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What can physics teach us about coronavirus control?


Much of the discussion to date about the novel coronavirus in the pharmaceutical and healthcare facility context has been led from the biological perspective (such as disinfection) (1) or from the engineering perspective (as with HVAC design) (2). There is an emerging role for physics as well, especially in relation to fluid dynamics (the importance relates to aerosols of respiratory droplet transmission being the primary vehicles for the rapid spread and continued circulation of viruses between humans). To highlight how, three examples are considered.

Temperature and humidity levels as essential elements for coronavirus control

Understanding how climate affects the spread of the coronavirus is necessary, given the relationship between the virus and the ambient conditions provided by temperature and humidity.

Aerosol of respiratory droplet transmission is well-known to be a primary vehicle for the rapid spread and continued circulation of viruses in humans. There is also strong support that environmental conditions will affect rates of virus transmission. With the SARS-CoV-2 virus, medical data has consistently detected the virus in the saliva of infected people (3).

This is particularly so as winter virus infections are generally more common (and those of use based in the northern hemisphere) will experience cooler temperatures. To this end, physicists have studied the impact of relative humidity, environmental temperature, and wind speed in relation to the human respiratory cloud and also with virus viability.

The key aspect from the research relates to a critical factor for the transmission of the infectious particles or virions of nucleic acid material (especially when they are immersed in respiratory clouds of saliva droplets). This is the rate of evaporation. The importance of this is the way by which heat and mass transfer around and within respiratory droplets, affect the chance of infection, especially within a closed environment.

The optimal conditions to reduce the chance of infection from coronavirus are environments with high temperature and low relative humidity. This is because such environs lead to high evaporation rates of saliva-contaminated droplets, and this is critical for reducing the virus viability. Warmer temperatures, for instance, increase the energy of molecules, causing them to move and vibrate more quickly; whereas drier conditions decrease the droplet side and influence the ability of viruses to attach to surfaces (4).

The virologists call this factor the saliva liquid carrier-droplet evaporation rate. However, even under these optimal conditions the virus can still spread. This arises due to another factor: wind speed. This means the wind outdoors or air movement through air changes when indoors. The significance of such research could be towards a better understanding  of the evaporation and how this connects with climate effects. This could become the basis to a model that would enable scientists to better predict coronavirus concentration and hence to assess the viability of the virus or at least the potential for virus survival.

Such a model could be possible through the use of a computational fluid dynamics platform. The researchers used a very specific model, called the three-dimensional multiphase Eulerian–Lagrangian computational fluid dynamics solver (5).  This requires an understanding of the steady-state of heat and mass transfer in relation to flowing spherical particles, as viral particles would be in a stream of ejected saliva – such as when we talk, cough or spit.

What was most complicated with the model was assessing the vapor film of thickness that forms around the droplet containing viral particles. Such a model cannot predict if someone will become infected as that depends on the viral load and the individual, accounting for factors like age, gender, underlying medical conditions, and, possibly, genetics and so on (6).

But what the model can do is assist with building design and social distancing indoors and outdoors and provide advice for optimal environments to help lower transmission. The model shows how social distancing remains important both in the streamwise (wind direction) and spanwise direction – it’s not just a linear concept.

Of concern for those living in the northern hemisphere as winter approaches are the combination of low temperatures and high wind speeds , which are set to increase airborne virus survival and transmission – at least based on the new research.

Optimizing social distancing

Social distancing is one of the key measures to take when seeking to minimize the risk of coronavirus transmission. The generally accepted safe distance is two-meters. But is this enough? Physicists have begun to express doubts.

The most effective way to avoid coronavirus transmission is to stay away from an area altogether, but that is not really practical when it comes to making medicines. Social distancing (sometimes, and more accurately, called ‘physical distancing') is the next most important thing a person can practice avoiding coronavirus. This is followed by regular hand washing – using hot water and soap or an alcohol-based hand sanitizer - and lastly by wearing a face mask.

Social distancing is about observing a set distance apart from another person and avoiding all forms of physical contact such as hugs and handshakes. The general advice is that 2 meters (or 6 feet) guidance works, this is the medical consensus. This is because two meters is generally is outside of the range of droplet projection. In terms of the effectiveness of this distance, consider the fact that at one meter (3 feet), the chance of becoming infected by someone with COVID-19 is 13 percent; whereas, at two meters, this drops to 3 percent. While social distancing works, not everyone in society is predisposed to practice it. With this, there are some demographic variations, with social distancing more likely to be followed by seniors compared with younger people.

Assuming most people do maintain 2 meters, is two meters enough? A new study suggests that a safe distance might need to be 5 meters. This hospital-based study looks for viable virus particles. By sequencing the genome of the virus University of Florida researchers found and showed that it came from that patient and not some other source. The patient was identified as having active respiratory infection with a nasopharyngeal swab positive for the coronavirus - SARS-CoV-2 (7).

What is most of interest is that the virologists have detected viable virus up to 4.8 meters away, from a patient. This is over twice the recommended 2 meters (6 foot) spacing recommended by most governments. The genome sequence of the SARS-CoV-2 strain isolated from the material collected at this distance was identical to that isolated from the nasal swab from the patient with an active infection. This finding also adds weight to the importance of the aerosol transmission route (that is transmission in air not simply projected by a cough or a sneeze, but droplets produced by a person speaking and then free-floating in the air).

