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Duck and Cover is a method of personal protection against the effects of a nuclear explosion, which the United States government taught to generations of United States school children from the early 1950s until the end of the Cold War in the late 1980s. It was intended to protect them in the event of both an unexpected nuclear attack, which, they were told, might come at any time without warning (although with the Ballistic Missile Early Warning System, and the Pinetree Line of the era, a successful surprise attack was becoming less likely) and in the event sufficient warning is given. Under the conditions of a surprise attack, immediately after they saw a flash they had to stop what they were doing and get on the ground under some cover—such as a table, or at least next to a wall—and assume a prone like position, lying face-down and covering their exposed skin and back of their heads with their clothes, or if no excess clothes such as a coat was available, to cover the back of their heads with their hands. Similar instructions were given in 1964 in the United Kingdom by Civil Defence Information Bulletin No. 5. and, in the 1980s, by the Protect and Survive series. Under the conditions where sufficient warning is given, they were told to find the nearest fallout shelter, or if one could not be found, any well built building to stay and shelter in.
Proponents of duck and cover argue that thousands could be saved through following the advice's precautions, without which, for example, people would instead run to windows to find the source of the bright nuclear weapon flash. During which time the slower moving blast wave would soon arrive causing a window glass implosion, shredding onlookers.
According to the 1946 book Hiroshima, in the days between the Hiroshima and Nagasaki atomic bombings in Japan, one Hiroshima policeman went to Nagasaki to teach police about ducking after the atomic flash. As a result of this timely warning, not a single Nagasaki policeman died in the initial blast. This allowed more surviving Nagasaki police to organize relief efforts than in Hiroshima. Unfortunately, the general population was not warned of the heat/blast danger following an atomic flash because of the bomb’s unknown nature. Many people in Hiroshima and Nagasaki died while searching the skies for the source of the brilliant flash.
When people are indoors, running to windows to investigate the source of bright flashes in the sky still remains a common inquisitive human response to experiencing a flash, so although the advice to duck and cover is over half a century old, ballistic glass lacerations caused the majority of the 1000 human injuries following the Chelyabinsk meteor air burst of February 15, 2013. This natural human behavior was also observed amongst people in the vicinity of Hiroshima and Nagasaki, and therefore a human behavior advised against in Duck and Cover advice, and in the film of the same name.
Similar to duck and cover is the advice to "Drop, Cover and Hold On" which is taught in areas prone to earthquakes. Schools in some tornado-prone areas of the United States also practice tornado drills that involve children squatting and covering the backs of their heads.
The United States' monopoly on nuclear weapons was broken in 1949 when the Soviet Union tested its first nuclear device, and many in the government and public perceived that the United States was more vulnerable than it had ever been before. Duck-and-cover exercises quickly became a part of Civil Defense drills that every American citizen, from children to the elderly, practiced to be ready in the event of nuclear war. In 1950, during the first big Civil Defense push of the Cold War; the movie Duck and Cover was produced (by the Federal Civil Defense Administration) for school showings in 1951.
Within a considerable radius from the surface of the nuclear fireball, 0–3 kilometers—largely depending on the explosion's height, yield and position of personnel—ducking and covering would offer negligible protection against the intense heat, blast and prompt ionizing radiation following a nuclear explosion. Beyond that range however many lives would be saved by following the simple advice, especially since at that range the main hazard is not from ionizing radiation but from blast injuries and sustaining thermal flash burns to unprotected skin. Furthermore, following the bright flash of light of the nuclear fireball, the explosion's blast wave would take from first light, 7 to 10 seconds to reach a person standing 3 km from the surface of the nuclear fireball, with the exact time of arrival being dependent on the speed of sound in air in their area. The time delay between the moment of an explosions flash and the arrival of the slower moving blast wave is analogous to the commonly experienced time delay between the observation of a flash of lightning and the arrival of thunder during a lightning storm, thus at the distances that the advice would be most effective, there would be more than ample amounts of time to take the prompt countermeasure of 'duck and cover' against the blast's direct effects and flying debris. For very large explosions it can take 30 seconds or more, after the silent moment of flash, for a potentially dangerous blast wave over-pressure to arrive at, or hit, your position.
