An air quality index (AQI) forecast is a planning tool that estimates how clean or polluted the air may be at a particular time and place. By reading the number, color, pollutant, forecast timing, and local conditions together, you can make better decisions about exercise, commuting, outdoor work, and indoor protection.
What the AQI forecast is telling you
The AQI converts measurements or predictions for one or more air pollutants into a simpler scale. Instead of requiring you to interpret concentrations of ozone, fine particles, carbon monoxide, nitrogen dioxide, or sulfur dioxide separately, the index presents an overall health-oriented rating.
Most AQI systems use a scale in which a lower number means cleaner air and a higher number means greater potential health concern. The exact thresholds and labels can vary by country, so always check which national or regional system the forecast uses. A forecast from one website may not be directly comparable with a number from another service if they use different calculations, averaging periods, or pollutant standards.
A forecast usually includes five pieces of information:
- The predicted AQI number or category.
- The color associated with that category.
- The forecast date and time.
- The pollutant expected to drive the rating.
- The location, monitoring station, or modeled area covered by the prediction.
Do not interpret the AQI as a measurement of every possible contaminant. It is a summary of selected pollutants, and a low overall number does not guarantee that every indoor or outdoor air-quality issue is absent.
Understand the AQI categories
The familiar category structure used by many public AQI systems is shown below. Treat the table as a general guide rather than a substitute for the local agency’s definitions.
| AQI range | Common category | General meaning | Practical response |
|---|---|---|---|
| 0–50 | Good | Air pollution poses little risk for most people | Normal outdoor activities are generally reasonable |
| 51–100 | Moderate | Some unusually sensitive people may notice effects | Sensitive people should watch symptoms during prolonged exertion |
| 101–150 | Unhealthy for sensitive groups | Higher risk for children, older adults, and people with heart or lung conditions | Reduce intense or lengthy outdoor activity if sensitive |
| 151–200 | Unhealthy | More people may experience symptoms | Consider moving strenuous activities indoors and limit exposure |
| 201–300 | Very unhealthy | Health risk is increased for everyone | Avoid prolonged outdoor exertion and follow local guidance |
| 301+ | Hazardous | Emergency-level conditions may affect the population | Stay indoors as much as practical and follow official alerts |
The category is more useful than the number alone. For example, an AQI of 98 and an AQI of 102 are numerically close, but they may fall into different advisory categories. A boundary crossing does not mean the air suddenly changed from safe to dangerous at that exact number; it means the recommended precautions become more conservative.
Likewise, an AQI of 150 is not necessarily twice as harmful as an AQI of 75. AQI scales are designed to communicate relative health concern, not to function as a simple dose meter.
Check which pollutant is driving the forecast
The overall AQI is commonly controlled by whichever pollutant has the highest individual index value. The forecast may therefore say that the AQI is driven by ozone, PM2.5, PM10, smoke, or another pollutant. This detail matters because pollutants behave differently and require different precautions.
Fine particulate matter, often called PM2.5, consists of very small particles that can travel deep into the respiratory system. It commonly becomes a concern during wildfire smoke, wood burning, heavy traffic, or stagnant weather. When PM2.5 is the main pollutant, reduce outdoor exertion, close windows during the worst periods, and consider filtration indoors.
Ozone is a reactive gas that often becomes more problematic on sunny, warm days. It may be highest in the afternoon or early evening, depending on local weather and emissions. If ozone is driving the forecast, moving exercise to an earlier or later time may help, but check the hourly forecast rather than assuming morning is always best.
PM10 includes larger inhalable particles that may rise during dust, construction, agricultural activity, or strong winds. Avoiding dusty areas and using appropriate respiratory protection for necessary work may be more relevant than focusing only on indoor filtration.
Carbon monoxide can increase near combustion sources, enclosed parking areas, traffic, or poorly ventilated appliances. Outdoor AQI information does not replace carbon monoxide alarms inside a home. If an indoor alarm sounds, leave the area and follow emergency guidance.
Nitrogen dioxide and sulfur dioxide are often associated with combustion and industrial sources. People with asthma or other respiratory conditions may be particularly sensitive. Look for local health recommendations when these pollutants are elevated.
Read the forecast by time, not just by day
A daily AQI value can hide substantial changes during the day. Before making plans, open the hourly view if one is available and compare the forecast during the exact hours you expect to be outside.
Use this process:
- Identify your planned activity and its start and end time.
- Check the hourly AQI for those hours, not only the daily maximum.
- Note whether the pollutant changes during the day.
- Look at the next available days for a cleaner alternative.
- Recheck shortly before leaving if the activity is important or strenuous.
Outdoor pollution often changes with wind, temperature, sunlight, traffic, smoke movement, and atmospheric mixing. A morning reading may not describe afternoon conditions. A forecast marked “moderate” for the day could still include a short period that reaches an unhealthy category.
Pay attention to forecast wording such as “daily maximum,” “current AQI,” “24-hour average,” or “nowcast.” These are not interchangeable. A current or nowcast value describes conditions around the present time, while a daily forecast may represent a predicted maximum or a longer averaging period.
Adjust plans for people and activities
Your response should depend on both the AQI category and the person exposed. Someone who is healthy and taking a brief walk may have a different risk than someone with asthma doing intense outdoor exercise for two hours.
