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PM2.5 is the pollutant measurement; AQI is the health-communication scale built from that measurement. During most wildfire-smoke events, the AQI shown by AirNow is driven by PM2.5, but the two numbers are not interchangeable. PM2.5 may be reported as a concentration in micrograms per cubic meter (µg/m³), while AQI is a normalized number with color-coded health categories.
To decide what to do, identify the pollutant, check whether the value is an hourly concentration, a current NowCast estimate, or a daily forecast, then use the AQI category and your personal risk factors. Smoke conditions can change quickly, so one reading is not a permanent description of the air around you.
PM2.5 and AQI answer different questions
What PM2.5 measures
PM2.5 means fine particulate matter with an aerodynamic diameter of 2.5 micrometers or smaller. It is a physical pollutant in smoke and can enter deep into the lungs. Monitoring systems commonly report its concentration in micrograms per cubic meter (µg/m³). A concentration tells you how much particulate matter was measured in a volume of air over a stated sampling period.
An hourly PM2.5 value, for example, describes the concentration measured during that hour at a particular monitor. It is not automatically a health category, a daily average, or a prediction of what every person in the area inhaled.
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What AQI communicates
The Air Quality Index converts a pollutant concentration into a common 0-to-500-plus scale and assigns a color and health message. The number is not a concentration and should not be read as µg/m³. AQI lets people interpret different pollution levels using consistent action categories.
The U.S. Environmental Protection Agency explains the relationship this way: “The higher the AQI value, the greater the level of pollution and the greater the health concern.” An AQI can be calculated for several pollutants, so always check which pollutant produced the displayed index. In wildfire smoke, that pollutant is most often PM2.5.
| Value type | What it represents | Typical unit or label | How to use it |
|---|---|---|---|
| PM2.5 concentration | Measured fine-particle mass in air over a stated period | µg/m³ | Useful for examining the actual pollutant level, provided you know the time period and monitor location |
| Current particle AQI | A health-category index based on a current estimate of particle pollution | AQI number, color and category | Use the category and recommended actions for present conditions |
| Daily AQI | An index based on the pollutant’s daily averaging period | AQI number, color and category | Use for the day-level health outlook, not as an hourly snapshot |
| Forecast AQI | A prediction for a future period | Forecast AQI number or category | Use for planning; confirm conditions with current observations |
Why the same smoke event can show different numbers
Time basis changes the meaning
PM2.5 and AQI values depend on the period being summarized. A single hourly concentration can rise sharply when a smoke plume arrives. A daily particle AQI uses a 24-hour measure, so it changes more gradually and incorporates pollution from the broader averaging period. Comparing an hourly concentration with a daily AQI as though they were equivalent can produce a misleading conclusion.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsEPA says an AQI of 100 generally corresponds to a pollutant’s short-term primary health-based standard. For PM2.5, that short-term standard is based on a 24-hour average. This does not mean that an AQI below 100 guarantees safety for every person or that an individual outcome can be predicted from a single number.
How the NowCast represents current particle conditions
AirNow’s Fire and Smoke Map uses a current particle AQI based on the NowCast. The NowCast combines multiple hours of monitor readings rather than treating one hourly value as the whole story. Its averaging behavior can adjust when conditions change rapidly, allowing the estimate to respond more quickly to an incoming or departing smoke plume.
A NowCast AQI is therefore neither a single-hour PM2.5 concentration nor the same thing as a full-day AQI. When you record a value, write down whether it is labeled current, NowCast, observed, daily, or forecast, along with the time and location.
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U.S. AQI categories and what they imply
The official U.S. categories provide the action framework used by AirNow and EPA. They describe population-level health concern, not a guarantee about an individual’s response.
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|---|---|---|---|
| 0–50 | Good | Air quality is satisfactory and pollution poses little risk under normal conditions | Normal activities are generally reasonable; continue checking conditions if a fire is nearby because smoke can change quickly |
| 51–100 | Moderate | Air quality is acceptable, but some unusually sensitive people may experience effects | People who are sensitive should watch symptoms and consider reducing prolonged or strenuous outdoor activity if smoke is noticeable |
| 101–150 | Unhealthy for Sensitive Groups | Sensitive groups may experience health effects; the general public is less likely to be affected | People at higher risk should reduce exposure, especially strenuous outdoor exertion; others should pay attention to symptoms |
| 151–200 | Unhealthy | Some members of the general public may experience health effects, with greater effects for sensitive groups | Reduce outdoor exposure and avoid strenuous activity; move activities indoors where the air is cleaner |
| 201–300 | Very Unhealthy | Health alert: the risk of health effects is increased for everyone | Avoid optional outdoor activity, keep indoor air cleaner and follow public-health instructions |
| 301+ | Hazardous | Emergency conditions; the entire population is more likely to be affected | Stay out of smoke as much as possible, follow emergency guidance and use the cleanest available indoor air |
Above 100, air quality is unhealthy first for sensitive groups, and the concern expands to the general population as the AQI rises. A category boundary is not a switch that makes one side safe and the other side dangerous; risk changes along a continuum, and people respond differently.