It must be kept in mind that the graphs of lethal ranges as a function of yield, that are commonly encountered, are the unobstructed "open air", or "free air" ranges that assume amongst other things, a perfectly level target area, no passive shielding such as attenuating effects from urban terrain masking, e.g. skyscraper shadowing, and so on. Therefore they are thus considered to present an overestimate of the lethal ranges that would be encountered in an urban setting in the real world.
To highlight the effect that being indoors, and especially below ground can make, despite the lethal open air radiation, blast and thermal zone extending well past her position at Hiroshima, Akiko Takakura survived the effects of the 16 kt atomic bomb at a distance of 300 meters from ground zero, sustaining only minor injuries, due in greatest part to her position in the lobby of the Bank of Japan, a reinforced concrete building, at the time of the nuclear explosion, and to highlight the protection conferred to an individual who is below ground during a nuclear air burst, Eizo Nomura survived the same blast at Hiroshima at a distance of 170 meters from ground zero. Nomura, who was in the basement of what is now known as the rest house, also a reinforced concrete building, lived into his early 80s.
In contrast to these cases of survival, the unknown person sitting outside on the steps of the Sumitomo Bank next door to the Bank of Japan on the morning of the bombing—and therefore fully exposed—suffered what would have eventually been lethal third degree burns from the near instant nuclear weapon flash if they hadn't then promptly been killed by the slower moving blast wave, when it reached them, ~2 seconds later.
To elucidate the effects on laying flat on the ground in attenuating a weapons blast, Miyoko Matsubara, one of the Hiroshima maidens, when recounting the bombing in an interview in 1999, said that she was outdoors and less than 1 mile from the hypocenter of the Little Boy bomb. Upon observing the nuclear weapons silent flash she quickly lay flat on the ground, while those who were standing directly next to her, and her other fellow students, had simply disappeared from her sight when the blast wave arrived and blew them away.
During the 2013 Chelyabinsk meteor explosion, a fourth-grade teacher in Chelyabinsk, Yulia Karbysheva, saved 44 children from potentially life threatening ballistic window glass cuts by ordering them to hide under their desks when she saw the flash. Despite not knowing the origin of the intense flash of light, she ordered her students to execute a duck and cover drill. Ms. Karbysheva, who herself did not duck and cover but remained standing was seriously lacerated when the explosions blast wave arrived and window glass severed a tendon in one of her arms, however not one of her students, who she ordered to hide under their desks, suffered a cut.
The advice to cover one's exposed skin with anything that can cast a shadow, like the picnic blanket and newspaper used by the family in the film, may seem absurd at first when one considers the capabilities of a nuclear weapon, but even the thinnest of barriers such as cloth or plant leaves would reduce the severity of burns on the skin from the thermal radiation—which is primarily light rays in the ultraviolet, visible, and infrared range, and it is this combination of light rays that delivers the burning energy to exposed skin areas. This burning thermal radiation would be experienced by people within range for several seconds after the explosion. A photograph taken about 1.3 km from the hypocenter of the Hiroshima bomb explosion showed that the shadowing effect of leaves from a nearby shrub protected a wooden utilities pole from charring discoloration due to thermal radiation, while the rest of the telephone pole, not under the protection of the leaves, was charred almost completely black.
