Air Quality Frequently Asked Questions

Expert answers to common questions about air quality, the EPA Air Quality Index (AQI), pollutants like PM2.5 and ozone, and how to protect your health from air pollution.

Data Source: All air quality information on this page is based on official guidance from the U.S. Environmental Protection Agency (EPA) and the AirNow.gov program.

Air Quality Index (AQI) Basics

Understanding the EPA Air Quality Index scale and what it means for your health.

QWhat 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. The AQI runs on a scale from 0 to 500, where lower numbers indicate cleaner air. An AQI of 50 or below is considered good, while values above 100 become increasingly unhealthy. The AQI is calculated for five major pollutants: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide.

QHow is the AQI calculated?

The AQI is calculated by measuring concentrations of five major air pollutants at monitoring stations across the country. Each pollutant concentration is converted to an AQI value using EPA-established breakpoint tables. The highest AQI value among all measured pollutants becomes the reported AQI for that location, and that pollutant is identified as the "primary" or "dominant" pollutant. For example, if ozone has an AQI of 85 and PM2.5 has an AQI of 65, the reported AQI would be 85 with ozone as the primary pollutant.

QHow often is the AQI updated?

The AQI is updated hourly based on real-time data from EPA monitoring stations across the United States. The EPA AirNow system collects data from over 2,000 monitoring stations operated by federal, state, local, and tribal agencies. During significant air quality events like wildfires, updates may be issued more frequently. Air quality forecasts are typically issued twice daily, in the morning and afternoon, to help you plan outdoor activities.

QWhat AQI level is safe for outdoor activities?

An AQI of 0-50 (Green/Good) is considered safe for all outdoor activities with no restrictions. An AQI of 51-100 (Yellow/Moderate) is acceptable for most people, though unusually sensitive individuals should watch for symptoms. Once AQI reaches 101-150 (Orange/Unhealthy for Sensitive Groups), children, older adults, and people with respiratory or heart conditions should limit prolonged outdoor exertion. At AQI 151+ (Red/Unhealthy), everyone should reduce outdoor activity.

QWhat do the AQI colors mean?

The EPA uses six color-coded categories to make air quality easy to understand at a glance. Green (0-50) means good air quality. Yellow (51-100) indicates moderate conditions. Orange (101-150) is unhealthy for sensitive groups. Red (151-200) is unhealthy for everyone. Purple (201-300) represents very unhealthy conditions. Maroon (301-500) indicates hazardous, emergency-level air quality. These colors appear on maps, apps, and forecasts to help you quickly assess conditions.

QCan air quality vary within the same city?

Yes, air quality can vary significantly within a single city or metropolitan area. Factors that cause local variation include proximity to highways and major roads, industrial facilities, geographic features like valleys that trap pollution, elevation differences, and local weather patterns. Areas near busy intersections or industrial zones typically have worse air quality than parks or residential areas. This is why it is important to check AQI data from monitoring stations closest to your specific location.

Air Pollutants Explained

Learn about PM2.5, ozone, and other common air pollutants that affect your health.

QWhat is PM2.5 and why is it dangerous?

PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometers or smaller, about 30 times smaller than a human hair. These microscopic particles are especially dangerous because they can penetrate deep into your lungs and even enter your bloodstream, causing cardiovascular and respiratory problems. PM2.5 comes from vehicle exhaust, power plants, wildfires, industrial processes, and wood burning. According to the EPA, PM2.5 is one of the most significant threats to public health from air pollution.

QWhat is the difference between PM2.5 and PM10?

PM2.5 (fine particles) are 2.5 micrometers or smaller, while PM10 (coarse particles) include particles up to 10 micrometers. PM2.5 is considered more dangerous because its smaller size allows it to penetrate deeper into the respiratory system and potentially enter the bloodstream. PM10 sources include dust from roads, construction sites, agriculture, and natural sources like pollen. Both can cause health problems, but PM2.5 has been linked to more serious cardiovascular and respiratory effects.

