Thanks everyone for your replies so far. It's making me feel like I am not completely paranoid!

I think I will try to suggest that any smokers have either a 'smoking jacket' they can remove before touching the baby or a clean top to put on before touching the baby. Washing hands will also be a must, as well as trying to get the smoke out of their breath.

I think perhaps going down the allergy-ridden/medical issues path might be a good spin on things too, as although my parents don't know this (I think anyway) there are lots of allergies/medical issues on DH's side (e.g. lactose intolerant, wheat allergies, nut allergies, coeliac disease, epilepsy (don't think my parents realise DH has this in fact!)).

I will also let them know that we will be mentioning the same things to other smokers (although admittedly the only ones on DH's side are a couple of his Aunties).

I hope it goes well