Staff at Edinburgh Royal Infirmary are fainting on the wards because of the heat, according to unions.
The hospital was not built with air conditioning
Unison said temporary measures like fans and portable air conditioning units were not enough.
The health workers' union said the problem had existed for four years at ERI and was not just a result of this summer's high temperatures.
NHS Lothian said the hospital was not built with air conditioning, but it was now being installed.
Unison said it was looking into the possibility of balloting workers at Edinburgh Royal Infirmary on strike action.
Spokesman Tom Waterson said staff had been fainting and feeling nauseous in the hospital.
A spokesman for the university hospitals division of NHS Lothian said it was endeavouring to improve things for workers.