As I understand it, the surface of the earth is commonly bombarded by muons (and lots of them). Surface muons are used for imaging the Egyptian pyramids, among

As I understand it, the surface of the earth is commonly bombarded by muons (and lots of them). Surface muons are used for imaging the Egyptian pyramids, among other things: https://www.nature.com/articles/d41586-018-05254-2 https://www.nature.com/articles/d41586-018-05254-2 Neutrons are much more dangerous, as they will readily be absorbed by atomic nuclei when moving slowly enough, and can form unstable isotopes that might decay by alpha/beta/gamma radiation. Neutron radiation must be at much lower levels than surface muon bombardment.