One metre or two? That is a question exercising politicians right now. Halving the current social distancing guideline of two metres would make a huge difference to the viability of the hospitality industry, which has taken a big hit during the lockdown.
Two metres is already not safe. The risk depends on many contingencies. Let’s assume your face is two metres from the face of someone already infected with the virus. Whether you catch it will depend on:
- How long you spend with them
- How infectious they are
- How many water droplets and aerosols they are expelling; in particular whether they cough or sneeze in your direction
- Whether you are facing each other and which way the air currents are moving
- Whether one or both of you are wearing masks
The transmission risk will be proportional to the time you are exposed. Evidence suggests there is a wide variation in individual infectiousness. It is higher if your contact is breathing heavily, speaking loudly or singing. It depends on what stage of infection they are at and how heavy their ‘viral load’ is. It will be much worse if you are in a crowded space with poor ventilation, such as a pub or night-club.
A recent meta-study - a study of studies - suggests that using one metre instead of two for your social distancing will double the risk of contagion, but that’s just an average and the confidence margins are wide. The real risk depends on the detailed circumstances of your encounter and these override the simplistic calculations derived from the meta-study.
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