Here is a page that explains the "MERV" rating.
http://www.furnacefiltercare.com/merv-ratings.php The air filter that came with my Designer's air unit 2 years ago was installed by Munn's and is rated MERV 8. Munn's sells replacements for about $50.00. They are also available from Shortys
http://shortyshvac.com/ for less. Mine lasted a year and the Munn's guy said it did not look that bad but it was a year old so it was changed. We do not have allergies and the filter seems to be fine. If we had allergies I would go to a MERV 10, also available from the above site. What is the MERV rating of the April Aire?