Ventilation and COVID-19

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Background

Transmission routes

SARS-CoV-2 seems to have caused many to rethink their understanding of droplet and airborne transmission. These two transmission mechanisms form a continuum, but the following is generally accepted:

  • Infectious particles <5μm in size can remain suspended and viable for many hours and these contribute to the risk of airborne transmission.
  • Droplet transmission involves larger particles which can also spread through the air for some distance, but the range of transmission is generally considered to be less than 2 meters where after particles fall out of the breathing zone. It is important to remember that within this 2 m distance these larger droplets are essentially 'airborne' and diluting ventilation systems have little effect on reducing the risk of near-range droplet transmission[1].

Droplet precautions, therefore, include standard precautions like PPE, hand washing and distancing while airborne precautions include negative pressure isolation, respiratory protection, special exhaust or filtration regimes, etc.

Diseases seldom obey only one mode of transmission (obligatory transmission) but often have preferences (preferential transmission) while occasionally exploiting circumstances which provide rare opportunities for transmission (opportunistic routes). SARS-COV-2 is understood to be preferentially droplet and contact spread (a form of droplet spread where droplets can settle on fomites) with possible rare and opportunistic airborne spread, although an extensive outbreak review revealed no indication of airborne spread[2].

Airborne Transmission

There is still little strong evidence of common long-range airborne transmission in the sense of droplet nucleation, as with TB and measles[3]. Where evidence of airborne transmission has been reported, this can be seen in the context of opportunistic long-range droplet spread[4]. A discussion contextualizing the reported cases of airborne transmission is discussed below.

Van Doremalen's SARS-CoV-2 survival study is often incorrectly reported to have shown that SARS-CoV-2 can remain viable in air for extended periods[5]. No evidence for airborne viability has yet been found outside of lab settings. SARS-CoV-2 virus found dispersed at long range has not been cultured to prove viability and many studies have failed to detect it directly in air in quantities substantial enough to culture[6][7]. Correlations between culture viability, particle size and the real world infectious quantum were not described in this study[5] and it was not the study's intention to claim COVID-19 was airborne. More recent studies have made similar findings[8]. More importantly, similar lab studies have also demonstrated a 3h airborne survival for viral strains not thought to be airborne[9]. This makes the direct application of this lab study in real-world settings problematic.

Therefore, the understanding of the mechanisms of COVID-19 transmission is still largely reliant on what is understood of SARS (SARS-CoV-1)[10]

The CDC's advice regarding SARS-CoV-2 transmission is still nearly identical to it's guidance for SARS-CoV-1: 

"The primary transmission of COVID-19 is from person-to-person through respiratory droplets. These droplets are released when someone with COVID-19 sneezes or coughs. COVID-19 can also be spread when you are in close contact with someone who is sick (e.g., shaking hands or talking). A physical distance of at least 1 meter (3ft) between persons is suggested by the World Health Organization (WHO) to avoid infection, although some WHO member states have recommended maintaining greater distances whenever possible.  Respiratory droplets can land on objects or surfaces around the person when they cough or talk, and people can then become infected with COVID-19 from touching these objects or surfaces and then touching their eyes, nose or mouth. Recent data suggests that there can be transmission of COVID-19 through droplets of those with mild symptoms or those who do not feel ill" [11][12]

Studies which have found real-world SARS-CoV-2 in air, ducting and on extraction fans have so far failed to prove that the virus found was still viable[13][14]. It has been suggested that high temperature and humidity would reduce the spread of the virus[15][16]. The temperature ranges suggested (>50°C) are beyond what anyone could endure in an ICU but the humidity ranges of between 40-60% are achievable. The high humidity slows the nucleation of the viral droplet and increases its settling speed, thereby reducing its range.

High Risk Settings

Much of the work being done to understand the transmission mechanism of COVID-19 is focused on community transmission. It is important to remember that transmission risk in an ICU will not be the same as in homes and workplaces. The conditions and procedures in ICUs could promote transmission - see WHO 2020 below[17]. Firstly, in a COVID ICU unit, the contamination source strength is much higher than other spaces since infected patients are congregated there. These are presumably ill patients with high viral shedding. Secondly, procedures like intubation are understood to release high quantities of aerosolized particles, unlike with general talking or coughing. Additionally, viral shedding through talking and coughing can be more readily mitigated than from intubation.

Fecal-Oral Transmission

Fecal oral route of transmission is acknowledged for COVID-19[18] and considerations for waste water management are discussed here and here[19]

This transmission route indirectly affects ventilation system design as special consideration should be given to common scenarios where the aerosolisation of contaminated wastewater is a possibility such as in bathrooms, sluice rooms and slurry pumping. These spaces should be well-ventilated and kept under negative pressure relative to adjacent spaces.

