Two types of drops
Let’s start with a simple question: how can an infection be transmitted through the air? When we speak, cough, sneeze, or simply breathe with our mouths open, we expel two types of drops that contain saliva and other respiratory fluids.
The larger ones are called Flügge drops, in honor of the German hygienist Karl Flügge, who described them in the late 19th century, and are sometimes easily observable. These drops remain in suspension for a limited time and do not disperse beyond 1.5 or 2 meters from the emitter. Hence, this separation between people has been established as a safety distance.
Being relatively large, heavy and with a good water content, Flügge droplets tend to fall within a few minutes, settling on surfaces for relatively long times. In the case of SARS-CoV-2, the viruses contained in these particles can remain viable, depending on the type of surface, from hours to days. Of course, the amount of virus decreases rapidly over time, which limits the possibility of infection. For this reason we must take extreme precautions for hygiene and general cleaning. And, of course, wash our hands, as we may have touched contaminated objects.
What happens is that, in addition to the Flügge drops, we also emit a second type of much smaller drops. They are practically negligible, below 5 microns, and they disperse like aerosols.
And there the controversy arises. Because while the role of Flügge drops in disease transmission has never been in doubt, the actual role of aerosols in disease transmission has been highly controversial. Already in March a study was published in which the presence of the virus in these aerosols was demonstrated for quite some time. So why the doubts? Well, because for us to develop the disease it is necessary for a sufficiently high amount of virus to penetrate our respiratory tract.
In other words, we are not going to get sick because a single virus particle invades us. Many more are needed, and experts questioned whether this minimum infective dose could be achieved in everyday situations. The doubts began to be cleared in April when the detailed study of three epidemic outbreaks originating in a bus, in a call center and in a restaurant revealed a very clear pattern of spread by aerosols.
By reconstructing the place where each infected person was with respect to the patient or each outbreak (or originating from it), as well as the time in which they had remained in contact, it was found that all the episodes had three things in common. Namely: a confined space, a prolonged and close contact, and the presence of air conditioning, which agitated the air.
Cleared then? Not at all. There were still doubts because for many years it has been thought that this possibility did not suppose a significant viral transmission mechanism in real circumstances. Furthermore, in no other situation had such a clear pattern of transmission been established, or even data clearly against it.
Studies that contradict
In mid-May, another revealing study of a major outbreak (119 infected, 39 of them patients and 80 health workers) occurred in a South African hospital was released. The epidemiologists managed to reconstruct with detective precision the chain of transmission of the virus within each of the hospital units. At that time it was very clear that the main method of transmission had been contaminated objects, called fomites, and not the airway.
Furthermore, epidemiologists were surprised in their report that aerosol transmission seemed to have played no relevant role. Not even in a situation in which an emergency intubation had to be performed on a patient with COVID-19 and in which the health workers did not have the appropriate personal protective equipment. Therefore, the debate as to the real effect of aerosols as a transmission mechanism continued.
The last chapter of this debate is featured in a letter published by two experts and endorsed by 239 other scientists in which it is insisted that transmission by drops or by fomites does not sufficiently explain all infections, proposing a model of dispersal of the virus in spaces closed. These accumulated evidence, together with the permanence of SARS-CoV-2 in aerosols and the demonstrated participation of this pathway in the transmission of similar viruses, such as influenza or respiratory syncytial, have made the World Health Organization modify its initial position in relation to the importance of this transmission mechanism in the current pandemic.
Ventilation and air conditioning
The change of position of the WHO implies that, from now on, we should maximize the use of masks, social distance and hand washing and general hygiene. But it also urges us to pay extra attention to indoor air circulation patterns.
If, indeed, the virus travels in sufficiently infectious quantities beyond a safe distance of 2 meters, the only possible answer is to urgently implement two easy measures. The first is a frequent ventilation of the interior spaces, to ensure the renewal of air (even if this means a loss of air conditioning in public buildings). The second, a rigorous maintenance of the air conditioning filters. As well as, of course, minimizing the capacity of people in these closed spaces.
We should be clear that each of the possible transmission routes is neither more nor less important, but that the circumstances of each moment are those that will tip the balance towards one of them.
Ignacio J. Molina Pineda de las Infantas. Professor of Immunology, Center for Biomedical Research. University of Granada
This article was originally published on The Conversation. Read the original.
