Non-Smoker living with heavy smokers

I'm not really sure what I can do about this situation, its a bit difficult.

I recently moved into a new house (about 6 months ago) and two of my three housemates are heavy smokers. I didn't know how much they smoked until after I'd already moved in. I was diagnosed with (very mild) asthma when I was 8, although I grew out of it by the age of 10.

However I still get a really bad cough if I am around cigarette smoke.
Since I've moved in I've noticed that I am getting a lot more in the way of coughs and colds than I did before, and I find the smell when they are smoking disgusting. I can't stop coughing at times, and it actually makes me gag and feel sick after a while.

Many of the people they invite round also smoke, and we had a party at our house a few weeks back where I started coughing so badly that I had to go and stand in the back doorway because my throat was killing and felt like it was closing. If they start smoking around me now, I tend to leave the room, although at parties I don't like doing this because I want to spend time with the other people. I don't want to go and sit somewhere else on my own because they are all smoking.
I have just gotten over another cold and I actually cannot stop coughing at the moment. I'm reluctant to ask them not to smoke around me, because they have been living in the house much longer than I have (about seven years), and it is pretty much their house. I can't ask them to stop smoking because of me.

My other housemate won't back me up either, because she said that she doesn't have a problem with them smoking around her. (She isn't asthmatic. She also thinks that smoking around children and babies actually improves their immune system and isn't harmful, which of course is complete and utter B-S!)

One of my friends is highly asthmatic due to her relatives smoking around her when she was a child, and I lost three relatives to smoking-related diseases. So I'm very wary of cigarettes and I don't like being around them when they smoke because I don't want to be subjected to the risks of passive smoking. However I can't tell them to stop because as I say, despite it being rented, it's generally seen as pretty much their house.
As I live here also I shouldn't have to leave the living room every time they light up, and I don't want to leave. I just can't see any way of dealing with this, apart from just leaving the room, How can I speak to them about it without offending them or sounding like I'm bossing them around? I need to sort this out because I'm actually starting to feel quite ill.

Thanks.
Grey.