Hundreds of commuters have fainted on the London Underground during morning rush hours, the BBC has found.
Since 2016, 825 people have fainted or felt faint while travelling on the Tube between 08:00 and 08:59, Transport for London (TfL) data shows.
The RMT union described the numbers of people fainting as "alarming".
Passengers raised concerns about the heat and overcrowding, but TfL insisted it "constantly" tried to keep temperatures down.
Figures obtained by the BBC showed the most common day of the week for people fainting was Monday, closely followed by Thursday.
King's Cross St Pancras, the busiest station on the Tube network, recorded the highest number of fainting incidents between January 2016 and May 2018, followed by Green Park and Liverpool Street.
During the summers of 2017 and 2018, temperatures on the Tube soared to more than 35°C (95F) - hotter than the European Union limit of 30°C (86F) for transporting cattle.
The RMT said transporting humans in such excessive heat was dangerous and claimed a lack of air conditioning on Tube trains was a "huge contributor" to the amount of people fainting.
General secretary Mick Cash added: "Overcrowding on Tube and rail services is a daily nightmare for thousands of passengers and its no surprise that more people are passing out."
