On the BAe146 the problem usually was a slight leak in the APU seal which allowed traces of oil to seep into the air conditioning system. We were always told to start the packs in Full Manual Cold and to run them like that for two minutes. This would usually ensure that any contamination present would be dumped overboard before any heat was applied.
However, I have seen pilots start the packs with a Hot selection and that, from time to time, would create a thin grey mist and a bit of a pong.
We had two pilots who had to be moved on to other aircraft on the fleet after suffering some symptoms.
I have always been fascinated to learn why it is that a small minority suffers so badly and the rest of us have no problems. I survived nearly 20 years on the BAe146 and I am pleased to report that I am still in rude health.