Air Quality Index Calculator
Check current air quality levels and understand the health impact. Enter an AQI value or pollutant concentration (PM2.5 or PM10) to see the risk category and recommended actions.
Air quality is acceptable. Some pollutants may be a moderate concern for a very small number of unusually sensitive people.
Based on the US EPA AQI standard. AQI 101–150 = Unhealthy for Sensitive Groups (USG).
Understanding the Air Quality Index: A Complete Guide
The Air Quality Index, or AQI, is a standardized numerical scale used to communicate how clean or polluted the air is and what associated health effects might be a concern. Developed by the United States Environmental Protection Agency (US EPA), the AQI has become the most widely recognized air quality reporting system in the world. A higher AQI value indicates greater levels of air pollution and a greater concern for public health. An AQI of 0–50 is considered Good, while values above 300 are classified as Hazardous and represent emergency conditions.
Air quality varies constantly throughout the day and across locations. Major contributing factors include vehicle emissions, industrial activity, wildfires, dust storms, pollen, and weather patterns. Understanding the AQI helps individuals make informed decisions about outdoor activities, ventilation, and health precautions — especially for vulnerable groups such as children, elderly adults, and those with respiratory or cardiovascular conditions.
How the AQI Scale Works
The US EPA AQI scale runs from 0 to 500 and is divided into six color-coded categories. Each category corresponds to a different level of health concern. Values of 0–50 indicate Good air quality where the risk is minimal. Values of 51–100 indicate Moderate air quality, which is acceptable but may cause issues for a very small number of unusually sensitive individuals.
When the AQI reaches 101–150, it is classified as Unhealthy for Sensitive Groups. People with lung disease, heart disease, children, and older adults may experience health effects, although the general public is unlikely to be affected. An AQI of 151–200 is Unhealthy — everyone may begin to experience adverse health effects, and sensitive groups may suffer more serious effects.
An AQI between 201 and 300 is Very Unhealthy, representing a health alert where everyone may experience significant health effects. Values above 300 are Hazardous, meaning emergency health conditions exist. Everyone is at risk, and outdoor activities should be avoided entirely.
Key Pollutants Measured by the AQI
The AQI is calculated based on five major air pollutants regulated under the US Clean Air Act: ground-level ozone (O₃), particle pollution (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Each pollutant has its own set of EPA breakpoints, and the overall AQI reported for an area is the highest AQI value calculated for any individual pollutant.
Particulate matter — especially fine particles known as PM2.5 (particles smaller than 2.5 micrometers in diameter) — is one of the most common and harmful pollutants. These particles are so small they can penetrate deep into the lungs and enter the bloodstream, causing cardiovascular and respiratory problems. PM2.5 is the primary pollutant of concern during wildfires and in heavily industrialized areas.
PM10 refers to coarser particles smaller than 10 micrometers, including dust, pollen, and mold spores. While larger than PM2.5, PM10 particles can still reach the lungs and trigger asthma attacks, coughing, and breathing difficulty. Monitoring both PM2.5 and PM10 provides a fuller picture of particulate pollution.
How PM2.5 and PM10 Are Converted to AQI
The EPA uses a linear interpolation formula to convert pollutant concentrations into AQI values. The formula finds where a given concentration falls within predefined breakpoints and scales it linearly to the corresponding AQI range. For PM2.5, concentrations from 0 to 12.0 µg/m³ map to AQI values of 0–50 (Good). Concentrations from 12.1 to 35.4 µg/m³ map to AQI 51–100 (Moderate), and so on up to 500.4 µg/m³, which corresponds to the maximum AQI of 500.
PM10 uses different concentration breakpoints. Values from 0 to 54 µg/m³ map to AQI 0–50. The scale extends to 604 µg/m³, which maps to AQI 500. This calculator supports both PM2.5 and PM10 inputs using the official EPA breakpoints, giving you accurate AQI conversions for both pollutant types.
Who Is Most Vulnerable to Poor Air Quality?
Certain groups of people are more sensitive to the effects of air pollution than others. Children are particularly vulnerable because their lungs are still developing and they breathe more air per pound of body weight than adults. Prolonged exposure to poor air quality during childhood can permanently reduce lung capacity and increase the risk of respiratory diseases throughout life.
Older adults face heightened risk because of the natural decline in respiratory and cardiovascular function that comes with aging. People with pre-existing conditions such as asthma, chronic obstructive pulmonary disease (COPD), heart disease, or diabetes are especially susceptible to the effects of elevated pollutant levels. Even short-term exposure to high AQI levels can trigger asthma attacks, hospitalizations, or cardiac events in these groups.
