Its basic tunnel infrastructure has changed very little since it was constructed 150 years ago – long before air-conditioning was even invented.

The tunnels were designed and built with only enough room for trains. This means, on the deep-level tubes, there is very little space for the addition of air-conditioners on the trains, inside or outside.

Transport for London (TFL) has released a map of the underground which indicates the hottest spots for commuters. If you live in London, or if you have visited, you will not be surprised to find that the Central line has some of the worst spots, with the Bakerloo line also feeling the heat.

Tourist favourites Oxford Circus, Paddington and St Paul's are also in the highest band. According to the research by the TFL, the underground network has recorded figures above 35C in these areas.

With the tube running more trains and carrying more people than ever before, the increased energy and heat from the underground's trains, lighting system, escalators and the passengers ultimately has an effect on the temprature within the tunnels and trains. The tube uses a total of 600 gigawatts of electricity each year, and on top of that, passengers emit 50 gigawatts of heat every day, which is equivalent to over 63 million light bulbs burning continuously.

Millions of pounds and lots of time have been invested over the years to make life cooler underground in the summer for passengers including new air-conditioned trains. In 2012, London Mayor, Boris Johnson announced that the first air-conditioned trains are expected to run in summer 2013 and that by 2015, all trains running on the District, Circle, Metropolitan and Hammersmith and City lines will be air-conditioned.

Phew!

Ways to beat the heat

■ Always carry a bottle of cold water with you.
■ If you feel faint, go above ground for some fresh air. Don't hesitate to ask a member of staff for help if you need it.
■ If trains are stopped in the tunnels for whatever reason on a hot day, don't panic. If a train breaks down getting passengers out is a top priority for the TFL and the underground staff.