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A new wave of Covid-19 infections casts light on air quality and management

Is it in the air? As the world faces yet another Covid-19 wave, experts continue to debate how the SARS-CoV-2 virus mainly spreads: via visible “respiratory droplets” such as when a person sneezes or coughs, or from aerosols that are inhaled from the air.


Aerosols are produced during normal exhalation, in addition to activities such as speaking, shouting, singing, working out and so on. “Direct measurements show that speaking produces thousands of aerosol particles and few large droplets, which supports the airborne route,” according to a recent article published in The Lancet medical journal, ‘Ten scientific reasons in support of airborne transmission of SARS-CoV-2’.


The authors say this explains Australia’s hotel quarantine ‘leaks’,among other cases, when travellers under quarantine became infected by the virus carried across air-conditioning ducts even when they were not in the same room.


Current public health advisories issued by the WHO and the US Centers for Disease Control and Prevention (CDC) are informed by respiratory droplets being the main mode of transmission of SARS-CoV-2.


While CDC concedes that “(a)irborne transmission of SARS-CoV-2 can occur under special circumstances,” [1] both WHO and CDC believe Covid-19 is spread primarily through respiratory droplets during close contact such as those produced when people cough, shout or sneeze. These are thought to mostly fall to the ground beyond six feet.


According to the CDC, “The epidemiology of SARS-CoV-2 indicates that most infections are spread through close contact, not airborne transmission ... because if it were spread primarily through airborne transmission like measles, experts would expect to have observed considerably more rapid global spread of infection in early 2020”.


However, the authors of The Lancet article say superspreading events are very likely the pandemic's primary drivers because “detailed analyses of human behaviours and interactions, room sizes, ventilation, and other variables in choir concerts, cruise ships, slaughterhouses, care homes, and correctional facilities, among other settings … are consistent with airborne spread of SARS-CoV-2 that cannot be adequately explained by droplets or fomites (transmission via surfaces)” and “strongly suggest the dominance of aerosol transmission.”


They also note that the evidence shows asymptomatic or presymptomatic[2] transmission of SARS-CoV-2 likely accounts for at least a third or up to 59 percent of all transmission globally and is “a key way SARS-CoV-2 has spread around the world”. Because asymptomatic and presymptomatic patients do not exhibit symptoms such as coughing or sneezing, this implies the virus is less likely to be transmitted via respiratory droplets but aerosols in the air.

As to transmission occurring from close contact conducive to infections via droplets, it is flawed thinking to assume aerosol transmission does not also play a part. Also, despite being physically larger, droplets do not always have a higher concentration of pathogens than smaller aerosols.


On the significance of managing air quality and air flow, the authors point out that “transmission of SARS-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation. Both observations support a predominantly airborne route of transmission.”


SARS-CoV-2 has also been identified in air filters and building ducts in hospitals with COVID-19 patients, which could be reached only by aerosols.


Further, nosocomial or hospital-acquired infections have been documented in healthcare contexts despite strict use of personal protective equipment (PPE) designed to shield against droplet but not aerosol exposure.


However, is there direct evidence of the SARS-CoV-2 virus in the air? The authors draw on studies where viable SARS-CoV-2 was detected in the air in laboratory experiments where it stayed infectious for up to three hours.


Viable or still-infectious SARS-CoV-2 have also been found in rooms occupied by COVID-19 patients where no aerosol-generating health-care procedures (such as intubation) were being conducted, implying that aerosol transmission can happen as a matter of course. Viable samples have also been detected in the air of an infected person's car.


However, WHO has been circumspect about accepting evidence of airborne transmission of SARS-CoV-2 on the grounds that the laboratory conditions do not correspond with real-life situations (“human cough conditions”). Also, there are other studies where viable samples of the virus were not successfully cultivated from the air in healthcare settings with COVID-19 patients, according to its own systematic review in March.


The authors address this discrepancy by explaining in detail that virus sampling is fraught because it is a delicate process where samples can often be destroyed, lost or compromised. They go on to point out that measles and tuberculosis, two primarily airborne diseases, have never been cultivated from room air.


They conclude that the lack of direct evidence of SARS-CoV-2 in some air samples is not enough to cast doubt on its airborne transmission because of the quality and consistency of the overall evidence in favour of it. Accordingly, the authors see fit to exhort the public health community to “act accordingly and without further delay”.

[1] Such as prolonged exposure to an increased concentration of respiratory droplets in the air (e.g., from shouting, singing, exercising) and inadequate ventilation that allows a build-up of respiratory droplets and particles [2] The difference between asymptomatic and presymptomatic patients is that asymptomatic patients do not develop symptoms, while presymptomatic patients do.

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