As the escalator descends below ground at King's Cross St Pancras station, the shift from a merely hot station entrance to the furnace-like depths is perceptible. On the tube it's worse: a man leans back, eyes closed, sweltering; people hold electric fans an inch from their faces. London commuters, known for their stoicism, appear to accept this as another tribulation. They'll need to: heatwaves in the capital are becoming routine.

"We're quite lucky this platform is almost empty, because when it's packed it's like a sauna," says Anna, a passenger at Oxford Circus. "When it's peak hours, it's quite difficult." Anna usually adapts well to hot temperatures, but even she finds it hard to bear. Craig, another passenger, says he travels in gym clothes and changes at the office because of the heat.

London's underground isn't adapted for the 30°C+ heatwaves that have hit the city recently. Lines like the Victoria line - the deepest on the network - and the Bakerloo line - with some of the oldest trains in passenger use in the country - are particularly bad. Sharmin, a barista at Pret a Manger by the barriers at King's Cross, says she's seen people faint in and around the station. She finds the heat so oppressive she's asked to go home early during some shifts. "I've felt like I was going to faint," she says.

A quick glance at a thermometer shows the station at about 30°C. On platforms and tubes it crawls to 32°C, and at the Victoria line platform at Finsbury Park it hits 34°C. In the UK, it's illegal to transport cattle above 30°C; transporting people at 34°C, though, might be becoming the norm. It's ten degrees higher underground than outside, according to an iPhone's weather app. Between 8am and 9am, readings show 34°C on Victoria line platforms at Finsbury Park, Victoria, and Oxford Circus.

Asher Minns, executive director of the Tyndall Centre for Climate Change Research, says tube tunnels are "basically radiators," taking on heat from clay and concrete, plus hundreds of kilowatts from trains braking. The warmer it is outside, the worse it gets underground. But infrastructure is difficult to adapt due to age and clay; it'll likely be years before the network is better suited. "It can't go on like this, and it's not going to get any better," Minns says. He suggests limiting passenger numbers or reducing trains during heatwaves.

Nick Dent, TfL's director of customer operations, said TfL is investing in making the network more resilient and comfortable, introducing new air-conditioned trains on the Piccadilly line and DLR. He added that "short-term and stop-start nature of funding over recent years has meant that TfL has had to carefully prioritise its investment." In other words: we'll get air conditioning sometime before the next ice age, but don't hold your breath - you might faint.