ROI and Fallow Time Calculator

Fallow time Calculator
To calculate fallow time using the CDC’s formula, you require the room volume, air flow and a multiplier called K-factor.
k-Factor is the multiplier that represents the efficiency of air exchange for when calculating fallow time.
  • 0: Perfect elimination of generated aerosols from their source (Viral-Vac™ DentalVac)
  • 0.25: Most aerosols removed from their source (Viral-Vac™ DentalVac with safety margin)
  • 1: A perfect exchange. i.e. air entering one end of a tube and then exiting out the other.
  • 3: CDC uses 3 to represent isolation rooms in hospitals.
  • 4-6: Floor standing HEPA filters with high air flow in the centre of the room. (Use 4 for the Vortex)
  • 7-9: HEPA filters placed in corners depending on CFM and the ability to push air to the opposite corners.
  • 10: Wall mounted HEPA filters generally do not have enough air flow to be effective at remote corners of a room.
The RCDSO uses a k-factor value of 1 for calculating fallow time. The CDC does not recommend this. CDC does recommend negative pressure isolation rooms with 12 air exchanges/hour (ACH).



Please enter the dimensions of your room and the number air exchanges (ACH) at your office.

( m)

( m)

( m)

Room Volume: ft3 ( m3)


Fallow Time (pre COVID): 0 minutes

Fallow Time (COVID, no changes to office): minutes (as per RCDSO assume 180 minutes if no knowledge of HVAC system)

Fallow Time (with Viral-Vac™ DentalVac): minutes (ACH=)

Fallow Time (with Viral-Vac™ Vortex): minutes (ACH=)

Compare to your system. Please select the airflow of your filter and its efficiency (k-factor)

CFM of comparison filter ( l/min)

k-factor of comparison filter and room configuration:

Fallow Time of comparison filter: minutes (ACH=)


Revenue (pre COVID): $ per day

Revenue (COVID, no changes to office): $ per day

Revenue Potential (with Viral-Vac™ DentalVac): $ per day

Revenue Potentail (with Viral-Vac™ Vortex): $ per day


ROI of Viral-Vac™ DentalVac: (days)

ROI of Viral-Vac™ Vortex: (days)

Assumes only 1 operatory. Adapted from: Centers for Disease Control and Prevent, Guidelines for Environmental Infection Control in Health-Care Facilities (2003): Table B.1. Air changes/hour (ACH) and time required for airborne-contaminant removal by efficiency. Available at:
This page assumes using True HEPA filtration units that are 99.97% efficient at removing particles down to 0.3 microns