Exactly. Few things can cross the intestine wall, for example we don't have a problem with the absolutely insane amount of bacteria that process food inside of

Exactly. Few things can cross the intestine wall, for example we don't have a problem with the absolutely insane amount of bacteria that process food inside of us and produce, well, shit. If you were to make a small cut in the intestine somewhere, well, then chances of getting septic (i.e. the bacteria overpowering your immune system) are high. So I'm not worried about things larger than a bacteria, unless proven otherwise. The other issue is with quantity. We all have lead or radioactive isotopes embedded in the body. Or, we can breath in small quantities of carbon monoxide, form example from a passing by car. That doesn't immediately cause problems, the quantity matters.