In a millisecond, on the 16 July, 1945, the evolution of the human species took a remarkable turn. Until the explosion of the world's first nuclear weapon, the human body coexisted albeit uneasily with sources of radioactivity. We're surrounded by them-- the sky, rocks in the ground, form within our very bodies. In fact, our body cells are designed to naturally repair the ravages of radioactivity. But as the innocent at Hiroshima will attest, a cataclysmic blast of radiation in an instant is beyond the body's repair mechanisms. Half a century on, with X-rays and nuclear medicine, it's clear that radioactivity is increasingly becoming a part of life. But the question remains-- just how much radioactivity can we really cope with? This episode of Quantum explores the ultimate scientific irony: setting safe doses of radioactivity depends almost entirely on the victims of nuclear devastation. Scientists tell us that the survivors of Hiroshima and Chernobyl provide the only reliable source of data as to how much radioactivity constitutes a dangerous dose. In the centenary of the discovery of radioactivity, Quantum asks the question: how do we know we're safe?