The most common types of air filters are fiberglass media and pleated media filters.
What you use and how often you change them depends on the type and size of filter, number of people in the home, pets, air filter location, etc.
A good rule of thumb on how often to replace residential HVAC air filters is with a “1 month per inch” schedule. For example, a 1” filter should be replaced each month, 2” filters every 2 months, 4” filter every 4 months.
HVAC air filters use a MERV (Minimum Efficiency Reporting Values) rating of 1-20. The higher the MERV rating, the better it is at removing small particles. Fiberglass filters are generally MERV 1 – MERV 4 and do well at catching dust bunnies but that’s about it. At the high end are HEPA filters (MERV 17+), they are capable of capturing particles as small as viruses.
But don’t run out the door and buy a HEPA filter for your furnace to catch all the viruses in your home without doing some homework. If your system was designed for a MERV 6 filter a HEPA filter could do more harm than good. The higher the MERV rating the more restrictive it is to airflow and restricted airflow is one of the top causes of damage to a heating and cooling system.
You should use the type and size that the system was originally designed for. And never run the system without a filter.