Plutonium is not particularly dangerous externally as it emits alpha particles which are stopped by of air, or the dead layer of skin cells on the body, and is not a very intensive source of radiation, due to its long half-life of 24,000 years. It is dangerous when it enters the body, in the worst case by breathing and lodging in the lungs, and therefore tiny particles, often the result of such explosion testing, are the worst threat. The extreme biological persistence of plutonium's radioactive contamination and the cancer threat posed by alpha radiation occurring internally together establish plutonium's dangers.