QWhat causes ground-level ozone pollution?

Ground-level ozone, commonly known as smog, forms when nitrogen oxides (NOx) from vehicles and power plants react with volatile organic compounds (VOCs) in the presence of sunlight and heat. This is why ozone pollution is typically worse on hot, sunny summer afternoons. Unlike the protective ozone layer in the upper atmosphere, ground-level ozone is harmful to breathe. It can trigger asthma attacks, reduce lung function, and cause chest pain and coughing. Ozone levels usually decrease at night when the sun sets.

QIs wildfire smoke more dangerous than regular air pollution?

Yes, wildfire smoke is generally more harmful than typical urban pollution. Wildfire smoke contains extremely high concentrations of PM2.5, along with toxic gases and chemicals from burned vegetation and structures. During major wildfire events, AQI can spike into the "Hazardous" range (300+), which is far worse than typical city pollution that usually stays below 150. Wildfire smoke particles may also carry additional toxic compounds that make them more harmful than particles from other sources. The EPA recommends staying indoors during wildfire smoke events.

QWhat other pollutants does the AQI measure?

Besides PM2.5 and ozone, the AQI measures four additional pollutants: Carbon monoxide (CO) is a colorless, odorless gas from vehicle exhaust and fuel burning. Sulfur dioxide (SO2) comes primarily from power plants burning coal and oil. Nitrogen dioxide (NO2) is produced by burning fuel, especially from vehicles and power plants. PM10 includes larger particles from dust, pollen, and mold. Each pollutant has different health effects and sources, which is why the AQI reports the "dominant pollutant" that is most elevated at any given time.

Health Effects & Protection

How air pollution affects your health and what you can do to protect yourself.

QWho is most at risk from air pollution?

The EPA identifies several "sensitive groups" who are most vulnerable to air pollution: children under 18 (whose lungs are still developing), adults 65 and older, people with asthma, COPD, or other lung diseases, people with heart disease or diabetes, pregnant women (air pollution can affect fetal development), and people who work or exercise outdoors. These groups should pay close attention to AQI forecasts and take precautions at lower pollution levels than the general population.

QCan air pollution affect healthy people with no respiratory conditions?

Yes. While sensitive groups experience effects at lower pollution levels, healthy people can also be affected by air pollution, especially during prolonged exposure or at higher AQI levels. Common symptoms include throat irritation, coughing, shortness of breath, reduced lung function, and increased fatigue during exercise. Studies have shown that even healthy adults can experience temporary inflammation and reduced lung capacity during high pollution events. Long-term exposure may increase the risk of developing respiratory and cardiovascular diseases.

QDo face masks protect against air pollution?

N95 and KN95 respirator masks are highly effective at filtering PM2.5 and other fine particles when worn correctly with a tight seal around the face. They can filter at least 95% of airborne particles. However, surgical masks and cloth face coverings provide minimal protection against air pollution particles because they do not seal around the face and are not designed to filter fine particles. For effective protection during poor air quality, ensure your N95/KN95 mask fits snugly with no gaps around the edges.

QDo air purifiers really help with indoor air quality?

Yes, portable air purifiers with true HEPA filters are very effective at removing PM2.5 and other airborne particles from indoor air. To be effective, the purifier must be properly sized for your room (check the CADR rating), run continuously during poor air quality events, and have filters replaced according to manufacturer recommendations. Note that HEPA purifiers do not remove gases like ozone or carbon monoxide, only particles. For whole-home protection, consider upgrading your HVAC filter to MERV-13 or higher.

QWhat should I do if I have asthma and air quality is poor?

People with asthma should follow their asthma action plan during poor air quality days. Key steps include: staying indoors as much as possible with windows and doors closed, using air conditioning or running HEPA air purifiers, keeping your rescue inhaler readily accessible, avoiding strenuous activities that increase breathing rate, monitoring symptoms closely, and contacting your healthcare provider if symptoms worsen despite precautions. If you experience severe symptoms like difficulty breathing, chest tightness, or your rescue inhaler is not helping, seek medical attention immediately.

