Having flown types which are notorious for fumes for over 20 years and now suffering from the effects, my view is that it’s the small cumilative effects that are the main problem. Many on here will be familiar “sweaty socks” odour, often shortly after the start of descent. Evidence of such is obtainable, but little effort seems to have been expended over obtaining it.
This is not to say that toxins do not come in other ways, but in my view having experienced it daily for many years the relatively small leaks are the biggest problem.