The following document is an overview of the scientific evidence to-date as it relates to COVID-19 as well as an update on the international guidance on infection prevention and control of COVID-19. It does not purport to be a comprehensive list of all the research available. Our understanding of COVID-19 is changing daily in response to emerging science.
This research is provided for your information, but the CFNU cannot guarantee its accuracy or completeness, as the CFNU was not involved in reviewing or producing these resource materials.
Guidance in Canada for health care professionals recommends wearing a surgical mask for routine care of suspected and confirmed COVID-19 patients. Airborne precautions – requiring the use of N-95 masks for respiratory protection – are only recommended for aerosol-generating medical procedures.
Infections with respiratory viruses are principally transmitted through three modes: contact, droplet, and airborne.
Close-range transmission – COVID-19 can remain in the air in small droplets and particles (aerosols) and be ‘inhaled’:
How COVID-19 spreads (updated by the Public Health Agency of Canada on November 3, 2020): “SARS-CoV-2, the virus that causes COVID-19, spreads from an infected person to others through respiratory droplets and aerosols created when an infected person coughs, sneezes, sings, shouts, or talks. The droplets vary in size from large droplets that fall to the ground rapidly (within seconds or minutes) near the infected person, to smaller droplets, sometimes called aerosols, which linger in the air under some circumstances. The relative infectiousness of droplets of different sizes is not clear. Infectious droplets or aerosols may come into direct contact with the mucous membranes of another person’s nose, mouth or eyes, or they may be inhaled into their nose, mouth, airways and lungs.”
Long-range transmission – COVID-19 in small droplets and particles can sometimes be ‘airborne’ (see definition above):
Contact transmission – spread from touching surfaces is not thought to be a common way that COVID-19 spreads:
From The Washington Post. Sun, L.; Guarino, B.“CDC says airborne transmission plays a role in coronavirus spread in a long-awaited update after a website error last month”, October 5, 2020
“Experts welcomed the CDC’s new guidance on airborne transmission – Linsey Marr, who studies aerosols at Virginia Tech and was an author of the Science letter: “I would like to emphasize that short-range airborne transmission when people are in close contact, meaning inhalation of aerosols, probably is more important than transmission by large droplets that are sprayed onto mucous membranes.”
We are learning more and more about COVID-19 each day, and many of the assumptions we made about COVID-19 just a few months ago have been proven wrong.
The U.S. CDC guidance now recognizes that COVID-19 may be spread at close range through respiratory droplets “when they are inhaled”, and that sometimes airborne transmission (which is the way measles, tuberculosis and chickenpox have spread) long-range transmission can occur under certain conditions.
Guidance in Canada and the U.S. has also changed in favour of the public wearing homemade masks as we have discovered that asymptomatic and presymptomatic transmission account for a significant percentage of the spread of this novel coronavirus.
In a nutshell, the precautionary principle, as applied to a novel, highly transmissable, virus such as this coronavirus, with a significant public health impact, requires governments and employers to begin with the highest level of protection, not the lowest, for health care workers, and then reduce the level of protection as the science emerges to justify this measure.
Therefore, Canada must change its infection prevention and control guidance for health care professionals to recognize that the virus is being spread both through the inhalation of aerosolized particles at close range (less than 6 feet) and through long-range airborne transmission. In changing the guidance to recognize the risk of health care worker exposure to the aerosolized COVID-19 virus when infected patients/residents/clients breathe, talk, yell, sing, cough or sneeze, it must, at a minimum, mandate fit-tested NIOSH-approved N95 respirator masks or preferably better (i.e. elastomeric respirators, powered air-purifying respirators (PAPRs)) for those working in clinical areas considered aerosol-generating medical procedure area ‘hot zones’ (e.g., intensive care units (ICU), emergency rooms, operating rooms, post-anaesthetic care units and trauma centres) and for all workers caring for suspected or confirmed cases in designated COVID-19 units.
The U.S. CDC’s recognition of both close-range inhalation of aerosolized particles and airborne transmission of the virus is based on months of evidence compiled by researchers around the world. While the PHAC has recognized close-range inhalation of aerosolized particles from both symptomatic and non-symptomatic individuals as a main route of transmission, unlike the U.S. CDC, it has failed to formally recognize long-range airborne transmission despite mounting evidence.
From Occupational Health Clinics for Ontario Workers (OHCOW). Airborne Transmission: Risk and Control: Presentations by: Oudyk, J.; Hedges, K.; Marr, L.; Milton, D.; Smith, S.; Brosseau, L. Webinar topics include: “Ventilation and COVID-19”, “The role of infectious dose in COVID-19”, Selecting effective respiratory equipment for COVID-19”, “The role of droplets and aerosols in SARS CoV-2 transmission”, “Understanding and controlling SARS CoV-2 transmission”, and “Applying the hierarchy of controls in point-of-care risk assessments (PCRAs)”.
From the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST). Government of Quebec. “Respiratory Protection for Health Care Workers in the Context of SARS CoV-2 Transmission through Inhalation,” February 10, 2021
“The advice provided by international organizations is converging towards a model of COVID-19 transmission through inhalation of respiratory particles […]Medical masks are not respirators and do not protect workers from inhalable particles. Regardless of the level of protection of the medical mask (1, 2 or 3), even if the worker tries to adjust it properly, it is impossible to avoid leaks and to prevent inhalation of particles. […] To adequately protect healthcare workers, the IRSST recommends that respirators should be worn in areas where there is a risk of exposure to SARS-CoV-2.”
From Nature. Editorial. “Coronavirus is in the air – there’s too much emphasis on surfaces,” February 2, 2021
“Now that it is agreed that the virus transmits through the air, in both large and small droplets, efforts to prevent spread should focus on improving ventilation or installing rigorously tested air purifiers. People must also be reminded to wear masks and maintain a safe distance. At the same time, agencies such as the WHO and the CDC need to update their guidance on the basis of current knowledge. Research on the virus and on COVID-19 moves quickly, so public-health agencies have a responsibility to present clear, up-to-date information that provides what people need to keep themselves and others safe.”