How to check wildfire smoke conditions in your community
Use the AirNow Fire and Smoke Map
AirNow’s Fire and Smoke Map brings several pieces of information together: current PM2.5 information, fire locations, smoke-plume displays and Smoke Forecast Outlooks when available. It also provides recommended actions. Use the map’s current reading for what is happening now and its forecast layer for planning later in the day or the next day.
- Open the map and allow it to use your location, or search for your community.
- Identify the particle AQI value and its category. Confirm that the pollutant is PM2.5 rather than another pollutant.
- Check the timestamp and whether the value is labeled current, NowCast or forecast.
- Look at nearby monitor locations and smoke-plume information. A monitor several miles away may not represent the air beside your home, school or workplace.
- Check the Smoke Forecast Outlook and local public-health advisories before scheduling outdoor work, exercise or travel.
- Recheck during an active fire. Wind shifts, fire behavior and changing plume direction can move smoke into or out of a community quickly.
Interpret other sensors cautiously
Consumer or community sensors can help show neighborhood variation, but their estimates are not automatically interchangeable with regulatory-monitor values. Before comparing a sensor with AirNow, check the pollutant represented, averaging period, calibration or quality information, location and whether the source supplies official health-category guidance.
| Comparison question | Why it matters |
|---|---|
| Is it PM2.5 or another pollutant? | AQI and concentration values are pollutant-specific |
| Is it an hourly value, NowCast, daily value or forecast? | Different time bases summarize different conditions |
| How far is the monitor or sensor from you? | Smoke can vary over short distances, especially near a plume or changing wind |
| Does the source provide health categories? | Official guidance helps translate a measurement into an action level |
When to act: a practical exposure-reduction plan
Inhaled dose is influenced by three controllable factors: the concentration of pollution you encounter, how hard you breathe and how long you remain exposed. You can lower dose by choosing cleaner air, reducing breathing intensity and shortening time in smoky conditions.
When the AQI is 0–100
Most people can continue normal activities, but do not assume a morning value will last all day during a fire. Sensitive people should monitor symptoms and avoid prolonged, strenuous exercise if smoke is visible or irritating. Keep the map or local alert service available so you notice a rapid change.
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When the AQI is 101–150
People with heart or lung disease, older adults, children and teenagers, pregnant people and anyone with greater exposure should reduce time and exertion outdoors. Move exercise, recess or other demanding activity indoors when practical. People outside those groups should still reduce strenuous activity if smoke is thick or symptoms appear.
When the AQI is 151–200
Postpone optional outdoor exercise and physically demanding work. If you must be outside, keep the activity brief, take frequent breaks in cleaner air and watch for coughing, wheezing, chest discomfort or shortness of breath. Employers, schools and event organizers should use the current category and local advisories when deciding whether to change schedules.
When the AQI is 201 or higher
Very Unhealthy and Hazardous conditions call for avoiding optional outdoor exposure for everyone. Stay in a cleaner indoor environment, close windows and doors when outdoor smoke is worse, and follow evacuation or emergency instructions from local authorities. Do not rely on a forecast alone; verify the current reading before leaving a protected space.
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Make indoor air cleaner
Choose a cleaner-air room
Select a room that can be closed off from outdoor air and use it as a clean-air space during the smoke event. Keep windows and doors closed when outdoor air is more polluted. If your building has central heating, ventilation and air-conditioning, follow the equipment manufacturer’s guidance for filtration and outdoor-air settings; do not disable safety-critical ventilation without qualified advice.
Use a portable HEPA air cleaner appropriately
A portable HEPA air cleaner can reduce airborne particles in a room, but its usefulness depends on airflow, room size, filter condition and how well smoke is kept from entering. Check the unit’s stated room-sizing information, run it long enough to circulate the room air and replace filters when they are loaded or according to the manufacturer’s instructions. Confirm that replacement filters remain available for the model you choose. A purifier does not make outdoor exposure safe and is not a substitute for AQI guidance.
