Use the local Air Quality Index (AQI), not a raw PM2.5 number alone, to decide what to do. In the U.S. EPA system, AQI 101–150 calls for less prolonged or heavy exertion by sensitive people; 151–200 means everyone should limit that exertion; and 201–300 means sensitive people should stay indoors with low activity while everyone else avoids prolonged or heavy exertion. Keep windows closed, filter indoor air, avoid creating particles indoors, and follow local emergency instructions. If closing up makes the indoor space dangerously hot, move to a cooler cleaner-air location instead.
What PM2.5 is and why it matters
PM2.5 means fine particulate matter with an aerodynamic diameter of 2.5 micrometres or less. The particles can come from wildfire smoke, traffic, industrial combustion, wood burning, and other sources. Their small size lets them travel deep into the respiratory system. Particle pollution can affect both the lungs and the cardiovascular system, so exposure is not only an asthma concern.
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Children, older adults, and people with heart or lung disease generally have less physiological reserve and can be affected sooner. A person who is pregnant, has asthma or chronic obstructive pulmonary disease, or has previously reacted strongly to smoke should use their clinician’s action plan and take precautions early. Someone who feels well is not guaranteed to be unaffected: the U.S. Environmental Protection Agency (EPA) says studies show little evidence of a population threshold below which particle-pollution effects are unlikely.
The World Health Organization’s 2021 global air-quality guideline reflects a stronger evidence base that health effects occur at lower concentrations than earlier guidelines assumed. That does not mean every sensor display is a medical diagnosis; it means there is no universal “safe for everyone” PM2.5 reading.
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- Track and measure – Our indoor air quality monitor keeps tabs on 5 key factors: particulate matter (PM 2.5), volatile organic compounds (VOCs), carbon monoxide (CO), humidity, and temperature.
- Stay informed – Get an indication of current indoor air quality from the color-coded LED, and detailed information and an easy-to-understand air quality score in the Alexa app.
- Real-time alerts - Get notifications on your phone or announcements on Echo devices when Alexa detects poor indoor air quality.
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Why the AQI is more useful than a raw PM2.5 number
A PM2.5 concentration is a measurement, usually expressed in micrograms per cubic metre. The AQI converts pollutant data into a health-oriented scale that includes concentration breakpoints, a defined averaging period, and action advice. For short-term PM2.5 interpretation in the U.S. EPA framework, the relevant averaging period is 24 hours. A real-time spike on a home monitor and a regulatory AQI value therefore are not interchangeable.
| U.S. EPA AQI | Color/category | Practical meaning |
|---|---|---|
| 0–50 | Good | Air quality is generally satisfactory. |
| 51–100 | Moderate | Generally acceptable, although unusually sensitive people may notice effects. |
| 101–150 | Unhealthy for Sensitive Groups | Sensitive people should reduce prolonged or heavy outdoor exertion. |
| 151–200 | Unhealthy | Everyone should limit prolonged or heavy exertion; sensitive people should avoid strenuous outdoor activity. |
| 201–300 | Very Unhealthy | Sensitive people should remain indoors and keep activity low; everyone else should avoid prolonged or heavy exertion. |
These bands describe the U.S. EPA system. Other countries can use different breakpoints, pollutant averaging periods, colors, or advice. Read the health message from your own national or local air agency rather than importing U.S. cutoffs into another system.
The AQI is designed around health effects that may occur within hours or days of breathing polluted air. It is not a promise that a lower value eliminates risk, nor does it predict exactly how one individual will respond. When an agency issues a shelter-in-place, evacuation, or cleaner-air-shelter instruction, that instruction takes priority over an app’s simplified label.
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AQI 0–100: ordinary plans, with attention to symptoms
At Good or Moderate levels, most people can follow normal outdoor plans. A person with asthma, heart disease, or a history of smoke sensitivity should still watch symptoms and use the action plan supplied by a clinician. A local agency can also issue a special advisory even while the broad AQI category remains lower.
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AQI 101–150: sensitive people cut back first
People in sensitive groups should shorten or slow outdoor exercise, especially running, strenuous work, and activities lasting a long time. Move exercise indoors or choose a time when the AQI improves. People outside sensitive groups should pay attention to coughing, throat irritation, unusual fatigue, or breathing discomfort rather than treating the number as a guarantee of safety.
AQI 151–200: everyone limits hard, prolonged exertion
At this level, strenuous outdoor workouts and physically demanding jobs create more inhaled dose because breathing is faster and deeper. Everyone should limit prolonged or heavy exertion. Sensitive people should avoid strenuous outdoor activity altogether and choose a filtered indoor environment when possible.
