Any of the identified interferent substances could, in principle, be used to mask a narcotic, or dirt. If you put a lot of dirt into a machine, it doesn't operate very well.
As I mentioned, these are incredibly sensitive machines. They're looking for traces. If you have a bunch of junk in there, then it's going to render them less effective. We've been working on this for almost a couple of decades now, and I think we have a pretty good handle on how the machines operate and how they need to be looked after.