Unhealthy Air Days
Days when the AQI exceeds 100, meaning air quality may pose health risks for sensitive groups or the general population.
Detailed Explanation
Unhealthy air days are days when the AQI surpasses 100, entering the "Unhealthy for Sensitive Groups" category or higher. The EPA defines sensitive groups as including children, older adults, people with heart or lung disease, and active adults who exercise or work outdoors. As AQI climbs beyond 150 (Unhealthy), 200 (Very Unhealthy), and 300 (Hazardous), the health risk extends to the entire population. AirHistory tracks unhealthy air days as a key component of the Air Quality Grade because the number of days with poor air quality is often more relevant to health outcomes than average AQI alone. A city with an average AQI of 55 but 30 unhealthy days per year presents different health challenges than a city with the same average but only 5 unhealthy days — the spikes matter. The primary drivers of unhealthy air days vary by region: wildfire smoke dominates in the West, ozone in the Sun Belt and Southeast, and a mix of PM2.5 from industry and transportation in the Midwest and Northeast. AirHistory data shows that unhealthy air days have increased in many western cities over the past decade, even as overall average AQI has improved nationally, largely due to the increasing frequency and severity of wildfire seasons.
Related Terms
Air Quality Index (AQI)
A standardized EPA scale from 0 to 500 that communicates daily air quality and associated health risks.
AQI Categories
The six color-coded health concern levels — Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous — used by the EPA to describe air quality.
Sensitive Groups
Population groups more vulnerable to air pollution, including children, older adults, pregnant women, and people with heart or lung conditions.
Air Quality Grade
AirHistory's proprietary A-F scoring system that rates cities based on average AQI, trend direction, unhealthy days, and dominant pollutant.
Frequently Asked Questions
Days when the AQI exceeds 100, meaning air quality may pose health risks for sensitive groups or the general population.
Unhealthy air days are days when the AQI surpasses 100, entering the "Unhealthy for Sensitive Groups" category or higher. The EPA defines sensitive groups as including children, older adults, people with heart or lung disease, and active adults who exercise or work outdoors. As AQI climbs beyond 150 (Unhealthy), 200 (Very Unhealthy), and 300 (Hazardous), the health risk extends to the entire population.