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GENEVA, Switzerland: When it comes to airborne pathogens, size matters. Scientists have written an open letter urging the World Health Organization (WHO) to recognise that SARS-CoV-2 could be spread through microdroplets that are small enough to remain airborne for extended periods. WHO responded by acknowledging that evidence of airborne transmission is emerging, but stopped short of changing its guidance or recognising the potential for airborne transmission outside of hospitals and dental settings.
The letter, signed by 239 scientists and engineers from 32 countries, cited emerging evidence of airborne transmission. There is every reason to expect that SARS-CoV-2 could be transmitted via aerosols, the scientists argued. WHO has previously only recognised the possibility of airborne transmission where aerosol-generating procedures are undertaken, such as in hospitals and dental clinics.
Current guidance from WHO focuses on precautions that inhibit the spread of the virus through close contact with infected persons, through larger pathogen-containing respiratory droplets and through contact with infected surfaces. The scientists argued, however, that this guidance was insufficient to protect people from becoming infected with SARS-CoV-2 via airborne virus-carrying microdroplets.
The letter read: “It is understood that there is not as yet universal acceptance of airborne transmission of SARS-CoV-2; 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.”
The authors continued: “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 […] This matter is of heightened significance now, when countries are reopening following lockdowns.”
The authors recommended that the following measures be taken in order to mitigate the risk posed by airborne transmission:
Provide sufficient and effective ventilation (supply clean outdoor air and minimise recirculating air) particularly in public buildings, workplace environments, schools, hospitals and aged care homes.
Supplement general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration and germicidal ultraviolet lights.
Avoid overcrowding, particularly in public transport and public buildings.
WHO officials admitted that evidence of airborne transmission was emerging, but cautioned that the evidence requires further assessment. The health body released updated guidance on the role of airborne droplets in the transmission of SARS-CoV-2 on 9 July, in which it was stated: “Further studies are needed to determine whether it is possible to detect viable SARS-CoV-2 in air samples from settings where no procedures that generate aerosols are performed and what role aerosols might play in transmission.”
SARS-CoV-2: The big problem with small particles
The size of particles is the most important factor in determining aerosol behaviour, according to a review of the scientific literature on aerosols generated by individuals with respiratory infections by Dr Kevin P. Fennelly, published online on 24 July 2020 in Lancet Respiratory Medicine.
The review pointed out that traditional infection control policies are based on the premise that respiratory infections are transmitted by pathogen-containing droplets that are larger than 5 μm. Airborne transmission is associated with smaller particles that are smaller than 5 μm in size. Unlike the larger droplets that are generated through coughing or sneezing, smaller particles are generated through speech or exhaled breath, are immediately respirable and remain suspended in the air for a longer period, travelling greater distances. Until now, airborne transmission was thought to occur only for tuberculosis and a small number of other pathogens.
Fennelly found that “infectious aerosols from humans exist in a wide range of particle sizes that are strikingly consistent across studies, methods, and pathogens”. He stated: “There is no evidence to support the concept that most respiratory infections are associated with primarily large droplet transmission. In fact, small particle aerosols are the rule, rather than the exception, contrary to current guidelines.”
The review concluded that data is accumulating which shows that SARS-CoV-2 is transmitted by both small and large particle aerosols. This data, Fennelly found, supports calls for the recognition of aerosol transmission of SARS-CoV-2 and the use of environmental controls such as air disinfection in congregate settings.