Its mostly the fault of one isotope of potassium and many hiker types eat bananas specifically to boost their blood K levels because otherwise they get really annoying muscle cramps. Its a moderately strong beta emitter and the range in water (close to human body) is about 2 cm or so. There are probably no parts of your innards more than 2 cm from the nearest bodily fluid (blood or whatever).

Humans are surprisingly radioactive BTW. We each have about a hundred grams of potassium happily radiating away. An interesting technological challenge for a startup might be using purely passive radiation monitoring to detect humans. For security sensing or something. Maybe in a 100 years automatic supermarket style doors will sense humans radioactively rather than current microwave doppler and infrared systems. Maybe.