Probably thinking of radioactive Carbon-14. Uranium-238 and -235 are the most common isotopes of that element. Uranium-14 would be tricky, since the negative 86 protons (electons?) would make it highly unstable, and also a different element.

Also you are correct that asbestos is dangerous when you breathe it in, but it's banned widely because it's brittle and very easy to get particles of it into the air through accidental damage.