I smoked for 22 years and would never have considered smoking near a baby. Remember that it is an addiction that smokers suffer from, most realise this and I feel that they will accomodate you, obviously we don't live in an ideal world so you will have people who will ignore this, my advice is to ignore them.
If you can smell smoke, or anything else for that matter, it is because the smoke particles are entering your airways, so it follows that they don't need to be physically smoking for any harm to be done. As the say, smoke lingers.
We are lucky that here in Scotland there is now a complete ban on smoking in all enclosed pulic places. The health of people in Scotland is considered to be not good, so this measure was brought in to try and brting about a change in our health. It has helped me I didn't give up straight away, it took me some months, infact it was nearer a year for me to give up, you guys on here helped alot.
I can only reiterate what has been said, be firm and polite and if that dosen't work, well be rude.