Understanding Air Quality: A Complete Guide to the AQI
Published: October 5, 2025 | Reading Time: 10 minutes
Air quality affects every breath you take, yet many people don't fully understand what the Air Quality Index (AQI) means or how to use it to protect their health. This comprehensive guide will help you understand air quality measurements, interpret AQI readings, and make informed decisions about outdoor activities based on current air conditions.
What is the Air Quality Index (AQI)?
The Air Quality Index is a standardized indicator developed by environmental agencies to communicate how polluted the air currently is or how polluted it is forecast to become. Think of it as a weather report for air quality—it translates complex air pollution data into a simple numerical scale that anyone can understand.
In the United States, the Environmental Protection Agency (EPA) developed the AQI system that has become the standard used by many countries worldwide. The index runs from 0 to 500, with higher numbers indicating greater levels of air pollution and greater health concerns.
The AQI is calculated based on five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant is measured separately, and the AQI value reported is based on whichever pollutant has the highest concentration at that time.
The Six AQI Categories
The AQI is divided into six categories, each corresponding to a different level of health concern. These categories are color-coded to make them easy to understand at a glance:
Good (0-50): Green
Air quality is considered satisfactory, and air pollution poses little or no risk. This is the optimal condition for outdoor activities. At this level, everyone can enjoy outdoor activities normally without any health concerns. The air is clean and fresh, making it an ideal time for exercise, sports, and other outdoor pursuits.
Moderate (51-100): Yellow
Air quality is acceptable for most people. However, unusually sensitive individuals may experience minor respiratory symptoms. The majority of people can continue normal outdoor activities, but those who are extremely sensitive to air pollution might notice minor effects. This level is still generally safe for exercise and outdoor recreation.
Unhealthy for Sensitive Groups (101-150): Orange
Members of sensitive groups may experience health effects, while the general public is less likely to be affected. Sensitive groups include children, older adults, people with respiratory diseases like asthma, and people with heart disease. At this level, sensitive individuals should consider reducing prolonged or heavy outdoor exertion. The general public can still engage in outdoor activities, but sensitive groups should take precautions.
Unhealthy (151-200): Red
Everyone may begin to experience health effects, and members of sensitive groups may experience more serious health effects. At this level, everyone should reduce prolonged or heavy outdoor exertion. People with respiratory or heart conditions should limit outdoor activities. This is when air quality becomes a concern for the general population, not just sensitive groups.
Very Unhealthy (201-300): Purple
Health alert—everyone may experience more serious health effects. At this level, everyone should avoid prolonged or heavy outdoor exertion. Sensitive groups should remain indoors and keep activity levels low. This is a significant air quality event that requires serious precautions from all members of the community.
Hazardous (301-500): Maroon
Health warnings of emergency conditions—the entire population is more likely to be affected. Everyone should avoid all outdoor exertion and remain indoors as much as possible. This represents a serious environmental health emergency. Schools and workplaces may close, and authorities typically issue health advisories.
Key Air Pollutants Explained
Understanding what causes poor air quality helps you make sense of AQI readings and their health implications:
Particulate Matter (PM2.5 and PM10)
Particulate matter consists of tiny particles suspended in the air. PM2.5 refers to fine particles with a diameter of 2.5 micrometers or less—about 30 times smaller than a human hair. PM10 includes particles up to 10 micrometers in diameter.
These particles are particularly dangerous because they can penetrate deep into your lungs and even enter your bloodstream. PM2.5 is often the pollutant that drives AQI readings in many locations because it's so prevalent and harmful.
Sources include vehicle exhaust, power plants, industrial emissions, wildfires, and even cooking. Long-term exposure to particulate matter is linked to respiratory diseases, cardiovascular problems, and reduced life expectancy.
Ground-Level Ozone
Unlike the protective ozone layer in the upper atmosphere, ground-level ozone is a harmful pollutant formed when nitrogen oxides and volatile organic compounds react in sunlight. This is why ozone levels typically peak on hot, sunny days.
Ozone can trigger asthma attacks, reduce lung function, and increase susceptibility to respiratory infections. Even healthy individuals may experience reduced lung function and throat irritation when exercising outdoors on high-ozone days.
Major sources include vehicle emissions, industrial facilities, and chemical solvents. Ozone is often worse in suburban areas downwind from urban centers, where pollutants have had time to react in sunlight.
Nitrogen Dioxide (NO2)
This reddish-brown gas primarily comes from vehicle emissions and power plants. It can irritate airways and exacerbate respiratory diseases. Long-term exposure is linked to increased risk of respiratory infections and reduced lung function, particularly in children.
Sulfur Dioxide (SO2)
Produced by burning fossil fuels, particularly coal and oil, sulfur dioxide can cause breathing problems and aggravate existing respiratory conditions. It's less commonly a concern in areas that have transitioned away from high-sulfur fuels, but it remains significant near industrial sources.
Carbon Monoxide (CO)
This colorless, odorless gas reduces oxygen delivery to the body's organs. While it's rarely the pollutant driving AQI readings in outdoor air, it's a concern in areas with heavy traffic congestion and in indoor environments with poor ventilation.
