Key Takeaways
- AQI ranges from 0-500, with higher values indicating worse air quality
- PM2.5 (fine particulate matter) is the most dangerous common pollutant
- AQI above 100 poses health risks for sensitive groups (children, elderly, respiratory conditions)
- The EPA defines 6 AQI categories from "Good" (0-50) to "Hazardous" (301-500)
- Indoor air quality can be 2-5x worse than outdoor air in many buildings
What Is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a standardized measurement developed by the U.S. Environmental Protection Agency (EPA) to communicate how polluted the air is and what associated health effects might be a concern. The AQI translates complex air quality data into a simple number from 0 to 500, making it easy for the public to understand current air pollution levels.
Think of the AQI as a "yardstick" for air quality. Just as you might check the temperature before going outside, the AQI tells you whether it's safe to breathe the outdoor air. An AQI value of 50 represents good air quality with little potential to affect public health, while an AQI value over 300 represents hazardous air quality that poses serious health risks for the entire population.
Understanding AQI Categories
| AQI Range | Category | Color | Health Implications |
|---|---|---|---|
| 0-50 | Good | Green | Air quality is satisfactory, and air pollution poses little or no risk |
| 51-100 | Moderate | Yellow | Air quality is acceptable; some pollutants may pose moderate health concern for very few people |
| 101-150 | Unhealthy for Sensitive Groups | Orange | Members of sensitive groups may experience health effects; general public less likely affected |
| 151-200 | Unhealthy | Red | Everyone may begin to experience health effects; sensitive groups more seriously affected |
| 201-300 | Very Unhealthy | Purple | Health alert: everyone may experience more serious health effects |
| 301-500 | Hazardous | Maroon | Health warnings of emergency conditions; entire population likely affected |
How AQI Is Calculated: The EPA Formula
The AQI is calculated using a piecewise linear function that converts pollutant concentrations to a standardized index value. Each pollutant has its own set of breakpoints defined by the EPA.
AQI = ((Ihigh - Ilow) / (Chigh - Clow)) x (C - Clow) + Ilow
Key Air Pollutants Measured by AQI
PM2.5 - Fine Particulate Matter
PM2.5 refers to particles less than 2.5 micrometers in diameter (about 30 times smaller than a human hair). These particles can penetrate deep into the lungs and even enter the bloodstream. Sources include vehicle exhaust, power plants, wildfires, and industrial processes. PM2.5 is considered the most dangerous common pollutant due to its ability to cause cardiovascular and respiratory diseases.
PM10 - Coarse Particulate Matter
PM10 includes particles between 2.5 and 10 micrometers in diameter. Sources include dust from roads, construction sites, and agricultural operations. While less dangerous than PM2.5, PM10 can still cause respiratory problems, especially for those with asthma or other lung conditions.
Ozone (O3)
Ground-level ozone is formed when pollutants from cars, power plants, and industrial facilities react with sunlight. Unlike the protective ozone layer in the upper atmosphere, ground-level ozone is harmful to breathe and causes respiratory problems, particularly on hot, sunny days.
Carbon Monoxide (CO)
Carbon monoxide is an odorless, colorless gas produced by incomplete combustion of fossil fuels. High levels can reduce oxygen delivery to the body's organs and tissues, causing headaches, dizziness, and in extreme cases, death.
Sulfur Dioxide (SO2)
Sulfur dioxide is produced primarily by burning fossil fuels containing sulfur, particularly coal. It can cause respiratory problems and aggravate existing heart and lung diseases.
Nitrogen Dioxide (NO2)
Nitrogen dioxide comes mainly from motor vehicle exhaust and power plants. It contributes to the formation of ground-level ozone and fine particulate matter, and can cause respiratory problems on its own.
Pro Tip: Understanding Multi-Pollutant AQI
When multiple pollutants are measured, the overall AQI is determined by the highest individual pollutant AQI. This ensures the reported value reflects the worst air quality condition present, providing maximum protection for public health.
Who Are Sensitive Groups?
The EPA identifies several groups as particularly vulnerable to air pollution:
- Children and teenagers - Their lungs are still developing, and they breathe more air relative to body size
- Older adults - May have undiagnosed respiratory or cardiovascular conditions
- People with asthma - Airways are more reactive to air pollution
- People with heart or lung disease - Existing conditions can be worsened
- Outdoor workers - Higher exposure due to extended time outdoors
- Athletes and active people - Deeper breathing brings more pollution into lungs
Important Health Warning
When AQI exceeds 100, sensitive groups should reduce prolonged outdoor exertion. Above 150, everyone should limit outdoor activities. At "Hazardous" levels (301+), all outdoor activities should be avoided and indoor air filtration is recommended.
How to Protect Yourself from Poor Air Quality
Check AQI Before Going Outdoors
Make checking the AQI part of your daily routine, especially during wildfire season or in urban areas. Many weather apps now include AQI readings, or you can check AirNow.gov for real-time data.
Reduce Outdoor Exertion When AQI Is High
During high AQI days, consider moving workouts indoors, walking instead of running, or exercising during cooler parts of the day when ozone levels are typically lower.
Use Air Purifiers Indoors
HEPA air purifiers can remove PM2.5 and other particles from indoor air. Look for purifiers with a Clean Air Delivery Rate (CADR) appropriate for your room size.
Keep Windows Closed During Poor Air Quality
While ventilation is normally important, keeping windows closed during high AQI events prevents outdoor pollution from entering your home.
Wear N95 Masks When Necessary
During severe air quality events (AQI above 200), properly fitted N95 or KN95 masks can filter out harmful particles. Cloth masks and surgical masks do not provide adequate protection from air pollution.
Frequently Asked Questions
For most people, an AQI of 0-50 (Good) is ideal for outdoor exercise. At 51-100 (Moderate), unusually sensitive individuals should consider reducing prolonged exertion. Above 100, sensitive groups should limit outdoor activities, and above 150, everyone should reduce prolonged outdoor exertion.
PM2.5 particles are so small (less than 2.5 micrometers) that they can bypass the body's natural defenses, penetrate deep into the lungs, and even enter the bloodstream. This allows them to affect the heart, brain, and other organs. Long-term exposure is linked to heart attacks, strokes, lung cancer, and reduced life expectancy.
Health effects can occur within minutes to hours of exposure. Short-term exposure to high AQI can cause immediate symptoms like coughing, throat irritation, and shortness of breath. Sensitive individuals may experience asthma attacks or heart symptoms. Long-term health effects develop from chronic exposure over months to years.
Wildfire smoke contains a mixture of gases and fine particles from burning vegetation and structures. The primary concern is PM2.5, which can reach extremely high levels during active fires. Smoke can travel hundreds of miles, affecting air quality far from the fire itself. During major wildfires, AQI can exceed 500, which is considered "Beyond the AQI" and extremely hazardous.
Not always. Indoor air can be 2-5 times more polluted than outdoor air due to cooking emissions, cleaning products, off-gassing from furniture, and poor ventilation. However, during high outdoor AQI events like wildfires, staying indoors with windows closed and running air purifiers provides significant protection. The key is maintaining good indoor air quality through filtration and ventilation when outdoor air is clean.
EPA monitoring stations typically report hourly data, but the official AQI is usually calculated as a 24-hour average for PM2.5 and PM10, and 8-hour averages for ozone. NowCast AQI provides more responsive hourly readings by weighting recent hours more heavily. Real-time sensors and apps may update more frequently but should be considered estimates.