As Mr. Carignan explained, a bus running on natural gas has a vent that allows gas to escape. There is no exhaust on an electric bus. Consequently operating an electric bus in downtown Montreal generates no pollution.
And there is not just air pollution; there is also the noise. An electric bus makes no noise, nor does a trolleybus or a tramway. A bus running on natural gas uses a combustion engine, similar to a diesel engine: that makes noise. The noise levels of the STM's buses are a problem. The STM cannot offer certain routes at night because they are too noisy. We will be able to offer them with electric buses.
So there are a lot of benefits for us in switching to electric buses.