Air filters are rated from 1-12 based on their minimum efficiency reporting value, or MERV.  The higher the MERV, the better the filtration it provides. But a higher number isn’t always better. The better the filtration, the more energy needed to pull air through, the higher your electricity bill. You’re probably going to want to compromise a bit here.  Certified Heating and Cooling recommends a rating between 5 and 8.