Why the AQI Matters

The Air Quality Index condenses complex pollution data into a single number. Higher values indicate a greater chance of irritation or respiratory problems, making the index a quick way to judge whether outdoor air is healthy.

How This Tool Computes AQI

Enter concentrations for fine particles (PM2.5), coarse particles (PM10), and ground-level ozone (O₃). The calculator compares each value with EPA breakpoints and applies a linear formula to produce a sub‑index for each pollutant. The highest sub‑index becomes the overall AQI and reveals which pollutant is currently driving air quality concerns.

AQI Categories

AQI	Category
0–50	Good
51–100	Moderate
101–150	Unhealthy for Sensitive Groups
151–200	Unhealthy
201–300	Very Unhealthy
301–500	Hazardous

Understanding Particulate Pollution

Fine particles known as PM2.5 measure less than 2.5 micrometers in diameter—about one‑thirtieth the width of a human hair. They originate from vehicle exhaust, wildfire smoke, and industrial processes. Their small size allows them to penetrate deep into the lungs and even pass into the bloodstream, where they can aggravate asthma or contribute to cardiovascular problems. Coarser PM10 particles, though larger, are still easily inhaled and can irritate the nose, throat, and upper airways.

In the United States, the Environmental Protection Agency (EPA) established breakpoints that map concentration ranges of each pollutant to an index scale from 0 to 500. A lower index indicates cleaner air, while higher values signal more severe pollution. For instance, a PM2.5 concentration between 0 and 12 µg/m³ corresponds to an AQI range of 0 to 50, labeled “Good.” When concentrations rise to around 35 µg/m³, the AQI jumps into the “Moderate” or even “Unhealthy for Sensitive Groups” category. These breakpoints are based on epidemiological studies linking pollutant exposure to health outcomes, ensuring that the index has real‑world relevance.

The Role of Ozone

While particulates often dominate headlines, ozone is another major component of air pollution. Ground‑level ozone forms when sunlight triggers chemical reactions between nitrogen oxides and volatile organic compounds emitted by vehicles and industrial activities. Unlike the protective ozone layer high in the stratosphere, ozone near the surface is a powerful oxidant that can inflame airways and reduce lung function within hours of exposure. Hot, sunny days with little wind tend to produce the highest ozone readings, which is why summer smog alerts are common in many cities.

This calculator estimates ozone’s contribution by using the EPA’s eight‑hour concentration breakpoints. If the ozone sub‑index exceeds the particulate sub‑indices, the resulting AQI highlights that ozone is the principal pollutant. Knowing which pollutant dominates helps tailor mitigation strategies—limiting vehicle idling can curb ozone precursors, while filtering indoor air targets particulate matter.

Using the Results

When the AQI exceeds 100, children, older adults, and people with lung or heart conditions should limit outdoor activity. Very high readings may require everyone to stay indoors or wear respirators. Because guidelines vary internationally, consult local authorities for region‑specific advice. Many communities update the AQI hourly, so revisiting the numbers throughout the day can guide decisions about exercise, commuting, or opening windows for ventilation.

Interpreting Changes Over Time

Pollution levels fluctuate due to weather, traffic patterns, and local industry. Temperature inversions can trap pollutants near the ground, spiking the index, while strong winds or rainfall can disperse particles and improve air quality. Wildfire smoke may travel hundreds of miles, turning a previously clean day into a hazardous one. Tracking daily AQI trends helps you spot patterns—perhaps weekday mornings are consistently worse due to rush hour traffic, or summer afternoons bring ozone spikes.

Health Effects and Protective Measures

Understanding AQI categories can influence daily decisions. When the index is below 50, outdoor activities pose minimal health risks. As the AQI climbs above 100, individuals with respiratory conditions or heart problems may begin to experience symptoms such as coughing or shortness of breath. At 150 or higher, even healthy individuals can feel the effects of poor air quality. Long‑term exposure to high particulate levels has been linked to increased rates of asthma, reduced lung function in children, and heightened mortality. By checking the AQI, you can decide whether to limit time outside, close windows, or use air filtration indoors.

Protective actions vary by category. On moderate days, sensitive groups might reduce prolonged or heavy exertion. During unhealthy conditions, everyone should scale back outdoor activity and consider wearing N95 or P100 respirators if exposure is unavoidable. Very unhealthy or hazardous levels warrant staying indoors with windows closed and using high‑efficiency particulate air (HEPA) filters or air purifiers. Individuals who must work outside may require employer-provided protective gear and additional rest breaks.

Beyond Particulates

The official AQI also considers nitrogen dioxide, sulfur dioxide, and carbon monoxide. These gases typically stem from combustion sources and can irritate the respiratory system or reduce the blood’s ability to carry oxygen. If your sensor provides these values, you can extend the calculator with additional breakpoint tables to identify which pollutant is driving poor air quality. Understanding the pollutant mix also clarifies which emission sources need attention—traffic, industry, or natural events like volcanic activity.

Data Sources and Sensor Quality

Accurate AQI calculations depend on reliable measurements. Government monitoring stations use calibrated instruments and strict protocols, but low‑cost consumer sensors vary in precision. Factors such as humidity, sensor aging, and placement near vents or walls can skew readings. Periodically comparing a personal sensor against official data improves confidence in the numbers. Many communities host calibration events where residents bring devices for cross‑checks.

Global Perspectives

Different countries employ distinct AQI scales and breakpoints. For example, India’s Central Pollution Control Board uses a 0–500 scale similar to the U.S. but with different concentration ranges, reflecting local health research and policy priorities. The European Union reports pollutant concentrations directly and classifies air quality in qualitative bands. When traveling or reading international news, be aware of these differences to avoid misinterpreting values.

Improving Air Quality

Individuals can take steps to reduce personal contributions to pollution. Carpooling, using public transit, and maintaining vehicles decrease emissions. Indoors, avoiding candles or wood‑burning stoves limits particulate buildup. On high‑AQI days, delay lawn mowing or refueling until evening when ozone formation slows. Communities can advocate for greener energy sources, stricter industrial controls, and expanded tree canopies that absorb pollutants.

Planning With Forecasts

Weather agencies often publish AQI forecasts days in advance. Combining this calculator with forecasts allows you to plan hikes, outdoor parties, or construction projects for cleaner periods. If you live in an area prone to seasonal smoke or dust, stocking up on filters and masks before the peak season ensures you’re prepared. Schools and sports leagues increasingly rely on AQI thresholds to decide when to cancel practices or move activities indoors.

Conclusion

This calculator demystifies how the AQI is derived and highlights the health implications of airborne pollutants. By translating raw concentrations into an easy‑to‑interpret scale, it empowers you to make informed choices about daily activities and long‑term mitigation strategies. Bookmark the tool, share it with friends, and use it alongside local reports to stay mindful of the air you breathe.