Armed with a thermometer, ITV London reporter Rags Martel stepped underground to find out London's hottest tube.
On board the Bakerloo line the heat rises fast, climbing from 28C to 32C by the end of a 5 minute journey.
Commuters on the tube called it "inhumane" and "like a sauna".
The Victoria line took the top spot for highest monthly temperature, pushing the mercury to an average of more than 31C in August last year. This is only set to increase as temperatures soar in the capital.
Mark Evers from TFL said "when the tunnels were built, in some cases over a century ago, the ventilation shafts weren't put in place to deal with the intensity of how we operate today".
When quizzed about air conditioning TFL said " We are working really hard to get air conditioning across all of our services".
Last year the Victoria line took the top spot for the hottest tube line, with an average year-round temperature of over 27C. The average monthly temperature on the line never dropped below 24C.
The Bakerloo and Northern lines weren't far behind, averaging 26C and 25C respectively.
If commuters want to stay cool during the late-summer heatwave, they're best off sticking to the Waterloo and City or Jubilee lines - which in the summer months last year averaged less than 25C.
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