The main way of transfer of COVID-19 virus is aerogenic. Viruses move by means of aerosols which arise in respiratory tracts of the infected person. In this article basics of physics of aerosols and transfer of infectious particles in the context of transfer of a virus from the person to the person will be covered.
The following aspects are important for distribution of aerosols from lungs of the person: emergence of aerosols in source respiratory tracts, their allocation to the environment and their behavior in free space and also deposition after its aspiration in respiratory tracts of the recipient.
Release of aerosol from respiratory tracts of a source requires energy. For this purpose there is just enough energy which is formed at normal respiration of the person at rest. In comparison with breath at rest acts of cough and sneezing increase the speed of outgoing air flow approximately by 4 times, however the maximum distance which overcomes an aerosol cloud before a human face, increases slightly: this distance makes 0.6 m at usual breath, 0.6 m through sneezing, 0.8 m at cough.
Aerosol represents the solid or liquid particles weighed in the gas environment. Diameter of these particles varies from from 0.001 to 100 microns. Aerodynamic diameter of a particle which is equal to a ratio of the size and particle density is necessary for sedimentation and an impaktion for calculation of the main mechanisms of sedimentation — —. For classification of aerosols mass and median aerodynamic diameter is used. The ability of aerosol particles to have viruses depends on the mass of particles, at the same time the mass of a particle is equal to its diameter built in the third degree. At an exhalation of a particle of aerosol change the size depending on relative humidity of the environment. At lower humidity of a particle contract, at higher can increase, changing at the same time physical properties. In free space under the influence of force of an attraction there is particle sedimentation. In case the particle size less than 0.5-1 microns, force of internal friction of Stokes interferes with sedimentation. Aerosols with particles of such size remain in air almost not movably and can get into a human body with a breath again.
The aerosols consisting of more coarse particles quicker accumulate, so the risk to face such aerosols is highest near their source (patient). At the same time it is necessary to consider viability of a virus in aerosol. The last experiments on a coronavirus showed that the Sars-CoV-2 virus semi-life period is equal in aerosols 1.1 hours. Whether the virus will be able to reach stable dangerous concentration in air, depends on virus load of the person (low or high), the minute volume of breath, the size and ventilation of the room. In general, transfer of a virus is represented in the aerosol way quite probable. Therefore it makes sense to provide constant airing of the room and to use masks.
Aerosol particles of 3-4 microns in size move together with the gas carrier. However if the air flow changes the direction, because of inertia of masses the particle seeks for movement directly. If it at the same time encounters a surface, there is an impaktion. The sharpest change of the direction of an air flow at an exhalation in an upper part of respiratory tracts happens in a glottis. Thus, fine particles more likely can get into the lower airways and in cells of an alveolar epithelium, but there are 50% the probability that the particles having mass and median aerodynamic diameter less than 10 microns can be deposited in bronchial tubes.
At use of methods of respiratory support: a high-line nasal oxygenotherapy, CPAP therapy and noninvasive and invasive ventilation of the lungs the system of a respiratory organs is exposed to additional external pressure or giving of a stream of respiratory mix is provided. The research on IVL showed that at increase in positive pressure in respiratory tracts on an exhalation (in this case> 5 cm of H2O) at an exhalation the quantity of aerosol particles increases. As the respirabelny fraction of aerosol is formed at the level of alveoluses, the increased alveolar recruitment, most likely, causes the increased formation of aerosols. Higher probability of transfer of a disease is also explained by it at stay near the patient who receives the specified respiratory therapy.
If to talk about standard IVL, then even prior to invasive ventilation the intubation when which carrying out use of appropriate SIZ is absolutely necessary is necessary. At endotracheal aspiration formation of virus aerosols at the patient also is high. In literature there are no data concerning transfer of a virus at patients on invasive IVL beyond the scope of carrying out medical manipulations. From the point of view of authors, when comparing data concerning an intubation and endotracheal aspiration the last represents much higher risk of spread of an infection in the aerosol way. Therefore the closed aspiration contours have to be applied to aspiration. At invasive ventilation of the lungs it is necessary to pay attention that the air exhaled by the patient was exposed to the corresponding filtration.