Generally, a negative air unit/air scrubber will be placed inside the contained area to exhaust out the negative air.

To calculate how many air scrubbers you need - you will need to do an Air changes per hour calculation -

The air scrubber flow rate CFM is divided by the cubic volume of the contaminant space and then multiplied by 60 to convert minutes to hours.

Let’s say the enclosure is 25’ x 32’ x 12’ and that 4 air exchanges per hour are required.


  • Calculate the cubic volume - 25’ x 32’ x 12’ = 9600 ft³
  • Thus 1 air change is = 9600 ft³
  • Therefore 4 air exchanges is = 38400 ft³
  • Convert 38 400 ft³ per hour to ft³ per minute (divide by 60)
  • = 640 ft³ per minute (cfm)

If the machine you are using is rated at 500 CFM, you will need 2 machines to provide the correct air filtration.