A related strand of research looked at 58 global cities and found that implementing social distancing measures late enables the SARS-CoV-2 virus to spread further compared with cities that acted more promptly. The overall epidemiological assessment indicated that every day a city delayed implementing social distancing measures, following the appearance of a first case added, 2.4 days, on average, to the length of an outbreak (8).

This also carries risks should a ‘second wave’ occur. The research infers that waiting a week once early signs of resurgence appear, could add 17 further days of social distancing and other supporting measures, like regular handwashing, in order to slow the spread of the pandemic. With the easing of lockdowns, if measures backfire and doubles the infection rate, a second wave can be expected. In contrast, if the infection rate balances the recovery rate, the new infections stay approximately constant. Or if infection rates fall, this could push out the possibility of a second wave considerably.

Surface survival

Physicists have also been able to apply mathematical equations for the assessment of coronavirus survival on surfaces. This included an examination of the drying time of respiratory droplets from COVID-19-infected subjects. This related to a differing range of surfaces, with samples taken from six cities located in different parts of the world, including North America, Asia and Australia.

The research focus was to look at the droplets expelled from the mouth or nose, as a person speaks, coughs or sneezes. If a person is infected with the coronavirus., then a proportion of these droplets will contain viral particles (virions). The droplets acts to protect the relatively fragile enveloped virus and also affect the aerodynamics, in terms of how far the droplets are likely to travel and with the direction they will take.

The typical droplet size, when viewed at any position, is approximately similar to hair width. As these droplets settle on surfaces, some of the viral material will remain and become attached, leading to the prospect of a person becoming infected by touching the contaminated surface and then touching their nose or mouth.

The model developed drew on interface science (a field that considers the boundary between two spatial regions occupied by different matter). This approach enabled the scientists to run drying time calculations. The model could be adapted according to variations with ambient air temperature, different surface materials, and variations with indoor relative humidity (4).

The model showed, for example, how a higher ambient temperature dried out a droplet faster than under conditions of cooler air. A faster air-drying time reduced the possibility of virus survival. This approach also showed that where humidity is higher, the additional moisture enabled the droplet to remain on a surface for longer. The consequence of this was that virus survival was greater.

The findings were then examined alongside epidemiological data relating to the infection rates in the cities studied. It was found that the model had good predictive power in that with cities with a higher COVID-19 infection rates, the ambient conditions were such that the drying time for respiratory droplets was considerably longer.

The study also revealed the types of surfaces of greater concern, in relation to both virus survival and viral viability. From this, surfaces like smartphone screens, cotton and wood were at a greater risk that surfaces of glass or steel surfaces. The key factor here was the degree to which surfaces are hydrophilic. These types of surfaces enable droplets to evaporate faster and hence they present a lower risk (9).


This article has considered three areas where modern physics can help to advance our understanding of the novel coronavirus, adding an additional scientific perspective to the mix and hence aiding those who manage healthcare facilities with additional guidance. Simply taking action to increase temperature and lowering humidity, for example, can go some way to reducing infectivity. Physics can also help to consider the most suitable distances to adopt and to appreciate that when governments recommend a two-meter distance that this is the minima and not the maximal safe level.

Physics can offer further insights and research teams are looking at many aspects of droplet formation and dispersal, for example, as well as mechanisms to treat different surface materials. These approaches, coupled with more conventional contamination control approaches, illustrate why a joined-up approach for science is essential.


  1. Sandle, T. (2020) The Survival of Coronavirus Sars-Cov-2 On Surfaces and Designing Disinfection Strategies to Eliminate the Virus, Journal of GxP Compliance, 24 (3) at:  
  2. Sandle, T. (2020) Consideration of Covid-19 Prevention Measures For Those Working In GMP Pharmaceuticals And Healthcare Facilities, Journal of Validation Technology, 26 (2) at:  
  3. Wei, J et al (2020) Homologous recombination within the spike glycoprotein of the newly identified coronavirus 2019-nCoV may boost cross-species transmission from snake to human, Journal of Medical Virology, doi: 10.1002/fut.22099
  4. Bhardwaj, R. and Agrawal, A. (2020) Likelihood of survival of coronavirus in a respiratory droplet deposited on a solid surface. Physics of Fluids, 32 (6): 061704 DOI: 10.1063/5.0012009
  5. Badin, G.; Crisciani, F. (2018). Variational Formulation of Fluid and Geophysical Fluid Dynamics - Mechanics, Symmetries and Conservation Laws -. Springer. p. 218. doi:10.1007/978-3-319-59695-2
  6. Dbouka, T. and  Drikakisb, D. (2020) Weather impact on airborne coronavirus survival, Physics of Fluids, 32, 093312 (2020);
  7. Ledniky, J. et al (2020) Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients, medRxiv, at:
  8. Meter, L. (2020) For Each Day’s Delay in Social Distancing, a COVID-19 Outbreak Lasts Days Longer, University of Austin, at:  
  9. Bhardwaj, R. and Agrawal, A. (2020) Tailoring surface wettability to reduce chances of infection of COVID-19 by a respiratory droplet and to improve the effectiveness of personal protection equipment. Physics of Fluids, 2020; 32 (8): 081702 DOI: 10.1063/5.0020249

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