From Journal of Hospital Infection. Tang, J. et al. “Dismantling myths on the airborne transmission of severe acute respiratory syndrome coronavirus (SARS CoV-2),” January 12, 2021
“This article gathers together and explores some of the most commonly held dogmas on airborne transmission in order to stimulate revision of the science in the light of current evidence. Six ‘myths’ are presented, explained, and ultimately refuted on the basis of recently published papers and expert opinion from previous work related to similar viruses. There is little doubt that SARS-CoV-2 is transmitted via a range of airborne particle sizes subject to all the usual ventilation parameters and human behaviour. Experts from specialties encompassing aerosol studies, ventilation, engineering, physics, virology and clinical medicine have joined together to present this review, in order to consolidate the evidence for airborne transmission mechanisms and offer justification for modern strategies for prevention and control of Covid-19 in healthcare and community.”
From the Ontario Society of Professional Engineers. “Engineers call on Ontario to refocus efforts on airborne transmission of COVID-19,” January 12, 2021
“OSPE and its engineers believe there is a key piece to our defence against this virus that has not been properly addressed by the Ontario government – the need for proper ventilation and air filtration to stop the spread of the virus via infected aerosol particles in the air.”
From the PHAC. “COVID-19: Guidance on indoor ventilation during the pandemic,” January 2021
“The virus causing COVID-19 is known to spread through droplets and aerosols, which represent a risk particularly to people who are in: enclosed spaces; indoor situations where people are in close proximity.
The most important elements in reducing the risk of COVID-19 are preventive measures, such as: minimizing the number of persons in a place at the same time; maintaining a physical distance of at least 2 metres; using well-constructed, well-fitting masks; practising good hand and respiratory hygiene. In addition to these practices, adequate ventilation can contribute to reducing the risk of COVID-19 transmission in indoor settings.”
From Masks4Canada.org. Signed by more than 363 physicians, scientists, occupational health and safety experts, engineers and nursing professionals.“Open letter to Dr. Theresa Tam, Minister Patty Hadju, Premiers and Medical Officers: There Is Still Time to Address Aerosol Transmission of COVID-19,” January 4, 2021
“We are deeply concerned by the recent increase in cases and hospitalizations across Canada. We urge to update the provincial COVID-19 guidelines workplace regulations and public communications to reflect the science – COVID-19 spreads through inhaled aerosols.”
From International Journal of Infectious Disease. Hwang, S.E. et al. “Possible Aerosol Transmisison of COVID-19 Associated with an Outbreak in an Apartment in Seoul, South Korea, 2020,” December 17, 2020
From EurekAlert! TROPOS. “Corona pandemic could be better tackled by reducing aerosol transmission.” December 8, 2020
“Aerosols and their spread play an essential role in the transmission of COVID-19. However, the risk of transmission could be significantly reduced if more could be done to reduce indoor airborne viruses. The Working committee particulate matter (AAF) has therefore issued an statement with concrete recommendations. These include window ventilation, exhaust ventilation, air purification systems and CO2 measuring devices for indoor areas such as classrooms or transportation, and the increased use of N95 and FFP2 masks.”
From Canadian Agency for Drugs and Technologies in Health (CADTH). Mr. William Dean, Dr. David Fisman, Dr. Jason Kindrachuk, Heather Logan. “CADTH COVID-19. Webinar – Community-Based Aerosol Transmission of COVID-19 and HVAC Systems.” December 3, 2020
“The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes COVID-19, spreads when an infected person breathes out, sneezes, coughs, or talks, thus putting virus-carrying liquid particles into the air where they can infect other people. Larger particles fall to the ground rapidly, while smaller particles, sometimes called aerosols, can linger in the air. Some infectious diseases — such as measles, tuberculosis, and influenza — are known to spread through heating, ventilation, and air conditioning (HVAC) systems, raising concern that the same may be true of COVID-19. In this webinar, a panel of experts will outline what is known about how COVID-19 is transmitted and discuss what the latest research shows about the potential role HVAC systems play in spreading or mitigating the risk of transmitting COVID-19.”
From first10em.com. Morgenstern, J. “COVID-19 is spread by aerosols: an evidence review,” December 2, 2020
“The equally fallacious corollary to the Ro argument is that “if COVID-19 is transmitted through aerosols, we should see a lot of infections occurring over long distances”. Although it is true that aerosols will disperse much further than droplets, it is faulty logic to define the mode of transmission by the distance of transmission. The concentration of infectious particles falls dramatically with distance, even when those infectious particles are carried by aerosols. They are spread out through 3 dimensional space, and therefore decrease exponentially with distance. Although aerosols can transmit disease over long distances, they are much more likely to transmit disease over a short distance. (Chen 2020)
This illogical step is so ingrained in the infectious diseases literature that most studies just assume droplet spread if there was close contact. This illogical assumption undermines a great deal of the existing infectious disease literature.”
From the Canadian Institutes of Health Research. “CIHR: PHAC-CADTH-Best Brains Exchange – Transmission Routes for COVID-19: Implications for Public Health,” November 26, 2020
“All would agree that COVID-19 is transmitted by respiratory particles. That being said, a division exists among experts on defining the spectrum of respiratory particles, what is an aerosol, the degree of infectivity of the virus in aerosols, and what role aerosols may be playing in human-to-human transmission [….] if guidance is written based upon a model in which a given activity does not produce an aerosol, then measures put in place to mitigate transmission will reflect that model. However, if the model is shown to be incorrect and in fact aerosols are generated by that activity, and under certain circumstances are infectious, then the prevention advice will need to reflect the new model. Achieving an understanding of the nature of respiratory particles produced by humans and their potential to transmit COVID-19, is therefore fundamental to producing accurate guidance on transmission prevention.”
