Transmission of SARS-CoV-2 by inhalation of aerosols: how can we lower the risk of infection?
Julian Gelman Constantin, Natalia Quici, Nahuel Montesinos
Contributions from atmospheric sciences
In less than a year since the start of its spread, COVID-19 has caused unprecedented changes in the daily life of a substantial part of the population worldwide. Even though vaccination has already started in some countries, there is still an urgent need for other health actions to prevent further infections and deaths, until a sufficient degree of immunity has been achieved in risk groups and society in general. The innumerable amount of studies carried out last year by different scientific disciplines, showed strong evidence that links atmospheric chemistry and physics with the transmission of the SARS-CoV-2 virus. This article presents a perspective on the transmission of the virus from the atmospheric and environmental chemistry, together with a series of recommendations to prevent contagion. The aim is to foster knowledge exchange with experts from other scientific disciplines as medicine, virology, epidemiology, and others.
We hope this article will contribute to understanding which exposure situations are riskier and should be avoided to limit the transmission of SARS-CoV-2, in its different variants (lineages), and other respiratory viruses of frequent circulation, which public health impact could be reduced following some of the recommendations of this article.
How are respiratory viruses transmitted?
Human respiratory activities generate droplets emission. These droplets have a wide size distribution, starting from droplets smaller than a micrometer (1 µm = 0.001 mm) up to several hundred micrometers. The lower respiratory tract (bronchioles) generates liquid particles in diameters close to 1.6 µm, the ones produced by the larynx are about 2.5 µm, while the upper respiratory tract (including the oral cavity) generates much larger particles, with a distribution centered at 145 µm. It is worthy to emphasize that the emission is higher for activities such as talking, yelling, or singing (compared to silent breathing). Largest particles (drops with a diameter > 100 µm) fall to the ground in a few seconds (Table 1). However, the smallest droplets persist longer in the air (from tens of seconds to hours), and they are called aerosols. Also, the size of the emitted droplets is reduced by partial water evaporation (up to 50%), which increases the time and the proportion of originally emitted particles that persist in air and, therefore, the distance that they can travel before reaching the ground.
Table 1. Approximate calculations (Stokes´ Law) on the persistence of aerosols in the air according to their diameter. The limit velocity of fall reached by the aerosols (in absence of air currents) and the time they will travel before falling 1.5 m were estimated. Note that the size of the aerosols emitted can be reduced by evaporation and, therefore, can take longer to settle on surfaces.
At least three relevant routes of transmission of the virus can be distinguished:
- “Drop spray”. When an infected person is coughing or sneezing (and to a lesser extent in other respiratory activities) a drop spray can be easily noticed: drops larger than 100 µm which move like projectiles and fall to the ground or other surfaces few meters after emission. Infection through this mechanism requires that these drops impact the mucous membranes (eyes, mouth, nostrils) of another person, and hence is relevant at short distances (usually < 2 m).
- "Aerosols inhalation", or “airborne”. The small droplets (aerosols, smaller than 100 µm) emitted by coughing, sneezing, or even singing, speaking, or breathing can last longer in the air and, therefore, reach greater distances or accumulate in a room. If a person inhales enough of these small droplets, they could become infected. Risk of aerosols inhalation is maximum in close proximity (< 2 m), but there is also evidence of transmission at larger distances (> 5 m) when ventilation is not appropriate.
- “Surface touch”, or by "fomites". Infected people can contaminate surfaces through the respiratory droplets (and perhaps through other routes). If a person touches these surfaces and then touches their own mucous membranes, they could become infected. There is some degree of evidence on this transmission route, but today its relevance is thought to be secondary.
It is important to note that the three routes of transmission are likely to take place both, in the case of symptomatic and asymptomatic people, since the virus is also present in drops and aerosols emitted by respiratory activities of the two types of manifestations of COVID-19.