People who may need extra caution include:
- Children, especially during vigorous play.
- Older adults.
- People with asthma, chronic obstructive pulmonary disease, or other lung conditions.
- People with heart disease or other cardiovascular conditions.
- People who are pregnant or otherwise medically advised to limit pollution exposure.
- Outdoor workers, athletes, cyclists, and anyone exercising intensely.
When AQI rises, start by reducing the dose of exposure. You can shorten the activity, lower the intensity, choose a less polluted route, take more breaks, or move the activity indoors. A short, easy walk is generally a smaller exposure than a long run, but local health advice should take priority.
If you must work outdoors, ask whether the task can be rescheduled, rotated among workers, or performed with additional breaks. Follow workplace respiratory-protection rules, and do not assume that any face covering provides reliable protection from fine particles. A properly selected and correctly fitted particulate respirator is different from a loose cloth covering, but fit, comfort, heat, and workplace requirements still matter.
Do not ignore symptoms. Coughing, wheezing, chest tightness, unusual shortness of breath, throat irritation, dizziness, or palpitations are reasons to stop exertion and move to cleaner air. Seek urgent medical help for severe breathing difficulty, chest pain, confusion, fainting, or symptoms that do not improve.
Compare outdoor and indoor conditions
Going indoors can reduce exposure, but it does not automatically create clean air. Outdoor smoke or pollution can enter through open windows, doors, cracks, and ventilation systems. Indoor sources such as cooking, candles, fireplaces, tobacco smoke, cleaning products, and poorly vented appliances can also make air worse.
When outdoor particulate pollution is high:
- Close windows and exterior doors when practical.
- Set heating, ventilation, and air-conditioning systems to recirculate if that is appropriate for the equipment.
- Use a portable air cleaner sized for the room, preferably with effective particle filtration.
- Avoid adding indoor particles through smoking, candles, frying, or unnecessary combustion.
- Keep one room as a cleaner-air space if the pollution event is prolonged.
- Replace or maintain filters according to the equipment instructions.
Do not use an air cleaner as a reason to disregard an evacuation order, severe smoke warning, or other official emergency instruction. Filtration has capacity limits, and it may not address gases or odors unless the device is specifically designed for them.
If you use an indoor air-quality monitor, treat it as a local indicator rather than a universal authority. Sensor readings can be affected by placement, humidity, dust, cooking, calibration, and the device’s detection range. Compare patterns over time and avoid placing the sensor directly beside a stove, window, purifier outlet, or air vent.
Account for location and local conditions
AQI forecasts are only as specific as their location. A citywide estimate may not represent a roadside corridor, valley, rural area, industrial zone, or neighborhood near a wildfire plume. Conditions can differ across a short distance when wind direction, elevation, traffic, and local emissions change.
Choose the forecast location closest to where you will actually spend time. If the service offers multiple monitoring stations, compare nearby stations and look for a consistent pattern. A single station may be temporarily influenced by a nearby source or may not reflect conditions at your destination.
Weather can explain unexpected changes. Wind may disperse pollution or carry smoke into a new area. Rain can temporarily reduce airborne particles, while hot, sunny, stagnant conditions may increase ozone. Temperature inversions can trap pollutants near the ground, especially in valleys or urban areas.
For wildfire smoke or other fast-moving events, consult official emergency-management or public-health alerts in addition to an AQI app. If the forecast and local observation disagree, investigate the timestamp, location, pollutant, and averaging method before deciding which number to trust.
Know the limitations of an AQI forecast
A forecast is an estimate, not a guarantee. It may be based on monitoring data, weather models, emissions information, satellite observations, or a combination of sources. Forecast accuracy can decrease when pollution moves quickly, fires change direction, emissions are unusual, or local measurements are sparse.
Important limitations include:
- The AQI may summarize only selected pollutants.
- A single overall number can hide a pollutant-specific concern.
- Forecasts may use different scales and averaging periods.
- A daily value can conceal short periods of poor air.
- Modeled data may not match conditions at street level.
- Low outdoor AQI does not rule out indoor pollution.
- Personal health risk varies with sensitivity, duration, and exertion.
Use the forecast as one part of a decision. Combine it with the pollutant detail, hourly trend, weather, local alerts, your health status, and the intensity of the activity. If you have a chronic heart or lung condition, ask your clinician for a personal action plan rather than relying on a generic AQI threshold.
A practical decision routine
Before an outdoor activity, run through this short checklist:
- Confirm the location and timestamp of the forecast.
- Read the category and numeric AQI.
- Identify the pollutant responsible for the rating.
- Check the hourly forecast for the planned activity period.
- Consider who will be exposed and how hard they will exert themselves.
- Look for official smoke, weather, or public-health alerts.
- Decide whether to continue, shorten, reschedule, move indoors, or choose a cleaner location.
- Recheck conditions if the event lasts several hours or the weather changes.
- Stop and seek cleaner air if symptoms develop.
For routine planning, save a reliable local AQI source and learn how it labels current conditions, forecasts, and alerts. The most useful interpretation is not simply “Is the air good?” but “Which pollutant is present, when will it be worst, who is exposed, and what practical change will reduce that exposure?”