Use the AQI as a daily decision tool
The Air Quality Index, or AQI, turns air-pollution measurements into one number and one color. The higher the number, the greater the health concern. The AQI is most useful when you read four details together: the current number, the color category, the primary pollutant, and the time the reading was updated.
During wildfire season, air can change quickly. A forecast helps you plan, but the current reading helps you decide what to do now. Check conditions before outdoor exercise, before children play outside, and before opening windows to air out your home.
What the AQI measures
The U.S. Environmental Protection Agency maintains an AQI for five major pollutants regulated under the Clean Air Act:
- Particle pollution, including PM2.5 and PM10
- Ground-level ozone
- Carbon monoxide
- Nitrogen dioxide
- Sulfur dioxide
Each pollutant has its own AQI calculation. The AQI shown for a location is generally the highest AQI among the pollutants being reported there. This is why the primary pollutant matters. A high AQI may be driven by PM2.5 from smoke, by ozone on a hot sunny day, or by another pollutant.
The six AQI categories
| AQI | Category | What it means |
|---|---|---|
| 0-50 | Good | Air quality is satisfactory and pollution poses little or no risk. |
| 51-100 | Moderate | Air quality is acceptable, although unusually sensitive people may be affected. |
| 101-150 | Unhealthy for Sensitive Groups | People at higher risk may experience health effects. The general public is less likely to be affected. |
| 151-200 | Unhealthy | Some people in the general public may experience effects, while higher-risk groups may experience more serious effects. |
| 201-300 | Very Unhealthy | The risk of health effects is increased for everyone. |
| 301+ | Hazardous | Emergency conditions. Everyone is more likely to be affected. |
The color is a quick signal, but the number gives more detail. An AQI of 105 and an AQI of 149 are both orange, yet the second represents a higher level of pollution. If conditions are rising quickly, act before the AQI enters the next category.
Why wildfire smoke often means PM2.5
Wildfire smoke is a changing mixture of gases and particles produced by burning vegetation, buildings, vehicles, and other materials. Fine particle pollution called PM2.5 is usually the main pollutant used to track smoke exposure. PM2.5 particles are 2.5 micrometers wide or smaller. They are small enough to travel deep into the lungs.
Smoke can irritate the eyes, nose, throat, and lungs and can cause coughing, wheezing, headaches, tiredness, chest discomfort, and difficulty breathing. Wildfire smoke can make anyone sick, but some people are more likely to experience serious effects.
Visible haze and smell are useful warning signs, but they are not reliable measurements. Smoke can affect air quality even when it is not strongly visible or easy to smell. Use current monitoring data rather than relying on your senses alone.
Current AQI, forecast AQI, and the NowCast
Current AQI
Current AQI on AirNow is usually a NowCast. The NowCast uses multiple hours of recent monitoring data to estimate present conditions. It gives more weight to recent readings when pollution is changing rapidly, as often happens during wildfire smoke.
Forecast AQI
A forecast predicts the AQI expected later today or tomorrow. State and local forecasters use monitoring data, weather models, satellite images, smoke movement, and local knowledge. Use forecasts to plan, but recheck current conditions before starting outdoor activity.
Daily AQI
The official daily AQI describes air quality over a longer averaging period. It is useful for reports and trends but may not describe a short smoke spike happening right now.
What the primary pollutant tells you
Always look for the pollutant listed next to the AQI. During a wildfire, PM2.5 is often the primary pollutant. On hot days in Southern California, ozone may be higher than PM2.5 and may drive the overall AQI.
This can explain why two Fire Season Wellness cards seem different. For example, the overall current AQI may be red because of ozone while the PM2.5 smoke forecast is yellow. Both can be correct because they describe different pollutants or time periods.
When smoke is present, use the AirNow Fire and Smoke Map in addition to the general AQI dial. The Fire and Smoke Map focuses on PM2.5 and includes permanent monitors, temporary monitors, selected low-cost sensors, smoke plumes, fire locations, trends, and smoke outlooks where available.
How to make household decisions
Before outdoor activity
- Check the current AQI and primary pollutant.
- Consider each person’s health, age, symptoms, and planned activity.
- Remember that running, sports, and heavy work increase the amount of air you breathe.
- Shorten the activity, lower the intensity, take more breaks, or move indoors when conditions worsen.
