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

by Joan Hutchings RN, CPN, BA, MSc, MPA

This article is based on a review of the literature, personal communications with heating, ventilation and air condition (HVAC) experts, building contractors as well as information obtained from regulatory bodies. The goal is to create awareness on how construction of temporary walls and doors for aerosol generating procedures (AGPs) can cause inadvertent problems with air changes per hour (ACH) and ultimately fallow time (FT).

Our business, SAFE Dentistry Inc., is all about infection prevention and control as it pertains to dentistry. Standards Achievement Facility Evaluation (SAFE) is what we do. Our goal is to assist dental teams to comply with the current infection prevention and control (IPAC) standards and practice guidelines published by the Royal College of Dental Surgeons of Ontario (RCDSO). Dentists have asked us for advice about COVID-19 and AGP standards and so it is important that we share this with you.

COVID-19 has had a profound impact on how dentists and their teams deliver safe care. The RCDSO released a series of publications, the latest dated January 14, 2021, titled, COVID-19: Managing Infection Risks During In-Person Dental Care. Relevant paragraphs are quoted below directly from this document.

A. “Without careful planning and appropriate guidance, dental offices are at a high risk for spreading COVID-19 given the aerosol generating nature of dental procedures, the proximity of the operating field to the upper respiratory tract, and the number of patients seen per day. Dentists providing any degree of in-person care must comply with the direction of government and the College to maintain the safety of patients and staff, and to not contribute to the transmission of COVID-19.” 1

B. “AGPs must be performed in an operatory that is capable of containing aerosol. This requires floor-to-ceiling walls and a door (or other barrier) that must remain closed during and after such procedures. Temporary walls and doors are permitted, provided they create an area to contain aerosols and are constructed of materials that can withstand repeated cleaning and disinfection.” 1

C. “Clearing the Air of Aerosol (Fallow Time) Following Aerosol-Generating Procedures Following an AGP, dentists must:
a. Ensure that the operatory is left empty (with the door closed) to permit the clearance and/or settling of aerosols.
b. Ensure that the operatory remains empty for a length of time that achieves 99.9% removal of airborne contaminants (dentists must use Table 2 to calculate the length of time necessary to achieve 99.9% removal of airborne contaminants based on air changes per hour [ACH]).
c. If dentists have not had the rate of air changes for their office confirmed by an HVAC professional, dentists must assume a rate of two air changes per hour and adhere to a minimum fallow time of 3 hours.” 1

D. “Options to improve ACH (and further reduce the fallow time) may be explored, including:
a. Consulting an HVAC professional to determine whether changes to the existing HVAC system are possible to improve ACH for the dental practice.
b. If changes to the existing HVAC system are not possible or adequate, dentists may consider the use of an in-operatory air cleaner (e.g. HEPA filtration) to increase the effective air changes per hour (eACH) for a specific operatory.
c. If an in-operatory air cleaner (e.g. HEPA filtration) will be used to increase the effective air changes per hour (eACH) for a specific operatory, the HVAC professional must also take into account several additional factors, including: i. any structural changes that may be necessary to contain the spread of aerosols, ii. the type of unit being considered, iii. the cubic feet of the operatory and airflow rate of the unit, and iv. the optimal placement and operation of the unit.”1

Dentists rely on the guidance provided by the RCDSO. It is important that the information is clear and comprehensive with sufficient detail to ensure predictable outcomes. When recommendations for change are presented, adverse impacts on the existing systems should be taken into consideration.

Installation of temporary walls and doors to contain aerosols has limitations. The barriers may cause exclusion, obstruction or interference of the air flow system. The result is a change in the calculated ACH for the space, most likely a reduction in ACH. The reduction in ACH not only impacts the fallow time required, but more importantly during the AGP staff are continuously exposed to a higher concentration of harmful aerosols. This environment is like running a gas vehicle in the garage with the doors closed.

It is extremely important that dentists consult an (HVAC) expert to evaluate the impact of barriers on air circulation, especially if the office design is open concept. Typically, if a structural change is made to a building, it will involve a building permit of some sort. Usually, part of the final inspection requires the contractor to assess the ventilation system to ensure that the ACH has not been affected by the renovation.

Regardless of whether the dentist does hire a HVAC professional or not, it is critical that the dentist clearly understands the basic functions of the office heating, ventilation, and air conditioning HVAC system. Virtually all systems have a control switch that staff can use to mange the temperature of the space.

Another important factor about the HVAC systems is the control option to run the system on automatic or continuous mode. This is critical to the ACH formula. When ACH is calculated by the HVAC professional it is assumed that the system remains on continuous mode. However, often the mode is changed to automatic either for economic reasons or to reduce constant fan noise. To maintain a reliable ACH as determined by the HVAC professional or following recommended guideline to assume a rate of 2 air changes per hour without professional ACH calculation, the HVAC system should remain on continuous mode. Otherwise, a HVAC system may provide less than required ACH during fallow time.

Fallow time is defined as the amount of time that the operatory is left empty (with the door closed) to permit the clearance and/or setting of aerosols. The operatory must remain empty for a length of time that achieves 99.9% removal of airborne contaminants.1

The calculation for a specific fallow time is dependent on an accurate ACH number. An accurate ACH is difficult to determine, especially when so many factors can affect it. It is extremely tricky and perhaps impossible to accurately determine the ACH in any given space. Especially if any of the circumstances discussed above have occurred.

The wording in section D(c) in the above insert from the RCDSO is cause for confusion. Does this mean that the dentist should monitor the HVAC professional to ensure process is completed properly? Will an in-operatory air cleaner increase the effective air changes per hour? Is air filtering through the air cleaner the same as air exchange?

Clearly this information is confusing. Simply put, it all depends on the quality of the air cleaner installed. It may be purchased from hardware stores at a low price or obtained through a company claiming to have the most effective air cleaner on the market at a significantly higher price. Without the assistance from a HVAC professional, it is difficult to compare the effectiveness of available systems.

The science behind reducing the risk of AGP is complicated. Furthermore, it is likely that the research on air quality was not conducted in dental settings but rather in healthcare facilities, extrapolated and applied to the dental office. This can be problematic.

Going forward, dentists will be tasked with making decisions to ensure compliance with ever-changing IPAC standards and guidelines. The purpose of this article is to bring awareness to some less obvious problems that may increase the risk of disease transmission. SAFE Dentistry Inc. is committed to assist dentists and their staff to provide dental services without harm.


  1. COVID-19: Managing Infection Risks During In-person Dental Care, January 14, 2021, published by RCDSO.

About the Author

Joan Hutchings RN, CPN, BA, MSc, MPA, has 35 years of experience in healthcare as a registered nurse, surgical suite specialist and epidemiologist. She is an expert on infection prevention and control practices. Joan was director and clinical instructor of the Operating Room Technique and Management program, a hospital-based surgical program. As a clinical consultant, she provided consulting services and audits to hospitals, outpatient clinics and dental offices across Canada. As an epidemiologist, Joan cofounded SAFE Dentistry Inc., which assists dentists and staff to deliver quality safe services in compliance with IPAC.

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