Why do we sometimes see higher levels of PM2.5 in the winter in Western Mass?
Many people use wood furnaces, stoves, or fireplaces for heating in the winter months, and smoke from wood burning contributes to higher PM2.5 levels. This can be a particular problem in river or stream valleys, which are prone to temperature inversions that can trap pollutants near the ground.
A temperature inversion is when a layer of cold and dense air gets trapped underneath a layer of warmer air (this is opposite from the usual situation in which air near the ground is warmer than higher up). Inversions tend to happen on clear cold nights with calm winds, which allow the ground to cool off quickly. The inversion acts like a lid, trapping moisture (which can cause fog) and pollutants (which worsen the air quality). Sometimes you can tell there is an inversion when you see smoke rising from vertically a chimney and then flowing horizontally when it hits the cold air layer.
During the day, the sun tends to warm up the ground and the air next to it, dispersing the inversion. As a result, air quality in the winter often worsens at night and improves during the day.
Keep an eye out for increased PM2.5 levels this winter season on healthyairnetwork.org.
Last modified: January 13, 2026
