The Air Quality Index appears daily on weather forecasts and environmental websites, yet many people misinterpret its meaning. The AQI scale has been used since 1976 by the EPA to report daily air quality, providing a standardized communication tool that converts complex pollution measurements into public health guidance. Understanding what AQI numbers mean is essential for protecting yourself and your family from air pollution.
The AQI Scale: From 0 to 500
The AQI runs from 0-500, with values of 50 or below indicating good air quality, and values over 300 indicating hazardous conditions. This simple numerical framework enables quick assessment of whether today’s air quality poses health risks for various populations.
The scale divides into six color-coded categories. The six color-coded categories make it easy for the public to understand air quality levels quickly. Green indicates good conditions (0-50). Yellow signals moderate air quality (51-100)—acceptable, but sensitive individuals should reduce strenuous activity. Orange (101-150) means unhealthy for sensitive groups. Red (151-200) indicates unhealthy conditions for general population. Purple (201-300) represents very unhealthy conditions. Maroon (301+) signals hazardous pollution—most people should remain indoors.
What Pollutants Does the AQI Measure?
The AQI is calculated by measuring six primary air pollutants, and the highest value (called the ‘dominant’ AQI) is reported. These six pollutants are ozone, PM2.5 (fine particulates), PM10 (coarse particulates), nitrogen dioxide, sulfur dioxide, and carbon monoxide.
The U.S. AQI measures five major pollutants regulated by the Clean Air Act: ozone, particulate matter, CO, NO2, and SO2. Each pollutant has established concentration limits. Each pollutant has concentration limits averaged over a specific time period, acknowledging that short-term spikes differ from sustained elevated levels.
Dominant Pollutant Variation by Season
The “dominant” AQI—the highest of the six pollutants—determines the reported AQI value. This means your AQI may be driven by different pollutants in different seasons. Summer AQI typically reflects ozone, which peaks during warm months when sunlight and heat accelerate its formation. Winter AQI more often reflects particulate matter from heating, vehicle emissions in stagnant air, and wildfire smoke in western regions.
Understanding what drives your specific AQI reading improves response appropriateness. If ozone dominates, limiting afternoon outdoor activity proves most effective since ozone concentrations peak mid-afternoon. If PM2.5 dominates, continuous outdoor exposure reduction throughout the day matters more since particulates remain suspended regardless of time.
Geographic and Regulatory Variations
While the U.S. system uses 0-500 AQI, AQI systems differ between the U.S. (0-500 scale) and UK (0-10 scale). This variation reflects different regulatory frameworks and public communication approaches. U.S. AQI emphasizes the familiar color coding approach, while UK’s 0-10 scale represents a different numerical system.
Countries outside the U.S. and UK use additional variations. China uses a 0-500 scale similar to the U.S. but with different concentration breakpoints. India uses a different calculation methodology. Australia uses color categories without numerical values. When traveling internationally or accessing air quality data from global sources, confirm which AQI system is being used.
Converting Measurements to AQI Values
Air quality monitors measure actual pollutant concentrations in parts per million (ppm) or micrograms per cubic meter (μg/m³). These raw measurements are converted to AQI values using standardized formulas. For example, 50 μg/m³ of PM2.5 converts to AQI 170 (unhealthy). The same concentration in different regions produces the same AQI value, enabling standardized interpretation.
The conversion uses breakpoint concentrations defined by EPA based on health research. At lower concentration ranges, the conversion is linear. At higher concentrations, breakpoints may shift, reflecting non-linear health effects at extreme pollution levels.
Beyond the AQI: Understanding What the Numbers Mean for Health
While AQI provides standardized communication, health impacts vary by individual susceptibility. A red AQI (unhealthy, 151-200) means different things for a healthy 30-year-old and an 80-year-old with heart disease. AQI values above 100 are generally considered unhealthy for general population, but vulnerable individuals experience effects at lower values.
Sensitive groups include children with developing lungs, elderly individuals with declining organ function, people with asthma or COPD, those with heart disease, pregnant women, and low-income populations with limited access to healthcare. These populations should take protective action starting at orange (101-150) AQI, while general population can wait until red (151-200).
Real-Time Monitoring and Forecast AQI
Two AQI values matter for daily planning: current air quality (based on recent monitoring) and forecast air quality (predicted for next day). Current AQI helps you decide whether to take protective measures right now. Forecast AQI enables you to plan tomorrow’s activities and take preventive measures in advance.
Most U.S. forecasts are issued in afternoon and evening, providing next-day predictions based on weather forecasts and pollution modeling. Forecast accuracy typically exceeds 75%, though unexpected weather changes or emission events can reduce reliability.
FAQ: Understanding Air Quality Index Values
Q: If AQI is 100, am I in danger?
A: AQI 100 falls on the moderate-unhealthy boundary. Sensitive individuals should limit outdoor activity, but general population faces minimal risk. Exact impacts depend on which pollutant drives the AQI and your personal health status.
Q: Why does my weather app show different AQI values?
A: Different data sources use monitoring stations in different locations. Depending on your exact location, nearby monitoring stations can show variation. Use the station closest to where you spend most time.
Q: How often does AQI update?
A: Real-time AQI values update hourly based on the latest monitoring data. Most agencies update values at the top of each hour, though some stations update more frequently.
Q: What does PM2.5 in the AQI mean?
A: PM2.5 refers to particulate matter particles 2.5 micrometers or smaller. These fine particles penetrate deep into lungs and can enter the bloodstream, causing the most serious health effects.
Q: Can I rely on AQI from websites other than AirNow?
A: Yes—IQAir, AirVisual, Plume Labs, and official state/local agency websites all provide reliable AQI based on EPA monitoring data. Ensure the source uses EPA-standard AQI calculations for consistency.
Using AQI for Personal and Community Health Protection
Effective air quality management starts with understanding AQI. Check today’s AQI before planning outdoor activities. Monitor next-day forecasts to prepare for high pollution. Adjust exercise intensity and timing based on AQI categories. Share air quality information with children and elderly family members who may not independently monitor conditions.
Communities use AQI data to advocate for pollution controls. High AQI episodes trigger media attention and policy discussions. Documenting patterns of unhealthy air quality demonstrates need for stronger emission standards and traffic management.
Accurate, real-time AQI values depend on reliable ambient air quality monitoring. EMSOL provides continuous monitoring systems that feed data into AQI calculations, enabling forecasters and public health agencies to communicate air quality conditions accurately. If you operate a local government, health agency, or facility requiring AQI-quality air quality data, contact us to discuss real-time monitoring networks that support public health protection through accessible, accurate air quality information.