From Scientific Reports. Nissen, K. et al. “Long-distance airborne dispersal of SARS CoV-2 in COVID-19 wards,” November 11, 2020
“…the apparent capability of the virus to be transported in air, as we present here, should raise concerns for the risk of infection in smaller, confined spaces in close proximity to contagious patients, i.e. all air in patients rooms, intensive care units, etc. during care for COVID-19 patients. This may be even more important concerning patients in earlier phases of disease, in which contagiousness may be high. This includes both symptomatic and asymptomatic SARS-CoV-2 infected persons in any confined space, such as homes, public transportation, restaurants, etc. The presented findings indicate airborne dissemination of SARS-CoV-2, especially considering the distance SARS-CoV-2 RNA was dispersed.”
From the Public Health Agency of Canada (PHAC). “Emerging Evidence on COVID-19: Evidence Brief on SARS-CoV-2 Aerosol Transmission.” November 6, 2020
“Many experts maintain expelled respiratory particles containing infectious pathogens can occur in a continuum of sizes, and smaller respiratory particles (often termed aerosols) can remain suspended in air and disperse further distances than large respiratory droplets. It has been established that other pathogens that are transmitted through large droplets (e.g., Influenza, SARS-CoV-1, streptococcus pneumonia, and legionella) can also spread by aerosols in some settings and conditions. As such, virus particles in aerosols may play a role in SARS-CoV-2 infection transmission. This evidence brief summarizes studies providing evidence of potential aerosol transmission of SARS-CoV-2 published up to November 6, 2020.”
From Environment International. Tang, S. et al. “Aerosol transmission of SARS CoV-2? Evidence, prevention and control,” November 2020
“Current evidence on SARS-CoV-2 has limitations, but is strongly indicative of aerosols as one of several routes of COVID-19 transmission. It should be noted that the equivalent evidence for contact and large droplet transmission is not available, but has been an unproven assumption from the outset.”
From Lancet Respiratory Medicine. Editorial. “COVID-19 transmission up-in-the-air,” October 29, 2020
“As cases of COVID-19 increase globally, we need to more fully understand the transmission routes. It is crucial that we embrace new research and do not rely on recommendations based on old data so that clearer and more effective infection control guidance can be provided in the face of pandemic fatigue.”
From National Academies of Sciences Engineering Medicine. “Airborne Transmission of SAR-CoV-2: Proceedings of a Workshop in Brief”, October 2020
Virtual Workshop held August 26-27, 2020 [all papers available] to address 4 critical questions:
1. What size aerosol particles and droplets are generated by people and how do they spread in air?
2. Which size aerosol particles and droplets are infectious and for how long?
3. What behavioral and environmental factors determine personal exposure to SARS-CoV-2?
4. What do we know about the relationship between infectious dose and disease for airborne SARS-CoV-2?
From Science. Prather, K., Marr, L., Schooley, R.T., McDiarmid, M.A., Wilson, M.E., Milton, D.K. “Airborne Transmission of SARS-CoV-2″, October 16, 2020
“There is overwhelming evidence that inhalation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) represents a major transmission route for coronavirus disease 2019 (COVID-19).
Individuals with COVID-19, many of whom have no symptoms, release thousands of virus-laden aerosols and far fewer droplets when breathing and talking. Thus, one is far more likely to inhale aerosols than be sprayed by a droplet, and so the balance of attention must be shifted to protecting against airborne transmission. In addition to existing mandates of mask-wearing, social distancing, and hygiene efforts, we urge public health officials to add clear guidance about the importance of moving activities outdoors, improving indoor air using ventilation and filtration, and improving protection for high-risk workers.”
From MedRxiv. Chen, P., Brobovitz, N., Premji, Z., Koopmans, M., Fisman, D.N., Gu, F.X. “Heterogenity in transmissibility and shedding SARS-CoV-2 via droplets and aerosols”, October 15, 2020
“Taken together, our findings provide a potential path forward for disease control. They highlight the disproportionate role of high-risk cases, settings and circumstances in propelling the COVID-19 pandemic. Since highly infectious cases, regardless of age or symptomatology, can rapidly shed SARS-CoV-2 via both droplets and aerosols, airborne spread should also be recognized as a transmission risk, including for superspreading. Strategies to abate infection should limit crowd numbers and duration of stay while reinforcing distancing and then widespread mask usage; well-ventilated settings can be recognized as lower risk venues.”
From CBC News. Miller, A. “Canada still downplays risk of airborne spread of coronavirus despite WHO, CDC guidance”, October 10, 2020
“…infectious diseases specialist and medical microbiologist Dr. Raymond Tellier, who is also an associate medical professor at McGill University in Montreal, says that by acknowledging ventilation plays a role in curbing transmission of COVID-19, PHAC is admitting that aerosols are a significant route of transmission. That’s because ventilation does not change the risk of transmission via larger respiratory droplets or contact with contaminated surfaces.
“If you promote avoiding a poorly ventilated indoor area, you implicitly admit that you accept aerosol transmission because the ventilation affects only aerosol transmission,” he said. “So if you are pushing ventilation, what are you talking about, if not aerosols?”
From REA Project No. 16832 – Health Sciences Association of British Columbia. Murphy, J. “Update on Evidence for Aerosol Transmission of COVID-19 and Implications for Health Care Worker Respiratory Protection”, October 2, 2020
“Based on the emerging picture on the significance and potential dominance of aerosol transmission mode for COVID-19, and the substantial evidence of elevated risk among health care workers, it seems likely that near field aerosol transmission is happening in certain care setting interactions. Given the overall picture of the evidence on airborne transmission, it is likely that is occurring as a result of exposure to non-visible aerosols, and not “droplets” as wrongly conceived by the world’s major public health authorities.”
From the Office of Chief Science Advisor. Government of Canada. “The role of bioaerosols and indoor ventilation in COVID-19 transmission,” September 28, 2020
“Close and prolonged contact is the most common route of SARS-CoV-2 transmission, which includes short-range inhalable particle transmission. While aerosol transmission over longer distances is possible, there are currently many unknowns about the conditions under which it could occur.”
