Your Personal Safety

February 8th, 2018 | National Director

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Common Misconceptions About Nuclear Fallout

The immediate effects of a nuclear detonation – tremendous heat, blast force and initial radiation – are relatively localized and limited in area. The bigger concern for most of Canada’s population is the fallout which can be blown many hundreds of miles to fall to the earth as radioactive dust particles.

When we have a nuclear incident like Chernobyl or Fukushima and a nuclear reactor leaks radioactive materials, these can remain highly dangerous for extended periods of time.

However, the fallout dust from a nuclear detonation generally follows the 7/10 Rule: Fallout loses roughly 90% of its radioactivity in the first 7 hours after detonation and an additional 90% for every 7-fold increase in time: 90% in the first seven hours; 99% in 49 hours (two days) and 99.9% in two weeks.

So it is the first couple of days after a nuclear detonation that are the most dangerous for the large majority of the population. This is the period when Canadians would most need protection from radiation.

During the Cold War years, a risk study conducted by the Canadian Defence Research Board considered many different attack patterns on US and Canadian cities, as well as on important military and industrial centres. More than a hundred different wind patterns were used to evaluate the potential distribution of fallout on Canadian territory. The results were plotted the map below.


Canada nuclear risk map based on a risk study conducted by the Canadian Defence Research Board before 1974. Because general wind patterns remain the same, it remains a fair estimation of danger zones today – the best we have at the moment..


In Risk Area 3, south of the broken line, there is one chance in five that total  exposure dose in the open would exceed 750 Roentgens in six weeks. (Roentgen or R is a unit of measurement for the exposure to X-rays and gamma rays. The acute effects of radiation over the short period of weeks is accumulative. The more of a total dose you receive over a few weeks or months, the more dangerous it is.)

In Risk Area 2, there is a one in five chance that the dose in six weeks would be between 100 and 750 R. Risk Area 1 would probably receive less than 100 R in six week’s exposure in the open.

Without adequate fallout protection, 750 Roentgens of exposure would be fatal for a majority of the population.






Contrary to popular belief, there is no Hazmat suit or mask that will protect you from the majority of gamma radiation emitted by the fallout dust scattered about in the environment after a nuclear or thermonuclear detonation. The emitted gamma rays would simply pass through the thin layers of a hazmat suit and into your body, just the way they do through any other clothing you wear. Gamma rays are part of the electromagnetic spectrum, like radio waves, X-rays and light. If you picture the fallout particles landing on trees, roofs and the ground being like tiny little light bulbs that shine like x-rays through the surrounding materials, you get the general idea.

Other than distance from the fallout particles themselves and the decay of the radiation over time, it is only the thickness and density of matter between you and the fallout that can significantly reduce the radiation levels reaching your body. Any mass will reduce or block some of the gamma rays, whether that mass is lead, dirt, wood, feathers or chocolate bars, as long as you have enough mass or thickness of material. Lead is far more dense than wood, so you would need less of a thickness of lead than other materials to block the same amount of radiation. However, because of their lower cost, concrete and dirt are the most common materials used to create fallout shelters.



Fortunately, what this means is that modern cities, with their massive structures, built of thick layers of concrete and steel and with deep basements, offer a great many areas that could easily serve as expedient, short-term fallout shelters. And indeed, this is what both the US and Canadian government’s civil defence plans, developed in the 1960s, capitalized upon. Existing buildings that had areas which offered a high degree of protection from outside radiation were designated to be used as community fallout shelters in the case of nuclear attack.


Some shelters offer limited fallout protection, particulary single-story wood-frame structures. Numbers in the graphic represent a radiation reduction factor. A dose reduction factor of 10 indicates that a person in that area would receive 1/10th of the dose of a person in the open. — FEMA graphic


Today, large numbers of people living in cities could easily find expedient shelter from fallout radiation in the many existing structures around them – that is if they knew where to go and could gain access. But this would really need to be pre-planned and possibly even prearranged. The vital first 7 hours following nuclear detonations would be, by far, the deadliest. (Remember, the effects of radiation are cumulative. )

Yet in just two days (without further detonations), radiation levels would already have dropped by 99%.  In the projected Canadian scenarios mentioned above, it might even be safe enough for people to come out of their make-shift community shelters and return home. (Having instruments on hand that measure radiation at the high levels produced by nuclear detonations, coupled with some simple basic training, would take much of the guesswork out of this. So supplying such instruments in community shelters as was once done in the US would make tremendous sense. )

To help to train people in skills related to fallout shelter safety, the League currently delivers two online courses. The first, the Fallout Shelter Safety Officer Course, provides training in basic radiation detection, how to ensure safety in a makeshift community shelter, and how to track radiation exposures and ensure that everyone around one remains as safe as possible. Much of this course is based on the 1980s FEMA handbook, Radiation Safety in Shelters.

 The second online course, the Radiological Defence Officer Course, teaches basic mathematical analysis using graphs and simple math to more accurately monitor and predict radiation dangers, past, present and future, to better ensure people’s safety in the days and weeks following an attack. It is largely based on the training that was delivered by the Canadian government to Radiological Defence Officers in the 1970s.

For more information on these courses, see here.

At current levels of nuclear armament,  Mutually Assured Destruction (MAD) no longer really exists as a deterrent. Far from it. The world’s nuclear arsenals are a small fraction of what they were in the 1980’s. And the need to target military infrastructure first would likely leave most cities in tact in a modern nuclear war.

As well. theories about a “Nuclear Winter” following war with nuclear weapons, developed originally more for a political disarmament agenda rather than peer reviewed science, are today considered unlikely. (See the excellent article on Wikipedia on the subject.)  For example, in a paper by the United States Department of Homeland Security finalized in 2010, fire experts stated that due to the nature of modern city design and construction, a firestorm is unlikely after a nuclear detonation in a modern city. Fires would not coalesce and form the stratospheric firestorm plume that the nuclear winter papers required in their climate computer models. 

So it is primarily radioactive fallout that the large majority of people would have to worry about after a nuclear attack. Unfortunately, these modern realities along with the modernization of nuclear arsenals have led some military planners to consider that the actual use of nuclear weapons, especially “low yield” ones, is now completely feasible under extreme circumstances. And the trouble is, in interstate wars, extreme circumstances do arise.

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