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Guidelines on COVID-19 and indoor ventilation – CDC, WHO and NICD

International agencies have issued reminders to people to be aware of the risk of COVID-19 spreading in enclosed spaces. They have also published guidelines on the benefits of indoor fans, and how they can help dissipate the virus’s potential to spread.

The US Centres for Disease Control and Prevention (CDC) has said that while fans alone cannot make up for a lack of outdoor air, they can help increase the effectiveness of open windows. Fans can also be used to improve room air mixing, which helps to distribute supplied clean air and dilute viral particle concentrations throughout the room, thereby reducing the likelihood of stagnant air pockets where viral concentrations can accumulate.

As with all fan use during the COVID-19 pandemic, care should be taken to minimise the potential to create air patterns that flow directly across one person on to another:

  • Avoid the use of the high-speed settings
  • Use ceiling fans at low velocity and potentially in the reverse-flow direction (so that air is pulled up toward the ceiling)
  • Direct the fan discharge towards an unoccupied corner and wall spaces or up above the occupied zone.

Fans can also enable clean-to-less-clean directional airflow. Such applications should be evaluated closely to avoid unintended consequences and only adopted when supported by a safety risk assessment.

The risk of COVID-19 spreading through ventilation systems “is not clear at this time”, added the CDC. Viral RNA has reportedly been found on return air grilles, in return air ducts, and on heating, ventilation, and air conditioning (HVAC) filters, but detecting viral RNA alone does not imply that the virus was capable of transmitting disease, it said.

A study in 2020, published in the International Journal of Infectious Diseases, reported that the use of a new air-sampling method allowed them to find viable viral particles within a COVID-19 patient’s hospital room with good ventilation, filtration and ultraviolet (UV) disinfection (at distances as far as about 4,8m from the patient). However, the concentration of viable virus detected was believed to be too low to cause disease transmission.

There may be some implications for HVAC systems associated with these findings, but it is too early to conclude that with certainty. While airflows within a particular space may help spread disease among people in that space, there is no definitive evidence to date that viable virus has been transmitted through an HVAC system to result in disease transmission to people in other spaces served by the same system.

The World Health Organization (WHO) advises opening a window when a fan is in use, and that if you want to use a pedestal fan, minimise as much as possible how much air blows from one person to another.

“You can place a fan in front of an open window to increase air flow and to push the indoor air outside.”

Ceiling fans avoid pockets of stagnant air forming indoors and improve air flow, but they should also be used with open windows and doors.

The WHO reiterated the CDC’s advice on opening doors and windows wherever possible when inside.

“For better ventilation, open windows/doors on opposite sides of a room to create a cross breeze. If creating a cross breeze is not possible, you can place a fan in front of an open window to increase air flow and push indoor air outside.

“If the temperature outside is extremely hot or cold, you can open windows for a few minutes every hour to bring in fresh air.”

In a directive to schools across South Africa, the National Institute for Communicable Diseases (NICD) said it is important to have doors and windows open during the school day to minimise the risk of infection spreading in classrooms.

“Using a fan in an enclosed space can increase the spread of the virus that causes COVID-19, so it is important to open windows and doors whenever using a fan, to replace indoor air with outdoor air.

“A small concentration of virus particles in poorly ventilated spaces, combined with humidity and high temperature, can result in infection spreading. It is important to have natural ventilation in school buildings by ensuring all windows and doors are left open during the day.”

Study details
Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients

John Lednickyab, Michael Lauzardo, Z. Hugh Fan, Antarpreet Jutla, Trevor Tilly, Mayank Gangwar, Moiz Usman, Sripriya Nannu Shankar Karim Mohamed, Arantza Eiguren-Fernandez, Caroline Stephenson, Mahbubul Alam, Maha Elbadry, Julia Loeb, Kuttichantran Subramaniam, Thomas Waltzek, Kartikeya Cherabudd, J. Glenn Morris, Chang-Yu Wuf

Published in the International Journal of Infectious Diseases on November 2020


Viable (infectious) SARS-CoV-2 was present in aerosols within the hospital room of COVID-19 patients.

Airborne virus was detected in the absence of healthcare aerosol-generating procedures.

The virus strain detected in the aerosols matched with the virus strain isolated from a patient with acute COVID-19.


Because the detection of SARS-CoV-2 RNA in aerosols but failure to isolate viable (infectious) virus are commonly reported, there is substantial controversy whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can be transmitted through aerosols. This conundrum occurs because common air samplers can inactivate virions through their harsh collection processes. We sought to resolve the question whether viable SARS-CoV-2 can occur in aerosols using VIVAS air samplers that operate on a gentle water vapor condensation principle.

Air samples collected in the hospital room of two coronavirus disease-2019 (COVID-19) patients, one ready for discharge and the other newly admitted, were subjected to RT-qPCR and virus culture. The genomes of the SARS-CoV-2 collected from the air and isolated in cell culture were sequenced.

Viable SARS-CoV-2 was isolated from air samples collected 2m to 4.8m away from the patients. The genome sequence of the SARS-CoV-2 strain isolated from the material collected by the air samplers was identical to that isolated from the newly admitted patient. Estimates of viable viral concentrations ranged from 6 to 74 TCID50 units/L of air.

Patients with respiratory manifestations of COVID-19 produce aerosols in the absence of aerosol-generating procedures that contain viable SARS-CoV-2, and these aerosols may serve as a source of transmission of the virus.


World Health Organization article – Coronavirus disease (COVID-19): Ventilation and air conditioning (Open access)


CDC article – Ventilation in buildings (Open access)


International Journal of Infectious Diseases article – Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients (Open access)


See more from MedicalBrief archives:


Limiting SARS-CoV-2 transmission at large events is possible — Experimental mass gathering


MIT modelling study challenges indoor social distancing guidelines


New CDC guideline admits that COVID-19 transmission is airborne


10 scientific reasons supporting airborne transmission of SARS-CoV-2



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