Is Helium crypto mining safe in terms of radiation exposure?

Helium router hotspots use sub-Ghz frequencies. WiFi has higher energy at 2.4Ghz. Helium is around 900MHz which is safer than WiFi.

Helium hotspots broadcast in milliseconds. They are mostly listening. Your city may have already had thousands of LoRaWAN devices pinging around you for years.

Just don't put the hotspot router in your bedroom. Best to keep it in the garage or attic away from kids.