Radon is a noble gas. It's not going to affect fiber or wiring... but yes, if you're going to spend much time down there, definitely a good idea to check for it.

Radon is radioactive. The alpha particles it emits can break down polymers and degrade plastics.

Interseting - I have high radon (mitigeted in the house via a fan), if I burry fibre what will I need to do to get something that will work? I have an outbuilding that I want to give internet.

No, radiation from radon at naturally occurring levels won't cause any damage to plastics.

If it were radioactive enough to do that, you'd have a much bigger problem than some networking flakiness.

I don’t disagree but your comment made it sound impossible which it isn’t.

Well technically a nuclear bomb would also degrade the jacket of fiber cabling pretty badly, but we don't really concern ourselves with that since it means you're dead and the house is gone anyways.

It absolutely is, for all practical purposes. Alpha radiation won't go more than 100 um or so into plastic.