From International Journal of Infectious Diseases. Lednicky, J.A. et al. “Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients”, September 15, 2020
From Harvard Medical Grand Rounds. Fauci, A. “Video: COVID-19: Public Health and Scientific Challenges”, September 10, 2020
From CIDRAP. Van Beusekom, M. “Yet more data support COVID-19 aerosol transmission”, August 31, 2020
“Two studies published late last week in Clinical Infectious Diseases highlight the role of airborne spread of COVID-19 and the importance of efficient ventilation systems. One study found that patients can exhale millions of viral RNA particles per hour in the early stages of disease, and the second tied an outbreak affecting 81% of residents and 50% of healthcare workers at a Dutch nursing home to inadequate ventilation.”
From Time Magazine. Jimenez, J-L. “COVID-19 is Transmitted Through Aerosols. We Have Enough Evidence. Now is the Time to Act”, August 25, 2020
“Droplets move ballistically—they fly like a cannonball from someone’s mouth and then travel through the air until they either hit something (worst case someone else’s eyes, mouth or nostrils) or fall to the ground. Aerosols on the other hand, act like smoke: after being expelled, they don’t fall to the ground, but rather disperse throughout the air, getting diluted by air currents, and being inhaled by others present in the same space. Contact tracing shows that, when it comes to COVID-19, being outdoors is 20 times safer than being indoors, which argues that aerosol transmission is much more important than droplets; outdoors, there’s plenty of air in which aerosols can become diluted; not so indoors.”
From BMJ. Wilson, N. Editorials. “Airborne Transmission of COVID-19. Guidelines and governments must acknowledge the evidence and take steps to protect the public”, August 20, 2020
“In July, 239 scientists signed an open letter “appealing to the medical community and relevant national and international bodies to recognise the potential for airborne spread of covid-19.” Although the World Health Organization conceded that “airborne transmission cannot be ruled out,” the response was reserved and arguably mistaken in continuing to suggest that airborne and droplet transmission are discrete categories and that airborne transmission occurs only during medical “aerosol generating procedures.”
From SSRN. Guenther, T. et al. “Investigation of a superspreading event preceding the largest meat processing plant-related SARS-Coronavirus 2 outbreak in Germany”, July 23, 2020
“Interpretation: Our results indicate climate conditions and airflow as factors that can promote efficient spread of SARS-CoV-2 via distances of more than 8 meters and provide insights into possible requirements for pandemic mitigation strategies in industrial workplace settings.”
From The Lancet Respiratory Medicine. Fennelly, K.P. “Particle sizes of infectious aerosols: Implications for infection control“, July 24, 2020
“Studies of cough aerosols and of exhaled breath from patients with various respiratory infections have shown striking similarities in aerosol size distributions, with a predominance of pathogens in small particles (<5 μm). These are immediately respirable, suggesting the need for personal respiratory protection (respirators) for individuals in close proximity to patients with potentially virulent pathogens. There is no evidence that some pathogens are carried only in large droplets. Surgical masks might offer some respiratory protection from inhalation of infectious aerosols, but not as much as respirators [N95s]. However, surgical masks worn by patients reduce exposures to infectious aerosols to health-care workers and other individuals. The variability of infectious aerosol production, with some so-called super-emitters producing much higher amounts of infectious aerosol than most, might help to explain the epidemiology of super-spreading.”
From medRxiv. Santarperia, J.L. et al. “The Infectious Nature of Patient-Generated SARS-CoV-2 Aerosol”, July 21, 2020
“Conclusion: Our results demonstrate that SARS-CoV-2 RNA exists in respired aerosols less than 5 µm in diameter; that aerosols containing SARS-CoV-2 RNA exist in particle modes that are produced during respiration, vocalization, and coughing; and that some fraction of the RNA-containing aerosols contain infectious virions. This study supports the use of efficient respiratory protection and airborne isolation precautions to protect from exposure to fine SARS-CoV-2 aerosol when interacting with infected individuals, regardless of symptoms or medical procedure being performed.”
From McMaster University National Collaborating Centre for Methods and Tools. “COVID-19 Summary SARS-CoV-2 Virus Airborne Transmission”, Prepared for the Public Health Agency of Canada, July 10, 2020 (completed); July 24, 2020 (submitted)
“Overview of the Evidence: Publications appearing in the emerging literature up to July 7, 2020 have informed this evidence brief. The available body of evidence is limited, largely theoretical, and does not specifically consider SARS-CoV-2 infectious dose or confirm the infectiousness of airborne particles. The theoretical and modeling evidence is of good quality. The available empirical and modeled evidence indicates there is some risk of SARS-CoV-2 virus laden aerosol and droplet dispersion at distances beyond two meters, while epidemiological evidence implicates airborne transmission of SARS-CoV-2 to have occurred in some indoor settings. Airborne infection transmission risks appear to be amplified in low temperature high humidity conditions, as well as in crowded and poorly ventilated areas where infected individuals may cough or speak loudly (i.e. sing, scream).”
From MIT Technology Review. Patel, Neel V. “If the coronavirus is really airborne, we might be fighting it the wrong way”, July 11, 2020
“One of the biggest questions we still have about COVID-19 is how much of a viral load is needed to cause infection. The answer changes if we think it is aerosols that we need to worry about. Smaller particles won’t carry as large a viral load as bigger ones, but because they can linger in the air for much longer, it may not matter—they’ll build up in larger concentrations and get distributed more widely the longer an infected person is around to expel aerosolized virus.
The more people you have coming in and out of an indoor space, the more likely it is that someone who is infected will show up. The longer those infected individuals spend in that space, the higher the concentration of virus in the air over time.”
From Oxford Academic: Clinical Infectious Diseases. Morawska, L., Milton, D. and 237 scientists from 32 countries who support this Commentary. “It is Time to Address Airborne Transmission of COVID-19.”, July 6, 2020
“Studies by the signatories and other scientists have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond 1 to 2 m from an infected individual. […] It is understood that there is not as yet universal acceptance of airborne transmission of SARS-CoV2; but in our collective assessment there is more than enough supporting evidence so that the precautionary principle should apply. In order to control the pandemic, pending the availability of a vaccine, all routes of transmission must be interrupted. We are concerned that the lack of recognition of the risk of airborne transmission of COVID-19 and the lack of clear recommendations on the control measures against the airborne virus will have significant consequences: people may think that they are fully protected by adhering to the current recommendations, but in fact, additional airborne interventions are needed for further reduction of infection risk. […] The evidence is admittedly incomplete for all the steps in COVID-19 microdroplet transmission, but it is similarly incomplete for the large droplet and fomite modes of transmission. The airborne transmission mechanism operates in parallel with the large droplet and fomite routes, that are now the basis of guidance. We appeal to the medical community and to the relevant national and international bodies to recognize the potential for airborne spread of COVID-19. There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several meters, or room scale), and we are advocating for the use of preventive measures to mitigate this route of airborne transmission.”
