The concerns were voiced to OC Transpo, the city agency that runs public transportation in Ottawa, on Wednesday when a driver fell ill and was sent to hospital, said Diane Deans, the city's transit commissioner.
There were two other complaints of nausea, she said, but there were not any complaints from passengers. The driver sent to hospital has since returned to work, she added.
Some drivers said they smelled gas fumes, which stemmed from a gasket seal failure on the exhaust system, according to OC Transpo engineers. It happened only "during extreme operating temperatures found during the regeneration process."
A memo sent to city councillors said the engineers have solved the issue temporarily by putting the emissions control system in override mode.
New seals are going to be installed on the fleet of 25 double-decker buses, which is expected to permanently fix the problem.