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This wall has strange horizontal and vertical white marks on it (see pics), and has done for some time. The marks will wipe off and I am about to do this for the whole wall. The wall is interior and behind it is the bathroom with the shower unit. I just assumed that this was condensation coming through? However the thin strip of wall where the light switch is also has these white marks.

Few points to note which may help with the answer:
The wall was painted black over 20 years ago and has not been touched up since
The gaps in the wall where there are no white marks had two posters on it
I was a smoker up until 6 years ago; I now vape
The room is particularly warm due to computer and tv giving off heat and the boiler being in a cupboard in the room (changed 3 times over the years since I painted) so I have the window open a lot and rarely have heating on, even in winter
The wall in question is hollow with those egg box fillings
The house is 47 years old
I have been taking antihitamines for about the last 4 years where before never required them

My main concern is that it may be mould. Can anyone give advice please?

Hi davebdiy
Almost certainly going to be mould forming on the walls.
Do you have an extractor in the bathroom and if yes is it functioning.
Regards S

Yes there is an extractor in the bathroom, wasn't working for a long time there but got fixed last May. Hardly use it at all though. Is this mould dangerous?

Hi davebdiy
I doubt mould is dangerous but I suggest you treat the wall with bleach and use the bathroom extractor.
Regards S

Thanks, I have today. Used a mould and mildew spray that contained bleach. Another piece of advice I got was to use water mist; if it disappears with that then it's not mould. It did disappear, so could it be efflorescence? Will also use extractor a lot more. Thing is though, this white stuff is also on another two walls in the room- one that is the outdoor facing wall and the other on the wall that joins onto next door. This wall has woodchip wallpaper that had been painted black (not much on this though just some bits up near ceiling and down side). Also read that this could be due to old cheap paint? It was around 25 years ago I painted it.

I guess you got the answer from the Project Manager.
By the way, sometimes this can happen for the damping of the wall...

Hi Davebdiy,
It would be most unusual for efflorescence to appear on the face of an egg box wall. It would also be most unusual for it to take 25 years before salts appear due to the paint you used.
Mould is most likely due to the proximity of the bathroom and lack of ventilation.
Regards S.

The white residue has been on the wall for some time actually, but hasn't appeared to have spread. Forgot to mention, there was water getting underneath from the shower through to my room a few years ago, not flooded by any means but you could see the floorboards were a bit damp. It was fixed. The wall had tiles at that point, now it has a plastic screen. It is a council house so if it comes back I can give them a call. I'll give it a bit of time see if it comes back, if not , I will repaint the room. At least the bleachy smell is gone now lol. Been using the extractor fan so hopefully that will help as you suggested. Thanks.