Terrorists have detonated a low-yield nuclear warhead in your city. How long should you hide, and where, to avoid the worst effects of radioactive fallout? We talked to Lawrence Livermore National Laboratory atmospheric scientist Michael Dillon to find out.

Yesterday Dillon published a paper on this topic in the Proceedings of the Royal Society A. He's spent his career researching how the government should respond to disasters with an airborne component, whether that's a chemical accident, an epidemic, or nuclear fallout. After poring over dozens of studies on how fallout behaves, and analyzing as many factors as possible related to urban detonations, he's come up with a disaster plan that he hopes can be implemented by first responders working with governments from the local to the federal level.


The best part of Dillon's fallout plan is that it's aimed at people like you and I, who won't have access to information about wind direction and blast magnitude. It's a plan that works even if all you know is that a nuclear bomb has gone off in your city.

This Is Not A Cold War Bomb

When I spoke to Dillon about his work, he was quick to point out that his disaster plan is still theoretical. Nobody has yet had a chance to study a low-yield nuclear blast in a real-world city — "thankfully, these are rare events," Dillon said. But as the threat of a terrorist nuclear attack grows more likely than a Cold War scenario, it's crucial for cities to have plans in place. And that means a major paradigm shift in how we think about nuclear attack.


The classic nuclear attack scenario that most of us imagine comes straight out of the Cold War — or movies like Terminator. Multiple megaton-class bombs go off all over the world. The results are catastrophic, with whole regions burned to a crisp, mass deaths, and a fallout plume that stretches hundreds of miles. But the scenario we're more likely to encounter today involves bombs that are anywhere from .1 kilotons to 10 kilotons. They're small compared to the bombs that leveled Hiroshima and Nagasaki, and infinitesimal compared to the warheads we had in our Cold War arsenals.

"These events are more like a Katrina-level disaster," Dillon said. "Your city has the potential to survive, and that's what we're planning for."

The chart below gives you a sense of the damage radius of the bombs that Dillon studies, as opposed to Cold War weapons. The most dangerous areas are in the dark blue (psi stands for pounds-force per square inch, and is used to measure blast force). People who are up to a kilometer beyond this blast zone are at risk for radiation doses and burn injuries. What's most important, though, is that the range of radiation danger is much smaller with an improvised nuclear bomb threat than a Cold War thermonuclear weapon. For example, 10 kiloton warhead will pose an immediate radiation danger up to 1 kilometer away from ground zero, and hazardous fallout contamination will go for another 10-20 miles.


So you can appreciate why a nuclear attack today doesn't have to mean instant death for everyone around — and could even be something that your city would recover from.

Taken from the Student Guide to Federal Nuclear Detonation Response Planning

What To Do When the Bomb Goes Off

"If you see a bright flash, don't go to the window, since it can break as you are looking around," Dillon advised. "Like thunder and lighting, the blast travels a lot slower than the light." Next you need to worry about protection from radioactive fallout. Because we're not in a Cold War world anymore, Dillon said, "You don't need a specific fallout shelter to get the protection you need." You just have to be aware of what kinds of buildings will provide adequate shelter and which won't.


Emergency responders measure the effectiveness of a fallout shelter on the "PF" scale (you can see a FEMA guide about that here), but Dillon is assuming you won't have PF numbers on all the buildings in your neighborhood. What you want to do is try to find what he calls "adequate shelter" about 30 minutes after the bomb goes off. After those 30 minutes, the initial radiation from the explosion is long gone, and your main danger is from the sand-sized particles that have fallen around you.

Dillon explained:

If you find yourself in a building that doesn't provide good protection and you know of one that provides adequate shelter less than 15 min away, wait up to a half hour and then go for it. Make sure to brush off the sand-sized fallout particles when you enter the shelter.


But what is adequate when it comes to shelters? Said Dillon:

Put as much mass and distance between you and radioactive fallout as possible. Look for heavy things - thick concrete walls and roofs, large stacks of books, earth - to hide in. Go underground if you can get there - a basement is the classic shelter. Go deep inside big buildings.

Think about your city. Where is the nearest adequate shelter to your home and your work? Is it a subway station? A library with thick concrete walls lined with books? Your basement? A large building with lots of interior rooms that are shielded by many walls? Dillon warns that you want to try to reach this place in 30 minutes, but don't count on being able to drive there. Traffic may be at a standstill. Make plans that will allow you to walk or possibly bicycle to your adequate shelter.


Then the question becomes how long to wait in this shelter until it's safe to go outside. In the movies, of course, we see all kinds of ridiculous scenarios, from people going outside within minutes to whole civilizations remaining underground for centuries. None of those are really accurate, said Dillon.

Your best bet is to stay until emergency responders come. Given that we're talking about a low-yield bomb, which may have a blast radius of less than a mile, this isn't a disaster that has taken out the nation's power structure. Help will arrive soon. But let's say nobody does come. Dillon says his personal preference would be to wait about 12-24 hours before going outside. But, he emphasized, "wait for emergency responders because they'll help with an evacuation route." You don't want to jump out of your fallout shelter and walk right into the path of the radiation.


How Does Fallout Work?

My first reaction to Dillon's advice was disbelief. I could be relatively safe walking out of a fallout shelter less than a day after the blast? The answer is yes, because the most immediate danger is from what's called early fallout, which is comprised of radioactive particles that are heavy enough to fall within hours of the blast. They usually fall in zones fairly close to the blast, depending on wind direction and intensity.


Taken From "Key Response Planning Factors for the Aftermath of Nuclear Terrorism"

Said Dillon, "It's going to be falling for hours after the blast. These large particles are the most dangerous and have the highest levels of radiation. This is the stuff that's going to make you physically sick immediately." He contrasts the radiation sickness you can get from this early fallout to other kinds of illnesses, like cancer, that you can get many years after radiation exposure. Sheltering from fallout may not prevent cancer in the future, but it will prevent you from dying immediately of radiation exposure.

The other thing to keep in mind is that fallout isn't a magical substance that floats everywhere and gets into everything. "There will exist a physical region that's contaminated with highly radioactive particles," he said. "After leaving the shelter, you want to exit that region." That's where emergency responders can help, of course — they'll be able to tell you how to avoid that zone, and how far away to go. Certainly there are lighter fallout particles that can stay airborne for much longer than the early fallout, but those particles are not going to cause immediate radiation sickness — which is what you're trying to avoid in the bomb's aftermath.


Dillon added that the early, dangerous fallout also "decays really fast." The "dangerous zone shrinks quickly, and it's a lot safer to be outside in 24 hours" than it is an hour after the blast.

Our pop culture is still straining to catch up with a world where nuclear blasts result in a scenario more like Katrina than On the Beach. We've been trained to think of nuclear attack as the end of the world, but it's like many other disasters: horrific, but something that we can survive. While we're waiting for a movie that realistically depicts a low-yield nuclear attack in the post-Cold War era, we can start planning our real-life escape routes and shelters in the citiscapes around us. One day, that big ugly building downtown with the thick concrete walls could save your life.


Read the full scientific study in Proceedings of the Royal Society A

Unless otherwise specified, all charts taken from the US national security staff publication Planning Guidance for Response to a Nuclear Detonation.

Annalee Newitz is the editor-in-chief of io9 and author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction.