dg wrote:

Cigarette smoke would have to be in a very large concentration to get in
through the duct from outside, plus I can't see how it could get in as to be
detectable if the fan is extracting out.

Is there any other way the smoke could be getting pulled into the room by
the fan - i.e. through any gaps in walls , behind units etc? If the fan is
extracting it is pulling air into the room, and this air could be containing
the smoke.

Some of our high-rise flats had problems with cooking odours getting in
through open joints and cracks between wall and floor concrete panels. These
were hidden behind timber panels and cupboard units and were hard to trace.
We initially thought it was due to close proximity of vents and open
windows, but this was not the case.

If you are in a flat then any ingress of smell is potentially serious as the
flats should be compartmentised in case of fire in an adjacent flat. If
smoke is getting in then so could smoke and flames from a fire.

You should have a service agreement with a management agency for repairs to
the structure, and I would contact them without delay to investigate.

dg




Same conclusion as me, DG, and your advice is better.