Unless you swallow the rear element of certain Yashinon or Takumar lenses. It’s the high levels of beta radiation they give off that’s dangerous. It cant spread more than a few centimeters in air , and any lens cap would stop 99.9% of it too. But if you held up the rear element directly to your eye for like an hour you’d probably have some problems next year haha. And yeah, don’t break the lens open and swallow the glass either.