Re: Aerosol Generating Procedures, Fallow Time, and Air Changes/Hour: What You Should Know

by John Hardie, BDS, MSc, PhD, FRCDC

I wish to thank Joan Hutchings for her timely article but perhaps not for the reasons that she might think. She offers a quote from the Royal College of Dental Surgeons of Ontario (RCDSO) which states adamantly that, “…dental offices are at high risk for spreading COVID-19 given the aerosol generating nature of dental procedures…” There are a number of faults with this RCDSO conclusion.

  1. Aerosols do not spread COVID-19 which is an infectious disease diagnosed on the basis of signs, symptoms and a positive PCR test. If aerosols are a route of transmission, it is because they spread SARS-CoV-2, the causative agent of COVID-19.
  2. The aerosol route for the transmission of SARS-CoV-2 requires that the virus remains infectious from its generating procedure until it successfully invades a new host.
  3. Proving that this occurs requires live viral studies in dental clinics and not by a reliance on laboratory cultures and simulated experiments.
  4. Such dental studies have not been done. In fact, it has been extremely difficult to culture anaerobes and viruses from dental aerosols.1
  5. Real life hospital based investigations suggest that transmission from aerosol generating medical procedures is likely exaggerated.2
  6. SARS-CoV-2 is a respiratory virus. There is every reason to believe that as an airborne pathogen it will behave as do its four endemic coronavirus cousins which cause 15-30% of common colds and flus.3
  7. The RCDSO has not suggested that aerosols from these coronaviruses demand fallow times and sophisticated air exchange equipment.

The RCDSO is not expressing an opinion on dental aerosols and COVID-19, it is making a frank declaration that there is a causal relationship between certain dental procedures and the acquisition of COVID-19. It cannot arrive at such a conviction without unequivocal evidence obtained from dental clinical investigations. Without such proof its recommendations on fallow times and HVAC are mere hot air. That conclusion has its genesis in the quote used by Ms. Hutchings which is why I thank her.


  1. Harrel SK. Are Ultrasonic Aerosols an Infection Control Risk? Dental Hygiene 2008; 6(6): 20-26.
  2. Chagla Z. It Is Time to Address Airborne Transmission of COVID-19. Clinical Infectious Diseases, 2020;, ciaa1118 Available at:
  3. Messel-Lemoine M. A Human Coronavirus Responsible for the Common Cold Massively Kills Dendritic Cells but Not Monocytes. J Virol 2012;86(14): 7577-7587.

A note from the original author, Joan Hutchings, President, SAFE Dentistry Inc.

I had two goals in mind when I wrote the article. The first goal was to increase awareness of the increased risk associated with the limited information provided by the RCDSO pertaining to AGPs and enclosed operatories. The second goal was to initiate important conversations about the RCDSO standards and guidelines as they relate to AGPs and COVID-19. Both of my goals have been achieved.

It is important that discussions on this topic continue, not only for the safety of the dental team, but also for the patients. Critical decisions must be evidenced-based, and science driven.