What AQI is safe to go outside?

Under 50: perfectly safe. 51-100: fine for most people, use judgment if you're sensitive. 101-150: OK for brief exposure for healthy adults, but sensitive groups (children, elderly, asthma) should limit exertion. Above 150: everyone should limit outdoor time and exertion. Above 200: stay indoors with windows closed.

What AQI level requires an air purifier indoors?

If outdoor AQI regularly exceeds 100 (especially during wildfire season), a HEPA purifier indoors is strongly recommended — it can keep indoor AQI under 50 even when outdoors is 150+. If your outdoor AQI rarely exceeds 50, a purifier is optional. Also consider one if indoor sources (cooking, pets, new furniture) are your problem.

AQI 100 is the boundary: 0-100 is 'good to moderate' (green/yellow), and 101+ is where 'unhealthy for sensitive groups' begins (orange). For healthy adults, 100 is mildly noticeable at most; for children, the elderly, and people with asthma or heart conditions, it's the threshold where they should start reducing prolonged outdoor exertion.

What does AQI 300 mean?

AQI 300+ is 'Hazardous' (maroon) — the highest category. Emergency conditions: everyone should stay indoors, windows closed, and run air purifiers. It occurs during extreme wildfire events or severe inversions. At this level, outdoor exposure is genuinely dangerous, and indoor air with filtration is dramatically safer than outdoor air.

What Is a Good AQI Level? The Complete Guide to Air Quality Numbers

Q: What is a good AQI level?

An AQI under 50 is 'Good' (green) — air quality that poses little or no risk. 51-100 is 'Moderate' (yellow) — acceptable for most people, with sensitive individuals advised to watch their activity. Under 50 is the number to aim for indoors; most homes with a HEPA purifier running can hold indoor AQI well under 50 even when outdoor AQI is high.

“Air quality is 87 today.” “AQI is 145.” “We’re under a purple air alert.”

These phrases float through weather reports every year — and most people have only a vague sense of what the numbers mean. Here’s the complete, practical guide: what’s good, what’s not, and exactly what to do at every level.

🧰 Interactive Tools

🌡️AQI ExplainerEnter any number, get the verdict→ 📏Room Size CalculatorIs your purifier big enough to hold the line?→

The Complete AQI Scale (EPA)

Checking air quality on a smartphone at a park

AQI	Color	Category	What to do
0-50	🟢 Green	Good	Nothing. Enjoy the outdoors.
51-100	🟡 Yellow	Moderate	Fine for most. Sensitive groups: watch activity.
101-150	🟠 Orange	Unhealthy for sensitive groups	Children, elderly, asthma: limit exertion. Healthy: OK for normal activity.
151-200	🔴 Red	Unhealthy	Everyone: limit prolonged outdoor exertion.
201-300	🟣 Purple	Very unhealthy	Everyone: avoid outdoor exertion. Sensitive: stay indoors.
301-500	🟤 Maroon	Hazardous	Everyone: stay indoors, windows closed, run purifiers.

The “Good” Answer, Precisely

A good AQI is 0-50 — the green zone where air pollution poses little or no risk. That’s the target for both outdoor days and indoor air.

For context, most US cities report AQI under 50 on clean days and spike to 100-300 during wildfire events or winter inversions. The WHO’s stricter guideline would classify some “Moderate” EPA days as unhealthy — so if you’re sensitive, treat 51-100 with more caution than the EPA suggests.

Indoor vs Outdoor: The Number That Matters Most

Here’s the part most people miss: the outdoor AQI is not your indoor AQI. With windows closed and a HEPA purifier running, indoor AQI can hold under 50 even when outdoor AQI is 150+.

Outdoor AQI	Indoor without purifier	Indoor with purifier
50 (Good)	~30-40	~10-20
100 (Moderate)	~70-85	~20-35
150 (Orange)	~110-130	~30-45
200 (Red)	~150-180	~40-60
300 (Purple)	~250+	~60-90

The purifier is the difference between “the air outside is terrible” and “my home is still fine.”

The Action Cheat Sheet

Situation	Action
Outdoor AQI < 50	Windows open, normal life
Outdoor AQI 51-100	Normal for healthy; sensitive: limit hard exercise
Outdoor AQI 101-150	Close windows if you’re sensitive; run purifier
Outdoor AQI 151-200	Windows closed, purifier on, limit outdoor time
Outdoor AQI 200+	Stay in, windows sealed, purifier on high
Indoor AQI > 50 (with purifier)	Check windows, filter age, unit size

When You Need a Purifier (Based on Your AQI)

The honest rule: if your area hits AQI 100+ more than a couple weeks a year, buy a purifier. If you live in wildfire country, a HEPA purifier is a seasonal necessity. If your city rarely exceeds 50, it’s optional.

The Bottom Line

A good AQI is under 50 — that’s the green zone for outdoor days and the realistic target for indoor air. The scale is simple: green is fine, yellow is watch, orange is sensitive-groups-only, red and above is stay-in. And remember the number that actually matters for your health: your indoor AQI, which a properly sized HEPA purifier can keep under 50 even on the worst outdoor days.