A Note on Amoy Gardens SARS studies[20]

Studies, which indicate the Amoy Gardens building's SARS outbreaks' transmission was via the airborne route, commonly reference the prevailing wind between buildings. It should be noted that, since these buildings are about 60m apart the environmental dilution and concentration decay effects are so strong it is not feasible that an infectious dose persists at that range. Similarly, the possibility that air can commute out of one window and into another needs to account for these dilution effects before assumptions of transmission can be drawn. These studies do not sufficiently account for dilution, infectious doses and pathogen survival rates. A more feasible hypothesis is that the Amoy Gardens intra-building spread was through re-aerosolisation of contaminated waste water coming from the faulty plumbing system. Similar outbreaks have more recently been found [21]

Institutional Guidance

WHO

The WHO's advice regarding SARS-CoV-2 transmission during clinical interventions is as follows: 

"In the context of COVID-19, airborne transmission may be possible in specific circumstances and settings in which procedures or support treatments that generate aerosols are performed; i.e., endotracheal intubation, bronchoscopy, open suctioning, administration of nebulized treatment, manual ventilation before intubation, turning the patient to the prone position, disconnecting the patient from the ventilator, non-invasive positive-pressure ventilation, tracheostomy, and cardiopulmonary resuscitation." - WHO 2020[17]

While the WHO's position acknowledges the increased risk of transmission in overcrowded and under-ventilated spaces, the appropriate response is not to increase prescribed general ventilation rates, but rather to avoid overcrowding and maintain ventilation systems correctly.

CDC

The US-CDC's recommendations regarding inpatient accommodation include the comment, 

"Experience in some settings in Taiwan and Toronto demonstrated that cohorting SARS patients, without use of AIIRs, effectively interrupted transmission"[22]

The CDC's guidance is consistant with the full context of hieracical risk-based infection control and takes is suitably cognisant of variously resourced settings. 

"Airborne Infection Isolation Rooms (AIIRs) (See definition of AIIR in appendix) should be reserved for patients who will be undergoing aerosol generating procedures (See Aerosol Generating Procedures Section)."[23]

ASHRAE

While the US-CDC and WHO maintains that the airborne transmission is possible but not common or of primary concern, ASHRAE (being an association dedicated to ventilation engineering) focuses on the airborne component. 

"Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures"[24]

ASHRAE makes usefule destinctions between guidance for healthcare[25], residential [26][27], commercial [28] and schools[29], but doesn't significantly address risk categories specifically in healthcare or resource limited settings.

REHVA

REHVA's temporary guidance is limited to commercial and public buildings[30] Similarly to ASHRAE, REHVA focusses on engineering controls for airborne transmission. REHVA acknowledges importance of droplet precautions and the lack of qualiy evidence for airborne transmission, but draws the conclusion that SARS-CoV-2 RNA found in ventilation ducting impies airborne transmission, even though these real world studies have not yet proven viability of these particles. REHVA also draws the airborne conclusion from the van Doremalen study [31] out of its intended comparative context.

Ventilation society guidance bears the risk of being biased toward over-prescribing solutions over which engineers have the greatest understanding and control. It is within this context that the valuable guidance published online by REHVA and ASHRAE should be considered. Revamping existing ventilation systems to meet admittedly overly cautious criteria should not be conducted without an informed investment case in resouce contrained healthcare settings. 

In these resource limited settings, it needs to be carefully considered whether resources are allocated to clinical capacity or to possibly unecessary ventilation when the benefits of these criteria may be comparitively marginal.

Engineering Controls

Without good viability studies of the viral particles found in ventilation systems, no firm guidance can be offered regarding the rate of reduction for SARS-CoV-2 viability with time and distance. Until that time it would be prudent to assume that the virus should only be considered as airborne under special and rare conditions, based on the guidance of the WHO. This would determine that we have different filtration and ventilation approaches between COVID-ICUs, general indoor public spaces and spaces with a potential for high density occupation.

For high-risk spaces it may be prudent to implement temporary measures to limit transmission risk to the minimum possible. In order of priorities, engineering interventions include:

  1. decongest indoor spaces to the minimum possible occupancy levels
  2. open windows to outside when occupational health, safety and security are not compromised
  3. increase HVAC fresh air rates to maximum possible levels
  4. reduce HVAC recirculation levels to minimum possible levels
  5. flush buildings with fresh air before and after daily occupancy

The following matrix is intended to guide our design responses for a sample of space types