QCan I exercise outdoors when air quality is unhealthy?

During unhealthy air quality (AQI 151+), you should avoid outdoor exercise entirely. At moderate levels (AQI 51-100), healthy individuals can usually exercise normally, but should monitor for symptoms. At "Unhealthy for Sensitive Groups" (AQI 101-150), reduce intensity and duration of outdoor workouts. If you must exercise on poor air quality days, try to go early morning when levels are typically lower, reduce workout intensity, shorten your session, avoid areas near traffic, and move activities indoors when possible. Listen to your body and stop if you experience symptoms.

QWhat are the long-term health effects of air pollution exposure?

Long-term exposure to air pollution has been linked to serious health conditions including increased risk of heart disease and stroke, development or worsening of asthma, reduced lung function and lung development in children, increased risk of lung cancer, premature death, and potential effects on cognitive function and brain health. The EPA has established National Ambient Air Quality Standards specifically to protect public health from these long-term effects. Reducing exposure over time can help minimize these risks.

Indoor Air Quality

Tips for maintaining healthy air quality inside your home.

QIs indoor air quality better than outdoor air quality?

Not necessarily. According to the EPA, indoor air can be 2-5 times more polluted than outdoor air due to sources like cooking, cleaning products, building materials, mold, and poor ventilation. However, during poor outdoor air quality events (wildfires, high ozone days), staying indoors with windows closed and running air purifiers provides significant protection. The key is to minimize indoor pollution sources while protecting against outdoor pollution when it is elevated.

QWhen should I open windows to improve air quality?

Open windows to ventilate your home only when outdoor air quality is good (AQI 0-50). Check the current AQI before opening windows, especially in summer when ozone can be elevated. When outdoor air quality is moderate or worse, keep windows closed and use air conditioning or air purifiers instead. Avoid opening windows during rush hour traffic times when pollution from vehicles is highest. The best times to ventilate are typically early morning or late evening when both temperature and pollution levels are lower.

QWhat HVAC filter should I use for better indoor air quality?

For improved indoor air quality, use HVAC filters rated MERV-13 or higher, which can capture PM2.5 and other fine particles. Standard fiberglass filters (MERV 1-4) only capture large particles like dust and lint. Before upgrading, check that your HVAC system can handle higher-rated filters without restricting airflow. Change filters according to manufacturer recommendations, typically every 3 months, or more frequently during wildfire season or high pollution periods. Consider running your HVAC fan continuously during poor air quality events to increase filtration.

QHow can I reduce indoor air pollution from cooking?

Cooking is a significant source of indoor PM2.5 and other pollutants, especially when using gas stoves or cooking at high temperatures. To reduce indoor pollution: always use your range hood exhaust fan and ensure it vents outside, open windows when outdoor air quality is good, use back burners which are closer to the exhaust hood, avoid burning food, consider an induction or electric cooktop which produces fewer pollutants, and run a portable air purifier in or near the kitchen.

QDo houseplants actually improve indoor air quality?

While houseplants can remove small amounts of certain volatile organic compounds (VOCs) in controlled laboratory settings, research shows they do not significantly improve indoor air quality in real-world home conditions. The air exchange in typical homes is too great for plants to make a meaningful difference. A single HEPA air purifier is far more effective at removing pollutants than even dozens of houseplants. However, plants provide other benefits like humidity regulation and psychological well-being.

Data & Monitoring

Understanding where air quality data comes from and how it is collected.

QWhere does air quality data come from?

Air quality data on this site comes from the EPA AirNow system, which aggregates real-time measurements from over 2,000 monitoring stations across the United States. These stations are operated by federal agencies (EPA), state environmental agencies, local air quality management districts, and tribal governments. Monitoring equipment measures pollutant concentrations continuously using EPA-approved methods. The data is transmitted to AirNow and made publicly available. We present this official EPA data without modification.

QHow accurate are air quality forecasts?

