Understanding AQI: What the Numbers Really Mean

What Is the Air Quality Index?

The Air Quality Index, or AQI, is the standardized system used in the United States to communicate how clean or polluted the air is. Developed by the Environmental Protection Agency (EPA), the AQI transforms complex air quality data into a simple, easy-to-understand scale that helps you make informed decisions about outdoor activities.

Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. It's designed to help you understand what local air quality means for your health.

How the AQI Scale Works

The AQI is divided into six color-coded categories, each representing a different level of health concern:

Green: Good (0-50) Air quality is satisfactory, and air pollution poses little or no risk. This is the ideal air quality for everyone, including outdoor exercise, activities with children, and people with respiratory conditions.

What you can do: Enjoy outdoor activities freely. This is a great time for running, cycling, outdoor sports, and playground time with kids.

Yellow: Moderate (51-100) Air quality is acceptable. However, there may be a risk for some people who are unusually sensitive to air pollution. Most people won't be affected at this level.

What you can do: Most people can continue normal activities. If you're unusually sensitive to pollution (you know who you are—you notice air quality before others do), consider reducing prolonged outdoor exertion.

Orange: Unhealthy for Sensitive Groups (101-150) Members of sensitive groups may experience health effects. The general public is less likely to be affected. "Sensitive groups" include: - People with asthma or other respiratory conditions - People with heart disease - Children under 14 - Adults over 65 - People who work or exercise vigorously outdoors

What you can do: Sensitive groups should limit prolonged outdoor exertion. Everyone else can generally continue normal activities but should watch for symptoms.

Red: Unhealthy (151-200) Everyone may begin to experience health effects. Members of sensitive groups may experience more serious health effects. This is when the general population starts to notice the air quality.

What you can do: Everyone should reduce prolonged or heavy exertion outdoors. Sensitive groups should avoid prolonged outdoor exertion entirely. Consider moving activities indoors.

Purple: Very Unhealthy (201-300) Health alert: The risk of health effects is increased for everyone. At this level, air quality is a significant health concern for the entire population.

What you can do: Everyone should avoid prolonged or heavy outdoor exertion. Sensitive groups should avoid all outdoor exertion. Stay indoors as much as possible with windows closed.

Maroon: Hazardous (301-500) Health warning of emergency conditions: everyone is more likely to be affected. This level triggers health warnings and may prompt official advisories to stay indoors.

What you can do: Everyone should avoid all outdoor physical activity. Remain indoors and keep activity levels low. Run air purifiers if available. This is a genuine health emergency.

What Pollutants Does the AQI Measure?

The AQI is calculated for five major air pollutants regulated by the Clean Air Act:

Ground-Level Ozone (O3) Ozone at ground level is a harmful pollutant created when sunlight reacts with emissions from vehicles and industry. It's the main component of smog and typically peaks in afternoon summer hours.

Particulate Matter (PM2.5 and PM10) - **PM2.5** - Fine particles smaller than 2.5 micrometers. These can penetrate deep into lungs and even enter the bloodstream. Major sources include vehicle exhaust, power plants, and wildfire smoke. - **PM10** - Coarse particles between 2.5 and 10 micrometers. Sources include dust, pollen, and construction activity.

Carbon Monoxide (CO) A colorless, odorless gas from incomplete combustion. High levels typically occur near busy roadways during rush hours.

Sulfur Dioxide (SO2) Produced by burning fossil fuels, especially coal. Can aggravate asthma and other respiratory conditions.

Nitrogen Dioxide (NO2) Primarily from vehicle emissions. Contributes to smog formation and can irritate airways.

How the AQI Is Calculated

The AQI is calculated separately for each pollutant. The highest individual AQI value becomes the overall AQI for that time period. This means if PM2.5 has an AQI of 75 and ozone has an AQI of 95, the reported AQI is 95.

Each pollutant has specific concentration breakpoints that correspond to AQI values. For example, for PM2.5: - 0-12 μg/m³ = AQI 0-50 (Good) - 12.1-35.4 μg/m³ = AQI 51-100 (Moderate) - 35.5-55.4 μg/m³ = AQI 101-150 (Unhealthy for Sensitive Groups) - And so on...

When you see an AQI report, it typically shows both the overall AQI and which pollutant is responsible for that value.

Why the AQI Matters for Your Health

Air pollution affects health in both the short term and long term:

Short-Term Effects (Hours to Days) - Eye, nose, and throat irritation - Coughing and wheezing - Shortness of breath - Chest tightness - Aggravation of existing respiratory conditions - Reduced lung function - Increased asthma attacks

Long-Term Effects (Months to Years) - Development of chronic respiratory diseases - Accelerated aging of the lungs - Decreased lung function - Reduced life expectancy - Increased risk of heart disease - Possible link to neurological effects

How to Use AQI Information

Check Regularly Make checking the AQI part of your daily routine, just like checking the weather. Air quality can vary significantly from day to day and even hour to hour.

Know Your Sensitivity If you or family members fall into sensitive groups, be more cautious. Track how you feel on different AQI days to understand your personal threshold.

Plan Activities Schedule strenuous outdoor activities for days with good air quality. On poor air quality days, shift to indoor activities or reduce intensity.

Understand Local Patterns Learn when air quality tends to be best in your area: - Ozone is usually lowest in early morning - PM2.5 from traffic peaks during rush hours - Temperature inversions can trap pollution, especially in valleys

Take Action at Higher Levels As AQI increases, take progressively stronger precautions: - Reduce outdoor time - Avoid strenuous outdoor activity - Keep windows closed - Use air purifiers indoors - Wear N95 masks if outdoor exposure is unavoidable

AQI Limitations to Understand

The AQI is a valuable tool, but understand its limitations:

1. Monitoring gaps - AQI is only as good as the monitoring network. Some areas have limited monitoring.

1. Averaging - AQI values are based on averaging periods (1-hour for ozone, 24-hour for PM2.5). Short-term spikes may not be captured.

1. Doesn't capture all pollutants - Some harmful pollutants aren't included in the AQI calculation.

1. Sensitivity varies - The category thresholds are population-based averages. Your personal sensitivity may be higher or lower.

For detailed, real-time air quality information in your area, explore our city air quality pages and view state-by-state data for all US states. You can also check our AQI FAQ for more common questions.