The overexposure to these metals you would need to receive to get really sick, is considerably higher than your ever going to get from smoking out of a brass or copper pipe. Granted if its some kind of brass or copper water pipe and its not cleaned regularly, the sitting water can cause copper to oxidize. This oxidation provides a film called "patina" over the surface. "Patina" is good for the copper because its protects itself from further corrosion. But bad for people as it will make you very sick, specially if you are inhaling it. Other than that You should have no problems with your pipes, they will not cause cancer lol.