I was a smoker before falling pregnant, and with a 6 y/o DSS if I ever smoked when he was about (less often) I would always ensure I was outside, around no open windows, I washed my hands and ensured I left it a good ten minutes before playing etc - I'd normally only smoke in the evening when he was in bed, as this was important to me - he has allergies and asthma.
I'm 5 months pregnant, and my fianceé's immediate family all smoke (parents, two sisters) - they've come to blows before because his family aren't very respectful of his request for them not to smoke around DSS - when we've been at the house, they smoke out the back door where smoke drifts in - however - it's their home, their rules. We like it or lump it - DSS is six, he's not a newborn. Plus he's exposed to smoke often enough on the school run, we used to smoke albeit in the evening, his Mum's boyfriend smokes, etc etc.
How this is going to work when the new baby is here is another matter - DP's parents are very much of the school that blowing smoke in a babies face = bad, but won't think anything of smoking and immediately picking up a child, or entering a car with a baby, or smoking within the car when a baby is due to be travelling. I hope me asking them to be a bit more mindful around the baby would be taken politely, but knowing MIL, she probably won't like it
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I'd like to think most people would either avoid smoking when visiting (unless it's more than 2+ hours) or take measures to not be passing over very recent inhaled cigarettes (washing hands, timing cigarettes appropriately) Anything other than that, I'm pretty laid back about.