From fields to floating pontoons, in horseboxes, barrels and beach huts, saunas are springing up across Britain.The British Sauna Society now lists about 640 saunas – up from 540 at the start of the year – while a recent report predicted that the UK could become the world’s largest sauna market by 2033, outpacing even Finland and Germany.“The continuing growth suggests that the peak has still yet to come – if there is one,” said Gabrielle Reason, the society’s director.But are saunas a tonic for the nation’s health – or a wellness fad with hidden risks?When it comes to measurable health effects, the strongest evidence relates to the cardiovascular benefits of sauna use.These are “substantial”, said Prof Setor Kunutsor, the Evelyn Wyrzykowski research chair in cardiology at the University of Manitoba in Canada.

“Comparing people who engage in four to seven sauna sessions per week with those who engage once a week, the risk reductions in cardiovascular outcomes range from 40 to 60%,” he said.“In our randomised controlled trial, we showed a systolic blood pressure reduction of 8mmHg after eight weeks of engaging in three sauna sessions per week.This is a huge reduction.”Evidence for the flip side of a trip to the sauna – the cold-water plunge – is thinner, he added, though it is used by athletes after exercise and is believed to enhance recovery and reduce muscle soreness.Precisely how taking a sauna triggers its benefits is still being studied, but the general idea is that it places the body under controlled heat stress, producing responses similar to moderate exercise, such as walking.

As body temperature rises, the heart beats faster and blood flow increases,This sets off a range of physiological responses, including improvements in blood vessel, heart and lung function, and reductions in blood pressure, inflammation and blood fats (including cholesterol),Researchers believe there may be many mechanisms at work, including effects on the immune system, hormones and cellular stress responses,Feelings of relaxation and wellbeing after a sauna may be linked to the release of endorphins, although this is less well established,When it comes to using a sauna to “sweat out” toxins, however, Kunutsor has his doubts.

“There is no strong evidence that sauna use meaningfully detoxifies the body,” he says.Sauna sessions can produce substantial sweating, and sweat does contain tiny amounts of metals such as aluminium, cobalt and lead and other substances, but sweating is primarily a cooling mechanism, he added.A new study suggests the benefits of sauna use may run deeper.Analysing data from more than 1,900 UK sauna users, researchers found that regular sauna use was linked to improved physical and mental wellbeing, with the strongest effects reported by those who used saunas weekly and felt the greatest sense of belonging to a sauna community.It suggested that much of the benefit may stem from the sense of connection and shared ritual that develops among bathers.

“A ritual is anything where you step out of your normal way of doing things and follow a pattern or a series of actions that have a meaning beyond their functional use,” said Martha Newson at the University of Greenwich in London, who led the research.“At its most basic, a sauna is people getting stripped off and sitting in a hot room.”According to Newson, the combination of shared ritual, physical intensity and vulnerability created by saunas can help to forge unusually strong social bonds.At Fire and Ice Wellness in north Bristol on a Wednesday lunchtime, the atmosphere is bucolic.The sauna sits in the grounds of a former wildlife park, where an old seal pool has been repurposed as an ice bath.

Inside the sauna, two strangers discuss redundancy and relationship strain in soft whispers.Another woman shows me her Oura Ring, a sleep and fitness tracker, and says she reliably gets more deep sleep after sauna bathing.Jamie Winn, Fire and Ice’s co-founder, describes the experience as an emotional “safety net”.“There’s something about people being in their bathers with nothing to hide, sort of forced to sit opposite each other, they’ll have a conversation and it all feels very natural,” he said, recalling one visitor who opened up about divorce and personal struggles.“He said ‘I’m going through a really tough time, and this place has quite literally saved my life’.

”Reason believes the rapid growth of community saunas reflects a search for connection in an increasingly digital world.“With more of our attention on screens, we’re away from people and increasingly indoors.That has meant less physical contact, fewer face-to-face conversations, and less time out in nature.”But there are physiological risks – particularly when saunas and cold-water immersion are combined, said Prof Mike Tipton, an extreme environments physiologist at Portsmouth University.Sweating in the sauna can cause fluid loss, while cold water shifts blood to the core and triggers urine production, a process known as cold-induced diuresis.

Together, these effects reduce blood volume, so when the body warms and blood vessels reopen, blood pressure can drop, causing dizziness, fainting and sometimes injury.The more cycles people do, the greater the risk.“We’re seeing this problem where people have booked a sauna for an hour, so they’re going to stay for an hour,” said Tipton.“By about the third time they leave the sauna, they’re feeling particularly lightheaded, and they’ll have a faint.”Drinking water can offset dehydration from sweating, though it does little to counter cold-induced diuresis, so Tipton advises starting well hydrated.

Gentle leg movement can help prevent blood pooling, and people should stand up slowly and watch for warning signs such as dizziness, nausea or tunnel vision.People with pre-existing cardiovascular disease should seek medical advice before using saunas or ice baths.For most people, moderation is probably the key.“The overall evidence suggests that reaping the optimal benefits from sauna sessions requires a frequency of 3-7 sessions per week, with each session lasting approximately 15-20 minutes,” Kunutsor said.If Reason is right, the UK’s sauna boom may only be beginning.

As I leave Fire and Ice, the air feels cool against skin still radiating heat.I smell faintly of woodsmoke.In these turbulent, technology-saturated times, the warmth, human connection and that lingering sense of floating on a cloud may be just the tonic we need.