From Atmosphere. Carducci, A. et al. “Covid-19 Airborne Transmission and Its Prevention: Waiting for Evidence or Applying the Precautionary Principle?” July 3, 2020
“Besides the predominant ways of transmission of SARS-CoV-2 (namely, contacts and large droplets) the airborne one is increasingly taken into consideration as a result of latest research findings. Nevertheless, this possibility has been already suggested by previous studies on other coronaviruses including SARS-CoV and MERS-CoV. To describe the state of the art of coronaviruses and airborne transmission, a systematic review was carried out using the PRISMA methodology. Overall, 64 papers were selected and classified into three main groups: laboratory experiments (12 papers), air monitoring (22) and epidemiological and airflow model studies (30). The airborne transmission of SARS-CoV-2 is suggested by the studies of the three groups, but none has yet obtained complete evidence. […] epidemiological investigations only hypothesize the airborne transmission as a possible explanation for some illness cases, but without estimating its attributable risk. Nevertheless, while waiting for more evidence, it is urgent to base advice on preventive measures, such as the use of masks, safe distancing and air ventilation, on the precautionary principle.”
From Emerging Infectious Diseases Journal (Centers for Disease Control and Prevention). Fears, A.C. et al. “Persistence of severe acute respiratory syndrome coronavirus 2 in aerosol suspensions”, June 22, 2020
“We aerosolized severe acute respiratory syndrome coronavirus 2 and determined that its dynamic aerosol efficiency surpassed those for severe acute respiratory syndrome coronavirus and Middle East respiratory syndrome. Although we performed [the] experiment only once across severeral laboratories, our findings suggest retained infectivity and virion integrity for up to 16 hours in respirable-sized aerosols.”
From Proceedings of the National Academy of Sciences of the United States of America. Zhang, R. et al. “Identifying airborne transmission as the dominant route for the spread of COVID-19”, June 11, 2020
“The inadequate knowledge on virus transmission has inevitably hindered development of effective mitigation policies and resulted in unstoppable propagation of the COVID-19 pandemic. In this work, we show that airborne transmission, particularly via nascent aerosols from human atomization, is highly virulent and represents the dominant route for the transmission of this disease.”
From medRxiv. Evans, M. “Avoiding COVID-19: Aerosol Guidelines”, June 4, 2020
“The COVID-19 pandemic has brought into sharp focus the need to understand respiratory virus transmission mechanisms. In preparation for an anticipated influenza pandemic, a substantial body of literature has developed over the last few decades showing that the short-range aerosol route is an important, though often neglected transmission path. We develop a simple mathematical model for COVID-19 transmission via aerosols, apply it to known outbreaks, and present quantitative guidelines for ventilation and occupancy in the workplace.”
From medRxiv. Santarpia, J.L. et al.”Aerosol and Surface Transmission Potential of SARS-CoV-2”, June 3, 2020
“During the initial isolation of 13 individuals with COVID-19 at the University of Nebraska Medical Center, we collected air and surface samples to examine viral shedding from isolated individuals. We detected viral contamination among all samples, indicating that SARS-CoV-2 may spread through both direct (droplet and person-to-person) as well as indirect mechanisms (contaminated objects and airborne transmission). Taken together, these finding support the use of airborne isolation precautions when caring for COVID-19 patients.”
From medRxiv. Ma, J. et al. “Exhaled breath is a significant source of SARS-CoV-2 emission”, June 2, 2020
“Here, 35 COVID-19 subjects were recruited; exhaled breath condensate (EBC), air samples and surface swabs were collected and analyzed for SARS-CoV-2 using reverse transcription-polymerase chain reaction (RT-PCR). EBC samples had the highest positive rate (16.7%, n=30), followed by surface swabs (5.4%, n=242), and air samples (3.8%, n=26). COVID-19 patients were shown to exhale SARS-CoV-2 into the air at an estimated rate of 103-105 RNA copies/min; while toilet and floor surfaces represented two important SARS-CoV-2 reservoirs. Our results imply that airborne transmission of SARS-CoV-2 plays a major role in COVID-19 spread, especially during the early stages of the disease.”
From Nature. Chia, P.Y. et al. “Detection of Air and Surface Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in Hospital Rooms of Infected Patients”, May 29, 2020.
“Understanding the particle size distribution in the air and patterns of environmental contamination of SARS-CoV-2 is essential for infection prevention policies. Here we screen surface and air samples from hospital rooms of COVID-19 patients for SARS-CoV-2 RNA. Environmental sampling is conducted in three airborne infection isolation rooms (AIIRs) in the ICU and 27 AIIRs in the general ward. 245 surface samples are collected. 56.7% of rooms have at least one environmental surface contaminated. High touch surface contamination is shown in ten (66.7%) out of 15 patients in the ﬁrst week of illness, and three (20%) beyond the ﬁrst week of illness (p=0.01, χ2 test). Air sampling is performed in three of the 27 AIIRs in the general ward, and detects SARS-CoV-2 PCR-positive particles of sizes >4µm and 1–4µm in two rooms, despite these rooms having 12 air changes per hour. This warrants further study of the airborne transmission potential of SARS-CoV-2.”
