Filters with a Merv rating of 17 to 20 are rarely needed in residential homes. A Merv rating of 13 to 16 is considered hospital-level air quality, so it's unlikely that your home will require more than that. An additional consequence of reduced air flow is seen on the cooling side. The air conditioner has a hot coil outside, the condenser, and a cold coil in the air handler inside.
This cold coil has an ideal operating temperature based on the amount of hot air flowing through it and the coolant passing through its copper tubing. With a higher Merv filter that reduces airflow through this essential part of the air conditioning system, the cold coil can eventually freeze into an ice ball, blocking airflow completely. Unprocessed, this leads to more expensive problems in the outdoor condenser coil. In general, filters with higher Merv ratings capture higher percentages of particles, as well as smaller particles.
A Merv 13 filter is usually sufficient. If the Merv rating is too high for the oven or air handler blower motor to handle, the result will be low airflow. Most modern units in recent years should be able to have a Merv 8 filter at least. To avoid confusion when buying an oven filter, purchase replacement air filters from Star Heating & Cooling.
Never cascade filters because the cumulative constraint on the system will be too great. Merv 13 filters also remove bacteria, tobacco smoke, car fumes, insecticide dust, pet dander and more. As an HVAC professional for over 40 years, I have seen numerous systems destroyed by using highly restrictive Merv filters. Merv ratings determine the capabilities of an air filter and its level of filtration efficiency.
If you're in a situation where you shouldn't use a high Merv filter (12+) for use in ovens or air conditioners and would like to research air purifiers, here are two suggestions. Buy washing alcohol that could create some strange chemicals, unless you are just washing cotton; then you would only use sunlight to disinfect them, even that could break the tall Merv filters a little and release unintentional, possibly dangerous chemical degassing. A high activated carbon filter could be your best option between you and any other exotic chemical action you have going on. Fortunately I have a 3D printer and have printed a face mask with a Merv 13 filter.
Before making any changes to the air filter of an HVAC system, users should consult their HVAC manual or an HVAC professional. Keep in mind that as the Merv rating increases, the filter becomes more restrictive and more pressure and energy will be needed to propel the air. Some HVAC technicians prefer these low-efficiency filters because of their low resistance to airflow. If you want to use pleated filters and superior Merv filters, the only safe way to do this is to have a 5-6 inch media filter installed by a professional.