Risk Response Matrix
Space Type Risk Initial Risk Engineering Response Residual Risk
ICU Transmission in ICU Severe
  1. Ventilate in accordance with IUSS Guidelines for ICUs
  2. Ensure compliance with contact, droplet and airborne precautions for staff and suspected cases
 Low
Transmission to Adjacent spaces Low
  1. Based on a risk assessment of adjacent spaces' occupancy and susceptibility rates
    1. Where high risks are associated with adjacent spaces, ventilate ICU in accordance with IUSS Guidelines for Airborne Precaution Rooms
    2. Extraction systems to discharge safely at high-level
    3. Exhaust air decontamination only prescribed for unsafe exhaust locations
 Low
Surgeries Transmission in Theatre Severe
  1. Ventilate in accordance with IUSS Guidelines for ICUs,
  2. Ensure compliance with contact, droplet and airborne precautions for staff and suspected cases
 Low
Transmission to Adjacent spaces Moderate
  1. Based on a risk assessment of adjacent spaces' occupancy and susceptibility rates
    1. Surgeries on identified COVID-19 patients in containment or sepsis theatres
    2. No recirculation to adjacent spaces
    3. Ensure compliance with contact, droplet and airborne precautions for staff
    4. Where high risks are associated with adjacent spaces, ventilate the operating room in accordance with IUSS Guidelines for Airborne Precaution Rooms or sepsis theatres
    5. Extraction systems to discharge safely at high-level
    6. Exhaust air decontamination only prescribed for unsafe exhaust locations
 Low
COVID Wards Transmission within COVID-19 Ward Low
  1. Ventilate in accordance with IUSS Guidelines for general wards
  2. Keep available windows open when safe and secure
  3. Ensure compliance with contact, droplet and airborne precautions
 Low
Transmission to Adjacent spaces High
  1. Based on a risk assessment of adjacent spaces' occupancy and susceptibility rate
    1. Increase ventilation rates in adjacent areas (passages)
    2. Positive pressure relative to COVID wards
    3. Ensure compliance with contact, droplet and airborne precautions for staff and suspected cases
    4. Extraction systems to discharge safely at high-level
 Low
General wards Transmission within and from Ward Low
  1. Ventilate in accordance with IUSS Guidelines for General Wards
 Low
Emergency centre Transmission within EC High
  1. Reduce number of occupants to only essential staff and caregivers
  2. Screen and fast-track patients with respiratory illness symptoms
  3. Isolate persons under investigation for COVID-19
    1. Isolation rooms ventilated in accordance with IUSS guidance for airborne precaution rooms
 Moderate
Hospital Waiting Areas Transmission within waiting room High
  1. Ensure compliance with contact, droplet and airborne precautions for staff and suspected cases
  2. Reduce waiting time and occupancy densities
  3. Introduce appointment and automated queueing systems
  4. Screen and fast-track patients with respiratory illness symptoms
  5. Relocate waiting areas to outdoors when possible
  6. Based on a risk assessment of adjacent spaces' occupancy and susceptibility rate
  7. Keep available windows open when safe and secure
  8. Ventilate in accordance with IUSS Guidelines for Waiting Areas
 Moderate
Other public waiting spaces Transmission within waiting room Moderate
  1. Ensure compliance with contact, droplet and airborne precautions for staff and suspected cases
  2. Reduce waiting time and occupancy densities
  3. Introduce appointment and automated queueing systems
  4. Screen and fast-track patients with respiratory illness symptoms
  5. Relocate waiting areas to outdoors when possible
  6. Based on a risk assessment of adjacent spaces' occupancy and susceptibility rate
  7. Keep available windows open when safe and secure
  8. Ventilate in accordance with Building Regulations
 Moderate

Therefore, assuming systems have been designed in accordance with the IUSS guidance, there should be little reason to change their configuration or pressurization unless general areas are repurposed as airborne precaution rooms. Risk assessments should be conducted for ICUs and COVID-19 wards immediately adjacent to public waiting areas or other high traffic areas, with corrective actions including but not limited to reducing occupancy times and rates for these areas.


