2021 saw the number of days with moderate air pollution and air quality alerts return to conditions sometimes seen prior to the COVID-19 pandemic. However, the air quality alerts occurred not due to usual sources of pollution such as transport from upwind states in summer or local woodsmoke in winter, but from wildfire smoke in summer 2021 that affected much of the northeast and primarily originated in Alberta, Canada. Wildfire smoke events as seen in Vermont in 2021 generally only occur about once a decade. While the target number of days with poor air quality is zero, in reality this is unrealistic because air quality is affected by natural phenomenon (such as wildfire smoke) and emissions from upwind of Vermont's borders. These factors, in addition to changes in meteorology from year to year, result in great year-to-year variability, but there is a significant improvement in air quality on a decadal scale
This metric counts the number of days per year that the “air quality index” (AQI) was categorized as “Moderate” or “Unhealthy for Sensitive Groups” (USG) for at least one pollutant at one monitoring site based on the National Ambient Air Quality Standards (NAAQS) for ozone and fine particulate matter (PM2.5). Air quality in the USG range exceeds the federal air quality standards (i.e., is worse than the standard); Moderate air quality still poses some risk to those with respiratory illnesses and can have additional environmental and visibility impacts. Ozone is monitored in Bennington, Rutland, and Underhill; PM2.5 is monitored in these locations, as well as in Burlington.