Since reading your post, I have researched this topic and now feel I have new information to be able to pass along to my customers.
Knowledge is an important piece of serving the public in a manner in which it should be treated.
Jimmy (below) is spot on. It has to do with the age of your home. Actually, when listing a home (or working with a buyer) I do not have to ask, as when I preview your home, I know what I am looking for in regards to material used during construction.
What I am looking for is a gray plastic pipe under sinks or in exposed areas in the basement or attic (easy to spot if you know what you are looking for)
If I see an issue, we would have a discussion about how to proceed.
Best of luck