Avoid adding indoor pollution
During a smoke episode, avoid activities that create particles indoors, such as smoking, burning candles or incense, and frying without effective ventilation. If outdoor air is temporarily cleaner, ventilate then; otherwise, opening windows can increase indoor smoke.
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Respirators for necessary outdoor exposure
If you must spend time outdoors in smoky air, a properly fitted NIOSH-approved N95 respirator can reduce inhalation of fine particles. Fit matters: gaps around the nose, cheeks or chin allow smoke to bypass the filter. Facial hair, the wrong size and an improvised face covering can reduce protection.
An N95 is not complete protection. It does not remove every hazard in wildfire smoke, does not replace reducing time outdoors and may be difficult for some people to wear. People with heart or lung conditions should ask a health professional whether strenuous activity or respirator use is appropriate for them. Follow local emergency guidance even when wearing one.
Who may be more vulnerable
EPA identifies several groups at increased risk from particle pollution:
- People with chronic heart disease or lung disease, including asthma and chronic obstructive pulmonary disease
- Older adults
- Children and teenagers, whose lungs are still developing and who may breathe more air relative to body size during activity
- People who are more heavily exposed because of outdoor work, housing conditions or other circumstances
- Pregnant people, for whom EPA notes evidence of added risk
- People with limited access to exposure-reduction measures such as filtered indoor air, transportation or time away from outdoor work
These categories describe higher population risk, not an individual diagnosis. Follow your clinician’s plan for asthma, heart disease or other conditions, and ask in advance what symptoms require medical attention.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Symptoms that mean you should reduce exposure
Coughing, wheezing and shortness of breath are signals to move away from smoke, stop strenuous activity and seek cleaner air. Chest pain, severe breathing difficulty, confusion, fainting or symptoms that do not improve require urgent medical attention; call emergency services when appropriate. Do not wait for the AQI to enter a higher color category if your body is reacting now.
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Common mistakes when reading smoke data
Treating AQI as a concentration
AQI 150 does not mean 150 µg/m³. AQI is a normalized index derived from a pollutant concentration and the applicable breakpoints. Use the units and labels shown by the monitoring source.
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Comparing unlike time periods
An hourly PM2.5 concentration, a NowCast AQI and a daily forecast summarize different periods. Label notes or screenshots with the time basis before deciding that one source disagrees with another.
Assuming one monitor represents an entire county
Smoke can vary by neighborhood, elevation, wind and distance from a fire. Treat a nearby official monitor as useful evidence, not a guarantee that every location in the region has identical air.
Waiting for symptoms before changing plans
Symptoms are an important warning, but exposure reduction is most effective when planned from the current category and forecast. Sensitive people should act earlier, especially before exercise or outdoor work.
Using a forecast as proof of current conditions
Forecasts help schedule activities; current observations tell you what is happening now. Recheck the current NowCast before leaving home, beginning a shift or starting outdoor exercise.
A simple decision checklist
- Find the pollutant: confirm that the reading is PM2.5 if wildfire smoke is the concern.
- Find the time basis: distinguish hourly concentration, current NowCast, daily AQI and forecast.
- Find the category: use the U.S. AQI band rather than the number alone.
- Check your situation: consider heart or lung disease, age, pregnancy, outdoor work, children’s activities and access to cleaner indoor air.
- Adjust the three exposure factors: lower the concentration around you, breathe less hard and spend less time outside.
- Recheck: during an active fire, monitor current conditions repeatedly because smoke can change quickly.
- Escalate for symptoms: stop exposure and seek medical help when symptoms are severe or persistent.
Sources and date context
This guidance follows U.S. EPA and AirNow materials. EPA’s public-health guide identifies its 2026 edition as an interagency collaboration and was last updated June 25, 2026. AirNow’s NowCast FAQ used to explain the multi-hour method is dated July 2021; the map’s current labels and documentation should take precedence when they differ. The AQI category breakpoints listed here are the U.S. system and may not match indexes used in other countries.
Frequently Asked Questions
Is PM2.5 of 100 the same as AQI 100?
No. PM2.5 is a concentration in µg/m³, while AQI 100 is an index value. The conversion depends on the pollutant breakpoint table and the averaging period, so the two numbers should not be treated as equal.
Should I use the current AQI or the forecast when planning outdoor exercise?
Use the forecast to plan the timing, then check the current PM2.5 NowCast immediately before exercising. If the current category is worse than expected, postpone or move the activity indoors.
Can an air purifier remove all wildfire-smoke risk?
No. A properly sized, maintained HEPA cleaner can reduce particles in a room, but outdoor exposure, leaks, filter condition and room airflow still matter. Continue following AQI guidance and public-health instructions.
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