AQI 201–300: indoor protection becomes the default for sensitive groups
Sensitive people should remain indoors and keep activity low. Others should avoid prolonged or heavy exertion and should move routine exercise inside a cleaner-air space. Keep checking official updates because smoke can rise or fall quickly as wind and fire conditions change.
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Emergency instructions and values above the displayed bands
If the local authority advises sheltering indoors, evacuating, or using a cleaner-air shelter, follow that instruction even if a personal monitor shows a different value. Do not infer a universal action rule for values beyond the table; agencies may issue location-specific directions based on smoke, fire behavior, heat, or other hazards.
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How to keep indoor air cleaner during a smoke episode
- Close openings. Shut windows and exterior doors while outdoor particle levels are high. Check that they latch and that obvious gaps are not admitting smoke. Do not seal a room so tightly that a fuel-burning appliance creates a carbon-monoxide danger; use normal safety precautions and alarms.
- Set heating or cooling to recirculate. If your air conditioner has a fresh-air or outside-air intake, close it when the equipment allows. Keep the system’s filter clean and replace it according to the equipment maker’s instructions. If the system cannot safely operate with the intake closed, follow the manufacturer’s guidance.
- Run a correctly sized portable HEPA purifier. Choose a unit intended for the room’s floor area, place it where air can circulate, and operate it at the speed recommended for the episode. A purifier lowers particle concentration in the room it serves; it is not a guarantee that every pollutant is removed or that the whole home is protected. Maintain and replace the filter as directed.
- Use central filtration when available. A central HVAC filter can treat air moved through the duct system, but coverage depends on whether the system is running, its airflow, the filter, and which rooms are connected. Keep interior doors positioned to support the intended airflow and do not block supply or return grilles.
- Stop adding particles indoors. Do not smoke or vape inside. Avoid candles, incense, fireplaces, and other combustion. Postpone cooking that produces smoke, or use an exhaust fan that vents outdoors rather than recirculating.
- Reduce strenuous indoor activity during severe smoke. Indoor air can still contain particles that entered before windows were closed or leaked through the building. Heavy exercise increases breathing rate and therefore inhaled dose, even indoors.
- Designate a cleaner room. Select a room with few openings, place the portable purifier there, and keep the door closed when practical. This gives sensitive household members a predictable refuge if smoke worsens.
“If you are advised to stay indoors, take steps to keep indoor air as clean as possible.” — U.S. Environmental Protection Agency
Portable purifier or central HVAC filtration?
| Option | Coverage | Air movement and control | Important limits |
|---|---|---|---|
| Portable HEPA purifier | Usually one room or a defined zone, based on the model’s stated room size | Can run continuously at a selected fan speed and be placed in the cleaner room | Needs a correctly sized unit, clear airflow, and filter maintenance; does not automatically clean adjacent rooms |
| Central HVAC filtration | Rooms reached by the duct system while the blower operates | Uses the home’s existing air circulation; outside-air intake may need to be closed during smoke | Performance depends on system design, filter condition, run time, and airflow; a central system may not reach every room equally |
For a single bedroom or living area, a portable unit provides direct control. For a home with a suitable central system, filtration can complement a portable unit, especially when occupants spend time in several connected rooms. Neither approach replaces an official warning, evacuation order, or medical advice.
Official AQI versus a personal indoor sensor
| Question | Official AQI or regulatory monitor | Personal indoor sensor |
|---|---|---|
| What it represents | A health category calculated by an air agency from monitored pollutant data and defined breakpoints | A measurement at one location inside a building |
| Averaging period | PM2.5 AQI interpretation in the U.S. uses a 24-hour average | May show an instantaneous value, a short rolling average, or another interval stated by the manufacturer |
| Authority | Used for public alerts and coordinated action guidance | Useful for seeing indoor changes, but not a substitute for the local agency’s alert |
| Why values differ | Outdoor monitor location, regional averaging, and reporting timing | Room location, nearby cooking or candles, sensor calibration, humidity, and the chosen averaging period |
When a home display conflicts with the public AQI, first check whether both values cover the same time period and pollutant. Move the sensor away from a stove, purifier outlet, window, or direct smoke source; confirm its maintenance and calibration information; then use the official AQI and emergency messages for decisions affecting health and safety.
