US AQI Scale — What the Number Means for Your Health
The US AQI is calculated by the EPA from the worst pollutant reading in your area at the time — typically PM2.5 during wildfire or high-traffic periods, ozone on hot summer days.
0–50
Good
No health concern. Outdoor activity unrestricted. Air purifier on auto or low is fine.
51–100
Moderate
Acceptable. Unusually sensitive individuals may notice effects. Run purifier at medium indoors.
101–150
Unhealthy — Sensitive Groups
Asthma, allergy, COPD, elderly, children: limit outdoor exertion. Run HEPA purifier at medium-high indoors. Close windows.
151–200
Unhealthy
Everyone may experience effects. Keep windows closed. Run purifier at high continuously.
201–300
Very Unhealthy
Avoid outdoor activity. Seal door gaps. High-CADR purifier running at max speed indoors is essential.
301+
Hazardous
Stay indoors. Seal all gaps. N95 mask if movement required. Run the largest available purifier at max speed.
When to Run Your Air Purifier Based on AQI
A HEPA air purifier reduces indoor PM2.5 by 80–95% within 30 minutes in a closed room when correctly sized. Here is how to calibrate it to the outdoor AQI reading:
AQI 0–50: Auto mode or low speed. Focus on indoor sources (cooking, cleaning, pets) rather than outdoor infiltration.
AQI 51–100: Medium speed. Close windows during peak traffic hours. A CADR 150+ unit maintains clean indoor air even with moderate outdoor PM2.5 infiltration.
AQI 101–150: Medium-high, continuous. Close windows and doors. For sensitive individuals, this is the threshold where indoor purification becomes clinically meaningful.
AQI 151+: High speed, continuous. Close every window and door. Seal gaps under doors with a towel if AQI is 200+. A correctly sized purifier (CADR ≥ 4× room volume ÷ 60) reduces indoor PM2.5 to near-zero within an hour even at AQI 200.
See our CADR guide to calculate the right purifier size for your room and the AQI levels you typically experience. For wildfire-specific guidance, see air purifier for wildfire smoke.
Why Indoor PM2.5 Can Be Worse Than the Outdoor AQI Shows
The AQI reading above reflects outdoor conditions at the nearest monitoring station. Your indoor PM2.5 can be significantly higher even on low-AQI days due to indoor sources:
Gas cooking: A 10-minute frying session on a gas stove can spike indoor PM2.5 to 150–400 µg/m³ — equivalent to AQI 200+ — even when outdoor air is Good. Running the range hood and a nearby HEPA purifier simultaneously reduces this by 70–90%.
Candles and incense: Burning a single paraffin candle raises indoor PM2.5 to 50–100 µg/m³ in a closed room within 20 minutes.
Pet dander: Households with cats or dogs have baseline indoor PM2.5 20–40% higher than the outdoor reading due to continuous dander generation. See our best air purifier for pets for sizing recommendations.
Running a HEPA purifier continuously at low speed addresses these indoor sources independent of what this tool shows for outdoor conditions.
Frequently Asked Questions
What is a good AQI level?
AQI 0–50 is Good — no health concern for any group. AQI 51–100 is Moderate — acceptable for most people. AQI 101–150 is Unhealthy for Sensitive Groups — people with asthma, allergies, COPD, the elderly, and children should reduce outdoor exertion and run a HEPA air purifier indoors. Above 150, everyone is at risk and indoor purification should run continuously.
When should I run my air purifier based on this AQI reading?
Run your purifier at medium speed when AQI exceeds 50. Run it at high continuously when AQI exceeds 100. Close all windows and doors when AQI exceeds 100 — opening windows at AQI 150 defeats the purifier by constantly introducing polluted outdoor air. At AQI 200+, seal gaps under doors. A correctly sized HEPA purifier (CADR ≥ your room's sq ft × 0.75) reduces indoor PM2.5 by 80–90% even at AQI 150 in a closed room.
What does PM2.5 mean on this air quality checker?
PM2.5 is fine particulate matter — airborne particles 2.5 microns or smaller. These particles are small enough to penetrate deep into the lungs and enter the bloodstream. The WHO 24-hour safe limit is 15 µg/m³. The EPA annual standard is 9 µg/m³. PM2.5 is the metric with the strongest health evidence and is what a True HEPA air purifier is specifically designed to capture (at 99.97% efficiency for particles 0.3 microns and larger).
Why does indoor air quality differ from this outdoor AQI reading?
Indoor PM2.5 sources — gas cooking, candles, cleaning sprays, off-gassing furniture, pet dander, and mold — raise indoor concentrations 2–5× above outdoor levels independently of outdoor conditions. A household with cats in a city with AQI 30 can have indoor PM2.5 of 20–35 µg/m³ from dander alone. Running a HEPA purifier continuously at low speed is recommended for sensitive individuals regardless of what the outdoor AQI shows.
How accurate is the AQI data on this tool?
Data comes from WAQI (World Air Quality Index), which aggregates readings from certified government monitoring stations — the same networks that feed official EPA and local authority air quality alerts. These are regulatory-grade reference instruments, not consumer sensors. Readings represent the nearest station to your city, which may be several kilometres from your exact address. For hyperlocal real-time PM2.5 at your location, a dedicated indoor monitor like the IQAir AirVisual Pro provides on-site measurement. See our air quality monitor buying guide for options.
Find the Right Air Purifier for Your AQI Level
Top picks ranked by AHAM-verified CADR — sized to maintain clean indoor air even when outdoor AQI is elevated.