Really this is all overreaction after the fact. I doubt that any of these people complaining had a clue of what the zoning was for the area when they moved in or before the bus garage was announced.
When you move into an area near industrial zones you're accepting that you may be putting yourselves at higher risk of health problems owing to that proximity. The garage is not the only industrial use nearby, and it certainly wouldn't be right to re-zone some of the dwindling employment lands in the city for yet more residential.
Some NIMBY causes I'm sympathetic to, when the developer really is putting their foot in their mouth with a dumb, unnecessary, or detrimental development. Not this one though - the garage will do wonders for bus service in a region of the city where ridership is high anyways.