Hepa Filters - How to Figure Out True Air Cleaning Capacity

Q. The air cleaning capacity of HEPA air filter machines confuses me. The same top rated machine is listed in several different catalogs with stated air changes per hour that vary from two to twenty. So what is the truth?

A. Disregard the air changes per hour data on a hepa filter - it can be misleading. The critical information is CFM (cubic feet per minute) of air flow. Put an air filter in a bigger room and the air changes per hour are smaller; in a smaller room, air changes will be more frequent. Two air changes per hour for a hepa air filter is absolutely minimal. We like to recommend a minimum of four per hour for allergy sufferers. At night you will want to put your air filter fan on low speed for noise level comfort. 400 CFM hepa filter machines, (the top rated domestic use air filters) generally run at 75 CFM on low speed. On high fan speed, they will clean 750 sq. ft. with four air changes per hour; but on low fan speed, hepa air filters will only clean 140 sq. ft. (12'x12' moderate size bedroom). Don't even bother with machines with less then 400 CFM cubic feet per minute air delivery.