Before opening windows
Compare outdoor air with your indoor conditions. If outdoor AQI is poor, keep windows and doors closed as much as practical and use recirculation and filtration. When outdoor air improves, use that cleaner period to air out the home if it is safe and temperatures allow.
When driving
Keep windows closed and set the vehicle ventilation system to recirculate during smoke. Replace the cabin air filter according to the vehicle manufacturer’s instructions. Recirculation is temporary; periodically bring in outside air when outdoor conditions improve to reduce carbon-dioxide buildup and drowsiness.
When you must be outdoors
Reduce time and exertion. A well-fitting NIOSH-approved N95 respirator can reduce exposure to airborne particles, but it does not filter gases and vapors and does not replace evacuation orders. Cloth masks and loose surgical masks are not designed to filter wildfire smoke particles effectively.
Who should take extra care
Wildfire smoke can affect anyone. People who may need earlier or stronger precautions include:
- Children and teenagers
- Older adults
- People who are pregnant
- People with asthma, COPD, heart disease, diabetes, chronic kidney disease, or other chronic conditions
- People who work or exercise outdoors
- People without reliable access to filtered indoor air
People with asthma or other chronic conditions should follow their care plan and keep prescribed medications available. The AQI is a population-level guide, not a personal diagnosis. Symptoms and medical advice matter even when the AQI category seems moderate.
Symptoms to watch for
Common smoke-related symptoms can include burning or stinging eyes, scratchy throat, runny nose, sinus irritation, cough, wheezing, headache, tiredness, chest discomfort, or a fast heartbeat.
Move to cleaner air and reduce activity if symptoms begin. Contact a medical professional for persistent or worsening symptoms, especially if the person has heart or lung disease. Call 911 for severe trouble breathing, chest pain, fainting, confusion, or blue or gray lips or face.
Why monitors and sensors may disagree
Air quality varies from block to block and hour to hour. Official regulatory monitors are carefully maintained but may be several miles away. Temporary monitors and lower-cost sensors provide more local detail but can be affected by humidity, placement, and device quality.
Different apps may also use different averaging methods, correction formulas, map layers, or pollutant priorities. During wildfire smoke, EPA recommends using the AirNow Fire and Smoke Map because it combines several PM2.5 data sources and shows smoke-specific information.
Look for the overall pattern rather than expecting every device to show the same number. If several nearby readings are rising, smoke is visible, or you are developing symptoms, take protective action.
What the AQI does not tell you
The AQI is an important tool, but it does not measure every substance in the air. It reports five major regulated pollutants. Smoke from burned structures may contain additional chemicals that are not represented by the AQI. The AQI also does not tell you the quality of the air inside your home.
Near an active fire, evacuation orders, ash, falling debris, damaged utilities, and rapidly changing fire behavior can be more urgent than the AQI. Always follow local emergency instructions. After a structure fire, use cleanup guidance specific to ash and debris rather than relying only on an AQI reading.
A simple 60-second AQI routine
- Check now: Read the current AQI, color, pollutant, location, and update time.
- Check later: Review today’s and tomorrow’s forecast.
- Check smoke: Open the Fire and Smoke Map if wildfire smoke is present or expected.
- Check your household: Identify anyone at higher risk and ask whether anyone has symptoms.
- Choose one action: Change activity, close windows, run filtration, use a cleaner room, or seek a cleaner-air location.
Checking daily helps you learn your local pattern. In some areas, mornings may be cleaner; in others, smoke settles overnight. Wind shifts can change conditions in minutes, so keep notifications and local alerts active during fire events.
Key terms
- AQI
- A color-coded index used by EPA to communicate outdoor air quality and health concern.
- PM2.5
- Fine inhalable particles 2.5 micrometers wide or smaller; often the key pollutant in wildfire smoke.
- Primary pollutant
- The pollutant producing the highest reported AQI at that time and location.
- NowCast
- EPA’s method for estimating current ozone or particle-pollution AQI from recent hourly data.
- Sensitive or higher-risk groups
- People more likely to experience effects at lower pollution levels because of age, health, pregnancy, exposure, or other factors.
Important
This resource provides general educational information. It does not replace medical advice, official public-health guidance, evacuation orders, or emergency instructions. Local agencies may recommend stronger precautions based on smoke, heat, fire behavior, or community conditions.