key: cord-339329-8yvre7qc
authors: Kumar, Prashant; Morawska, Lidia
title: Could fighting airborne transmission be the next line of defence against COVID-19 spread?
date: 2020-05-23
journal: nan
DOI: 10.1016/j.cacint.2020.100033
sha: 
doc_id: 339329
cord_uid: 8yvre7qc

Abstract The World Health Organization declared the infectious spread of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) an epidemic during its initial outbreak in Wuhan (China) and has since declared it a pandemic and, more recently, an endemic infection that may remain in our communities. A vaccine for COVID-19 is expected to take several months, meaning that the spread may continue in future, in the absence of the most effective measures of social distancing and self-isolation. While these measures have worked well under lockdowns, the potential of airborne transmission of COVID-19 under the eased restrictions has not been considered important enough. We discuss the need to acknowledge the airborne spread of COVID-19 inside built spaces under eased movement restrictions and the potential steps that can be taken to control it.

Social distancing, self-isolation, handwashing, provision of hand sanitisers in public buildings, frequent disinfection of high-touch surfaces and the use of face masks have been recommended as effective mitigation measures against the spread of COVID-19 by the SARS-CoV-2 virus. Their effectiveness has been demonstrated by the flattening curves for new cases identified, hospital admissions and deaths across time. However, such gains have come at great expense, with sacrifices including freezing people's movement and closing down academic institutions, schools and businesses, all of which has negatively impacted national and international economies so severely that they have already touched a record low in many countries. A long-term lockdown is therefore unlikely to be affordable. Some countries, such as Denmark and Germany, have already started to ease restrictions, while others, including the United States and the United Kingdom, have followed suit.

The rate of reproduction (R) of the coronavirus is a simple means of understanding the spread of COVID-19 against response measures in a particular country, city or region. The spread of new infections will multiply if the value of R increases over one, and will fall and eventually disappear as the value of R goes below one. A diverse range of R was experienced across the world during the early phases of infectious spread (e.g. approaching a value of eight in New York, United States) before the curve was generally flattened by the introduction of lockdown measures such as strict social distancing and clossures of schools, churches, bars, J o u r n a l P r e -p r o o f lockdowns such as those of Europe, Asia or the United States. The driving factors of their success included 'swift action, widespread testing and contact tracing, and critical support from citizens', which were underpinned by both the 'political will', in terms of imposing arduous measures in the absence of a crisis-level outbreak, and the 'public will', in terms of social trust of the people to support government decisions. As the curve flattens in many countries across the world and cross-country measures including international border control remain important, it remains vital to protect people from this novel virus inside built environments and public places within our cities.

How do we manage this under eased movement restrictions? Significant infection pathways are considered to include coming into direct contact with the sneeze/cough of an infected person and touching an infected person or a surface containing virus-laden droplets.

Restricting airborne transmission of COVID-19 has not been high on the list of control measures. However, numerous cases of airborne transmission of its predecessor, SARS-COV-1, such as in Hong Kong's Prince of Wales Hospital, health care facilities in Toronto, and aircraft, have been reported [1] . Evidence of airborne transmission of SARS-COV-2 has already started to emerge. For example, a recent study reported the probability of COVID-19 being spread by an extended short-range aerosol on 24 January 2020 in a poorly ventilated restaurant in Guangzhou, China [2] . Another study has reported a high concentration of viral RNA peaks in sub-and super-micrometre particle ranges and highlighted the potential transmission of SARS-CoV-2 via aerosols inside two Wuhan hospitals [3] . Most recently, researchers have assessed the COVID-19 outbreak in two buseswith and without a COVID-19 source patientas well as separately inside conference rooms [4] . The air conditioners were set on indoor recirculation mode in all cases, and the authors reported a greater than 40-times higher risk of COVID-19 infection in the bus with a source patient compared with the other bus, and an overall COVID-19 attack rate of over 48% in conference J o u r n a l P r e -p r o o f rooms [4] . This work highlighted the probability of a much higher COVID-19 infection rate in closed environments with re-circulated air, and substantiates our below point regarding airborne transmission. Another work reported the presence of SARS-CoV-2 viral RNA on the outdoor particulate matter ≤10µm samples collected at an industrial site in the Bergamo Province of Italy between 21 February and 13 March 2020 [5] . The basic principles of aerosol science substantiate the claim that airborne transmission would be a significant pathway of spread in indoor environments that provide conducive conditions, for the reasons discussed below.

Most viruses, including COVID-19, are less than 100 nanometres in size. However, the expiratory droplets containing the virus also include water, salts and organic material. If, upon expiration, some of the water content evaporates, the microscopic droplet becomes small and light enough to stay suspended in the air [1, 6] . Their concentration in the air will 

Airborne transmission of SARS-CoV-2: The world should face the reality

Evidence for probable aerosol transmission of SARS-CoV-2 in a poorly ventilated restaurant. medRxiv

Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals

SARS-Cov-2 RNA found on particulate matter of

Airborne transmission of COVID-19: epidemiologic evidence from two outbreak investigations

Dynamics and dispersion modelling of nanoparticles from road traffic in the urban atmospheric environment -a review