Physical distancing guidelines suggesting people stay one or two metres apart when outside their bubble are outdated and do little on their own to stop the spread of COVID-19, UK scientists say.
There's also no real evidence for considering only those who have spent at least 15 minutes near a confirmed case as a close contact, as our Ministry of Health does, they say.
Nicholas Jones, a clinical researcher at the University of Oxford, says the 1-2m rule dates back to research done in the late 19th century, but newer studies have found droplets can go much further - even from people just breathing and speaking.
"Breathing out, singing, coughing, and sneezing generate warm, moist, high momentum gas clouds of exhaled air containing respiratory droplets," he and others wrote in a new analysis for the British Medical Journal.
"This moves the droplets faster than typical background air ventilation flows, keeps them concentrated, and can extend their range up to 7-8 m within a few seconds."
Dr Jones says this explains how one person managed to infect dozens of others at a choir practise in the US "despite physical distancing". In one China superspreader event, 10 people in three families managed to be infected despite sitting nearly 5m apart - an investigation suggesting they were sitting in the path of the indoor air flow.
"Though it is widely assumed that duration of exposure to a person with COVID-19 influences transmission risk (studies of contact tracing, for example, consider thresholds of 5-15 minutes beyond which risk increases), we are not aware of studies that quantified this variable."
Instead, Dr Jones says the risk of transmission is determined by several factors - distance and time only being two of a complex stew of influences.
"Physical distancing rules would be most effective if they reflected graded levels of risk."
His team drew up a chart dividing different activities, at different levels of occupancy, into three categories of risk - low, medium and high.
For example, if you were outdoors, silent and there weren't too many people around, the risk of transmission would be low even if you weren't wearing a mask. Talking to your co-workers, even in a busy office, would also be low-risk provided you were wearing a mask - but medium risk if it had poor ventilation, and high risk if it involved a lot of shouting.
"This is shown, for example, in meat packing plants, where outbreaks have been attributed to the combination of high levels of worker contagion, poor ventilation, cramped working conditions, background noise (which leads to shouting), and low compliance with mask wearing. Similar compound risk situations might occur in other crowded, noisy, indoor environments, such as pubs or live music venues."
Other factors people need to think about include "individuals' susceptibility to infection, shedding level from an infected person, indoor airflow patterns, and where someone is placed in relation to the infected person.
"Humidity may also be important, but this is yet to be rigorously established."
A prior study found humidity might help stop the virus spreading, as particles collect water molecules in the air, get heavy and fall.
"Urgent research is needed to examine three areas of uncertainty: the cut-off duration of exposures in relation to the indoor condition, occupancy, and level of viral shedding (5-15 minute current ad-hoc rules), which does not seem to be supported by evidence; the detailed study of airflow patterns with respect to the infected source and its competition with average venting; and the patterns and properties of respiratory emissions and droplet infectivity within them during various physical activities.
"Physical distancing should be seen as only one part of a wider public health approach to containing the COVID-19 pandemic."
Health advice from both national and global authorities has been criticised for failing to keep up with the science. It took the World Health Organization until June to formally recommend the use of masks, for example, and as the virus spread worldwide towards the end of February, still recommended against travel bans to affected areas.
