Air Quality Index Lookup - Online AQI by City

The Air Quality Index (AQI) is a standardized indicator used worldwide to measure and communicate how polluted the air currently is or how polluted it is forecast to become. It converts complex pollutant concentration data into a simple numerical scale (typically 0-500), where higher values indicate worse air quality and greater health risks. The AQI is calculated based on six major air pollutants regulated by environmental agencies.

AQI is calculated by measuring the concentration of individual pollutants (PM2.5, PM10, O₃, NO₂, SO₂, CO) in the air over a specific averaging period. Each pollutant concentration is converted to a sub-index using standard breakpoint tables. The highest sub-index among all measured pollutants becomes the overall AQI value. This means the AQI represents the worst-performing pollutant at any given time.

The six criteria air pollutants monitored for AQI calculation are: PM2.5 (fine particulate matter with diameter ≤2.5μm), PM10 (coarse particulate matter ≤10μm), O₃ (ground-level ozone), NO₂ (nitrogen dioxide), SO₂ (sulfur dioxide), and CO (carbon monoxide). Each has different health effects and sources. PM2.5 is often the primary pollutant of concern due to its ability to penetrate deep into the lungs.

0-50 (Good): Air quality is satisfactory, no health risk.
51-100 (Moderate): Acceptable, but sensitive individuals may experience minor effects.
101-150 (Unhealthy for Sensitive Groups): Sensitive people may experience health effects.
151-200 (Unhealthy): Everyone may begin to experience health effects.
201-300 (Very Unhealthy): Health alert—everyone may experience serious effects.
301+ (Hazardous): Emergency conditions—entire population is likely affected.

PM2.5 refers to fine inhalable particles with diameters of 2.5 micrometers or smaller—about 30 times smaller than a human hair. These particles can penetrate deep into the lungs and enter the bloodstream. PM10 includes larger particles up to 10 micrometers, which primarily affect the upper respiratory tract. PM2.5 is considered more dangerous due to its ability to bypass the body's natural defenses and cause systemic health issues.

It's recommended to check AQI daily, especially if you belong to sensitive groups (children, elderly, pregnant women, or those with respiratory/cardiovascular conditions). AQI can change throughout the day based on weather conditions, traffic patterns, and industrial activity. During wildfire seasons or in heavily polluted areas, checking multiple times a day is advisable. Many weather apps now include AQI data for convenient daily monitoring.

Yes, AQI can fluctuate considerably within a single day. Morning and evening rush hours often see spikes due to increased vehicle emissions. Temperature inversions can trap pollutants near the ground overnight. Ozone levels typically peak in the afternoon on hot, sunny days. Wind and rain can rapidly clear pollutants, while stagnant air allows them to accumulate. This is why real-time monitoring is valuable.

When AQI exceeds 150 (Unhealthy), limit outdoor activities. Wear an N95 or KN95 mask if you must go outside. Keep windows and doors closed. Use an air purifier with HEPA filter indoors. Avoid physical exertion outdoors. For AQI above 200 (Very Unhealthy), stay indoors as much as possible. If you have respiratory conditions, ensure medications are accessible and consult your doctor if symptoms worsen.

Yes, outdoor air pollution significantly affects indoor air quality. Pollutants infiltrate buildings through windows, doors, ventilation systems, and small cracks. Studies show that indoor air can be 50-80% as polluted as outdoor air without proper filtration. Using HEPA air purifiers, sealing windows, and maintaining HVAC systems can help reduce indoor exposure during high AQI periods.

Weather plays a crucial role in air quality. Rain washes pollutants from the air (wet deposition). Strong winds disperse pollutants, improving AQI. Temperature inversions trap cold air—and pollutants—near the ground. Hot, sunny weather accelerates photochemical reactions that produce ground-level ozone. Humidity can enhance particulate matter formation. Understanding local weather patterns helps predict AQI trends.

Persistent high AQI in certain cities results from a combination of factors: high population density, heavy vehicular traffic, industrial emissions, coal-fired power plants, construction dust, agricultural burning, and geographical features (valleys that trap pollution). Cities like New Delhi, Beijing, and Mexico City face additional challenges from temperature inversions and surrounding mountain ranges that limit air circulation.

Long-term exposure to poor air quality is linked to chronic respiratory diseases (asthma, COPD), cardiovascular disease, lung cancer, stroke, and premature death. The WHO estimates that air pollution causes approximately 7 million premature deaths annually worldwide. Children exposed to high pollution levels may develop reduced lung function. Pregnant women face increased risks of preterm birth and low birth weight. Reducing exposure is critical for long-term health.