Key Takeaways
- AQI ranges from 0-500, with higher values indicating worse air quality and greater health risks
- PM2.5 (fine particulate matter) is the most dangerous common pollutant because it can penetrate deep into lungs and bloodstream
- AQI above 100 poses health risks for sensitive groups including children, elderly, and those with respiratory conditions
- The EPA defines 6 AQI categories from "Good" (0-50) to "Hazardous" (301-500) with color-coded warnings
- Indoor air quality can be 2-5x worse than outdoor air in many buildings without proper filtration
- Check AQI daily before outdoor activities, especially during wildfire season or in urban areas with heavy traffic
What Is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a standardized measurement system 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 for the general population. The AQI translates complex air quality monitoring data into a simple, easy-to-understand number ranging from 0 to 500, making it accessible for everyone to assess current air pollution levels in their area.
Think of the AQI as a "yardstick" for air quality that works similar to a thermometer for temperature. Just as you might check the temperature forecast before deciding what to wear, checking the AQI helps you decide whether it is safe to engage in outdoor activities. An AQI value of 50 or below represents good air quality with minimal potential to affect public health, while values above 300 represent hazardous conditions that pose serious health risks for the entire population.
The AQI is calculated separately for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The highest AQI value among these pollutants becomes the official AQI for that location, ensuring that public health warnings reflect the worst current air quality condition.
Step-by-Step Guide: How to Calculate AQI
Understanding how to calculate the Air Quality Index helps you interpret readings from monitoring stations and personal air quality sensors. Follow these steps to calculate AQI from pollutant concentration measurements.
Select the Pollutant Type
Choose which pollutant you are measuring. The most common is PM2.5, but you may also measure PM10, ozone, carbon monoxide, sulfur dioxide, or nitrogen dioxide. Each pollutant has different concentration units and breakpoint tables.
Measure or Obtain the Concentration
Get the pollutant concentration from an air quality monitor, weather service, or local monitoring station. For PM2.5, this is typically measured in micrograms per cubic meter (ug/m3). Ensure you are using the correct averaging period (24-hour average for PM2.5, 8-hour for ozone).
Find the Breakpoint Range
Look up the EPA breakpoint table for your pollutant to find which concentration range your measurement falls into. Each range corresponds to an AQI category (Good, Moderate, Unhealthy for Sensitive Groups, etc.).
Apply the AQI Formula
Use the EPA linear interpolation formula to calculate the exact AQI value within that category range. The formula converts your concentration to a standardized index value between 0 and 500.
Interpret Results and Take Action
Based on the calculated AQI, determine the appropriate health precautions. Values 0-50 are good, 51-100 moderate, and anything above 100 requires limiting outdoor activities for sensitive groups.
Understanding AQI Categories
The EPA established six distinct AQI categories, each associated with a specific level of health concern and corresponding actions to protect your health. These categories use a universal color-coding system recognized worldwide.
| 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 may experience more serious effects |
| 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 measured pollutant concentrations to a standardized index value. Each pollutant has its own set of breakpoints defined by the EPA based on health research.
AQI = ((Ihigh - Ilow) / (Chigh - Clow)) x (C - Clow) + Ilow
Example Calculation: PM2.5
If you measure a PM2.5 concentration of 35.5 ug/m3, here is how to calculate the AQI:
Clow = 35.5, Chigh = 55.4, Ilow = 101, Ihigh = 150
AQI = ((150-101)/(55.4-35.5)) x (35.5-35.5) + 101 = 101
Result: AQI 101 (Unhealthy for Sensitive Groups)
Key Air Pollutants Measured by AQI
PM2.5 - Fine Particulate Matter
PM2.5 refers to particles less than 2.5 micrometers in diameter, approximately 30 times smaller than a human hair. These microscopic particles can bypass the body's natural defenses, penetrate deep into the lungs, and even enter the bloodstream. Major sources include vehicle exhaust, power plants, wildfires, and industrial processes. PM2.5 is considered the most dangerous common pollutant due to its direct link to cardiovascular disease, respiratory illness, and premature death.
PM10 - Coarse Particulate Matter
PM10 includes particles between 2.5 and 10 micrometers in diameter. Common sources include dust from roads, construction sites, agricultural operations, and windblown soil. While larger than PM2.5 and typically filtered by the nose and throat, PM10 can still aggravate asthma, cause coughing, and irritate airways, especially during prolonged exposure.