Air quality forecasts are generally reliable for predicting whether conditions will be good, moderate, or unhealthy. They are produced by trained meteorologists who analyze weather patterns, emissions data, and historical trends. However, exact AQI values may vary from forecasts due to unexpected weather changes, wildfire starts, or industrial releases. Forecasts 1-2 days out are most accurate, while extended forecasts become less precise. Always check real-time AQI data before making decisions on poor air quality days.

QCan I measure air quality at home with a consumer device?

Yes, consumer air quality monitors can measure PM2.5 and sometimes other pollutants in your home. Popular devices include PurpleAir, IQAir AirVisual, Awair, and others. However, accuracy varies significantly between devices. For the most reliable consumer data, look for monitors that have been evaluated by the EPA Air Sensor Toolbox or validated against reference monitors. Use consumer sensors to understand trends and relative changes, but rely on official EPA AirNow data for health decisions and regional air quality assessment.

QWhy does my area not have air quality data?

Not all areas have air quality monitoring stations due to the cost and logistics of operating them. Rural areas and small towns often lack coverage. Monitoring stations are typically located in urban areas and places with known air quality concerns. If your area lacks a local monitor, you can use data from the nearest available station as a general guide, understanding that local conditions may differ. Some areas may be covered by temporary monitors during wildfire season or air quality studies.

Seasonal & Regional Patterns

How air quality varies by season, region, and weather conditions.

QWhy is air quality often worse in summer?

Summer typically brings worse air quality for ozone pollution because ground-level ozone forms when emissions react with sunlight and heat. Higher temperatures and longer daylight hours increase ozone formation. Additionally, hot weather increases electricity demand for air conditioning, which can increase power plant emissions. Wildfires are also more common in summer and early fall, creating significant PM2.5 pollution. However, PM2.5 can be elevated in winter too due to wood burning and temperature inversions.

QWhat is a temperature inversion and how does it affect air quality?

A temperature inversion occurs when a layer of warm air traps cooler air near the ground, preventing pollutants from dispersing vertically into the atmosphere. Normally, warm air rises and carries pollution away, but during an inversion, pollutants accumulate near ground level. Inversions are common in winter, especially in valleys and basins, and can cause pollution to build up to unhealthy levels over several days. They often persist until weather patterns change or precipitation "washes" the air clean.

QWhy does California have more air quality problems than other states?

California faces multiple air quality challenges. Geographic factors include valleys (like the Central Valley and Los Angeles Basin) that trap pollution, mountains that block air flow, and climate conditions favorable to ozone formation. California also has the largest population of any U.S. state, with corresponding vehicle emissions. Wildfires have become more frequent and severe. Agricultural activities contribute to PM pollution. Despite these challenges, California has made significant progress, reducing pollution levels dramatically since the 1970s through strict emissions regulations.

QDoes rain improve air quality?

Yes, rain can temporarily improve air quality by washing particles out of the atmosphere in a process called "wet deposition" or "rainout." This is especially effective for PM2.5 and PM10. However, rain has minimal effect on gaseous pollutants like ozone. The improvement from rain is usually temporary, lasting only until new pollutants accumulate. Heavy or prolonged rain tends to provide longer-lasting improvement than light showers. Thunderstorms can sometimes worsen ozone levels due to lightning-generated chemicals.

QHow do wildfires affect air quality hundreds of miles away?

Wildfire smoke can travel hundreds or even thousands of miles from the source, affecting air quality in distant cities. The smoke contains PM2.5 and other pollutants that remain suspended in the atmosphere and are carried by wind patterns. In recent years, West Coast wildfires have caused poor air quality as far away as the East Coast. Smoke plumes can also travel at different altitudes, sometimes passing overhead without affecting ground-level air quality, or descending to affect surface conditions. During major wildfire events, even areas far from fires should monitor AQI closely.

Official EPA Data

All air quality data comes directly from the EPA's official monitoring network.

AirNow.gov EPA Air Quality Data Our Methodology