Roommates come with habits, both good and bad. As a roommate you will have to accept both. While the good ones are welcoming, the bad ones will have to be tolerated. If you are a non-smoker living with a smoker, chances are smoking falls under the bad habit category for you. While it may not be absolutely impossible to live with a smoking roommate, there are certain ground rules that need to be followed for peace to prevail in your domestic life.
If you are not comfortable around smoking be up front about it. Mention it in the beginning rather than a month later when you have passively smoked enough to reduce your life expectancy by a year. Most smokers I have come across are considerate when you tell them you are not open to puffs of smoke on your face.
You not being open to smoking is not going to convert your smoker roommate in to a non-smoker. He or she will continue to do it which is why you should be particular about what you expect them to do when they smoke in the apartment. Opening windows to allow fresh air is one way around it. You can also encourage room freshner usage.
Smokers come with the menace of ashes. Unless you want random cups and saucers to get converted to ash trays, it is best to get one and be adamant about seeing ashes only on it and nowhere else. This is one area you need to be strict with.
Stubbing is another not so agreeable side of smoking. Unless you have a roommate who does not worry about the security deposit he or she might just go stubbing on the floor leaving ugly marks. Be upfront about your scare and mention that stubbing is allowed only in the ash tray. Flushing stubs in the toilet is also not acceptable in any form, even if they have done it for every year of their life as a smoker.