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| Situation | Best next step | Why |
|---|---|---|
| Moderate AQI, no symptoms | Normal activity with routine awareness | Most people can tolerate the conditions, but individual responses differ. |
| AQI 101–150 and you are in a sensitive group | Reduce prolonged or heavy outdoor exertion | Less intense breathing lowers the amount of polluted air inhaled. |
| AQI 151–200 | Everyone limits prolonged or heavy exertion; sensitive people avoid strenuous outdoor activity | The category signals a broader population risk. |
| AQI 201–300 | Sensitive people stay indoors with low activity; others avoid prolonged or heavy exertion | Outdoor exposure is high enough that ordinary exertion can substantially increase inhaled dose. |
| Official shelter, evacuation, or cleaner-air-shelter instruction | Follow the agency’s location-specific direction | The instruction may account for fire behavior, heat, and hazards not captured by one AQI number. |
| Closed windows cause dangerous indoor heat | Move to a cooler cleaner-air shelter or another safer location | Heat illness can make remaining in a sealed, overheated room more dangerous. |
Heat, symptoms, and higher-risk households
Heat can override the closed-window plan
Keeping smoke out is not a reason to remain in an dangerously hot room. If air conditioning is unavailable or cannot cool the space safely, use a designated cleaner-air shelter, a public facility identified by local authorities, or another location with safer temperature and air quality. Take water, medicines, chargers, and any needed medical equipment when relocating.
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Know when to seek medical help
Breathing difficulty, chest pain, severe wheezing, bluish lips, confusion, or rapidly worsening symptoms require urgent medical attention. Follow your asthma or heart-disease action plan; do not wait for a sensor to reach a particular number. For less severe but persistent irritation, contact a clinician, especially if you are in a high-risk group.
Plan before smoke arrives
- Identify the local air-agency AQI page and emergency-alert system.
- Check that windows, doors, air-conditioner settings, and HVAC filters are ready.
- Keep a suitable portable purifier and replacement filter available if your household relies on one.
- Decide which room will be the cleaner-air room and where you would go if heat makes staying home unsafe.
- Write down medication and medical-device needs for children, older adults, and people with heart or lung disease.
A practical smoke-day checklist
Before outdoor exposure
- Look up the current AQI category and the agency’s health message, noting the reporting time.
- Check whether the value is an official 24-hour PM2.5 AQI or a short-term sensor reading.
- Reschedule strenuous exercise and outdoor work when the category calls for reduced activity.
While smoke is present
- Close windows and doors, set HVAC to recirculation when possible, and run filtration.
- Avoid smoking, candles, fireplaces, and smoky cooking indoors.
- Keep activity light indoors during severe episodes and monitor vulnerable household members.
- Recheck official alerts rather than relying only on an indoor display.
When conditions improve
- Wait for the official AQI and local guidance to improve before reopening windows or resuming hard outdoor exercise.
- Ventilate only when outdoor air is cleaner than indoor air and heat conditions permit.
- Inspect purifier and HVAC filters and replace them according to their instructions.
Key points to remember
- PM2.5 is fine particle pollution small enough to penetrate deep into the respiratory system.
- The AQI supplies the practical health category; averaging period and breakpoints determine what a displayed value means.
- In the U.S. EPA system, sensitive people change behavior at AQI 101, and the general population faces risk above AQI 150.
- Closed windows, recirculated air, clean filters, portable or central filtration, and avoiding indoor particle sources can reduce exposure.
- Heat safety, symptoms, and official emergency instructions can outweigh a simple stay-inside rule.
Frequently Asked Questions
Why can my indoor PM2.5 sensor stay high after I close the windows?
Particles that entered earlier can remain suspended, and indoor sources such as cooking, candles, fireplaces, smoking, or a purifier placed in a poorly ventilated position can continue adding or redistributing them. Check the sensor’s averaging period and location, stop indoor particle sources, run appropriately sized filtration, and compare the trend with the official outdoor AQI.
Should I exercise indoors when the outdoor AQI is very unhealthy?
Only if the indoor space is genuinely cleaner and you can keep exertion light. During AQI 201–300, sensitive people should remain indoors and keep activity low, while everyone else should avoid prolonged or heavy exertion. Indoor air is not automatically clean if smoke has entered or indoor sources are active.
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What should I do if smoke and extreme heat occur together?
Do not remain in a dangerously hot sealed room. Seek a cooler cleaner-air shelter or another safer location, and follow local emergency instructions. Heat illness can become an immediate hazard even while you are trying to reduce particle exposure.
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