Why are isotopes bad for us?

Answer 1

In general they aren't bad in any way, they are unavoidable and you could not exist at all without them. Every element has isotopes (if it didn't it could not be an element), the usual ones we encounter (and that make up our bodies) are harmless (e.g. carbon-12, carbon-13, carbon-14).

However some isotopes are unstable and thus radioactive, giving off rays that can harm living tissue. Carbon-14 is one, but it occurs at such tiny levels (1 PPT) that the body is usually able to repair any damage that does happen.

Answer 2

Isotopes themselves are not inherently bad for us. Many isotopes occur naturally and are essential for various biological processes. However, some radioactive isotopes can be harmful in high amounts, as they can cause damage to cells and tissues leading to health issues such as cancer or radiation sickness. It's the type and amount of isotopes that determine whether they are harmful or beneficial to us.