How long before nuclear fallout is safe?

For survivors of nuclear war, this persistent radiation hazard could pose a serious threat for up to 1 to 5 years after the attack. Predictions of the amount and levels of radioactive fallout are difficult due to several factors. The U.S. Department of Health and Human Services recommends staying home for at least 24 hours in the event of a nuclear explosion.

After 48 hours, the exposure rate to a 10-kiloton explosion (the kind that could damage but not destroy a city) drops to just 1%. After the detonation of a weapon at or above the consequence-free altitude (a burst of air), fission products, non-fissioned nuclear material and weapon debris vaporized by the heat of the fireball condense into a suspension of particles 10 nm to 20 µm in diameter. EMP was first observed in the United States in the 1950s, when electronic equipment failed due to currents and voltages induced during some nuclear tests. The development of EMP is determined by the initial nuclear radiation of the explosion, specifically by gamma radiation.

Most of the radiation hazard of nuclear explosions comes from short-lived radionuclides external to the body; these are generally limited to the location downwind of the weapon's explosion point. The Centers for Disease Control and Prevention (CDC) has developed this fact sheet to describe what happens when a nuclear explosion occurs, the potential health effects, and what you can do to protect yourself in this type of emergency. In 1991, WANO concluded (using a probabilistic safety approach) that not all former communism-controlled nuclear reactors could be trusted and needed to be shut down. In 1986, the Chernobyl disaster was proof that the international safety of nuclear reactors should not be taken lightly.

High atmospheric radioactivity remains measurable after widespread nuclear testing in the 1950s. The ACS regulations against the potential consequences of nuclear reactors focused on the power plant's capacity for the maximum credible accident (MCA). As a rule, it is best to use only products on the body that are designed to be rinsed after a nuclear disaster. The 1963 Limited Nuclear-Test-Ban Treaty ended atmospheric testing for the United States, Great Britain and the Soviet Union, but two major non-signatories, France and China, continued nuclear testing at a rate of approximately 5 megatons per year.

A nuclear weapon detonated in the air, called an air blast, produces less rain than a comparable explosion near the ground. Sensors can fail and the results of a lack of preventive measures would cause local nuclear fallout. However, a nuclear explosion would likely cause great destruction, death and injury, and would have a wide area of impact.