Recently, part of the scientific community is highlighting the relevance of the transmission of the SARS-CoV-2 virus by aerosols and its implication in the prevention measures of the COVID-19 disease. The governments of Japan, Germany, the United Kingdom, Spain, and the United States, among others, have recognized the relevance of aerosol transmission and emphasized certain prevention measures, mainly the ventilation of interior spaces and the universal use of masks.
Superspreading Events: Relevance of Aerosols
The so-called superspreading events are characterized by a high number of infections from a few initial cases (high value of R0; https://vis.sciencemag.org/covid-clusters/). In some regions it is estimated that 8% of the people infected could produce up to 60% of secondary infections. Therefore, understanding and preventing superspreading events could be crucial in stopping the epidemic from progressing and moving towards a “new normal”.
Almost all documented cases (from a choir rehearsal in the US or a wedding in Jordan, a call center in South Korea, to a restaurant or a medium-distance bus in China) occur indoors, in spaces with poor ventilation, overcrowding, and, in some cases, with recirculation of air. There is evidence of a large number of infections that cannot be explained by close contact or contact with contaminated surfaces and, in some cases, transmission over long distances or infections by asymptomatic or pre-symptomatic individuals is demonstrated. These facts suggest that the most important contagion route in these events is the inhalation of aerosols.
SARS-CoV-2 virus drops and aerosols
There is strong evidence for the presence of SARS-CoV-2 RNA in air samples. Indoors, the virus has been detected even at distances greater than 2 m (the suggested physical distancing rule) and, on occasions, it could be found outside of patient rooms or even in outdoor areas near hospitals. The presence of SARS-CoV-2 has been demonstrated in submicron aerosols, sizes between 1 and 4 µm, and sizes greater than 4 µm, but there is still little certainty regarding the variation of the viral load in the different sizes of aerosols. There is also evidence regarding the ability of these aerosols to cause infection even at distances of 4.8 m from an infected person.
It has been shown that in dark conditions, aerosols with SARS-CoV-2 maintain their infectious capacity for hours (being reduced by half in approximately 1.1-1.2 hours), while in the presence of solar radiation the virus is inactivated much faster (being reduced by half in less than 8 minutes). The effect of temperature and humidity is known with less certainty, but it seems that viability is reduced with high temperatures and intermediate humidity (greater inactivation at 65% humidity, as compared with that at 40% or 85%). These studies show that the virus can survive indoors long enough so its accumulation in poorly ventilated spaces poses a risk of infection. Likewise, it confirms that outdoor spaces carry fewer risks (although not null), not only due to more efficient ventilation, but also due to the effect of solar radiation.
Implications for prevention measures
The relevance of aerosols in the transmission of the disease generates a clear picture of which activities are riskier (Figure 1) and what prevention measures should be recommended. It is important to note that none of these measures completely reduces the risk, and therefore it is necessary to always implement as many preventive measures as possible, simultaneously.
Figure 1. Relative risks of different activities regarding the transmission of SARS-CoV-2 by inhalation of aerosols. It is assumed that, in all cases, a distance of 2 m is maintained. Reproduced from N. R. Jones, Z. U. Qureshi, R. J. Temple, J. P. J. Larwood, T. Greenhalgh, and L. Bourouiba, “Two meters or one: what is the evidence for physical distancing in covid-19?,” BMJ, vol. 370, p. 3223, Aug. 2020.
Face masks
The use of face masks was recommended since the beginning of the pandemic to avoid the emission of large drops when coughing or sneezing, but they can also be very effective in containing aerosols produced by talking, breathing, singing, etc. To reduce aerosols emissions, it is essential that masks (commercial or homemade fit tightly to the face, covering the mouth and nose (https://youtu.be/mJ81IBTMvcU), and that a good choice of clothing materials is done (suitable for filtering out the smallest particles, see http://jv.colostate.edu/masktesting/). Although the recommendation in the first months of the pandemic indicated that the use of cloth masks was a better alternative to not using a mask at all, today, many experts agree that it is time to move to better quality masks. Prioritizing that high quality commercial masks (N95, KF94, KN95s) are reserved to health care and essential workers from high-risk jobs, the general population is suggested to wear double mask when performing risky activities (a surgical mask covered by a cloth mask, to improve the fit) or a mask fitter (https://making.engr.wisc.edu/mask-fitter/ or https://www.fixthemask.com/) over a surgical mask or over a two- or three-layer cloth mask.
