Tag Archives: ACH

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Reducing COVID-19 Exposure Risk from aerosol generating patient medical procedures

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The generation of aerosol particles from patient procedures posses great risk for exposure to COVID-19 to dentistry workers and employers (1). Other commonly performed medical procedures that generate aerosols or that create uncontrolled respiratory secretions, include (4):

  • open suctioning of airways
  • sputum induction
  • cardiopulmonary resuscitation
  • endotracheal intubation and extubation
  • non-invasive ventilation (e.g., BiPAP, CPAP)
  • bronchoscopy
  • manual ventilation
  • nebulizer administration (uncertain)
  • high flow O2 delivery (uncertain)

Aerosol particles in a treatment room may remain in suspension for up to 13 to 20 hours (99% to 99.9% deposition) after the procedure for particles between 0.3 to 10 microns (2). This posses a risk of exposure for any patient or personnel that enters the room where the procedure was previously performed if proper Environmental Infection Control measures are not put into place.

CDC Interim Infection Prevention and Control Recommendations for Patients with Suspected or Confirmed Coronavirus Disease 2019 (COVID-19) in Healthcare Settings (4) recommends the following:

“Once the patient has been discharged or transferred, HCP, including environmental services personnel, should refrain from entering the vacated room until sufficient time has elapsed for enough air changes to remove potentially infectious particles. After this time has elapsed, the room should undergo appropriate cleaning and surface disinfection before it is returned to routine use.”

The only way to minimize the risk of exposure from aerosol particles is to maintain the room negative pressure at all times, close the treatment room after the procedure is finished and the patient and personnel is no longer in the room and provide proper filtration for a prescribed duration.

Calculation of the Room Air Changes (ACH) and the elapsed time for aerosol particles count to be reduced to less than one percent of the initial concentration is necessary to properly size and select in-room air cleaners.

Current CDC guidelines (5) provides in Table B.1 the elapse time for removal of aerosol particles as follows:

Room Air Changes
Rate per Hour
99% Removal
Elapse Time (mins)
99.9% Removal
Elapse Time (mins)
* Most commonly design guidelines and codes prescribe rates for patient-care areas.

Notes to this table emphasize that these values apply for perfect mixing conditions of the air within the space. It also warns that perfect mixing does not usually occurs and that removal times will be longer in rooms or areas with imperfect mixing or air stagnation.

A study done by the US Environmental Protection Agency on In-Room Air Cleaners (2) shows that for a room with a 2:1:1 (L:W:H) aspect ratio with central furniture and an air cleaner in a corner at an angle, the mixing efficiency or air change effectiveness (ACE) can be as low as 44%. This means that the amount of air obtained from the above table Room Air Changes Rate would have to be multiplied by a factor greater than 2.25.

As we can see great care must be taken when using In-Room Air Cleaners to reduce the risk of exposure to COVID-19 from aerosol particles. Air distribution factors are important and can only be assessed by a competent professional engineer.


  1. COVID-10 – Control and Prevention, Dentistry Workers and Employers. US Department of Labor, Occupational Health and Safety Administration Website. Obtained May 29, 2020.
  2. Evaluation of In-Room Air Cleaners for Building Protection – Final Report. US Environmental Protection Agency, EPA/600/R-08/012, January 2008.
  3. Guidelines for Environmental Infection Control in Health-Care Facilities. US Center for Disease Control and Prevention, July 2019.
  4. Healthcare Infection Prevention and Control FAQs for COVID-19. US Center for Disease Control and Prevention, May 29, 2020.
  5. Interim Infection Prevention and Control Recommendations for Patients with Suspected or Confirmed Coronavirus Disease 2019 (COVID-19) in Healthcare Settings, Appendix B, Table B.1. US Center for Disease Control and Prevention, May 18, 2020.


Aerosol Generation during Dental Procedure Photo from ADS Dental System. The Author declares that there is no conflict of interest.