At a glance
Your quick answer
The practical decisions most people need to make when the AQI reaches 300.
- AQI
- 300
- Category
- Very unhealthy
- Outdoor exercise
- Avoid entirely
- Running
- Do not run outdoors
- Children
- Keep indoors
- Older adults
- Keep indoors
- Asthma
- High attack risk
- Heart disease
- Seek advice on symptoms
- Pets
- Indoors
- Windows open
- No — seal gaps
- Air purifier
- Essential
- Mask
- N95 required outdoors
AQI scale
300 sits in the Very Unhealthy range
The AQI runs from 0 to 500. A value of 300 triggers a health alert: the risk of serious effects applies to the whole population, not just sensitive groups. At 300 it sits near the top of the Very unhealthy band — close to tipping into the next category, so watch the trend rather than the single reading.
The meaning
What does AQI 300 mean?
What AQI measures
AQI translates pollution levels into one health-focused number. The higher the number, the greater the potential health concern.
Why 300 is Very Unhealthy
AQI 300 falls between 201 and 300. The EPA classifies this as a health alert, with everyone at risk of experiencing serious health effects.
What drives the number
Readings this high usually mean a severe PM2.5 episode — winter inversions, crop-residue burning or wildfire smoke holding particles at ground level.
Treat this as an emergency for vulnerable people
Anyone with asthma, COPD, heart disease or a recent respiratory infection is at real risk of an acute event. Keep medication within reach, do not wait to see whether symptoms settle, and get medical help for chest pain, severe breathlessness or a reliever inhaler that stops working.
Personal guidance
Is AQI 300 safe for you?
There is no group for whom this level is safe outdoors. The differences below are about how urgently to act.
General population
Stay indoors with filtered air. Cancel outdoor plans and keep any trip outside to minutes.
Children
Keep children indoors. Schools should cancel all outdoor activity at this level.
Pregnant women
Stay in filtered air. Exposure at this level is linked to adverse outcomes for both mother and baby.
Older adults
Remain indoors and avoid exertion. This band drives measurable rises in hospital admissions.
Asthma
High risk of an attack. Stay in filtered air, keep your reliever on you and escalate early.
COPD
Exacerbation risk is significant. Contact your clinician if your usual symptoms worsen.
Heart disease
Heart attack and arrhythmia risk rises sharply in this band. Treat chest symptoms as an emergency.
Pets
Keep pets indoors except for the briefest toilet break.
Plan your day
Outdoor activity guide
At AQI 300 there is no safe outdoor activity level. Every entry below assumes you have already moved what you can indoors.
| Activity | Recommendation |
|---|---|
| Walking | Essential trips only |
| Running | Do not run outdoors |
| Cycling | Do not cycle outdoors |
| Football | Cancel |
| Hiking | Cancel |
| Gardening | Postpone |
| Outdoor work | N95 and shortened shifts |
At home
Indoor recommendations
Open windows?
No
Keep everything shut at AQI 300 and seal obvious gaps under doors. Use recirculate on air conditioning.
Use an air purifier?
Essential
Run True HEPA units continuously in every occupied room. Under-sizing the unit is the most common mistake.
Run ceiling fans?
Yes, with purifiers
Fans mix filtered air through the room. Without a purifier they simply stir particles around.
Vacuum?
Postpone
Damp-mop and damp-dust instead. Vacuuming re-suspends particles you cannot afford to add.
Dry clothes outside?
No
Everything left outside becomes a carrier for particles. Dry indoors.
Keep pets indoors?
Yes
Minutes outdoors, not walks. Watch for coughing or laboured breathing.
Do you need an air purifier?
Yes — this is the level a purifier is built for. At AQI 300 a correctly sized True HEPA unit running continuously behind closed windows is the difference between a clean room and a polluted one. Cover the bedroom first, then wherever the household spends the day.
Browse recommended air purifiersDo you need a mask?
Yes — a well-fitted N95, KN95 or FFP2 for any time outdoors, without exception. Check the seal across the bridge of the nose; a leaking mask offers close to no protection. Cloth and surgical masks are not adequate at this level.
Health effects
What you might notice
At this level symptoms appear quickly, in people who normally have none, and vulnerable people can deteriorate fast.
Healthy adults
Headache, sore throat, burning eyes, cough and noticeable fatigue.
Children
Coughing and wheezing at rest; play alone is enough to trigger symptoms.
Asthma
Attacks are likely; reliever use rises sharply and can prove insufficient.
COPD
Severe breathlessness and a real risk of an exacerbation needing treatment.
Allergies
Persistent irritation of eyes, nose and throat throughout the day.
Heart disease
Elevated risk of heart attack and arrhythmia — seek urgent care for chest symptoms.
Simple actions
Practical tips for AQI 300
- Stay indoors and keep every window and door shut.
- Run HEPA purifiers continuously in occupied rooms.
- Cancel all outdoor exercise and outdoor school activity.
- Keep asthma and heart medication within reach, and escalate early.
FAQ
AQI 300 questions, answered
Is AQI 300 dangerous?
Yes. AQI 300 is classified as Very Unhealthy, a health alert under which everyone — not only sensitive groups — is at risk of serious health effects. Stay indoors with filtered air and avoid outdoor exertion completely.
Can I run at AQI 300?
No. Running at AQI 300 would multiply your intake of fine particles at a level already flagged as a health alert. Train indoors in a filtered space, or skip the session.
Is AQI 300 safe for babies?
No. AQI 300 is unsafe for babies and toddlers. Keep them indoors with a HEPA purifier running, keep windows shut, and seek medical advice for coughing, wheezing or laboured breathing.
Should I wear an N95 mask at AQI 300?
Yes, always, for any time spent outdoors at AQI 300. Use a well-fitted N95, KN95 or FFP2 and check the seal over the nose. Cloth and surgical masks do not protect against PM2.5.
Should I use an air purifier at AQI 300?
Yes — continuously, in every room you occupy. At AQI 300 a True HEPA purifier sized correctly for the room is the most important indoor protection you have.
Is AQI 300 unhealthy for asthma patients?
Dangerously so. At AQI 300 the risk of an asthma attack is high. Stay in filtered indoor air, keep your reliever with you, follow your action plan and seek urgent medical help if the inhaler is not working.
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