The Earth has been naturally radioactive since its formation. This natural radioactivity is found in the soil, rocks, air, and food, generating internal heat and contributing to environmental radioactivity.

15 3

37 4

25 2

19 1

20 1

21 2

6 0

14 1

17 1

13 1

34 1

2 0

16 3

18 2

6 0

2 0