ACH stands for “Air Changes per Hour” and is one of the least understood ratings on air purifiers. The ACH rating indicates how many times per hour and air purifying device can exchange the air within a room. For example, an ACH rating of 5x means that the full air volume is exchanged five times every hour. All air purifiers display a Cubic Feet per Minute (CFM) number under the specifications. If you multiple the CFM by 60 (minutes per hour) and divide by 8 feet (average ceiling height). Then divide that total by the ACH rating, you’ll end up with the maximum square footage number for the air purifier.

Air changes per hour, or air change rate, abbreviated ACH or ACPH, is a measure of the air volume added to or removed from a space (normally a room or house) divided by the volume of the space. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced.