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If you want to be prepared for a nuclear attack, here’s a science-based guide to help you get there.

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Let’s start by saying the odds of a nuclear bomb hitting anywhere near you is very unlikely. But if you’re the type of person who always wants to be prepared, it might help to know a little bit about how explosions and radioactivity work.

So, even though you probably won’t need to be hunkering down anytime in the near future, here’s a science-based guide to how to ride out a nuclear attack. A nuclear detonation releases a massive amount of energy. And depending on the size of the bomb and how high it is in the atmosphere when it goes off, that energy can affect you even if you’re tens of kilometers from the detonation site.

The size of nuclear bombs is measured by kilotons, or how many thousands of tons of TNT you’d have to blow up to release the same amount of energy. A 10 kiloton bomb is considered small, way smaller than most of what you’d find in a nuclear arsenal today. Instead, missiles tend to carry medium-sized bombs capable of 100 to 350 kilotons of blast energy.

But tests have also been conducted with bombs that are way bigger, ones measured in millions of tons of TNT instead of thousands. These would probably be really hard for anyone to actually use, though, because they’re too heavy for long-range missiles to carry. So small to medium blasts are what most government agencies have in mind when they come up with emergency plans.

In the seconds after the explosion, the most immediate dangers are the heat and pressure waves that ripple outward, which account for about 85% of the total energy of the blast. Since light travels fastest, the first thing you’d see is a flash. Hopefully you’d have enough early warning about an incoming missile to get to a shelter before that, because with the light comes a wave of heat, or thermal radiation, which can cause flash burns if it’s absorbed by your clothing and skin.

Think instant, severe sunburn. Thermal radiation can be so intense that flash burns would happen up to 3 kilometers from a small, 10 kiloton explosion near the ground in addition to starting fires all around you. For medium-sized bombs, a lot depends on how high in the atmosphere it is when it detonates and just how big the bomb is.

But a 100 kiloton bomb could cause burns up to 10 kilometers or so away. If you did see a flash and avoided the worst of the thermal radiation, you might have a few seconds to protect yourself. Following on the light’s heels would be the shock wave created when the rapid expansion of hot gas from the blast rams into the surrounding air molecules.

This pressure wave would be traveling at the speed of sound and could cause injuries like ruptured eardrums, but only if you were very close to the blast. And in that case, the heat would be a bigger concern anyways. It’s more of a problem a bit further out, because it can rock buildings and other structures.

At 5 kilometers from the blast site of a 10 kiloton bomb, for instance, the shock wave could blow out windows with enough force to send glass flying. A 100 kiloton bomb could do that for 10 kilometers, give or take. So, the typical advice is if you see a flash of bright light, stop whatever you’re doing and duck for cover, or at least lie face down and cover as much skin as you can.

Of course, any bomb produces heat and shock waves. What sets nuclear explosions apart is that they also release ionizing radiation, energy strong enough to knock electrons out of atoms, which means it can also damage DNA and other molecules in cells. This can lead to illnesses like cancer in the long-term, but the short-term danger is acute radiation syndrome.

That’s what it’s called when radiation kills the stem cells that would normally be responsible for replenishing vital cells in your gut, blood, and immune system. The effects of a bomb’s ionizing radiation can extend much further because of fallout: the radioactive material lifted by the mushroom cloud, which then rains down after the explosion. One of the tricky things with fallout is that it’s hard to tell where it’ll go.

Winds can carry it for hundreds of kilometers, and the local landscape can also influence where it ends up. So if you were anywhere near the source of the blast, even tens of kilometers away, that fallout means you would really want to get to a good shelter, and fast. The severity would depend on how much radiation you were exposed to and how long you were exposed to it, which is why getting inside is so key.

Several different kinds of radiation are emitted from fallout, some of which are more dangerous than others. Beta particles, for example, can burn skin, but they can be stopped by clothing. Gamma rays, on the other hand, can penetrate walls.

So the best way to limit your exposure would be to find a basement in a large building made of dense materials, like brick or concrete. A few meters of concrete or earth could completely shield you from gamma rays, which is why the basement would be your best option. But if you couldn’t get to a basement, your next best bet would be to move to the center of a building to put as much distance and protection as possible between you and the fallout.

You’d also want to remove any contaminated clothing as soon as you could and seal them into a bag. They could have fallout on them, even if you can’t see it. It would be even better if you could take a shower with soap, but you’d want to be gentle about it.

Scrubbing so hard that your skin became irritated or broken could let radioactive particles inside your body, which kind of defeats the purpose of showering. And you’d definitely want to wash your hair, but pass on the conditioner. Conditioners smooth hair by binding together the loose dead skin cells hair is made of.

But in the process, they could trap any fallout particles that might have landed on your head, which also defeats the purpose of showering. Finally, you would need to be prepared to wait, and do what’s called sheltering in place. That’s because radiation levels are highest right after the blast, more than half of all radiation from fallout comes in the first hour, and after one day, 80% of the risk is gone.

If you’d only managed to make it to a poor-quality shelter, it might be worth it to move to a good quality shelter if one was close enough, like, less than five minutes away. But you’d run the risk of more radiation exposure if you ventured outside, so it would be a gamble. Once you were in a good shelter, you’d really want to stay there for at least a day, more likely two, before heading for somewhere that’s not contaminated.

Which means it would be helpful to have a few days’ supply of food and water, some soap and clothes, and a radio stashed wherever you’d most likely shelter. Of course, if you’re the kind of person preparing for a nuclear attack, you probably already have an emergency disaster kit packed. And again, the odds that you’ll need to use this information are really low.

But now you know it, just in case. Thanks for watching this episode of SciShow. If you want to know more about what would happen if a nuclear missile was launched, you might like our episode on how we might stop one before it hits. [♪ OUTRO].