From Environment International. Morawska, L. et al. “Correspondence: How can airborne transmission of COVID-19 indoors be minimised?” May 27, 2020
“Inhaling small airborne droplets is probable as a third route of infection, in addition to more widely recognized transmission via larger respiratory droplets and direct contact with infected people or contaminated surfaces. While uncertainties remain regarding the relative contributions of the different transmission pathways, we argue that existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors. Appropriate building engineering controls include sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection, avoiding air recirculation and avoiding overcrowding. Often, such measures can be easily implemented and without much cost, but if only they are recognised as significant in contributing to infection control goals. We believe that the use of engineering controls in public buildings, including hospitals, shops, offices, schools, kindergartens, libraries, restaurants, cruise ships, elevators, conference rooms or public transport, in parallel with effective application of other controls (including isolation and quarantine, social distancing and hand hygiene), would be an additional important measure globally to reduce the likelihood of transmission and thereby protect healthcare workers, patients and the general public.”
From Lancet. Somsen, G.A. et al. “Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission.” May 27, 2020
“This study shows that better ventilation of spaces substantially reduces the airborne time of respiratory droplets. This finding is relevant because typically poorly ventilated and populated spaces, like public transport and nursing homes, have been reported as sites of viral transmission despite preventive physical distancing. The persistence of small respiratory droplets in such poorly ventilated spaces could contribute to the spread of SARS-CoV-2. Our findings confirm that improving ventilation of public spaces will dilute and clear out potentially infectious aerosols. To suppress the spread of SARS-CoV-2 we believe health-care authorities should consider the recommendation to avoid poorly ventilated public spaces as much as possible. The implications are also important for hospital settings where aerosolisation by coughing and medical treatments and close contact with COVID-19 patients is very common.”
From Proceedings of the National Academy of Sciences of the United States of America. Stadnytskyi, V. et al. “The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission”, May 13, 2020.
“Abstract: Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are increasingly considered to be a likely mode of disease transmission. Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second. In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of 8 to 14 min, which corresponds to droplet nuclei of ca. 4 μm diameter, or 12- to 21-μm droplets prior to dehydration. These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.”
From BMC Human Genomics. Godri Pollitt, K.J. et al. “COVID-19 vulnerability: the potential impact of genetic susceptibility and airborne transmission”, May 12, 2020.
“Detection of SARS-CoV-2 in the air prompts questions about safe exposure levels. The high transmissivity of the virus suggests that a low dose might be sufficient to infect an individual; however, such studies have yet to evaluate the infectious dose of SARS-CoV-2. Until scientific evidence emerges, it is useful for individuals to follow approaches that minimize their risk of infection by reducing their exposure level and duration of exposure. Initial studies (as detailed above) report a range of airborne virus exposure levels in hospitals, as well as public spaces. The combined use of masks and physical distancing can be effective approaches for decreasing exposure to airborne forms of SARS-CoV-2. Avoiding or minimizing the time in contact with these potential aerosol exposures would also be a critical parameter in lowering risk.”
From Preprints. Allen, J.; Marr, L. “Re-thinking the Potential for Airborne Transmission of SARS-CoV-2”, May 7, 2020
“While the traditional distinction between “droplet” and “airborne” transmission of infectious disease has been useful for setting guidelines on the use of personal protective equipment, it has also established a false dichotomy in understanding the behavior of viruses in the air. Viruscontaining droplets that are released by breathing, talking, and coughing span a continuum of sizes, from 0.01 to hundreds of microns. It is impossible for someone to release “large droplets” (>5 microns) without also releasing smaller ones.xii Thus, transmission that is purported to occur via the spray of large droplets from a cough could in fact be occurring through inhalation of much smaller droplets at close range. In fact, a physics-based simulation suggests that the majority of exposure at close range occurs by inhalation of small droplets rather than by contact with large droplets that land on the mouth, nose, and eyes, unless the people are closer than 30 cm or the droplets are very large.”
From Risk Analysis. Anderson, E. et al. “Consideration of the Aerosol Transmission for COVID‐19 and Public Health”, May 1, 2020.
“This article analyzes the available evidence to address airborne, aerosol transmission of the SARS‐CoV‐2. We review and present three lines of evidence: case reports of transmission for asymptomatic individuals in association with studies that show that normal breathing and talking produce predominantly small droplets of the size that are subject to aerosol transport; limited empirical data that have recorded aerosolized SARS‐CoV‐2 particles that remain suspended in the air for hours and are subject to transport over distances including outside of rooms and intrabuilding, and the broader literature that further supports the importance of aerosol transmission of infectious diseases. The weight of the available evidence warrants immediate attention to address the significance of aerosols and implications for public health protection.”
From International Journal of Environmental Research and Public Health. Setti, L. et al. “Airborne Transmission Route of COVID-19: Why 2 Meters/6 Feet of Inter-Personal Distance Could Not Be Enough” April 23, 2020
“However, recently published studies support the hypothesis of virus transmission over a distance of 2 m from an infected person. Researchers have proved the higher aerosol and surface stability of SARS-COV-2 as compared with SARS-COV-1 (with the virus remaining viable and infectious in aerosol for hours) and that airborne transmission of SARS-CoV can occur besides close-distance contacts. Indeed, there is reasonable evidence about the possibility of SARS-COV-2 airborne transmission due to its persistence into aerosol droplets in a viable and infectious form. Based on the available knowledge and epidemiological observations, it is plausible that small particles containing the virus may diffuse in indoor environments covering distances up to 10 m from the emission sources, thus representing a kind of aerosol transmission.”
From Anaesthesia. Wilson, N.M. et al. “Review article: Airborne transmission of severe acute respiratory syndrome coronavirus‐2 to healthcare workers: a narrative review”, April 20, 2020
“The mechanism for SARS‐CoV‐2 transmission is unknown, but the evidence suggestive of airborne spread is growing. We speculate that infected patients who cough, have high work of breathing, increased closing capacity and altered respiratory tract lining fluid will be significant producers of pathogenic aerosols. We suggest several ‘aerosol‐generating procedures’ may in fact result in less pathogen aerosolization than a dyspnoeic and coughing patient. Health care workers should appraise the current evidence regarding transmission and apply this to the local infection prevalence. Measures to mitigate airborne transmission should be employed at times of risk. However, the mechanisms and risk factors for transmission are largely unconfirmed. Whilst awaiting robust evidence, a precautionary approach should be considered to assure health care worker safety.”
