THERE is increasing evidence that SARS-CoV-2, a coronavirus that has caused the outbreak of COVID-19, can be transmitted between individuals as small droplets defined as aerosols drift in the air and gather after a period of time. After months of debate about whether individuals can spread the infection through the air they breathe, researchers are increasingly concerned about the transmission process.
On July 6, a global team of 237 infectious disease specialists, clinicians, engineers, epidemiologists and aerosol scientists made comments in the Clinical Infectious Diseases journal urging the clinical community and public health departments to acknowledge the potential of transmission mechanism. They additionally call for preventive measures to diminish this sort of risk. But researchers are disappointed that key organisations, the World Health Organisation, for instance, has not been with respect to their suggestion in public information.
On July 9, the World Health Organisation released a scientific brief on the spread of the virus. It insists that more research is needed in view of the possible impact of this transmission route but admits that short-distance aerosol transmission in crowded, poorly ventilated spaces cannot be ruled out.
The World Health Organisation has been firmly opposed to a serious threat to the spread of coronavirus through aerosols, which can collect in inadequately ventilated places and spread in air flows. The organisation believes that the infection is for the most part spread through polluted surfaces and droplets that are bigger than aerosols generated by sneezing, coughing, and speaking. They are considered to have a short travel distance and will fall from the air very quickly.
Researchers comment that this sort of guidance, such as WHO’s, hinders efforts which will prevent airborne transmission mechanism, like improving indoor space ventilation and restricting indoor gathering measures. They also add that although people may think that following current recommendations can adequately protect them, inadequate awareness of the dangers of airborne transmission and an absence of clear suggestions for airborne infection control measures will have significant consequences. Therefore, scientists are urging aerial intervention measures to further reduce the risk of infection. This is especially important because lockdown measures have been eased and businesses have reopened.
Nonetheless, aerosol infection is not well known among certain specialists as it contradicts many years of considering respiratory diseases. Since the 1930s, general health analysts and authorities have generally underestimated the significance of aerosols of particle size under 5 microns in diameter in respiratory sicknesses such as influenza.
Conversely, the predominant view is that respiratory infection is shed by bigger particles, or through the touch with droplets which drop on the surface or spread through human hands. When COVID-19 appeared in the end of 2019, people thought that it spread similarly as other respiratory infections. So, airborne transmission is of little significance.
Benedetta Allegranzi, technical leader of the WHO Infection Control Task Force, said that the WHO was tracking existing evidence and alleviating its past resistance to the possibility that the infection may shed through aerosols. In spite of the fact that the World Health Organisation recognises that airborne transmission is reasonable, the current proof is lacking in demonstrating it. Recommendations in the community regarding physical distancing and isolation and wearing masks may help to control the spread of aerosols to a certain extent, she included.
The debate on the transmission route is of great significance in efforts to stop the spread of the virus. Lighter and smaller drops stay in the air for a longer period and travel longer in the airflow. William Wells concluded in a paper in 1934, published in the American Journal of Epidemiology, that diseases such as influenza, common cold, whooping cough and coronavirus are mainly spread through large droplets. In these diseases, only large droplets will quickly fall to the ground within about six feet; therefore, only the closest contacts to the infected person will be infected.
At the time of outbreak of COVID-19, health sector experts suggested washing hands frequently and keeping a specific distance to separate droplets and contact transmission mechanism. Some specialists and clinicians state that these methods are sufficient and contact tracking data support these measures. Kate Grabowski, an epidemiologist of Johns Hopkins University at Baltimore in Maryland, says that higher risk contact is the person who shares the house with others, or others have been in a confined room for a long period and it may be mainly driven by droplet transmission although aerosol transmission may occur in rare cases.
Other researchers point out that cases of large clusters are important for airborne transmission. The US-CDC recommends six feet (1.8 metres) social-distancing but scientists claim that indoor transmission indicates that the virus is spreading in a different way than the health authorities imagined.
The CDC usually excludes the possibility of aerosol transmission above two meters although a recent observation indicated that singing may cause 53 out of 61 Skagit Valley Choir members to become ill, who participated in choir rehearsal in Washington on March 10 and two of them died of COVID-19. The members of the Skagit Valley Choir believed that no one had any symptoms of illness. They used hand sanitiser, brought their own music score, and did not hug or shake hands. But research conducted by a team found that a singer did show symptoms and was later tested positive for infection.
Towards the beginning of May, after South Korea had started to loosen up limitations on social distancing, fresh infection showed up around Seoul. The infection group (cluster) is related to a 29-year-old man who tested positive after visiting five nightclubs and bars one night. These people are defined as ‘super-spreaders’ that can produce more aerosols, as experts say. Notwithstanding, Lidia Morawska, an aerosol researcher and professor in QUT, Australia, said that inadequate ventilation, delayed contact and singing are adequate to clarify the quantity of individuals infected.
Another study found that the cause of the epidemic was aerosols conveyed by airflow from air conditioning unit in a restaurant at Guangzhou, China, affecting 10 diners from three families. The uninfected person is not in close contact with any infected person. No other employees or customers near the air conditioning unit were infected.
Professor Lidia Morawska of Queensland University of Technology said that thousands of people were infected on many cruise ships such as the Diamond Princess Cruise; even with hand hygiene measures having been in place, many infections occurred after passengers had to be isolated in cabins. The ventilation system could have spread airborne viruses between cabins.
Several studies have detected SARS-CoV-2 virus RNA in hospital air more than two meters away from patients and in outdoor air in crowded areas near hospitals and department stores and found that most of the viruses encapsulated into aerosols with a diameter smaller or equal to 2.5 microns (0.0001 inch) can be suspended in the air for two hours or more.
Research by other scientists has undoubtedly showed that the virus is released in droplets when exhaling, speaking and coughing. They are small enough to fly high in the air and could risk exposure within a distance of more than one to two metres from an infected person.
Current guidelines from many international and national agencies focus on hand washing, social distancing and surface sanitising. Washing hands and keeping a long distance are appropriate but we believe that this is not enough to prevent the infected person from releasing virus-carrying respiratory droplets into the air.
This problem is particularly serious in indoor or closed environments, especially in crowded and insufficiently ventilated environments.
For all the steps of COVID-19 droplet transmission, the evidence is considered to be incomplete, but for the large droplet and fomites transmission methods, the evidence is also incomplete. Airborne transmission mechanism and large droplets and fomites routes are now the basis for a guidance. Following the precautionary principles, we must address all possible important ways to slow the spread of COVID-19:
— Adequate and effective ventilation, especially in public buildings, workplace environment, schools, hospitals and elderly care homes. Opening windows and doors is practical and easy to implement to increase ventilation.
— Increase in general ventilation through airborne infection control such as local exhaust, high-efficiency air filtration and sterilising ultraviolet rays.
— Avoidance of overcrowding, especially in public transports and public buildings. Avoidance singing or shouting loudly in public places.
Dr Mohammad Mainul Karim is a professor of applied chemistry and chemical engineering in the University of Dhaka. Raziya Sultana Chowdhury works with International Laboratory for Air Quality and Health, WHO Collaborating Centre, Queensland University of Technology, Australia.
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