Ozone (O3)
Ground-level ozone forms when pollutants from vehicles, power plants, and industrial facilities react chemically with sunlight. Unlike the beneficial ozone layer in the upper atmosphere, ground-level ozone is a respiratory irritant that can trigger asthma attacks, reduce lung function, and cause chest pain. Ozone levels typically peak on hot, sunny afternoons in urban areas.
Carbon Monoxide (CO)
Carbon monoxide is an odorless, colorless gas produced by incomplete combustion of fossil fuels. At elevated levels, CO reduces oxygen delivery to organs and tissues, causing headaches, dizziness, confusion, and in severe cases, death. Vehicle exhaust and poorly ventilated heating systems are primary sources.
Sulfur Dioxide (SO2)
Sulfur dioxide is produced primarily by burning fossil fuels containing sulfur, particularly coal and heavy oil. Short-term exposure can cause bronchoconstriction and aggravate existing respiratory and cardiovascular disease. SO2 also contributes to acid rain formation.
Nitrogen Dioxide (NO2)
Nitrogen dioxide comes mainly from motor vehicle exhaust and power plants. It irritates airways and can increase susceptibility to respiratory infections. NO2 also contributes to the formation of ground-level ozone and fine particulate matter.
Pro Tip: Understanding Multi-Pollutant AQI
When multiple pollutants are measured at a monitoring station, the overall AQI is determined by the highest individual pollutant AQI. This ensures the reported value always reflects the worst air quality condition present, providing maximum protection for public health. Many cities also report the "responsible pollutant" alongside the AQI.
Real-World AQI Examples
Understanding how AQI values translate to real-world conditions helps you make informed decisions about outdoor activities.
Good Air Quality Day (AQI 25)
On a clear day with light winds and no nearby pollution sources, you might see an AQI of 25. This is perfect weather for jogging, cycling, or any outdoor activity. Even sensitive individuals can exercise freely. This is typical for coastal areas with onshore breezes or after rainfall clears the air.
Moderate Smog Day (AQI 85)
During summer in a major city with heavy traffic, AQI often reaches 85 during afternoon rush hour. Most people can continue normal activities, but unusually sensitive individuals may notice mild respiratory symptoms. Consider exercising in the morning when ozone levels are lower.
Wildfire Smoke Event (AQI 175)
When wildfire smoke drifts into your area, AQI can quickly spike to 175 or higher. At this level, everyone should reduce prolonged outdoor exertion. Sensitive groups should avoid outdoor activities entirely. Keep windows closed, run air purifiers indoors, and consider wearing an N95 mask if you must go outside.
Common Mistakes to Avoid
- Ignoring AQI during exercise: Heavy breathing during workouts increases pollutant intake 10-20x compared to rest. Always check AQI before outdoor exercise.
- Assuming indoor air is safe: Without filtration, outdoor pollutants easily enter buildings. Indoor AQI can match or exceed outdoor levels during pollution events.
- Using cloth masks for smoke: Regular cloth or surgical masks provide minimal protection against PM2.5. Only N95/KN95 respirators filter fine particles effectively.
- Confusing AQI with pollen counts: AQI measures air pollution, not allergens. You can have good AQI but high pollen, or vice versa.
- Ignoring averaging periods: Official AQI uses 24-hour or 8-hour averages. Real-time sensors may show higher momentary spikes that do not reflect sustained exposure risk.
Who Are Sensitive Groups?
The EPA identifies several populations as particularly vulnerable to air pollution health effects. These groups should take extra precautions when AQI exceeds 100:
- Children and teenagers: Their lungs are still developing, they breathe more air relative to body size, and they spend more time outdoors playing and exercising
- Older adults (65+): May have undiagnosed respiratory or cardiovascular conditions that increase susceptibility
- People with asthma: Airways are hyperreactive to air pollution, leading to increased attacks and symptoms
- People with heart or lung disease: Pollution can trigger angina, arrhythmias, heart attacks, and COPD exacerbations
- Pregnant women: Air pollution exposure linked to low birth weight, preterm birth, and developmental effects
- Outdoor workers: Extended exposure during work hours significantly increases health risks
- Athletes and active people: Deeper, faster breathing brings more pollutants into the lungs
Important Health Warning
When AQI exceeds 100, sensitive groups should reduce prolonged outdoor exertion. Above 150, everyone should limit outdoor activities. At "Very Unhealthy" levels (201-300), avoid all outdoor exertion. At "Hazardous" levels (301+), stay indoors with air filtration running, seal windows and doors, and avoid all outdoor activities.
How to Protect Yourself from Poor Air Quality
Check AQI Before Going Outdoors
Make checking the AQI part of your daily routine, just like checking the weather forecast. Most weather apps now include real-time AQI readings, or you can check AirNow.gov for official EPA monitoring data. Set up AQI alerts on your phone to receive notifications when air quality deteriorates.
