Current guidance note: Fallow-time rules were introduced during the COVID-19 response and have since changed in many jurisdictions. Practices should follow current local infection-prevention and ventilation guidance rather than applying historical time intervals automatically.
The literal meaning of the word “Fallow” is Dormancy or Latency. In Dentistry, fallow time is also referred to as settle time or stand-down period.
Dentistry involves a lot of procedures that produces aerosols. Thus, in times of COVID-19 it becomes very crucial to reduce the number of infectious micro-organisms in a dental setting. This is where ‘fallow time’ comes into picture.
Why is ‘Fallow Time’ so important?
Aerosols are defined as particulate matters which are less than 50 micrometers in diameter. Although they seem to be trivial, they are large enough to house COVID-19 causing microorganism i e SARS-CoV-2. These microorganisms are likely to have travelled to various surfaces of a dental office after the completion of any aerosol generating procedure.
Thus, before allowing the next patient inside the operatory, it should be left in solitude for a certain period of time. It is mandatory after any Aerosol Generating Procedure (AGP) for the microorganisms suspended in air to settle down.
This period can last from, as short as ‘20 minutes’, to as long as ‘120 minutes’.
What is ACH?
ACH is defined as the air change per hour necessary to make the operatory free from any viral or bacterial pathogen. This is the amount of air that gets displaced by various means of ventilation in a given area.
The faster this form of air circulation takes place, the lesser will be the fallow time. There are certain equipment available that can increase the air exchange after the completion of an aerosol generating procedure.
Here are a few important points to be considered
- Heating, Ventilation and Air Conditioning (HVAC) units should be regularly filtered or be replaced if old
- High Efficiency Particulate Air (HEPA) filter needs to be installed in the dental clinic
- An Extraction Fan can be installed as a cheaper option, it can displace the air within the operatory but at a relatively slow speed
- UV-C Filters can also be used that emit UV rays to kill the microorganisms. The major drawback of this system is that it needs immense precision in its placement else these filter lamps won't be able to assert their maximum effect
Note
Air filtration systems can handle a variety of environments. You can calculate the size of your room by multiplying the length, width and height of the room. You can then divide the size of the room in to the flow rate to see how many times an hour the room’s air will change.
Thus, it is essential that amidst the pandemic, enough importance is given to the fallow time depending on the total surface area of the dental operatory and the type of air purifier or air filter installed.
Clinical update: fallow time should be risk based
Updated perspective: fallow time is a period in which an operatory is left vacant after an aerosol-generating procedure so airborne particles can be removed by ventilation and filtration. A single fixed interval is not universally appropriate. Follow current national, regional and institutional guidance and base clearance on documented air changes per hour (ACH), ventilation effectiveness, procedure, source control and patient respiratory risk.
Practical controls
- Screen symptomatic patients and apply Standard Precautions.
- Use procedure-appropriate respiratory protection and PPE.
- Reduce aerosol with rubber dam where suitable, high-volume evacuation and four-handed technique.
- Confirm ventilation performance; correctly sized portable HEPA filtration may supplement it.
- Clean and disinfect surfaces using the labelled contact time after safe re-entry.
Do not copy legacy COVID-era time values into policy without checking the issuing authority’s current guidance. Record the policy source, review date and responsible person.
Frequently asked questions
Is one fallow time mandatory for every procedure?
No; rules vary and clearance depends heavily on ventilation and clinical risk.
How does ACH affect clearance?
Higher effective ACH generally removes contaminants faster, although room mixing also matters.
Does high-volume evacuation eliminate fallow time?
No. It reduces aerosol at source but does not alone determine local requirements.
Can HEPA filtration replace ventilation?
It can add clean-air delivery but should complement a documented ventilation plan.
What should a practice document?
The risk assessment, ventilation basis, controls, applicable guidance and review date.
This educational update does not replace local public-health or occupational-safety rules.