These are the coronavirus rules as we know them: Keep a distance of 1.5 to 2 meters (5 to 6 feet) from others, observe good hygiene and wear a mask. But this does not do justice to the complex reality of how aerosols spread, researchers from Oxford and London (UK) and Cambridge MA (US) have written in an analysis published in the British Medical Journal in late August.

British Prime Minister Boris Johnson has tried to show schoolchildren how it should be done. But what does his gesture mean exactly? Do his fingertips have to be 1.5 meters away from the fingertips of another person? That would be a reasonable interpretation of the regulations. But two arm lengths alone measure 1.5 meters, so distances of 4.5 meters or more could easily result.

The Icelandic Association of Sheep Breeders has established its own rules: Two sheep lengths are appropriate to avoid infection. One may wonder if face masks are also supposed to be knitted from real sheep's wool. This young shepherd in Senegal may be trying to find out how long a sheep is by pulling its hind leg. The Icelanders already know — exactly 1 meter.

Of course, this could also work. The standard length of a dog leash corresponds pretty exactly to the current coronavirus rules. Could it be a coincidence that a six-foot leash is usually prescribed for places where leashes are compulsory?

The authors led by Lydia Bourouiba, an expert in fluid dynamics and disease transmission at MIT, writes that the rule is outdated. Two meters was the distance recommended by the German physician C. Flügge in 1897. Visible droplets that he had caught within this distance were still contagious. A 1948 study showed that 90% of streptococci coughed out in droplets flew no further than 1.7 meters.

The 1948 study was published in the American Medical Journal. It also showed that 10% of streptococci flew much further: up to 2.9 meters. If that were the case, perhaps the people on this lawn on the banks of the Rhine in Dusseldorf would be safe — if every other circle remained free. But wait a minute — we are not dealing with streptococci (bacteria) here, but with viruses.

Viruses are much smaller than bacteria, so they can float around for hours and spread better in the air. This is why the researchers recommend that the distance between people should not be the only safety criterion but that other factors should be considered, too: How well a room is ventilated, whether people are wearing masks, and whether they are silent, speaking softly or singing and shouting.

Numerous studies have also shown that coughing can propel veritable parcels of viruses up to 8 meters through the air. Speaking or singing loudly also spread a lot of aerosols and droplets about the room. If, however, people only speak quietly, as in a library, and sit in the fresh air, safe distances can be smaller again.

The duration of a stay in a contaminated room and how many people are in that room are also decisive factors when assessing the risk of infection. The researchers have used those factors to develop a traffic light model. The clear result: In rooms with a high occupancy, you should generally stay only for a short time, make sure they are well aired, wear a mask and speak quietly.

Even very brief contact can be enough to transmit SARS-CoV-2. The US Center for Disease Control (CDC) had to tighten its rules on October 21, after a prison guard caught SARS-CoV-2 from prisoners with whom he had only had contact with for a few minutes at a time. Now, "close contact" is defined as being within 2 metres of an infected person for at least 15 minutes cumulatively within 24 hours.

Here, however, the traffic light of the UK-US research team would show green. Outside, people can be safe for long periods of time even without a mask, provided there are few people around, everything is well ventilated and no one talks much. But even so, will the distance between deck chairs being measured here be enough?