When Is Air Quality Unhealthy? AQI Levels Explained

On the U.S. Air Quality Index (AQI), 101–150 is “Unhealthy for Sensitive Groups,” while 151–200 is “Unhealthy” for everyone. At 201–300, air quality is “Very Unhealthy,” and 301 or higher is “Hazardous,” so the number helps you judge both who may be affected and how urgently to limit exposure.

An AQI of 51–100 is “Moderate,” not unhealthy for most people, but someone who is unusually sensitive to air pollution may notice symptoms. If you have asthma, another heart or lung condition, or symptoms such as coughing or shortness of breath, pay attention to how you feel rather than treating a moderate reading as an all-clear.

At 101 or above, people in sensitive groups should consider shortening or easing prolonged outdoor activity; at 151 or above, everyone should consider doing so, especially if symptoms develop. You can also keep windows closed when outdoor air is poor, avoid adding indoor smoke, and use ventilation or air filtration when it is appropriate for your space and the outdoor conditions.

Outdoor AQI describes outdoor pollution, so it does not automatically tell you what the air is like in every room; cooking, candles, cleaning products, and moisture can create separate indoor concerns. Our team put together Breathe Better Indoors, Live Better: A Practical Guide to Improving Indoor Air Quality to help you think through everyday room-by-room habits, including reducing pollution at its source and choosing when to bring in outdoor air.