Time Your Outdoor Activities
Air quality typically varies throughout the day. Ozone levels usually peak in the afternoon on hot, sunny days. PM2.5 from traffic peaks during rush hours. Schedule exercise and outdoor activities for early morning when pollution levels are typically lowest.
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. During smoke events, run purifiers continuously in bedrooms and living areas.
Keep Windows Closed During Poor Air Quality
While ventilation is normally important for indoor air quality, keeping windows closed during high AQI events prevents outdoor pollution from entering your home. Use recirculated air settings on HVAC systems and avoid using window fans that draw in outside air.
Wear N95 Masks When Necessary
During severe air quality events (AQI above 200), properly fitted N95 or KN95 masks can filter out harmful particles. Ensure the mask seals tightly around your nose and mouth. Note that cloth masks and surgical masks do not provide adequate protection from air pollution particles.
Pro Tip: Create a Clean Air Room
Designate one room in your home as a clean air refuge during smoke or pollution events. Choose a room with few windows, seal gaps with towels or weather stripping, and run a HEPA air purifier sized for that room. This provides a safe space to sleep and spend time during extended poor air quality periods.
Frequently Asked Questions
For most healthy adults, an AQI of 0-50 (Good) is ideal for all outdoor exercise including intense activities like running and cycling. At 51-100 (Moderate), most people can exercise normally, but unusually sensitive individuals should consider reducing prolonged outdoor exertion. Above 100, sensitive groups should limit outdoor activity. Above 150, everyone should reduce prolonged outdoor exertion and consider moving workouts indoors. At 200+, avoid vigorous outdoor activity entirely.
PM2.5 particles are so small (less than 2.5 micrometers) that they bypass the body's natural defenses like nose hairs and mucus, penetrate deep into the lungs, and can even cross into the bloodstream. From there, they can affect the heart, brain, and other organs. Long-term exposure to elevated PM2.5 is linked to increased risk of heart attacks, strokes, lung cancer, and reduced life expectancy. Even short-term exposure can trigger asthma attacks, heart arrhythmias, and respiratory symptoms.
Health effects can occur within minutes to hours of exposure to high pollution levels. Short-term exposure to elevated AQI can cause immediate symptoms including coughing, throat irritation, eye burning, shortness of breath, and chest tightness. Sensitive individuals may experience asthma attacks or heart symptoms within an hour of exposure. Long-term health effects like cardiovascular disease, reduced lung function, and cancer risk develop from chronic exposure over months to years.
Wildfire smoke contains a complex mixture of gases and fine particles from burning vegetation, structures, and other materials. The primary health concern is PM2.5, which can reach extremely dangerous levels during active fires. Smoke can travel hundreds or even thousands of miles, affecting air quality in regions far from the fire itself. During major wildfire events, AQI can exceed 500 (considered "Beyond the AQI") creating emergency health conditions that require everyone to stay indoors with air filtration.
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 building materials, and poor ventilation. However, during high outdoor AQI events like wildfires or severe smog, staying indoors with windows closed and running HEPA air purifiers provides significant protection. The key is maintaining good indoor air quality through proper filtration, ventilation when outdoor air is clean, and reducing indoor pollution sources.
EPA monitoring stations typically report hourly data, but the official daily AQI is usually calculated as a 24-hour average for PM2.5 and PM10, and 8-hour averages for ozone and carbon monoxide. The NowCast AQI algorithm provides more responsive hourly readings by weighting recent hours more heavily, giving a better picture of current conditions. Consumer-grade air quality sensors and apps may update every few minutes but should be considered estimates rather than official readings.
Yes, HEPA air purifiers are highly effective at removing PM2.5 and other particles from indoor air. Studies show that running a HEPA purifier can reduce indoor PM2.5 concentrations by 50-80% during smoke events. Choose a purifier with a Clean Air Delivery Rate (CADR) appropriate for your room size. For a bedroom, aim for a CADR of at least 100 CFM. Run the purifier continuously on high during poor air quality events, and place it in rooms where you spend the most time.
AQI and pollen count measure completely different things. AQI measures chemical air pollution from combustion, industrial processes, and reactions in the atmosphere (ozone, PM2.5, etc.). Pollen count measures biological particles from plants that trigger allergies. You can have excellent air quality (low AQI) but high pollen counts during spring, or poor air quality during wildfires with low pollen. Allergy sufferers should check both measurements before planning outdoor activities.