'The four degrees difference could be attributed to the old-fashioned design of the station - the windows that can only be opened inwards let the hot air from the streets come in,' said Chai Wan district councillor Tsang Kin-shing, who was at the station collecting signatures for a petition urging the MTR Corporation to improve air circulation there.

Tsang said the MTR should install exhaust fans and ban shops from installing air conditioning systems that emit heat within the station. He had received one or two complaints on the issue from Chai Wan residents every week this summer, he said.

But MTR passengers sweltering in the heat of the concourse might want to spare a thought for those bus passengers - and, especially, drivers - who make journeys in vehicles with no air conditioning.

Kwok Chi-sing, a 53-year-old KMB bus driver, was taken ill with heatstroke on Saturday.

'Like me, many colleagues are not used to the sweltering environment when we first take the 'hot-dog bus' shift,' Kwok said.

There are 88 non-air-conditioned buses left in KMB's fleet. Chung Kin-wah, vice-chairman of the Motor Transport Workers Union, said the temperature inside such buses could reach up to 40 degrees.

A spokesman for the MTR said electric fans had been installed at Chai Wan, while KMB's spokeswoman said any change to the type of bus used on a route needed approval from the Transport Department.

The Observatory says there were four days where temperatures hit 33 degrees or above in July, with a maximum of 33.9 degrees.