I just gave blood for one. Do some due diligence and make sure that the folks you are working with have prior experience, you don't want a lung issue, especially if you don't have fantastic insurance. Do you know what they want to do with the tissue?
I'm told they want to compare the effects of vapor on the airways of e-cig users to the effects of smoke on the airways of cigarette smokers. At this point, I don't know who the people are conducting the study, but other than asking for credentials and looking them up, what else might I do?
More than anything, I would just make sure it is a quality medical center doing the sampling. Ask them how many people are in the study, and if they have done this before. There are always risks to sticking things in your body. If done correctly, the risk should be very small. Just trust your gut instinct. If they seem like they have their shit together, they probably do. If they don't, don't feel bad about walking.