Originally published 2018-03-31.
Image by tofoli.douglas.
If a nuclear weapon ever detonated within sight of you and you survived the flash and the blast, the single most useful thing you could do next is simple: get inside the most solid building you can reach quickly, go to its centre or basement, and stay there while you listen for official instructions. The danger that kills most people who live through the first minute is not the explosion itself but the radioactive dust, called fallout, that starts settling over the following hour.
That advice comes from US emergency agencies and from modelling work by Lawrence Livermore National Laboratory scientist Michael Dillon. Below is what the guidance says, why it works, and where the common assumptions go wrong.
How you would know a detonation has happened
The first sign is a flash of light far brighter than the sun. Looking towards it can cause temporary flash blindness that usually lasts seconds to a minute or so. People closer in may feel intense heat on exposed skin, and a shock wave follows the flash, arriving seconds later depending on distance, much as thunder trails lightning. The familiar mushroom cloud rises over the next few minutes.
How far each of these effects reaches depends heavily on the size of the weapon and whether it exploded in the air or at ground level. A ground burst throws huge amounts of soil and debris into the cloud, and that material becomes the fallout that later drifts downwind. If you see the flash, drop behind something solid and cover your head; windows can shatter well beyond the zone of serious damage.
Concern about this scenario rose sharply in 2017, when North Korea was testing missiles and its envoys spoke openly about speeding up the programme, as in this Reuters report from the time:
MORE: North Korea will pursue 'acceleration' of nuclear and missiles programs, including first-strike capability, ICBM development - envoy https://t.co/yKzmjctja6
— Reuters World (@ReutersWorld) March 21, 2017
Why fallout is the danger you can do something about
Fallout is made of particles that were pulled into the fireball, made radioactive, and then dropped back to earth. Some of it is as coarse as sand and lands within an hour near the blast area; finer material can travel many kilometres with the wind. It gives off gamma radiation that passes through ordinary walls and clothing, which is why simply staying indoors is not always enough: what matters is how much dense material lies between you and the dust outside and on the roof.
The good news is that fallout loses its strength quickly. A rule of thumb from Cold War civil defence researcher Cresson Kearny, cited in the Wikipedia overview of nuclear fallout, says the dose rate falls roughly tenfold for every sevenfold increase in time. In practice, radiation levels drop by about half in the first hour and by around 80% during the first day. Every hour you spend well shielded early on is worth far more than an hour a week later.
Where to shelter: density and depth
Shielding works because dense, thick materials absorb gamma rays. According to the same overview, about 6 cm of concrete or 9 cm of packed earth cuts gamma exposure in half, and those layers multiply: ten such halvings reduce exposure roughly a thousandfold. That is why emergency planners rank shelters by mass rather than by how safe a room feels.
The best ordinary options are:
- A basement or underground car park, ideally in a corner away from windows.
- The central floors and inner rooms of a large brick or concrete building, away from the roof and outer walls.
- Underground transit stations and purpose-built shelters, where these exist.
The US Federal Emergency Management Agency publishes a diagram that puts numbers on this. A protection factor of 10 means a person inside receives one-tenth of the dose they would get outdoors:
The chart shows how wide the gap is. The ground floor of a one-storey wood-frame house offers a protection factor of only about 2 or 3, while its basement reaches about 10. Inner rooms in the middle of a large office or apartment block rate around 50 to 100, and the deep basement of such a building can exceed 200, meaning less than one two-hundredth of the outdoor dose.
Stay put or move? What Michael Dillon's model found
In 2014 Dillon published a mathematical model in the Proceedings of the Royal Society A asking a practical question: if the first building you reach is flimsy, when is it worth going outside again to reach a better one? His model assumed a relatively small, low-yield weapon and full exposure while travelling.
His answer, as summarised in coverage at the time, comes down to how far away the better shelter is:
- If a much better shelter is only a few minutes away (around five minutes on foot), go there straight away, before most of the fallout arrives.
- If it is further, roughly a quarter of an hour, it can be better to stay in the poor shelter for about an hour, until the most intense early radiation has decayed, and then move.
- If the place you are already in is reasonably protective, such as a basement or the core of a large building, stay where you are.
An Associated Press report on the study said that following this approach could save tens of thousands of lives in a large city. It also noted the critics: Stanford's Lawrence Wein argued the model underestimated how uncertain travel times and crowd behaviour would be in a real emergency, and pointed out that official advice already called for sheltering for many hours. Treat the timings as guidance for a hard choice, not exact rules.
What to do while you wait
Once inside, the US Centers for Disease Control and Prevention sums up the plan as get inside, stay inside, stay tuned. The practical steps recommended by the CDC and the US Environmental Protection Agency are:
- Close and lock doors and windows, move away from them, and bring pets inside.
- If you were outside when the dust fell, take off your outer layer of clothing carefully, put it in a sealed plastic bag, and leave the bag away from people. Removing outer clothing gets rid of much of the contamination.
- Shower if you can, using soap and shampoo but not hair conditioner, which can hold radioactive particles in the hair. Do not scrub hard or scratch the skin. If there is no shower, wipe exposed skin with a damp cloth, and gently clean your eyelids, ears and nose.
- Drink bottled water and eat food from sealed packages or containers that were indoors.
- Listen to a battery or hand-crank radio, television or phone alerts. Officials will say when it is safe to leave and where to go for contamination screening and medical help.
US guidance generally asks people to expect to stay inside for at least the first 24 hours unless told otherwise, so a small store of water, food, medicines and a radio at home is worth having in any case. If you feel unwell afterwards, talk to a doctor or the medical teams at screening centres rather than guessing at treatment.
Common myths about surviving a nuclear blast
"There is nothing anyone can do"
For people at the centre of a large explosion that is true, but most of the population of a city struck by a single weapon would be outside the zone of immediate destruction. For them, the choices made in the first hour, above all getting into solid shelter, make a measurable difference.
"Get in the car and drive away"
Cars offer almost no shielding, roads jam within minutes, and you may drive straight into the path of the fallout. Unless officials order an evacuation, sheltering usually beats fleeing.
"Iodine tablets protect you"
Potassium iodide only protects the thyroid gland from one type of radioactive material and does nothing against external gamma radiation. Take it only if public health officials tell you to.
"Any building is good enough"
As the FEMA chart shows, a light wooden house gives very little protection above ground. Its basement, or a nearby concrete building, is many times better.
Sources: Associated Press via Cullman Times: Study on how to survive a nuclear explosion (2014); Dillon, Proceedings of the Royal Society A (2014); CDC: Radiation emergencies, what to do; EPA: Protecting yourself from radiation; Wikipedia: Nuclear fallout; Ready.gov: Nuclear explosion; shelter diagram by FEMA, shared by Business Insider; originally reported by Lifehacker; cover image by tofoli.douglas.
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