Mold is one possibility.... another might be a reaction to what is being burned for fuel in your area for power or heat.

Our Air quality randomly is randomly showing increased particulates, probably from either the power plant on the river or the Steam Plant that Harvard has that runs the centralized heating system for a bunch of their older buildings.

Another possibility would be local wood stoves or oil burners that don't have proper filter systems (or ones that haven't been properly maintained) and are throwing stuff into your air.