The advice to "duck and cover" holds well in many situations where structural destabilization or debris may be expected, such as during an earthquake or tornado. At a sufficient distance from a nuclear explosion, the blast wave would produce similar results and ducking and covering would perhaps prove adequate. It would also offer some protection from flying glass and other small, but dangerous, debris. Ducking and covering would also reduce exposure to the gamma rays. Since they are mostly emitted in a straight line, people on the ground will have more chance to have obstacles serving as radiation protection such as building foundations, cars, etc. between them and the source of radiation. The technique offers a small protection against fallout—people standing up could receive a large, possibly lethal, dose of radiation, while people protected will receive less of it. However it must be said that the technique assumes that after the initial blast, a person who ducks and covers will realize when it is wise to cease ducking and covering (as the blast danger has passed) and to then seek out a more sheltered area, like a fallout shelter to protect themselves from the ensuing local fallout danger, if such a shelter is unavailable, at this point the person is advised to follow the Shelter in Place protocol, or if given, evacuation advice. Evacuation orders would entail exiting the area completely by following a path perpendicular to the wind direction, and therefore perpendicular to the path of the fallout plume. While "sheltering in place" is staying indoors, in a preferably sealed tight basement, or internal room, for a number of hours, with the oxygen supply available in such a scenario being more than sufficient for 3+ hours in even the smallest average room, under the assumption that the improvised seal is perfect, until carbon dioxide levels begin to reach unsafe values and necessitate room unsealing for a number of minutes to create a room air change.
After all, "Duck and Cover" is a first response countermeasure only, in much the same way that "Drop, Cover and Hold On" is during an earthquake, with the advice having served its purpose once the earthquake has passed, and possibly other dangers—like a tsunami—may be looming.
In the era the advice was originally given, the most common nuclear weapons were weapons comparable to the Fat Man. The main dangers that initially come from the nuclear blast from weapons of this, and higher, yields are the initial heat and blast effects and not from fallout or radiation. To maximize building damage, an air-burst is the preferred nuclear fuzing height, which maximizes surface damage and results in fallout being dispersed into the stratosphere, instead of highly contaminating the area under the explosion. For example in the Operation Crossroads tests of 1946, Test Able (an air burst) had little local fallout, but the infamous Test Baker (an underwater burst) left the test targets badly contaminated with radioactive fallout.
Widespread radioactive fallout itself was not recognized as a threat amongst the public at large before 1954, until the widely publicized story of the 15 megaton surface burst of test shot Castle Bravo on the Marshall Islands occurred with fatal results. The explosive yield of the Castle Bravo device the Shrimp was unexpectedly high, and therefore correspondingly higher amounts of fallout were produced. This resulted in the accidental death of one of the members of a Japanese fishing boat known as the Lucky Dragon, which was fishing outside the planned fallout zone.
It is however unlikely that a belligerent with nuclear weapons would waste their weapons with fusing to explode below or on the surface, as both test shot Baker, and Castle Bravo were respectively. Instead, to maximize the range of blast destruction, an air burst is preferred, as the ~ 500 m explosion heights of the only nuclear weapons used on cities Little Boy and Fat Man also attests to. With air bursts the total amount of radiation contained in the fallout, in units of Becquerel, is somewhat less than the total that would be released from a surface or subsurface burst, as in comparison little to no neutron activation or Neutron induced gamma activity of soil occurs from air bursts. Therefore the initial danger from concentrated local fallout, which commonly takes on a pumice or ash-like appearance, which was experienced by the crew of the Lucky Dragon, remains low in a global nuclear war scenario. Instead the fallout likely to be encountered by most survivors in this scenario is expected to be the less dangerous global/'late' fallout. As an air burst at optimum height will produce a negligible amount of early fallout.
A notable comparison to underline this is found when one compares the 50 megaton airburst Tsar Bomba, which produced no concentrated local/early fallout, and thus no known deaths from radiation, with the surface burst of the 15 megaton Castle Bravo, which in comparison, due to the local fallout produced, killed one of the 23 strong crew of the Lucky Dragon.
Furthermore, regardless of if a nuclear attack on a city is of the surface or air-burst variety or a mixture of both, the advice to shelter in place, in the interior of well built homes, or if available, fallout shelters, as suggested in the film Duck and Cover, will drastically reduce ones chance of absorbing a hazardous dose of radiation. A real world example of this occurred after the Castle Bravo test where, in contrast to the crew of the Lucky Dragon, the firing crew that triggered the explosion safely sheltered in their firing station until after a number of hours had passed and the radiation levels outside fell to dose levels safe enough to travel in. The comparative safety experienced by the Castle Bravo firing crew served as a proof of concept to civil defense personnel that Shelter in place(or "buttoning up" as it was known then) is an effective strategy in mitigating the potentially serious health effects of local fallout.
