One meter or two ? The science behind social distancing
One meter or two ? The science behind social distancing. China, Denmark and France recommend social distancing of one meter; Australia, Germany and Italy recommend 1.5 meters, and the US recommends six feet, or 1.8 meters. The UK, meanwhile, is reconsidering its relatively large two-meter distancing rule, but has attracted criticism from top scientists for doing so.
The truth is, we don’t yet know how far is far enough when it comes to coronavirus/chinavirus. A recent study found the virus in air as far as 4 meters away from infected patients in a COVID-19 ward. But another study, touted by the WHO, concluded that the risk of transmission becomes significantly lower with a distance of 1 meter or more from an infected person, reducing further with increased distance.
Why such a range of “safe” distances? That’s because social distancing is a complex problem with many variable influencing factors. Here are four of the most important ones.
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 on earth. The smaller droplets, carrying fewer particles, can remain suspended in the air for hours.
The number and size of droplets vary depending on the activity. A cough produces more droplets overall and a greater proportion of them are larger. Breathing produces fewer droplets overall and they are generally smaller. The speed with which the droplets leave your mouth and nose also influences how far they travel around you – sneeze droplets will travel furthest.
Viral load refers to the number of copies of the virus in a sample (for example, in the droplets that leave our mouth and nose). We know the number of virus copies in the respiratory samples of COVID-19/coronavirus 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 coronavirus.
Knowing the viral load in respiratory droplets allows us to calculate how many virus particles people may be exposed to and whether this might be enough for them to become infected.
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 by infected person.
The infectious dose for influenza strains varies from thousands to millions of copies. We do not yet know this number for SARS-CoV-2.
In time, further research on how the virus behaves in humans and other animals, and comparisons to other viruses will help to hone this number. In any case, we can be certain that the infectious dose will vary between different people.
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.