If you happen to know an outdoor or wild swimmer – and you’re reading the Observer, so let’s face it, you almost certainly do – they may well have been especially euphoric last week. The recent cold snap has seen water temperatures in the sea, in lakes and rivers and in lidos drop a couple of degrees, into what many wild swimmers think is the sweet spot of the mid-teens Celsius. Bracing enough to get the buzz but not too cold to leave you chilled to the bone afterwards.

“Have you heard of the Pareto principle?” asks Dr Mark Harper, an anaesthetist, researcher and author of Chill: the Cold Water Swim Cure. “It’s where 20% of the effort produces 80% of the results. So we’re in that beautiful time now where, if the temperature is between 15 and 20C, you’re probably getting 80% of the benefits of the cold water for just 20% of the effort.”

Wild swimming was on the up already, but the Covid pandemic has sent numbers doing the activity into overdrive. Lidos, formerly unloved and anachronistic, are resurgent: Cleveland Pools in Bath, the UK’s oldest open-air pool, has just reopened after 40 years of neglect and a £9.3m makeover. The Outdoor Swimming Society (OSS) had 300 members when it was started in 2006; now it has 175,000 across its channels and a million visitors annually to its website. The society recently polled its members on why they swam outdoors: 94% responded that the main reason was “joy” and reported feeling happier and less stressed after a dip.

Such testimonies are anecdotal, of course, and even the OSS acknowledges the society is “a borderline cult built on enthusiasm”. And this remains a recurring question mark for wild swimming and cold-water immersion: despite all the evangelical claims made by fans, there has so far been minimal scientific evidence to confirm them. That’s not to say that the benefits do not exist, only that there have not been sufficient, rigorous clinical trials to prove them either way.

That, though, is starting to change, and in the past month academic papers have been coming thick and fast. Harper was part of a team that looked into whether sea swimming could be “a novel intervention for depression and anxiety”. The study enrolled 53 people – 47 women, five men, one non-binary – in an eight-session swimming course and tracked their wellbeing by questionnaire. Harper says there was a notable upturn in many of the participants’ mental health, and he is particularly heartened by the fact that, three months later, 80% were still swimming outdoors, reporting that they found the activity helpful.

“It’s only a feasibility study, but the figures are phenomenal,” says Harper. “If you said you had this effect from a drug, people wouldn’t believe you. It’s way better than any effect you get from SSRIs [selective serotonin reuptake inhibitors commonly used to treat depression], for example.”

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Harper also worked on a project this year with frontline NHS workers to see if outdoor swimming could improve symptoms of stress and work-related burnout. Participants swam in a pool in London or in the sea in Cornwall, and overall reported a 14.8% increase in wellbeing scores after six weeks.

Before you go and hurl yourself in the nearest lake, however, there were words of caution in the British Journal of Sports Medicine. There, Mike Tipton, professor of human and applied physiology and a global expert on extreme environments, points out there was a 52% swell in HM Coastguard callouts between 2018 and 2021 connected to open-water swimming. There’s also been a 79% increase in deaths – from 34 to 61 in the UK.

Tipton was encouraged to publish the paper after seeing the BBC reality series Freeze the Fear with Wim Hof about the Dutch extreme athlete who has spent more than three hours in direct, full-body contact with ice. “Although there was a safety message at the start, if you watch those programmes you would be inclined to go and put yourself into cold water,” says Tipton. “So we thought there was some need for just saying: ‘Look, we are a tropical animal and this is one of the largest stresses you can place upon the body.’ We’re not trying to stop people doing things; we’re not the Fun Police. But there are ways of maximising the potential benefits and minimising the risks.”

Here, Tipton and Harper are in full agreement. If you are contemplating dipping a toe into outdoor swimming, especially this winter, you should have a medical assessment first. Start in a spot with lifeguards and enter the water gradually: resist the urge to jump, dive, cannonball. Spend less than 10 minutes in the water even if you don’t feel cold. From personal experience of outdoor swimming, this is a key point: I’ve had dips where I’ve timed it right and felt giddy all day, and others where I’ve spent too long in the water and my teeth are still chattering two hours later.

Tipton and Harper are also both clear that more research needs to be done before we assign transformative powers to outdoor swimming. “I can recognise the anecdotal responses – what we don’t know about going open-water swimming, though, is what the active ingredient is,” says Tipton. “So when you go open-water swimming you meet up with friends, you go into a beautiful environment, you’re floating, you’re supported by the water, you do some exercise, you do get cold, you come out and you have cake.

“There are so many other factors,” Tipton goes on, “but we don’t know which one is actually responsible for any claimed beneficial effects.”