Notes and References

  1. ↑ Liu, L., Li, Y., Nielsen, P. V., Wei, J. & Jensen, R. L. Short-range airborne transmission of expiratory droplets between two people. Indoor Air 1–11 (2016) doi:10.1111/ina.12314.
  2. ↑ https://www.who.int/news-room/commentaries/detail/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations
  3. ↑ World Health Organization. Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) 16-24 February 2020 [Internet]. Geneva: World Health Organization; 2020 Available from: source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf https://www.who.int/docs/default- source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf
  4. ↑ Wenzhao Chen, Nan Zhang, Jianjian Wei, Hui-LingYen, and Yuguo Li, “Short-range airborne route dominates exposure of respiratory infection during close contact,” medRxiv preprint, https://doi.org/10.1101/2020.03.16.20037291
  5. ↑ 5.0 5.1 Neeltje van Doremalen, Trenton Bushmaker, Dylan H. Morris, Myndi G. Holbrook, Amandine Gamble, Brandi N. Williamson, Azaibi Tamin, Jennifer L. Harcourt, Natalie J. Thornburg, Susan I. Gerber, James O. LloydSmith, Emmie de Wit, and Vincent J. Munster, “Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1,” The New England Journal of Medicine (2020), DOI: 10.1056/NEJMc2004973 [1]
  6. ↑ Faridi, S. et al. A field indoor air measurement of SARS-CoV-2 in the patient rooms of the largest hospital in Iran. Sci. Total Environ. 725, 1–5 (2020).
  7. ↑ Liu, Y. et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature (2020) doi:10.1038/s41586-020-2271-3.
  8. ↑ Fears SC, Klimstra WB, Duprex P, Hartman A, Weaver SC, Plante KS, et al. Persistence of severe acute respiratory syndrome coronavirus 2 in aerosol suspensions. Emerg Infect Dis. 2020 Sep [date cited]. https://doi.org/10.3201/eid2609.201806
  9. ↑ Robert Comparison of the Aerosol Stability of 2 Strains of Zaire ebolavirus From the 1976 and 2013 Outbreaks Robert J. Fischer, Trenton Bushmaker, Seth Judson, Vincent J. Munster J Infect Dis. 2016 Oct 15; 214(Suppl 3): S290–S293. Published online 2016 Oct 4. doi: 10.1093/infdis/jiw193 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5050463/
  10. ↑ Isao Arita, Kazunobu Kojima, and Miyuki Nakane, “Transmission of severe acute respiratory syndrome,” Emerging. Infectious Diseases 9 No. 9 (2003):1183-84, [2].
  11. ↑ https://www.cdc.gov/sars/about/faq.html
  12. ↑ https://www.cdc.gov/sars/about/faq.html
  13. ↑ Santarpia et al, “Transmission Potential of SARS-CoV-2 in Viral Shedding Observed at the University of Nebraska Medical Center,. medRxiv preprint (2020), [3]
  14. ↑ Po Ying Chia et al, 2020 (Preprint) “Detection of Air and Surface Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in Hospital Rooms of Infective Patients,” medRxiv preprint (2020), https://doi.org/10.1101/2020.03.29.20046557 [4]
  15. ↑ Chin, A. W. H. et al. Stability of SARS-CoV-2 in different environmental conditions. The Lancet Microbe 0–4 (2020) doi:10.1016/s2666-5247(20)30003-3.
  16. ↑ Pyankov, O. V., Bodnev, S. A., Pyankova, O. G. & Agranovski, I. E. Survival of aerosolized coronavirus in the ambient air. J. Aerosol Sci. 115, (2018).
  17. ↑ 17.0 17.1 WHO 2020, Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations https://www.who.int/news-room/commentaries/detail/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations
  18. ↑ Pan Y, Zhang D, Yang P, Poon LLM, Wang Q. Viral load of SARS-CoV-2 in clinical samples. Lancet Infect Dis. 2020;20(4):411-2.
  19. ↑ Kitajima et al,SARS-CoV-2 in wastewater: State of the knowledge and research needs,Science of The Total Environment,Volume 739,2020,139076,ISSN 0048-9697,https://doi.org/10.1016/j.scitotenv.2020.139076
  20. ↑ McKinney KR, Gong YY, Lewis TG. Environmental transmission of SARS at Amoy Gardens. J Environ Health. 2006;68(9):26-52.
  21. ↑ Bhowmick, G.D., Dhar, D., Nath, D. et al. Coronavirus disease 2019 (COVID-19) outbreak: some serious consequences with urban and rural water cycle. npj Clean Water 3, 32 (2020). https://doi.org/10.1038/s41545-020-0079-1
  22. ↑ US-CDC,2005, https://www.cdc.gov/sars/guidance/i-infection/healthcare.html
  23. ↑ Interim Infection Prevention and Control Recommendations for Healthcare Personnel During the Coronavirus Disease 2019 (COVID-19) Pandemic (updated July 9, 2020)[5]
  24. ↑ Q: Does ASHRAE’s guidance agree with guidance from WHO and CDC?[6]
  25. ↑ ASHRAE healthcare C19 guidance (ASHRAE 2020) [7]
  26. ↑ ASHRAE residential c19 guidance (ASHRAE 2020)[8]
  27. ↑ COVID 19 guidance for multifamily building owners-managers (ASHRAE 2020)[9]
  28. ↑ ASHRAE commercial C19 guidance (ASHRAE 2020)[10]
  29. ↑ ASHRAE Schools C19 guidance (ASHRAE 2020)[https://www.ashrae.org/file%20library/technical%20resources/covid-19/ashrae-schools-c19-guidance.pdf
  30. ↑ REHVA COVID-19 guidance document, April 3, 2020[11]
  31. ↑ van Doremalen et al, 2020
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