In the event of a nuclear explosion, survivors would have up to 15 minutes before radioactive particles, known as nuclear fallout, reach the ground.
People should look for shelter immediately, but not all shelters are equally effective: The best kinds are usually schools or offices made of brick or concrete.
Once inside, a few life-saving measures could reduce your risk of contamination.
There are also things you should never do, like wash your hair with conditioner.
It's a doomsday scenario that some Americans have feared since the Cold War: A nuclear bomb hits the US, setting off a flash of light, a giant orange fireball, and building-toppling shockwaves.
Those closest to the bomb would face death, while anyone up to 5 miles away could suffer third-degree burns. People up to 53 miles away could experience temporary blindness.
But a longer-term threat would come in the minutes and hours after that explosion.
Nuclear explosions can produce clouds of dust and sandlike radioactive particles that disperse into the atmosphere — what's referred to as nuclear fallout. Exposure to this fallout can result in radiation poisoning, which could damage the body's cells and prove fatal.
The debris takes about 15 minutes to reach ground level after an explosion, so a person's response during that period could be a matter of life and death.
Here are the do's and don'ts to remember in the event of a nuclear attack.
DO: Drop to the ground with your face down and your hands tucked under your body.
The Centers for Disease Control and Prevention (CDC) recommends this position because it will keep your hands, arms, and face away from any flying debris or sweltering heat that could burn your skin. Once the shockwaves have subsided, you can get up and look for shelter.
DON'T: Stare directly at the blast.
Depending on how close you are to a nuclear explosion, it might be impossible to avoid the initial burst of light, which can blind you for about 15 seconds to a minute. But for those farther away, it's best to avert and cover your eyes, according to the CDC.
A 1-megaton bomb (that's about 80 times larger than the "Little Boy" atomic bomb dropped on Hiroshima, Japan) could temporarily blind people up to 13 miles away on a clear day, and up to 53 miles away on a clear night.