This is Kotiharjun, Helsinki’s last traditional wood-burning public sauna. I’m here because, around the world, saunas are having a moment. In London, Paris and New York, a new breed of hipster sauna is firing up on rooftops, in car parks and disused industrial spaces, providing a healthy alternative to alcohol-based socialising. Peckham’s Sauna Social Club, for example, offers bathers weekend DJ nights beneath a railway arch, while a two-week sauna festival has just wrapped up on the Williamsburg waterfront. The chic Sant Roch, France’s biggest sauna, opened earlier this week in Paris, while Wales’s first floating sauna launched in Penarth.