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Outside/Inbox: How deep does a fallout shelter need to be?

Fallout Shelter in Carroll Gardens
Jurgen Stemper [CC by 2.0]
Fallout Shelter in Carroll Gardens

This segment first aired on Sept. 2, 2022. It was re-aired on July 20, 2023.

Every other Friday on Morning Edition, the Outside/In team answers a question from a listener about the natural world. This week, Jordan White asked:

“How deep does a fallout shelter need to be in order to remain effective? I mean, a nuclear explosion is huge — and I would imagine that the particles would seep into the earth. So how deep can those radioactive particles go?”

Helter Shelter

If you’ve seen movies like 10 Cloverfield Lane (which has a terrifying trailer), Blast from the Past (terrifying for very different reasons) or played any of the video games from the popular Fallout franchise, you’d be forgiven for thinking that fallout shelters are, by definition, deep underground bunkers made of concrete and metal.

That’s what I thought, anyway.

But then I spoke with David Monteyne, an associate professor of architectural history at the University of Calgary in Canada. He’s also the author of“Fallout Shelter: Designing for Civil Defense in the Cold War.”

He told me that Jordan and I are both making a common mistake.

“To answer this question, you kind of have to distinguish between the bomb shelter and the fallout shelter,” Monteyne said. “The bomb shelter has to protect against more things… blast effects, fire, and things like that. Whereas a fallout shelter is only made to protect from radiation. So to design a fallout shelter is in some ways no different than designing protection against the sun — another form of radiation.”

While bomb shelters are typically placed underground, a fallout shelter can be just about anywhere, if you’re meeting the right conditions.

“In theory," Monteyne said, "a fallout shelter could be in a glass skyscraper.”

Particle Problems

If fallout shelter safety isn’t about depth, what other factors are important?

First, consider what kind of radiation you’re protecting against. And remember, radiation is something you’re encountering every day. When you apply sunblock in July, you’re protecting your skin from solar radiation. When you get your basement checked for radon, you’re protecting your lungs from radiation. And despite those precautions, your body gets hit with some amount of (harmless) radiation every day. Even bananas contain a tiny amount of radioactive potassium!

“In one year, the average person is exposed to 6.2 millisieverts,” said Shaheen Dewji, an assistant professor of nuclear and radiological engineering at the Georgia Institute of Technology. "So a sievert is a unit of dose."

According to the Nuclear Regulatory Commission, the drop in life expectancy from that amount of radiation is about the same as taking 300 puffs from a cigarette over the course of a year - not enough to actually cause any quantifiable harm.

“Half of this dose that you get just by existing, every year, is from the environment," Dewji said. "These are natural.”

But protecting against different quantities and types of radiation requires different kinds of shielding.

Tracks of Alpha Particles from Central Points (C. T. R. Wilson's Method).
Image from page 216 of "Annual report of the Board of Regents of the Smithsonian Institution" (1846) via Public Domain (CCO 1.0)
Tracks of Alpha Particles from Central Points (C. T. R. Wilson's Method).

For example, you don’t need to use sunscreen to protect yourself from alpha particles, a variety of radiation produced by radon gas that sometimes contaminates basements. Alpha particles can’t even penetrate your outer layer of dead skin.

But when you inhale radon gas, those particles are ejected directly into your sensitive lung tissue, whichcan cause serious damage. Radon is the second leading cause of lung cancer after smoking.

By comparison, another radioactive material called Cesium-137 (one of the many products of a nuclear explosion) produces gamma rays that can penetrate your skin, drywall — and even metal.

To reduce the danger posed by this type of radiation, different types of shielding are more or less effective.

“To reduce the intensity of Cesium-137 by fifty percent, you’d need about 0.7 centimeters of lead,” Dewji said. “Equivalently, you can also use about a centimeter and a half of steel. Or just under 5 centimeters of concrete.”

Ultimately, determining the safety of fallout shelter design has little to do with depth in particular, but everything to do with math — and mass.

“It’s an equation of mass and distance,” David Monteyne said. “So you want enough mass between you and the radiation, and you want enough distance, or some combination of the two.”

One of Boston's now defunct fallout shelters
Joerg Moellenkamp [CC BY 2.0]
One of Boston's now defunct fallout shelters

Radiation Nation

Many fallout shelters are placed in basements because the combination of concrete foundations and earth is often the most economical barrier available. But historically, fallout shelters have been located in lots of places that aren’t technically underground.

In the 1960s, The Army Corps of Engineers contracted with architects and architectural students to survey countless buildings. Shelters were designated all across the country, with as many as 19,000 in New York City alone.

Typically, they were in commonplace buildings: schools, hospitals and fire departments.

“I toured one that was in a basement, but it was in a basement of an early 20th century bank building, like a neoclassical bank,” Monteyne remembered.

Shelters were equipped with barrels of water and crackers or high-energy biscuits made of oil and flour. Buildings that held fallout shelters were also posted with now iconic black and yellow signage.

You can still find some of these today through projects like Fallout Five Zero, a photographic chronicle of the Boston area’s now defunct shelters.

In summary: A fallout shelter doesn’t have to be underground at all, and determining safety when it comes to radiation depends on the type of radiation, your distance from its source and the shielding you use. Because dirt is cheap, an underground bunker may be the most economical option in building a safe fallout shelter — if, that is, your air supply isn’t contaminated, too. Only one question remains: How many bananas would you have to eat to die of radiation sickness? Apparently, the answer is 10 million bananas, all at once. Thanks, internet.

To submit your question about the natural world to Outside/In, you can record it as a voice memo on your smartphone and send it to You can also leave a message on our hotline, 1-844-GO-OTTER.

Outside/In is a podcast! Subscribe wherever you get yours.

Outside/In is a show where curiosity and the natural world collide. Click here for podcast episodes and more.
Taylor Quimby is Supervising Senior Producer of the environmental podcast Outside/In, Producer/Reporter/Host of Patient Zero, and Senior Producer of the serialized true crime podcast Bear Brook.

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