Two-metre rule based on 1930s modelling

Sir, – A recent letter asserted that because some countries are specifying a smaller social distance than two metres, the UK could safely move to a shorter distance and make it easier to go shopping and use public transport.

This made me wonder where our two-metre rule comes from. It turns out to be from the solution of mathematical equations – or models – devised in the 1930s for predicting the spread of coughs and colds.

All models are just as good as the assumptions built into them, and need to be viewed with suspicion until they can be compared with reality.

For instance, unless you use a famous formula for air resistance, you would expect a raindrop to hit you at 300 miles per hour, whereas its actual velocity is usually only about 20.

In the 1930s, there was no way to actually track the motion of droplets expelled by a sneeze or a cough, or to know how quickly evaporation would take place.

Since the 1930s, however, experimental techniques have changed out of all recognition, and since 2016 Dr Lydia Bourouiba at MiT has been studying the way droplets from influenza sufferers spread by using one of the world’s fastest camera systems.

The photographs show what is going on; droplets do not evaporate as quickly as once thought and a cloud of droplets contains a wide range of sizes and behaves quite differently from isolated droplets.

The results are quite startling; for a cough, some droplets will travel up to five metres, and for a sneeze the distance is up to eight metres.

These are not the predictions from mathematical equations, they are not the result of politicians in different countries telling people what they wish to hear, or the claims of individuals who wish to dilute the present rules.

They are the clearly visible experimental evidence provided by remarkable high-speed photography.

There is an obvious moral from this; believe nothing at all until you can check.

Brian Lawrenson. 51 Bay Road, Wormit.