From medRxiv. Fears, A.C. et al. “Comparative dynamic aerosol efficiencies of three emergent coronaviruses and the unusual persistence of SARS-CoV-2 in aerosol suspensions”, April 18, 2020.
“Collectively, this preliminary dataset on the aerosol efficiency and persistence of SARSCoV- 2 suggest that this virus is remarkably resilient in aerosol form, even when aged for over 12 hours, and reinforces the conclusions reached in earlier studies of aerosol fitness by others. Aerosol transmission of SARS-CoV-2, whether through direct respiratory droplet transfer or fomite generation, may in fact be a more important exposure transmission pathway than previously considered.” […]
“Humans produce aerosols continuously through normal respiration. Production of aerosols increases during respiratory illnesses, and even during louder-than-normal oration. A fraction of naturally-generated aerosols fall within the size distribution used in our experimental studies (<5 μm), thus leading us to the conclusion that individuals infected with SARS-CoV-2 have the capacity to produce viral bioaerosols that may remain infectious over long periods of time after production via human shedding and airborne transport.”
From Journal of Infectious Diseases. Bahl, P. et al. “Airborne or Droplet Precautions for Health Workers Treating Coronavirus Disease 2019?” April 16, 2020
The authors undertook a review of current international and jurisdictional guidance as well as the emerging science and concluded: “Several studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) support aerosol transmission, and 1 study documented virus at a distance of 4 meters (≈13 feet) from the patient. Moreover, evidence suggests that infections cannot neatly be separated into the dichotomy of droplet versus airborne transmission routes. Available studies also show that SARS-CoV-2 can be detected in the air, and remain viable 3 hours after aerosolization. The weight of combined evidence supports airborne precautions for the occupational health and safety of health workers treating patients with COVID-19.”
From SciTech Daily. Queensland University of Technology. “Indoor Precautions Essential to Stem Airborne COVID-19 – The World Should Face the Reality”, April 16, 2020
“Airborne transmission of COVID-19 must be taken into account.”
“Likely COVID-19 spread to cruise ship passengers through ventilation system even when passengers confined to their cabins.”
“Viable airborne viruses can travel beyond 1.5m on airflow when exhaled by an infected person.”
“Virus air transmission research must begin now not retrospectively.”
From New England Journal of Medicine (NEJM). National Institutes of Health. Correspondence: “Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering”, April 15, 2020
“Speaking calmly and at a normal volume produces liquid droplets so small they can remain suspended in the air long enough to enter the airways of other people, potentially exposing them to viruses including the one that causes Covid-19, according to a new study led by scientists at the National Institutes of Health.”
From Emerging Infectious Diseases Journal (Centers for Disease Control and Prevention) Vol 26; No. 7. Guo, Z-D. et al. “Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China 2020”, April 10, 2020
“SARS-CoV-2 was widely distributed in the air and on object surfaces in both the ICU and GW [COVID-19 General Ward], implying a potentially high infection risk for medical staff and other close contacts.” The maximum transmission distance of SARS-CoV-2 aerosol was measured at up to 4 metres. […]
“Half of the samples from the soles of the ICU medical staff shoes tested positive. Therefore, the soles of medical staff shoes might function as carriers.”
From Aalto University, Finland. Aalto University; Finnish Meteorological Institute; VTT Technical Research Centre and University of Helsinki. “Researchers modelling the spread of the coronavirus emphasise the importance of avoiding busy indoor spaces”, April 6, 2020
“…extremely small airborne aerosol particles emitted from the respiratory tract when coughing, sneezing or even talking are transported in the air. Such particles can carry pathogens such as coronaviruses. The researchers modelled a scenario where a person coughs in an aisle between shelves, like those found in grocery stores; and taking into consideration the ventilation.
The researchers obtained the same preliminary result: in the situation under investigation, the aerosol cloud spreads outside the immediate vicinity of the coughing person and dilutes in the process. However, this can take up to several minutes. ‘Someone infected by the coronavirus, can cough and walk away, but then leave behind extremely small aerosol particles carrying the coronavirus. These particles could then end up in the respiratory tract of others in the vicinity’, explains Aalto University Assistant Professor Ville Vuorinen.”
From Aerosol Science and Technology. Asadi, S. et al. “The coronavirus pandemic and aerosols: Does COVID-19 transmit via expiratory particles?” April 3, 2020
Given the large numbers of expiratory particles known to be emitted during breathing and speech, and given the clearly high transmissibility of COVID-19, a plausible and important hypothesis is that a face-to-face conversation with an asymptomatic infected individual, even if both individuals take care not to touch, might be adequate to transmit COVID-19.
From Environment International. Vol. 139. Morawska, L.; Cao, J. “Airborne transmission of SARS-CoV-2: The world should face the reality”
“Therefore, all possible precautions against airborne transmission in indoor scenarios should be taken. Precautions include increased ventilation rate, using natural ventilation, avoiding air recirculation, avoiding staying in another person’s direct air flow, and minimizing the number of people sharing the same environment (Qian et al. 2018). Of significance is maximizing natural ventilation in buildings that are, or can be natural ventilation and ensuring that the ventilation rate is sufficiently high.” […]
“To summarize, based on the trend in the increase of infections, and understanding the basic science of viral infection spread, we strongly believe that the virus is likely to be spreading through the air. If this is the case, it will take at least several months for this to be confirmed by science. This is valuable time lost that could be used to properly control the epidemic by the measures outlined above and prevent more infections and loss of life. Therefore, we plead that the international and national authorities acknowledge the reality that the virus spreads through air, and recommend that adequate control measures, as discussed above be implemented to prevent further spread of the SARS-CoV-2 virus.We predict that this failure to immediately recognize and acknowledge the importance of airborne transmission and to take adequate actions against it will result in additional cases of infection in the coming weeks and months, which would not occur if these actions were taken. The air transmission issue should be taken seriously now, during the course of the epidemic.”
