Exposure to radioactive material from fallout may occur in several ways:

1. From direct skin contact with fallout particles in the air (external)
2. From fallout particles that fell on the ground and that later come in contact with the skin (external)
3. From eating plants, milk, or meat that had radioactive fallout on or in it (internal)
4. From breathing in radioactive material in the air (internal)

Some of these isotopes could affect people through external exposure (exposure to radiation outside the body) while some could affect people through internal exposure (exposure of a specific internal organ to radiation inside the body). Some radioactive materials remain for only a short time, while others remain for a long period. Because some of the isotopes in fallout from weapons testing were the long-lasting type, a small amount of radioactive fallout remains in the environment today, and people can continue to be exposed.