Physical distancing
Distancing over 2 m is a good measure to prevent contagion by close contact, since its combination with the use of face masks prevents us from receiving the drops spray and aerosols that are emitted through the mouth and nose (https://youtu.be/MX8InIv0sXs). However, this distance may be insufficient to avoid SARS-CoV-2 spreading by aerosols indoors, since these can accumulate in the air if they are not eliminated efficiently. When indoors, various measures must be taken to reduce the concentration of aerosols (reducing occupation, use of face masks, improving ventilation, etc.) and reduce the exposure time.
Ventilation
As a general recommendation, indoor spaces must be ventilated as much as possible, in order to incorporate outside air, virtually free of respiratory aerosols. In addition, the air currents should be directed (where and when possible) in such a way as to remove the emissions from one person or group of people outside the room. In this sense, it is not advisable to recirculate air (as does the use of air conditioning equipment in homes and offices), since it allows a homogenization of the internal air’s composition together with the virus load throughout a whole room. The implementation of glass barriers (useful to avoid contact with the larger drops) must be carefully studied, as they can interfere with ventilation. One option recommended by experts in indoor air quality is to measure the CO2 concentration to assess whether ventilation is or not sufficient (https://www.aireamos.org/).
Filtration
When ventilation of occupied closed environments is not an option, air filtration can be a convenient palliative. There are commercial filters ("air purifiers") capable of efficiently retaining the micrometric and sub-micrometric droplets that act as vectors of the virus. It should be noted that in circumstances where HEPA filters (with high efficiency, but also high cost) cannot be used, filters with a lower retention capacity (MERV-13, MERV-11) can also reduce the contagion risk, always in combination with other prevention measures. When used in central ventilation systems, it is necessary to ensure a good fitting of the filter to the support, proper maintenance, and ensure that a high quality filter does not greatly reduce the ventilation. In the case of portable filters, it is necessary to verify that they deliver a sufficient flow for the size of the room.
UV-B or UV-C radiation
Effective inactivation of the SARS-CoV-2 virus is possible by irradiation with UV-B or UV-C lamps. This measure is widely recommended for surfaces, especially those in which manual cleaning can be complicated, such as in the case of air ducts. Efficient inactivation (> 90%) has been achieved in aerosols with irradiation times of less than 1 h. However, UV-C radiation can lead to an increase in ozone concentration in the air, which can be harmful to occupants. For this reason, it is recommended to limit its use to the sterilization of surfaces in unoccupied environments (https://www.mdpi.com/1660-4601/17/22/8553).
Risk estimation using particle distribution models in closed environments
Contagion risk models have been used and they allow analyzing the impact of each of the mentioned prevention measures adapted to different scenarios of concentration, ventilation, number of infected persons (or population prevalence of the disease), residence time, etc. It should be noted that, although the physics of aerosol dispersion models is well known, there are still many parameters related to the COVID-19 disease for which there is some uncertainty that limits the precision of the modeling results: the rate of emission of aerosols with viruses, viral load as a function of aerosol size, dose-response curve, among others. However, they can be very useful in estimating relative risks and guiding the development of health protocols.
Additional Resources:
This report (in Spanish, developed by the authors of this post and other colleagues) contains more details and references on these topics:
Questions and Answers on COVID-19 Transmission by Aerosols (in English, with automatic translation into multiple languages): https://tinyurl.com/FAQ-aerosols
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