When shopping for air purifiers, look for the Clean Air Delivery Rate (CADR) rating. This tells you how much clean air the device actually delivers, accounting for not only the filter's capture efficiency but also -- most importantly -- the volume of air the fan can move through that filter.
While HEPA filters have higher single-pass efficiency for fine particles (99.97% vs. <75% for MERV-13), their much higher resistance significantly reduces airflow (CFM) through portable devices, meaning they process less total air volume over time. A MERV-13 filter in a portable unit can achieve higher air changes per hour in a classroom because it allows the fan to move substantially more air, and multiple passes through a less restrictive filter can ultimately clean more of the room's air than fewer passes through a more efficient but restrictive HEPA filter.
To calculate the CADR you need for your space:
Example: A 30 × 30 × 9 ft classroom = 8,100 ft³ × 6 ÷ 60 = 810 cfm needed
Here's a table to give you a rough sense of typical classroom requirements:
| Classroom Size | CADR Needed |
|---|---|
| Small (30 × 25 × 9 ft) | 675 cfm |
| Typical (30 × 30 × 9 ft) | 810 cfm |
| Large (35 × 30 × 9 ft) | 945 cfm |
Can't find a single unit with enough CADR? Use multiple units—their CADRs add together.
Product labels show CADR at the highest (loudest) setting. When you turn down the fan for quiet operation, the actual CADR can drop 50% or more. Choose units that are quiet enough to run at full speed, or accept that you'll get less cleaning than the label promises.