How can I politely ask someone not to smoke around my baby?

Health risks. No one should smoke around a baby, period. If they do it is inconsiderate. I would point out the risks to an infant such as increased risk for sids, respiratory issues etc. And the fact that you do not want second hand smoke around a baby. If it is your home, your rules. If it is not your home, do not go there. First person you have to be concerned about is the baby.