The minimum typical protection factor of the fallout shelters in US cities is 40 or more, in many cases these shelters are nothing more than the interior of pre-existing well built buildings that have been inspected, and following their protection factors being calculated, re-purposed as fallout shelters. A protection factor of 40 or more means that the Radiation shielding provided by the shelter reduces the radiation dose experienced by at least 40 times that which would be experienced outside the shelter with no shielding. "Protection factor" is equivalent to the modern term "dose reduction factor".
During the first hour after a nuclear explosion, radioactivity levels drop precipitously. Radioactivity levels are further reduced by about 90% after another 7 hours and by about 99% after 2 days. An accurate rule of thumb for approximating the radioactive dose rate produced by the decay of the myriad of isotopes present in nuclear fallout is the "7/10 rule". The rule states that for each 7 fold increase in time the dose rate drops by a factor of 10. For example, assuming the fallout process has ended and the dose rate is 500 roentgens per hour at one hour after detonation, then 7 hours after detonation the rate will be 50 R/hr, 49 hours after detonation (7×7 hours) the dose rate will be 5 R/hr, 343 hours after detonation (49×7—or about 2 weeks) the dose rate will be about 0.5 R/hr, at which point no special precautions would need to be taken and venturing into the fallout zone during the daylight hours would pose a negligible health hazard. Following a nuclear detonation approximately 80 percent of the fallout would be deposited on the ground during the first 24 hours.
Expert advice published in the 2010 document Planning Guidance for Response to a Nuclear Detonation is to shelter in place, in an area away from building fires, for at least 1 to 2 hours following a nuclear detonation and fallout arriving, with the greatest benefit, assuming personnel are in a building with a high protection factor, is sheltering for no less than 12 to 24 hours before evacuation. Therefore, sheltering for the first few hours can save lives. Indeed fallout deaths are regarded by experts as the most preventable of all the effects of a nuclear detonation, being simply dependant on if personnel know how to identify an adequate shelter when they see one and enter one quickly, with the number of potential people saved being cited as in the hundreds of thousands.
Some historians have thus far sought to dismiss civil defense advice as mere propaganda, despite detailed scientific research programs laying behind the much-mocked UK government civil defense pamphlets of the 1950s and 1960s, including the prompt advice of ducking and covering.
The exercises of civil defense are seen by Historian Guy Oakes as having less practical use than psychological use: to keep the danger of nuclear war high on the public mind, while also attempting to assure the American people that something could be done to defend against nuclear attack.
In U.S. Army basic training in the 1970s, soldiers were taught to fall immediately down, covering face and hands in much the same way as is described by the advice to duck and cover.
Ducking and covering does have certain applications in other, more natural disasters. In states prone to tornadoes, school children are urged to "duck and cover" against a solid inner wall of a school, if time does not permit seeking better shelter—such as a storm cellar—during a tornado warning. The tactic is also widely practiced in schools in states along the West Coast of the United States, where earthquakes are commonplace. Ducking and covering in either scenario would theoretically afford significant protection from falling or flying debris.
In an earthquake, people are encouraged to "drop, cover, and hold on": to get underneath a piece of furniture, cover their heads and hold on to the furniture. This advice also encourages people not to run out of a shaking building, because a large majority of earthquake injuries are due to broken bones from people falling and tripping during shaking. While it is unlikely that "drop, cover and hold on" will protect against a building collapse, buildings built in earthquake-prone areas in the United States are usually built to earthquake "Life Safety" codes, and a building collapse (even during an earthquake) is rare. "Drop, cover and hold on" may not be appropriate for all locations or building types, but the red cross advises, it is the appropriate emergency response to an earthquake in the United States.