Air filters
I frequently recommend at least a quality pleated air filter over the cheapie blue or green see through type as shown below.
MERV ratings range from 1 – 16 and measurements are in microns. Some of the common particles related to MERV ratings are pet dander, insecticide dust, smog, dust, viruses, wood, tobacco smoke, spores, bacteria and pollen. Some of the most common filters found in residential use only have a merv rating of 1 to 4. I call these “bird stoppers” These are typically disposable panel type filters and do not do a good job of filtering the air because they will not stop particles smaller than 10 microns.
There are some manufacturers that have specific recommendations for their equipment, which should be followed for warranty purposes.