We don’t know for sure that they are. Technically it could turn out that they protect us from cancer. However, in general introducing foreign elements into the body is a risk. Or they could be completely negligible.
The main problem is that they are becoming so prevalent that we can’t really afford to wait and see how dangerous (or not) they are even though we are not yet seeing clear, widespread damage. If they are bad, we will already be screwed by the time we find out for sure.
That is why we want to limit it as much as possible.
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