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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →PM2.5 is the pollution measurement; AQI is the health-communication scale built from that measurement. During most wildfire-smoke events, the relevant pollutant is fine particulate matter (PM2.5). A monitor may report PM2.5 as a concentration, while AirNow converts particle data into an AQI number and color category. The two readings describe the same air problem in different ways, but they are not interchangeable.
To decide what to do, identify the pollutant, the time basis and the location of the reading. A single-hour PM2.5 concentration, a current NowCast AQI and a daily AQI are different kinds of information. As the AQI rises above 100, begin reducing exposure; at higher categories, precautions apply to progressively more people. Check current conditions and forecasts often because smoke can change quickly.
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What PM2.5 and AQI each tell you
PM2.5 is a concentration
PM2.5 means particulate matter with an aerodynamic diameter of 2.5 micrometers or smaller. Wildfire smoke contains large numbers of these fine particles, which can travel deep into the respiratory system. A monitor reports PM2.5 as a concentration, commonly in micrograms per cubic meter (µg/m³). That number describes how much particle pollution is in a volume of air at a particular place and time.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesA concentration is not automatically a health category. Its meaning depends on the averaging period, the measurement method and the pollutant-specific standard being used. A one-hour value tells you about that hour; it is not the same as a 24-hour average.
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AQI translates pollution into an action scale
The U.S. Air Quality Index (AQI) converts a pollutant concentration into a normalized number and a color-coded health category. The U.S. Environmental Protection Agency says, “The higher the AQI value, the greater the level of pollution and the greater the health concern.” The number is an index, not a concentration and not a percentage of risk.
In wildfire smoke, the AQI displayed by AirNow most often represents PM2.5. Other pollutants can determine an AQI in other circumstances, so read the pollutant label rather than assuming every AQI number is a particle reading.
U.S. AQI categories and what they mean
These are the official U.S. AQI breakpoints used by EPA and AirNow. They are population-level communication categories, not guarantees about how any one person will feel.
| AQI | Category | Practical interpretation |
|---|---|---|
| 0–50 | Good | Air quality is generally satisfactory. |
| 51–100 | Moderate | Air quality is acceptable overall, although an unusually sensitive person may notice effects. |
| 101–150 | Unhealthy for Sensitive Groups | People in higher-risk groups may experience health effects; the general population is less likely to be affected. |
| 151–200 | Unhealthy | Some people may experience health effects; sensitive people may experience more serious effects. |
| 201–300 | Very Unhealthy | Health alert: the risk of health effects is increased for everyone. |
| 301+ | Hazardous | Health warning of emergency conditions; everyone is more likely to be affected. |
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. Crossing a breakpoint does not mean that exposure below it is harmless or that exposure above it guarantees illness; it signals that the recommended level of caution has changed.
Why the same smoke can produce different-looking readings
Single-hour PM2.5 concentration
A sensor or monitor can show the concentration measured during a recent hour. This is useful for seeing rapid changes, but it is a narrow snapshot. Wind shifts, a passing smoke plume, traffic, indoor filtration and the monitor’s distance from a fire can all make the hourly value rise or fall.
Current particle AQI and the NowCast
The current particle AQI on AirNow’s Fire and Smoke Map uses the NowCast. NowCast combines several hours of monitor observations and changes the effective averaging behavior when conditions are changing quickly. It is designed to represent current conditions without treating one unusually high or low hour as the entire story.
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A NowCast AQI is therefore not the same thing as a single hourly PM2.5 concentration. Do not convert an hourly concentration directly into an AQI category unless the reporting system explicitly provides that conversion and time basis.
Daily AQI and forecast AQI
A daily particle AQI is based on a 24-hour measure. A forecast AQI is an estimate of what conditions may be during a future period. Forecasts help you plan, but they cannot replace the current reading when smoke is moving or fire behavior is changing. When you read a number, note whether it is labeled current, NowCast, daily or forecast.
How to check wildfire-smoke conditions in your community
- Open AirNow and select the Fire and Smoke Map. The map combines PM2.5 information with fire locations and smoke-plume information.
- Search for your town or allow your location. Compare the displayed monitor or reporting area with where you actually spend time. Conditions can differ across a metropolitan area or between a valley and a nearby hillside.
- Read the pollutant and time label. Confirm that the value is PM2.5 and identify whether it is a current NowCast, a daily value or a forecast.
- Check the color category and recommended action. The category is the quickest way to understand the public-health recommendation; the number adds precision within that category.
- Review the smoke plume and forecast outlook. A plume map explains where smoke is moving, while an outlook helps with planning later today or tomorrow. Neither replaces the monitor reading at your location.
- Recheck during an active event. Smoke can arrive or clear in a few hours. Use the latest local reading before exercising outdoors, commuting, opening windows or sending children outside.
Air quality can vary substantially within a region. A single monitor is an indicator of nearby conditions, not a complete measurement of every room, neighborhood or individual exposure.
When to act: a practical smoke-response framework
AQI 0–50: Good
Routine outdoor activity is generally reasonable. People who are unusually sensitive should still pay attention to symptoms and local advisories, especially if a fire is nearby or the monitor is far away.
AQI 51–100: Moderate
Most people can continue normal activities. If you have heart or lung disease, are pregnant, care for a child or older adult, or know that smoke affects you quickly, watch the trend and be ready to shorten or move outdoor activities indoors if the category rises.
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AQI 101–150: Unhealthy for Sensitive Groups
This is the point at which sensitive groups should actively reduce exposure. Postpone optional strenuous outdoor exercise, choose a cleaner indoor space and keep outdoor time shorter. People outside the listed groups should reduce prolonged or heavy exertion if they notice irritation or breathing symptoms.
