM2.5 often matters most
PM2.5 receives special attention because these particles are smaller than 2.5 micrometers in diameter. They are tiny enough to travel deep into the lungs and, in some cases, pass into the bloodstream. Short-term exposure can aggravate asthma and trigger breathing symptoms. Long-term exposure is associated with increased risk of cardiovascular disease, stroke, chronic respiratory illness, and premature death.
The same AQI number can feel different depending on which pollutant is driving it. Ozone-heavy pollution often worsens on hot sunny afternoons and can irritate the airways during exercise. Particle-heavy pollution may linger during winter inversions, wildfire smoke events, construction activity, crop burning, or heavy traffic periods. PollutIQ shows the AQI together with context so users can understand both the headline risk and the likely source of concern.
How to use AQI in daily decisions
The most useful habit is checking AQI before activities that increase breathing rate. A short walk may be reasonable on a Moderate day, while a long run beside traffic may not be. On Unhealthy days, consider exercising indoors, wearing a well-fitted N95 mask during unavoidable outdoor travel, using recirculation mode in vehicles, and running a HEPA purifier in the room where you sleep.
AQI should not create panic, but it should guide practical choices. Small exposure reductions repeated over months and years can matter, especially for people with existing health vulnerabilities. By turning technical measurements into clear risk levels, PollutIQ helps households make better decisions about commuting, school activities, outdoor workouts, and indoor air protection.