As I squeeze myself on to the top bench of the sauna in my local lido, I’m grateful to have a spot. It’s rammed; standing room only. Next to the stove, cold water swimmers huddle and struggle to peel neoprene off shivering hands and feet, their teeth chattering. In fact, everyone is chattering. It’s as noisy as the pub before closing time and as ebullient; a steamy collective fuelled by extremes of temperature rather than extremely large amounts of booze. But, unlike the pub, everyone is semi-naked, sweating and stripped back – job title, wealth, celebrity and status left firmly by the pool.

“Since we opened the sauna at Parliament Hill Lido six years ago, we have seen visitor numbers shoot up,” says Paul Jeal, swimming facilities manager at Hampstead Heath for the Corporation of London. Once deserted and running at a loss in winter, the lido now sees queues at weekends and Jeal has to manage sell-out sauna sessions of 30 tickets an hour. “Since Covid, visitor numbers have increased fourfold,” he adds. “Many are new to cold water swimming and they say the sauna has encouraged them to come.” In spring, the sauna benches collapsed from heavy use.

Sell-out sessions are also the norm at beach saunas. According to the British Sauna Society (BSS), there are around 60 pop-up saunas coast-to-coast and these have been doubling year-on-year since 2018. Fashioned from horseboxes, barrels and ex-army trucks, they are manned by a new wave of entrepreneurs who’ve had a sauna epiphany. Everyone who is into them has an epiphany, sometimes in Scandinavia or the Baltics where sauna is a way of life, sometimes at a festival, sometimes next to a body of cold water. For what could be better after a freezing dip in a lake, river, pond or sea than the warm embrace of steam?

There may even be health benefits – there is some evidence that regular sauna sessions can, like cold water swimming, help lower blood pressure, reduce the risk of cardiovascular disease and dementia and boost immunity. Tom Cullen, of the University of Coventry, says, “We know heat can have a positive impact on inflammation, but it has by far the most positive effects on the cardiovascular system.”

Several studies in Scandinavia have shown that a 30 to 50-minute sauna improves cardiorespiratory fitness in the same way that 30-50 minutes of moderate exercise (a brisk walk, say,) does. Five years ago, Finnish researchers published a review of health benefits of sauna bathing in Mayo Clinic Proceedings, the peer-reviewed medical journal. Several studies linked regular sauna use to decreased blood pressure and risk of cardiovascular disease; authors concluded this could be down to improved cholesterol levels and less inflammation in the sauna users, and relaxation could play a part, too.

However, it is worth bearing in mind that a sauna is not the same as a proper workout. For a start, it does nothing to promote fat loss or muscle mass. “But,” says Cullen, “a sauna might even be better than exercise for high blood pressure. And we do know for sure that heating the body in baths, hot tubs and saunas creates a positive feedback loop in the brain which makes us feel good.”

So central is sauna culture to Finland that in 2019 it was added to Unesco’s list of intangible cultural heritage. Each year, this charter identifies esoteric traditions, such as beekeeping in Slovenia and rum master knowledge in Cuba, and declares them in need of protection. (It’s no coincidence that sauna-loving nations such as Finland, Sweden and Norway jostle for top position in the annual UN World Happiness Report.) So when Helsinki-based photographer Maija Astikainen and I were commissioned to write a book about sauna and its health benefits, we knew exactly where to start. We plotted road trips to Finland, Estonia, Norway and Sweden, pinning our route to windswept archipelagos where saunas are serviced by an honesty box and a pile of wood, to dense Nordic forests with treehouse saunas, to raft saunas floating on empty lakes.

We clocked up 10,000km in Maija’s trusted camper van and learned that for centuries, important rites of passage were held in the sauna; it was the bathhouse, the pub, the town hall, the pharmacy and a sacred space. Babies were born in its herb-infused steam and the dead were prepared for burial. In Finland, the sauna was known as the poor man’s pharmacy and in the clean, warm, private space, folk healers would practice herbalism, bone-setting, cupping, massage, energy healing. They would travel from village to village to treat patients, their pockets full of snakeskins, bear’s teeth, special stones, their heads full of magic. “Seers”, “verbalists” and “ecstatics” would cast spells to ward off evil spirits, grant wishes, bring love and good luck.

In the UK, it’s a different story, one that is nowhere near as evolved or rooted in our past. But unchartered territory spells opportunity and a new generation is appropriating and tweaking the best bits from other sauna cultures to create a unique British scene. And rather like our cuisine, it’s a melting pot of new ideas.

From Estonian-style whisking with birch branches to Aufguss – the art of wafting steam and essential oils around a sauna with a towel – we’re starting to embrace it all in new and novel ways.

For the first time this year, team GB competed in the Aufguss World Championships. There are sauna reading nights, sauna marathons and even sauna whisky tastings thanks to the Scottish Malt Whisky Society. Sauna is a digital detox, a place to gather with friends, family, loved ones and strangers, to turn off and tune into the moment. It can also be a place to open up and feel heard. This month, Smoke Sauna Sisterhood, a documentary following a group of Estonian women sharing stories and secrets in a smoke sauna, arrives in the UK. Depicting the sauna as a healing space, the film won an award at this year’s Sundance Film Festival. To many, the smoke sauna, a windowless wooden hut with an earth floor, no chimney, and a wood burning stove is still the ultimate experience. (We tried it and we think so too.) The ancients believed it was the portal to another world, a gateway to ancestors and spirits. The warm, moist, womb-like space was a liminal zone that transported them from one state to another.

“Sauna can take you into deep places if you want it to,” says Katie Bracher, vice-chair of the British Sauna Society and founder of Wild Spa Wowo. She is also the UK’s leading “sauna master”, a role new to our shores which involves “holding the space, caring for bathers and making sure they feel safe”. Grasping the basics means knowing how long you should stay (as long as you feel comfortable), what will happen to your body (your heart rate increases, you sweat and can feel faint) and how to throw water on the hot rocks (for aficionados, a sauna is not proper unless you throw water on the rocks). “Creating the microclimate, building the heat and dealing with emotions that often boil over when it peaks is complex,” she says. It also takes years of practice, as any sauna shaman will tell you. There are about 70 sauna masters in the UK; many from the fast-growing mobile sauna scene and want to learn how to create “experiences” such as whisking, smoke rituals and Aufguss.

Unless you have visited a central European spa, you are unlikely to have come across Aufguss. Its Eurovision-meets-performance-art style in which a sauna master performs a 15-minute ritual using towels to manipulate the heat in front of naked bathers is Not Very British. But an Aufguss ritual is addictive and entertaining, and the deep sweating it induces can be transformational. “I see Aufguss as a treatment to get people to understand what saunas are really like,” says Pavel Poliacek, a sauna master from Northern Ireland and the UK’s first Aufguss champion. “Aufguss rituals are entertaining and encourage people to stay in a bit longer.” Dr Nicola Stoke, medical officer at the BSS, is advocating for saunas to be introduced to the NHS’s social prescribing scheme, which offers patients non-clinical services such as gardening, befriending, cookery, and sport to aid health and wellbeing.

For now, saunas are nothing if not a fun – and healthy – way to spend time with friends. “We are seeing more and more people form social groups around sauna,” says Mika Meskanen, co-founder and chair of the BSS. “We hope it will take on an identity, like being a surfer or a wild swimmer.” ■

Sauna: The Power of Deep Heat by Emma O’Kelly is published by Welbeck at £22.99. Buy a copy for £20.23 at guardianbookshop.com