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AQI 151–200: Unhealthy
Reduce outdoor exposure for everyone. Move workouts indoors, avoid unnecessary time in smoky areas and use a cleaner-air room. Sensitive people should take more stringent steps and follow their clinician’s plan for respiratory or heart conditions.
AQI 201–300: Very Unhealthy
Health risk is elevated for everyone. Avoid strenuous outdoor activity and postpone optional outdoor tasks. Keep windows and doors closed when outdoor air is dirtier than indoor air, operate appropriately sized filtration and monitor household members for symptoms.
AQI 301 and above: Hazardous
Treat this as an emergency-level air-quality warning. Stay in cleaner indoor air, avoid outdoor exposure unless necessary and follow instructions from local emergency-management and public-health officials. If authorities issue evacuation or shelter directions, those instructions take priority over a map color.
How to reduce the amount of smoke you inhale
Exposure is driven by three levers: the concentration of pollution you encounter, how hard you breathe and how long you remain in it. You can lower the inhaled dose by changing any of them.
- Lower indoor concentration. Choose one room with the fewest leaks and keep windows and doors closed when outdoor smoke is worse. A portable HEPA air cleaner can help create a cleaner-air room. Check the unit’s recommended room size and make sure replacement filters are available; a purifier is not a substitute for following AQI guidance.
- Reduce breathing rate. Avoid running, heavy yard work and other strenuous outdoor activity when smoke is present. If you must go out, move slowly and take breaks.
- Shorten exposure time. Combine errands, postpone optional outdoor work and spend less time near visible smoke.
- Use a respirator for necessary outdoor exposure. A well-fitting NIOSH-approved N95 respirator can reduce inhalation of fine particles when worn correctly. Fit matters, facial hair can interfere with the seal and no respirator provides complete protection. People with heart or lung conditions should ask a health professional whether strenuous breathing through a respirator is appropriate.
- Keep indoor sources low. Avoid adding particles with smoking, candles, fireplaces, frying or other activities that produce smoke while outdoor air is already polluted.
Follow local public-health advisories and your own medical advice. If you develop coughing, wheezing or shortness of breath, reduce exposure immediately and seek medical help when symptoms are severe, persistent or accompanied by chest pain, confusion or serious breathing difficulty.
Who should take extra precautions
EPA identifies several groups with increased risk during particle-pollution events:
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- 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 spend more time active outdoors.
- People who are more heavily exposed because of outdoor work, housing conditions or limited ability to move to cleaner air.
- Pregnant people, for whom EPA notes evidence of added risk.
These categories describe population-level vulnerability, not an individual diagnosis. A person outside them can still have symptoms, and a person inside them may respond differently from someone else. Use your clinician’s action plan for asthma, heart disease or other conditions.
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When two apps, sensors or websites disagree, compare the details rather than just the numbers.
| Question | Why it matters |
|---|---|
| What pollutant is represented? | PM2.5 AQI is not interchangeable with an ozone AQI or another pollutant index. |
| What is the time basis? | A single-hour concentration, a multi-hour NowCast, a 24-hour daily AQI and a forecast describe different periods. |
| Where was it measured? | Distance from the monitor, elevation, wind and local terrain can produce different conditions across a region. |
| How was it measured? | A regulatory monitor, a low-cost sensor estimate and a model-based forecast have different uncertainty and should not be treated as identical. |
| Does the source provide official health guidance? | An official AQI category supplies an action framework; a raw sensor value may not. |
If a low-cost sensor reports an alarming hourly value while the official map shows a different category, do not average the numbers or assume one is automatically wrong. Check calibration information, placement, time stamps and the nearest official monitor, then use the official health category and local advisories for decisions.
Common mistakes during smoke events
Assuming AQI is a concentration
AQI 150 does not mean 150 µg/m³. AQI is a normalized index. The concentration that maps to a particular AQI depends on the pollutant and the applicable breakpoint table.
Using a daily value as if it were current
A 24-hour daily AQI can hide a recent plume or a recent improvement. For immediate decisions, look for the current particle NowCast and its time stamp.
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Forecasts are useful for scheduling and preparation, but wind and fire behavior can change. Recheck current conditions before leaving home or beginning strenuous work.
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Relying on visibility alone
Haze and odor can be obvious when particle levels are high, but visibility is not a calibrated substitute for a PM2.5 monitor. Conversely, dangerous particles may be present even when the sky looks clearer.
Opening windows automatically
Ventilation is helpful when outdoor air is cleaner than indoor air. During a smoke peak, opening windows can bring more particles inside. Check the outdoor AQI first.
Waiting for symptoms
Symptoms are a warning, not a required threshold for action. If the AQI is rising, reduce exposure before coughing or breathlessness begins.
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A simple decision checklist
- What is the current PM2.5 AQI where I am?
- Is the number a NowCast, a daily value or a forecast?
- Is the reading from a nearby official monitor or a more distant location?
- Am I, or is someone with me, in a higher-risk group?
- Can I move the activity indoors, reduce its intensity or postpone it?
- Can I create a cleaner indoor room with closed windows and suitable filtration?
- Do I need a properly fitted NIOSH-approved N95 for unavoidable outdoor exposure?
- Are there local public-health, emergency-management or evacuation instructions?
What the numbers can—and cannot—predict
AQIs are designed to communicate population-level health concern consistently. They do not predict exactly how much pollution entered your home, how much any person inhaled or whether an individual will become ill. Indoor filtration, time outdoors, breathing rate, work conditions and personal health all matter.
The most reliable use of the scale is comparative and practical: a higher category means more reason to reduce exposure, and a worsening trend means you should act sooner rather than waiting for the next category boundary. Continue checking the official map throughout a fire because conditions and recommendations can change rapidly.
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