Health Effects of Poor Air Quality
The health impacts of air pollution extend far beyond the obvious respiratory effects:
Short-Term Effects
Even brief exposure to elevated air pollution can cause immediate symptoms. These may include eye, nose, and throat irritation, coughing, shortness of breath, chest tightness, and headaches. People with asthma may experience increased symptoms and may need to use their rescue inhalers more frequently.
Cardiovascular effects can occur rapidly as well. Studies have shown increased heart attacks and strokes in the days following air pollution spikes. The inflammatory response triggered by air pollution can affect blood vessels and heart function within hours of exposure.
Athletic performance suffers in polluted air. Endurance athletes notice reduced performance when training in elevated pollution levels, as the lungs must work harder to deliver oxygen to muscles while also dealing with irritation from pollutants.
Long-Term Effects
Chronic exposure to poor air quality has serious long-term health consequences. Respiratory diseases, including chronic obstructive pulmonary disease (COPD) and asthma, are more common in areas with persistent air pollution. Lung development in children can be impaired, leading to reduced lung function that persists throughout life.
Cardiovascular disease risk increases with long-term pollution exposure. The inflammatory effects of air pollution contribute to atherosclerosis (hardening of the arteries), increasing risk of heart attacks and strokes. Some research suggests life expectancy is reduced by months or even years in heavily polluted areas.
Recent studies have also linked air pollution to cognitive decline, increased risk of dementia, and even mental health effects including depression and anxiety. While more research is needed, the evidence suggests air pollution affects far more than just the lungs.
Who Is Most Vulnerable?
Certain groups face higher risks from air pollution:
Children breathe more air relative to their body size, and their developing lungs are more susceptible to damage. They're also more likely to be outdoors during high-pollution periods.
Older adults often have underlying health conditions that make them more vulnerable. The cardiovascular effects of pollution are particularly concerning for this age group.
People with respiratory diseases, including asthma and COPD, experience worsening symptoms during poor air quality episodes.
People with heart disease face increased risk of cardiac events when pollution levels rise.
Pregnant women should be cautious, as air pollution has been linked to preterm birth, low birth weight, and developmental effects in children.
Outdoor workers and athletes face higher exposures due to increased breathing rates during physical activity and more time spent outdoors.
How to Check Your Local AQI
Staying informed about air quality is easier than ever with modern technology:
Real-Time Monitoring
Many websites and apps provide current AQI readings for your location. Government environmental agencies typically offer the most reliable data. In the United States, AirNow.gov provides real-time AQI data for locations across the country.
Weather apps increasingly include AQI information alongside traditional weather forecasts. Smart home devices and virtual assistants can also provide air quality updates on request.
For the most accurate local information, check for monitoring stations near you. AQI can vary significantly even within a single city, so a reading from a nearby station will be more relevant than a city-wide average.
Understanding Forecasts
Just as you check weather forecasts to plan your activities, air quality forecasts can help you prepare for upcoming pollution episodes. Forecasts typically look ahead 24-48 hours and are most accurate for ozone pollution, which follows predictable patterns based on weather conditions.
Wildfires, dust storms, and other episodic events can be harder to forecast accurately but are usually accompanied by public health advisories when they occur.
Setting Up Alerts
Many air quality services allow you to set up alerts for your area. You can receive notifications when the AQI reaches certain thresholds, allowing you to adjust your plans accordingly. This is particularly useful for parents of children with asthma or people with respiratory conditions who need to take precautions when pollution rises.
Practical Steps to Protect Your Health
Understanding the AQI is only useful if you act on that information. Here's how to adjust your activities based on current air quality:
When AQI is Good to Moderate (0-100)
Enjoy outdoor activities freely. This is optimal time for exercise, sports, and outdoor recreation. Even sensitive groups can participate normally, though the most sensitive individuals might notice minor effects at the upper end of the moderate range.
When AQI is Unhealthy for Sensitive Groups (101-150)
If you're in a sensitive group, consider these precautions:
- Reduce the intensity or duration of outdoor exercise
- Watch for symptoms and be ready to move indoors if needed
- Take prescribed medications as directed
- Schedule outdoor activities for times when air quality is better (often early morning)
If you're not in a sensitive group, you can generally continue normal activities but remain alert for any symptoms.
When AQI is Unhealthy (151-200)
Everyone should take precautions:
- Limit prolonged outdoor exertion
- If you must be outside, reduce intensity and take frequent breaks
- Move activities indoors when possible
- Keep windows closed to prevent outdoor air from entering
- People with respiratory or heart conditions should consult their doctors about additional precautions
When AQI is Very Unhealthy or Hazardous (201-500)
Treat this as a health emergency:
- Avoid all outdoor exertion
- Stay indoors with windows and doors closed
- Run air conditioning or air purifiers with HEPA filters
- Follow emergency protocols if you have respiratory conditions
- Check on vulnerable neighbors and family members
- If you must go outside, consider wearing an N95 mask
Indoor Air Quality Matters Too
When outdoor air quality is poor, your indoor environment becomes your refuge. However, indoor air can have its own pollution sources:
Improving Indoor Air
Use air purifiers with HEPA filters to remove particulate matter. Place them in rooms where you spend the most time. Keep windows and doors closed during poor outdoor air quality episodes, but ventilate when outdoor air quality improves.
