Why do I smell exhaust fumes during taxi before take-off on commercial airplanes?

I've noticed that after the doors are sealed shut on commercial airplanes, there is an incredibly strong smell of exhaust for a few minutes. I wouldn't think that it would be from the outside exhaust because the cabin should be sealed. I haven't been able to find any info online, which is surprising to me. I would think that the fact that you can kill yourself from running your car in a closed garage, would create widespread discomfort among airline pengers who smell exhaust inside a sealed space. I deduced from looking at some technical info that it could come from the compressors that are used to pump the ozone into the cabin to pressurize it, but it was all to technical and I didn't have the time to learn all the inner-workings of an airplane. Can anyone explain? Should frequent fliers be concerned for their health as a result of this issue?