Taxi, minibus and bus drivers have demanded that the government shelve plans for a law next year banning idling engines, citing the risk of heatstroke in summer.
Representatives from the Motor Transport Workers General Union said yesterday temperatures inside the vehicles could top 40 degrees Celsius within minutes of the air conditioning being turned off.
The union organised for Environment Bureau officials to sit inside an unventilated taxi on July 22, with the temperature rising from 25 degrees to 50 in less than half an hour after the driver turned off the air conditioner.
Union second vice-chairman Chung Lin-wah said overheated vehicles could become 'time bombs' threatening the safety of road users and the general public.
'We urge the government to take responsibility for ensuring road safety, as well as a decent, safe working environment for drivers,' Mr Chung said. 'The legislative plan for a ban on idling engines is unrealistic. It does not consider the particulars of Hong Kong's climate, and the potential dangers to drivers in overheated vehicles.
'Imagine what would happen if drivers should suffer from heatstroke when they are driving. It would be a very violent, bloody picture that none of us would want to see.'
He also called on the Kowloon Motor Bus Company to speed up the replacement of its 200 or so non-air-conditioned buses.
Heat was cited as a factor in the death of a man in his 80s on Saturday. On the same day, the driver of a non-air-conditioned KMB bus had to stop driving after being overcome by the heat and called an ambulance.
Even drivers of air-conditioned buses can be affected by the heat. Lam Miu-ling, another KMB driver, felt unwell after getting into a hot bus on Sunday and had to take four days of sick leave.
Chu Pun-din, director of the New World First Bus branch of the union, said more drivers than normal had been calling in sick over the past week because of the heatwave.
He urged bus companies to review staff policies to ensure drivers took adequate breaks in summer.
A KMB spokesman said all its non-air-conditioned buses would be retired by 2012. He also said the company had seen no significant rise in sick leave taken by drivers in the past two weeks.
A New World First Bus spokeswoman said drivers' break times were sufficient.
A Transport Department spokesman said it reviewed companies' bus-retirement plans every year and no bus could be in service for longer than 17 years.
An Environment Bureau spokesman said the government was still analysing opinion from the transport sector and other groups on the proposed ban on idling engines.