Our investigation found the worst conditions on the Northern line, where relative humidity - at 58 per cent - sent the apparent temperature soaring to 46C. Around the world, only the streets of Hanoi, Vietnam, had a higher apparent temperature, at 48C.
The temperatures are officially dangerous-The National Oceanic and Atmospheric Administration - America's version of the Met Office - classifies any apparent temperature above 41C as carrying the risk of heat exhaustion.
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