Avoid indoor pollution sources like smoking, burning candles, and using spray products. Even cooking can degrade indoor air quality, so use exhaust fans when preparing meals.
Many modern HVAC systems include air filtration. Upgrade to higher-quality filters during wildfire season or periods of persistent poor air quality. Change filters regularly according to manufacturer recommendations.
Monitoring Indoor Air
Indoor air quality monitors have become affordable and accessible. They can help you understand when indoor pollution sources are affecting your air and whether your ventilation strategy is working during outdoor air quality events.
Air Quality and Climate Change
Understanding the connection between air quality and climate change provides important context for long-term trends:
Increasing Wildfire Smoke
Climate change is extending fire seasons and increasing the severity of wildfires in many regions. This means more frequent and prolonged air quality episodes from wildfire smoke, which can affect areas hundreds of miles away from the fires themselves.
Wildfire smoke is particularly problematic because it's rich in fine particulate matter (PM2.5) that can travel long distances and penetrate indoors. Communities that historically had good air quality are increasingly experiencing smoke episodes.
Heat and Ozone
Higher temperatures accelerate the chemical reactions that create ground-level ozone. As climate change brings more frequent and intense heat waves, ozone pollution is likely to worsen in many areas, particularly during summer months.
Regional Variations
While air quality has improved in many developed countries over recent decades due to pollution controls, climate change may partially offset these gains. Some regions may see more frequent poor air quality days despite overall reductions in emissions.
Taking Action Beyond Personal Protection
While protecting yourself and your family is important, broader action can improve air quality for everyone:
Reduce Your Contribution
Transportation is a major source of air pollution. Consider walking, biking, carpooling, or using public transit. If you drive, combine trips and maintain your vehicle properly. Electric vehicles produce zero tailpipe emissions, significantly reducing local air pollution.
Energy use in homes and buildings contributes to power plant emissions. Improve energy efficiency, use renewable energy when possible, and adjust thermostats to reduce energy consumption.
Community Involvement
Support policies that improve air quality, including clean energy standards, public transit investment, and emissions regulations. Participate in local planning processes to advocate for development patterns that reduce vehicle dependence.
Stay informed about local pollution sources and join community efforts to address them. Industrial facilities, busy roadways, and other significant pollution sources may require collective action to address.
Advocate for Vulnerable Populations
Not everyone has equal access to clean air. Lower-income communities and communities of color often experience higher pollution exposure. Support environmental justice efforts to ensure everyone has access to clean air.
Conclusion
The Air Quality Index is a powerful tool that transforms complex atmospheric science into actionable information. By understanding what the AQI means and how to respond to different air quality conditions, you can protect your health and make informed decisions about outdoor activities.
Check your local AQI regularly, especially during seasons when air quality is typically poor in your area. Adjust your activities based on current conditions, and take extra precautions if you're in a sensitive group. Remember that even healthy individuals can be affected by poor air quality, particularly during exercise or prolonged outdoor exposure.
As climate change and other factors continue to influence air quality, staying informed and taking appropriate precautions becomes increasingly important. The air you breathe profoundly affects your health—use the tools available to ensure every breath supports your well-being.
Frequently Asked Questions
Q: Is the AQI the same everywhere in the world?
A: While the EPA's AQI system is widely used, some countries use different scales or calculation methods. China, India, and Europe, for example, use their own air quality indices. The basic principles are similar—higher numbers indicate worse air quality—but the specific thresholds and categories may differ. When traveling, familiarize yourself with the local air quality index system.
Q: Can indoor plants improve air quality?
A: While indoor plants can absorb some pollutants, research shows their effect is minimal compared to proper ventilation and mechanical air filtration. You would need an unrealistic number of plants to match the effectiveness of even a modest air purifier. That said, plants offer other benefits and can be part of a comprehensive approach to healthy indoor environments, but don't rely on them as your primary air quality strategy.
Q: Why does the AQI seem different on different weather apps?
A: Different apps may pull data from different monitoring stations, update at different intervals, or use slightly different calculation methods. For the most reliable information, use official government sources like AirNow.gov in the US. If you notice significant discrepancies, check which monitoring station each app is using—you may be seeing readings from different locations.
Q: Should I exercise indoors when the AQI is moderate (51-100)?
A: For most people, moderate air quality doesn't require moving exercise indoors. However, if you're in a sensitive group (children, older adults, people with respiratory or heart conditions), or if you're at the higher end of the moderate range (approaching 100), consider reducing intensity or duration of outdoor exercise, or move strenuous workouts indoors. Listen to your body—if you notice symptoms like coughing or shortness of breath beyond normal exertion, that's a sign to move inside.