Virus Decay Rates Should Not Be Used to Reduce Recommended Room Air Clearance Times

Details

• Personal Author:
  Hammond, Duane R. ; Lindsley, William G. ; Martin SB Jr. ; Mead, Kenneth R.
• Description:
  To the Editors - We read with concern the letter by Hurlburt et al proposing revisions to the recommended room air clearance times for infectious aerosols in healthcare facilities. We believe that the calculations performed to justify the changes are based on flawed assumptions and an erroneous calculation. Experimental data on the survival of airborne SARS-CoV-2 virus and the dynamics of room ventilation do not support their conclusions. Hurlburt et al based their proposed changes on data describing the effects of humidity on the viability of airborne influenza viruses, and on reports that influenza decays more rapidly at mid range humidities. They then assumed that these decay rates apply to SARS-CoV-2 as well. In fact, this is not the case. Schuit et al studied the decay in viability of airborne SARS-CoV-2 for relative humidities of 20% to 70% at 20 degrees C and found that SARS-CoV-2 was relatively stable in air in the absence of sunlight (kinfect = 0.008 per minute) and that humidity did not significantly affect the decay rate. Other researchers have also reported either no effect or a small effect of humidity on the decay rate of airborne SARS-CoV-2. Using data for influenza rather than SARS-CoV-2, Hurlburt et al assumed that a relative humidity of 40% to 60% would reduce the viability of SARS-CoV-2 by 30% to 50%. Unfortunately, these researchers miscalculated the effect that this would have on air clearance times. They simply multiplied the equation for the clearance time by their assumed reduction in viability, which has the mathematical effect of assuming that the reduction in viability occurs instantaneously. In fact, experimental data for SARS-CoV-2 and other viruses show that losses in viability are best modeled as an exponential decay. The correct version of the formula is shown here. ... In conclusion, the modifications to the calculation of room air clearance times proposed by the authors are not supported by current scientific evidence. Near the end of the letter, the authors write, "The interaction between viruses and relative humidity is complex, and large knowledge gaps exist." We agree wholeheartedly with this statement, and it serves as an excellent argument against the proposed reductions in room air clearance times until the stability and decay in viability of airborne viruses are better understood. [Description provided by NIOSH]
• Subjects:
  Aerosols Communicable Diseases Health Care Facilities Workforce And Services Humidity Occupational Health Safety Ventilation
• Keywords:
  Coronavirus COVID-19 Health Care Facilities Infection Control SARS-CoV-2
• ISSN:
  0899-823X
• Document Type:
  Text
• Genre:
  Journal Article
• Place as Subject:
  Ohio ; OSHA Region 3 ; OSHA Region 5 ; West Virginia
• CIO:
  National Institute for Occupational Safety and Health (NIOSH)
• Division:
  HELD - Health Effects Laboratory Division ; RHD - Respiratory Health Division ; DFSE - Division of Field Studies and Engineering
• Topic:
  Workplace Safety & Health
• Location:
  North America
• Volume:
  43
• Issue:
  12
• NIOSHTIC Number:
  nn:20064310
• Citation:
  Infect Control Hosp Epidemiol 2022 Dec; 43(12):1987-1989
• Contact Point Address:
  William G. Lindsley, National Institute for Occupational Safety and Health, Morgantown, West Virginia
• Email:
  wlindsley@cdc.gov
• Federal Fiscal Year:
  2023
• NORA Priority Area:
  Healthcare and Social Assistance ; Construction
• Peer Reviewed:
  False
• Source Full Name:
  Infection Control and Hospital Epidemiology
• Collection(s):
  National Institute for Occupational Safety and Health
• Main Document Checksum:
  urn:sha-512:6aa747687a3349ca015206f5931fc1a8a917a216e6f671d8ab79558a8ef8f3eb11c4ed0b8cb69de2689841db084f27bed6d9063677ccdea8d1711b68161006ff
• Download URL:
  https://stacks.cdc.gov/view/cdc/222801/cdc_222801_DS1.pdf
• File Type:
  [PDF - 342.82 KB ]