As simple as it may seem, most people have very little knowledge of how an air filter, home, car, industrial, commercial institutional varieties actually work. There is an assumption that the filter media itself does all the filtering and the more dense the media the better. While denser media is certainly more effective at removing finer, smaller particles, these types of filter media also have a much higher resistance or pressure drop both initially and when they load up with dust and dirt. Also, the filter media is really just the base for what really does all the filtering of the fine, sub-micron airborne particulate.
Most filters today use a MERV rating to indicate their level or efficiency with the higher number being more dense and more effective at initially removing fine particulate and the lower numbers being more of a general duty filtering capability. However, if a MERV 18 filter is best for fine particulate removal even a MERV 6 or 8 rated filter will eventually reach the same overall efficiency of the MERV 18 filter, and here is the reason.
Even with “see through” filter medias, the internal areas of the filter fibers will, over time, fill up with larger airborne particles and make the passage ways through the media smaller and smaller eventually capturing even sub-micron dust particles as they are trapped in the now mostly blocked filter media. Unfortunately as the filter loads up the air resistance through the filter increases to the level where is significantly restricts the airflow through the filter and will reduce the performance of the fan and/or air handler.
Therefore, most HVAC filters are relatively open or low on the MERV scale so they will last at least 3 months before needed to be changed. For applications where a MERV 18 filter or true HEPA filter is required, a more powerful blower/motor assembly will be required along with a prefilter to keep the final filters from loading up too quickly.
A good, medium duty, Merv 6 or 7 rated filter is fine for most general duty applications. These give you best filtration performance over the life of the filter.