The minutes to hours after a nuclear blast are a critical window. The potential for radiation exposure decreases 55% an hour after an explosion and 80% after 24 hours, according to the Johns Hopkins Center for Health Security.

Immediate actions during those first few hours, like covering your eyes or hunkering down in an indoor shelter, could mitigate your risk of death or serious injury. Here's how to protect yourself in a worst-case scenario.

• Survivors of a nuclear attack would have about 15 minutes before sand-like radioactive particles, known as nuclear fallout, reached the ground.
• People should look for shelter in the opposite direction of fallen buildings.
• The best shelters are buildings like schools or ...