Pregnant women are also considered sensitive, as air pollution exposure during pregnancy has been linked to preterm birth, low birth weight, and developmental issues in newborns. Athletes and outdoor workers who engage in prolonged physical activity outdoors inhale more air and thus absorb more pollutants, increasing their risk even when AQI values are in ranges considered safe for the general public.
Protecting Yourself on High-AQI Days
On days when the AQI is elevated, there are several practical steps you can take to reduce your exposure. For outdoor activity, check the AQI forecast before heading out and schedule exercise for times of day when pollution is lower — typically early morning before traffic builds up, or after rain which temporarily clears particulates from the air. Reduce the intensity and duration of outdoor exercise on high-AQI days.
Indoors, keep windows and doors closed and use air conditioning to filter incoming air when possible. High-efficiency particulate air (HEPA) purifiers can significantly reduce indoor particulate levels. Avoid burning wood, candles, or incense, and refrain from frying or grilling, which generate indoor particulates. Wearing an N95 respirator mask (not a standard cloth mask) outdoors when PM2.5 levels are very high can provide meaningful protection.
During wildfire smoke events — one of the most significant causes of extreme AQI levels in many regions — it is best to shelter indoors with purified air, seal gaps under doors and windows, and follow local health authority guidance. Community air quality alerts are broadcast via apps, websites, and local news, and it is important to respond promptly to these advisories.
Air Quality Monitoring and Data Sources
Air quality monitoring networks operate across most developed countries, with sensors placed in urban centers, industrial zones, and rural areas. In the United States, the EPA's AirNow system (airnow.gov) aggregates data from thousands of monitoring stations and provides real-time and forecast AQI maps. Similar national systems operate in Japan (Soramame-chan), the European Union, China, India, and many other countries.
In recent years, low-cost consumer air quality sensors have proliferated, enabling hyperlocal monitoring that complements official networks. Platforms like PurpleAir aggregate crowdsourced sensor data and display it on public maps, providing more granular geographic coverage than traditional networks, though with less accuracy than regulatory-grade monitors. When using AQI data, always consider the source and calibration of the sensor, as readings can vary significantly between high-quality regulatory monitors and consumer-grade devices.
Frequently Asked Questions
What is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a standardized numerical scale from 0 to 500 used to report daily air quality and communicate health risks to the public. It is calculated by the US EPA based on measured concentrations of five key pollutants: PM2.5, PM10, ground-level ozone, carbon monoxide, and sulfur dioxide. The higher the AQI, the greater the health risk. Values 0–50 are Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, 151–200 Unhealthy, 201–300 Very Unhealthy, and 301–500 Hazardous.
What AQI level is safe to go outside?
An AQI of 0–100 is generally considered safe for most people for outdoor activities. AQI 0–50 (Good) poses little to no risk. AQI 51–100 (Moderate) is acceptable for most people, though unusually sensitive individuals may want to limit prolonged exertion. When the AQI reaches 101 or above, sensitive groups should begin reducing outdoor activity. At AQI 151+, everyone should limit outdoor exertion, and at 201+, all outdoor activity should be avoided.
What is the difference between PM2.5 and PM10?
PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometers or less, while PM10 includes particles up to 10 micrometers in diameter. PM2.5 is more dangerous because its tiny size allows it to penetrate deep into the lungs and even enter the bloodstream, causing more severe cardiovascular and respiratory effects. PM10 includes larger particles such as dust, pollen, and mold that can irritate the upper respiratory tract. Both are measured in micrograms per cubic meter (µg/m³) and can be converted to AQI using the EPA's linear interpolation formula.
How often does the AQI change?
AQI values are typically updated hourly by official monitoring networks, as pollutant concentrations can change significantly throughout the day. AQI tends to be higher during rush hour due to vehicle emissions, during summer afternoons when ozone formation peaks, and during wildfire or dust events. Weather factors such as wind, rain, and atmospheric inversions also cause rapid changes. Always check the current or forecast AQI rather than relying on yesterday's reading.
Does wearing a mask help with poor air quality?
Standard cloth or surgical masks do not effectively filter fine particulate matter like PM2.5. N95 or KN95 respirators, when properly fitted, can filter out at least 95% of airborne particles and provide meaningful protection against particulate pollution. However, masks do not protect against gaseous pollutants like ozone or nitrogen dioxide. For very high or hazardous AQI levels, staying indoors with filtered air is more effective than wearing a mask outdoors.