The researchers of the Centre for Energy Research (Environmental Physics Department), the Department of Pulmonology of Semmelweis University, and the Wigner Research Centre for Physics (Department of Applied and Nonlinear Optics) sought answer to the question of - given we're talking about airborne transmission - what size of particles the virus is likely to be attached to. They also looked to find out what external conditions, such as air quality and airflow, could influence the spread of the virus.
The research follows on from an earlier study by the university, which analysed where the virus would be deposited in the lungs if it travelled on the surface of air particles of different sizes. The studies were carried out at hospital sits, in normal wards and in non-invasive ventilation units.
We were able to detect the virus over a wider range of aerosol particle diameters (70 nm–10 µm) than ever before, even at ultrasmall sizes,
said dr. Veronika Müller, Director of the Department of Pulmonology at Semmelweis University.
Our results revealed that SARS-CoV-2 RNA is most likely to exist in particles with 0.5–4 µm aerodynamic diameter, but also in ultrafine particles.
This is important because the smaller the particle, the easier it is for it to enter the body - and thus the easier it is for the virus to enter the body.
Overall, it was found that SARS-CoV-2 RNA could be detected in all size ranges, from which the positivity rate is higher in the larger size ranges (particulate matter or PM of aerodynamic diameter ranging from 2.5 to 10 µm).
The detected size distributions were typically unimodal in the 0.25–10 µm range, and significant quantities of SARS-CoV-2 RNA were detected even in the sub-300 nm size range. Particles of a few hundred nanometres in size can travel from the upper respiratory tract via the alveoli directly into the bloodstream.
This suggests that the enveloped SARS-CoV-2 virus might even be able to spread in the air without being attached to a carrier aerosol particle.
"The particle size distribution associated the virus is important, since the lung deposition probability has a minimum at around 400 nm, alveolar deposition increases with decreasing particle diameter. As fine (PM1) particles can reach the alveolar surface, they might cause direct alveolar infection with highly contagious pathogens," the study says.
In general, the aerosol sources in hospital wards can basically be (i) the mixing of outdoor particles due to room ventilation, (ii) the emission of people by breathing and speaking, etc.(iii) resuspension as a result of their or staff activities, and (iv) the direct and indirect particle emission of machines (respirator apparatus) operating in the room.
As the effect of outdoor sources was eliminated, and the quantity of particles emitted during breathing by the hospital staff wearing masks and by patients even with respiratory support was found to be orders of magnitude smaller than the concentration fluctuations due to human activities, it can be stated that
the resuspension effect of inside movements are determinant for indoor PM concentration increase.
"Although patients are the primary source of airborne SARS-CoV-2 RNA, these virus laden particles are transmitted to the surfaces by direct contact or by sedimentation. Direct contact might be responsible for SARS-CoV-2 RNA identified on the patients’ bed, nightstand and patients’ phone [...]," the study says.
As resuspension of all particles are significant it can be concluded that the medical stuff activities (such as movement and treatment) could result in an excess of virus-laden aerosols in the indoor atmosphere.
It is worth noting that the virus detection method used in this work could not distinguish between viable and non-viable viruses/virions.
Surface disinfection, therefore, plays a key role not only in preventing infection through contact but also in reducing the airborne spread of infectious particles,
the authors stressed.
"So far, the importance of the disinfection of surfaces and hands has been emphasised, which is indisputable, but the possibility of contamination with aerosols is just as important," explains Dr Veronika Müller.
The separation of patients - using separate bathrooms and toilets - was important mainly because of the stool bacteria as a source of infection.
"We now know from research that particles carrying viruses or other pathogens are very easy to inhale, and I admit that I have since changed my view on isolation."
The findings are also helpful for correct mask use.
The research is also highly useful for hospital-acquired infections. Better knowledge of the sources of infection can help reduce the incidence of hospital-acquired diseases such as pneumonia.
Although the subject of the research was coronavirus, the results can also be applied to other airborne pathogens.
"The real value of this research is that we have been able to use SARS-CoV-2 as a prototype to describe the spread of other viruses. We don't have much opportunity to do this otherwise, as patients with different respiratory diseases are usually not isolated, and the pathogens and the method may be different," says Dr Veronika Müller.
The results suggest that this is likely to be a common factor in the spread of influenza and respiratory syncytial virus (RSV). This could help to prevent such outbreaks in the future and to treat them more effectively in hospitals, nursing homes or even at home.
Cover photo: Shutterstock