From the National Academies of Science and Engineering.”Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic”, April 1, 2020
“While the current SARS-CoV-2 specific research is limited, the results of available studies are consistent with aerosolization of virus from normal breathing.”
“Individuals vary in the degree to which they produce bioaerosols through normal breathing. This may have a bearing on efficiency of transmission of SARS-CoV-2 by different infected but asymptomatic individuals.”
“…for no respiratory virus is the exact proportion of infections due to air droplet, aerosol, or fomite transmission fully established, and many individual factors and situations may contribute to the importance of each route of transmission.”
From The Lancet. Fennelly, K.P. “Particle sizes of infectious aerosols: implications for infection control”, July 24, 2020
“The global pandemic of COVID-19 has been associated with infections and deaths among health-care workers. This Viewpoint of infectious aerosols is intended to inform appropriate infection control measures to protect health-care workers. Studies of cough aerosols and of exhaled breath from patients with various respiratory infections have shown striking similarities in aerosol size distributions, with a predominance of pathogens in small particles (<5 µm). These are immediately respirable, suggesting the need for personal respiratory protection (respirators) for individuals in close proximity to patients with potentially virulent pathogens. There is no evidence that some pathogens are carried only in large droplets. Surgical masks might offer some respiratory protection from inhalation of infectious aerosols, but not as much as respirators. However, surgical masks worn by patients reduce exposures to infectious aerosols to health-care workers and other individuals. The variability of infectious aerosol production, with some socalled super-emitters producing much higher amounts of infectious aerosol than most, might help to explain the epidemiology of super-spreading. Airborne infection control measures are indicated for potentially lethal respiratory pathogens such as severe acute respiratory syndrome coronavirus 2.”
From World Health Organization. “Advice on the use of masks in the context of COVID-19”, June 5, 2020
“Whereas medical masks filter 3 micrometre droplets, respirators must filter more challenging 0.075 micrometre solid particles.”[…] “Therefore, the layers of the filtration material and the FFR [filtering facepiece respirator] shape, ensuring outer edges of the FFR seal around wearer’s face, result in a guaranteed claimed filtration when worn compared to the open shape, or leaking structure, of medical masks.”
From BMJ. Liu, M. et al. “Use of personal protective equipment against coronavirus diseases 2019 by healthcare professionals in Wuhan, China cross sectional study”, June 2, 2020
“OBJECTIVE: To examine the protective effects of appropriate personal protective equipment for frontline healthcare professionals who provided care for patients with coronavirus disease 2019 (covid-19). RESULTS During the deployment period in Wuhan, none of the study participants reported covid-19 related symptoms. When the participants returned home, they all tested negative for SARSCoV-2.”
From CBC. “Keep wearing masks and social distancing — it works, new McMaster study says” The Canadian Press. Refers to WHO funded research published in The Lancet by Chu, D.K. et al. “Physical distancing, face masks and eye protection to prevent person-to-person transmission of SARS-Cov-2 and COVID-19: a systemic review and metaanlysis”, June 1, 2020
“Researchers concluded single-layer cloth masks are less effective than surgical masks, while tight-fitting N95 masks provide the best protection. A distance of 1 metre (more than 3 feet) between people lowers the danger of catching the virus, while 2 metres (about 6 1/2 feet) is even better.”
From The Lancet. MacIntyre, R.; Wang, Q. “Comment: Physical distancing, face masks, and eye protecton for prevention of COVID-19.”, June 1, 2020
“For health-care workers on COVID-19 wards, a respirator should be the minimum standard of care. This study by Chu and colleagues should prompt a review of all guidelines that recommend a medical mask for health workers caring for COVID-19 patients. Although medical masks do protect, the occupational health and safety of health workers should be the highest priority and the precautionary principle should be applied. Preventable infections in health workers can result not only in deaths but also in large numbers of health workers being quarantined and nosocomial outbreaks which is an unacceptable risk for front-line workers. To address global shortages of PPE, countries should take responsibility for scaling up production rather than expecting health workers to work in suboptimum PPE.”
From a 2017 study: MacIntyre, R. et al. “The efficacy of medical masks and respirators against respiratory infection in healthcare workers”
“The results suggest that the classification of infections into droplet versus airborne transmission is an oversimplification. Most guidelines recommend masks for infections spread by droplets. N95 respirators, as “airborne precautions,” provide superior protection for droplet‐transmitted infections. To ensure the occupational health and safety of healthcare worker, the superiority of respirators in preventing respiratory infections should be reflected in infection control guidelines.”
From U.S. Food and Drug Administration (FDA) “N95 Respirators and Surgical Masks (Face Masks)”
“While a surgical mask may be effective in blocking splashes and large-particle droplets, a face mask, by design, does not filter or block very small particles in the air that may be transmitted by coughs, sneezes, or certain medical procedures. Surgical masks also do not provide complete protection from germs and other contaminants because of the loose fit between the surface of the face mask and your face.
An N95 respirator is a respiratory protective device designed to achieve a very close facial fit and very efficient filtration of airborne particles.
The ‘N95’ designation means that when subjected to careful testing, the respirator blocks at least 95 percent of very small (0.3 micron) test particles. If properly fitted, the filtration capabilities of N95 respirators exceed those of face masks.”
From the Institute of Medicine, on the role of surgical masks (2010):
“Face masks, including surgical and procedure masks, are loose-fitting coverings of the nose and mouth that are designed to protect the patient from secretions from the nose or mouth of the physician, nurse, or other healthcare professional. Face masks are not designed or certified to protect the wearer from exposure to respiratory hazards.”
From the Ontario SARS Commission Inquiry: Final Report (2006) referencing a study by the Institute of Medicine of the National Academies:
“The loose fit of most medical masks [i.e., surgical and procedure masks] leaves gaps that could allow substantial contaminant leakage into and from the mask … Medical masks may be used as barriers against disease transmission by fluids, especially blood, and some large droplets, and they are designed to prevent release to the environment of large droplets generated by the wearer. They are not designed or approved for the purpose of protecting the wearer against entry of infectious aerosolized particles potentially surrounding the wearer and his mask.”