Horizon explores the topical scientific issues investigates, the truth behind our fear of radioactivity and asks whether our nuclear nightmares really are based on reality. From Hiroshima to Chernobyl scientists have been studying the impact of exposure to radiation for over 60 years and have always assumed that any level of radiation is bad. But now some scientists are questioning the power of radiation to cause cancer and finding evidence to suggest that it may have beneficial health effects.