Respiratory Droplets
When we breathe, talk, cough, and sneeze, thousands of droplets are expelled from our mouth and nose. The size of these droplets varies—some may be millimeters in size and some might be many thousands of times smaller. The larger droplets, which carry more virus particles, settle more quickly due to gravity. The smaller droplets, carrying fewer particles, can remain suspended in the air for hours.Viral Load
Viral load refers to the number of copies of the virus in a sample, like the droplets that leave our mouth and nose. We know the number of virus copies in the respiratory samples of COVID-19 patients can vary from a few thousand to hundreds of billions per milliliter.The viral load varies from one person to another but also depends on what stage of the illness the patient is at. We also know that people without symptoms can shed the virus.
Infectious Dose
The infectious dose is the number of copies of the virus that your body needs to be exposed to in order to develop an infection. When it comes to calculating a safe distance, the closer you are to an infected person, the more likely you are to be exposed to the infectious dose by breathing in virus-laden droplets.The Environment
Whether we are indoors or outdoors, in school, at work, on public transport or in the supermarket, the flow of air, ventilation, temperature, and humidity will influence what happens to respiratory droplets.Complex Scenarios
With these four elements, we can begin to put together what makes a safe distance.Let’s start with this scenario: Three people are in a room that is not ventilated. One of them is infected and two are not. One of the healthy people is standing closer to the infected person—for example, 80 centimeters away—and one is further away, say two meters.
The infected person coughs, producing a cloud of droplets. The larger droplets carrying more virus particles settle more quickly due to gravity. The smaller droplets carrying less virus travel further. So the person standing closest to the infected patient is at higher risk of being exposed to infectious droplets than the one standing further away.
Of course, the above scenario is overly simple. People move around. An open window may blow air in a particular direction. The infected person may cough repeatedly during a period of time. An air conditioner might recirculate air around a room. Room temperature and humidity may result in drying leading to smaller particles carrying higher concentrations of virus. Exposure to many smaller droplets over a longer period of time may be equivalent to exposure to a few larger ones in a short period.
There is an infinite number of scenarios and having one rule that applies to them all is impossible.
This means that different countries’ rules are, ultimately, best guesses made on the basis of some of the factors described above. They cannot apply in all contexts.
It is very unlikely that you would be exposed to infectious droplets outdoors because of rapid air flow and dilution, but enclosed crowded indoor spaces are much more of a risk. We all need to do our bit to stop the spread of coronavirus, so keep your distance, preferably as far as you can.