During a surprise nuclear attack

Dropping immediately and covering exposed skin provide[s] protection against blast and thermal effects ... Immediately drop facedown. A log, a large rock, or any depression in the earth's surface provides some protection. Close eyes. Protect exposed skin from heat by putting hands and arms under or near the body and keeping the helmet on. Remain facedown until the blast wave passes and debris stops falling. Stay calm, check for injury, check weapons and equipment damage, and prepare to continue the mission.

— US Army field manual FM 3–4 Chapter 4.^[1]

Immediately after one sees the first flash of intense heat and light of the developing nuclear fireball, one should stop, get under some cover and drop/duck to the ground. There, one should assume a prone-like position, lying face-down, and to afford protection against the continuing heat of the explosion further cover exposed skin and the back of one's head with one's clothes; or, if no excess cover or cloth is available, one should cover the back of one's head and neck with one's hands.

Similar instructions, as presented in the Duck and Cover film, are contained in the British 1964 public information film Civil Defence Information Bulletin No. 5^[2] and in the 1980s Protect and Survive public information series.^[3] Children in the Soviet Union likewise received almost identical classes on countermeasures, according to Inside the Kremlin's Cold War authors Zubok and Pleshakov.^[4]

In U.S. Army training, soldiers are taught to fall down immediately and cover their face and hands in much the same way as is described above.^[1]

In the classroom scene of the film, the rapid employment of school desks, as an improvised shelter following the awareness of the initial light flash, is a countermeasure primarily to offer protection from potential ballistic window glass lacerations when the slower moving blast wave arrived. However, in higher blast pressure zones, where partial-to-total building collapse may occur, it would also serve a similar role to that borne out from experience in urban search and rescue, where voids under the debris of collapsed buildings are common places for survivors to be found. More rigid examples of void-forming-tables to shelter under include the "Morrison indoor shelter", which was widely distributed by the millions in Britain as a protective measure against building collapse, brought about by blast pressures generated during the conventional bombing of cities in World War II.^{[5][6][7][8][lower-alpha 2]}

Cursory analysis

The "duck and cover" countermeasure could save thousands. This is because people, being naturally inquisitive, would instead run to windows to try to locate the source of the immensely bright flash generated at the instant of the explosion. During this time, unbeknownst to them, the slower moving blast wave,^[9][10] would be rapidly advancing toward their position, only to arrive and cause the window glass to implode, shredding onlookers.^[11] In the testimony of Dr. Hiroshi Sawachika, although he was sufficiently far away from the Hiroshima bomb himself and was not behind a pane of window glass when the blast wave arrived, those in his company who were had serious blast injury wounds, with broken glass and pieces of wood stuck into them.^[12]

During earthquakes and tornadoes

Similar advice to "duck and cover" is given in many situations where structural destabilization or flying debris may be expected, such as during an earthquake or tornado. At a sufficient distance from a nuclear explosion, the blast wave produces similar results to these natural phenomena, so similar countermeasures are taken. In areas where earthquakes are common, a countermeasure known as "Drop, Cover, and Hold On!" is practiced.^[13][14][15] Likewise, in tornado-prone areas of the United States, especially those within Tornado Alley, tornado drills involve teaching children to move closer to the floor and to cover the backs of their heads to prevent injury from flying debris.^[16][17] Some US states also practice annual emergency tornado drills.^[18][19]

Blast effects

Flash & burn injuries

Anything that can cast a shadow will protect that which is shadowed from being burnt.^[65][66] In this case a valve protected a portion of the bitumen^[67] coated wall of a gas holder from having a line-of-sight with the nuclear fireball whereas all unshadowed surfaces were lightened, akin to a near instant "sun fading" of the coating.^[68] A large number of these permanent markings were used, by extrapolating backward, to determine the exact point of detonation in the sky.^[69][70][71]

A man who was present at Nagasaki on August 9, 1945 during the dropping of the 20-kiloton Fat Man bomb; this photo displays 1st- and 2nd-degree burn injuries he experienced on his unclothed skin, the shoulder and arm, while the thin vest garment of clothing, a radiant barrier, that he was wearing at the time of the explosion completely protected his stomach and lower chest from experiencing similar burns. A clearer color restored version of this picture is also available.^[72]

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. However, even the thinnest of barriers such as cloth^[73][74][75] or plant leaves would reduce the severity of burns on the skin from the thermal radiation with the flash light, similar in average emission spectrum/color to sunlight.^[76][77][78] The thermal radiation emits in the ultraviolet, visible light, and infrared range but with a higher light intensity than sunlight, and this combination of light rays is capable of delivering radiant burning energy to exposed skin areas.^[79][80][81] As the time-to-rise at peak and total duration of the emittance pulse of this burning thermal radiation is both prolonged and increases with larger explosive yield,^[82] it is usually at least a few seconds long for all high yield stockpiled weapons, creating the potential for protective countermeasures.^[83]

High importance is given to closing eyelids and covering the eyes as temporary or permanent flash blindness is a risk potential without this covering, especially at night.^[84]

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 the burst of thermal radiation; the rest of the telephone pole, which was not under the protection of the leaves, was charred almost completely black.^[85] The difference in required flash-energy necessary to produce essentially immediate, though transitory, non-propagating flaming, and that required to achieve a continued self-sustained propagating flaming are orders of magnitude in difference for most combustible materials. In the case of untreated timber it is largely dependent on the depth of char.^[86][87][88] While the propagating fires in both Japanese cities were almost exclusively ignited by the blast wave overturning charcoal cooking-braziers and similar secondary events, thermal flash-fires from untreated fabric and timber in the urban environment is considered potentially the widest destructive effect of the higher yield explosive devices.^[82] The Nevada test site used for testing nuclear devices had a dry desert environment with low humidity, which repeatedly demonstrated the flash-combustion effect during tests. Many investigative films made on location there, such as The House in the Middle and others, focused on the combustion of fabrics and clothing.^[73]

In the only human accounts at these high luminous intensities that are not of the more common Arc flash accidents, a number of the Hiroshima Maidens survived despite their close proximity to the explosion and in a range where the flash-fire of their customary Japanese summer attire, made of thin kimono cloth, was near instantaneous. As their clothing combusted, some of the Maidens performed an incomplete stop, drop and roll in an effort to extinguish the flames.^[89]

Initial nuclear radiation

While not designed for those faced with low-yield neutron bombs or for those who are, in general, so close to the nuclear fireball that prompt/initial radiation would be life-threatening in the short-medium term, ducking and covering would nevertheless slightly reduce exposure to the initial gamma rays, specifically the portion emitted after the first flash of visible light.^[90] The initial gamma rays are defined as those emitted from the fireball & following mushroom cloud which can reach personnel on the ground for a total of approximately 1 minute, at which point the intensity of the radiation has diminished and the atmosphere itself is thick enough to act as full shielding.^{[91][92][93][94]}

As approximately half of these gamma rays are emitted in the first second and the other half, over the following 59^[95] alongside gamma rays being mostly emitted in a straight line, people lying on the ground will more likely have obstacles serving as radiation protection such as building walls, foundations, car engines, etc. between their bodies and the radiation emitted from both the fireball and the accompanying lower levels of radiation that continue to arrive at the ground for about 1 minute, during the mushroom cloud phase, which is termed "cloudshine".^[96] It would also give protection from the even smaller fraction of radiation that changes direction and is randomly reflected and scattered by the air/"skyshine".^[97] Approximately "One and one half inches"/37 mm of steel will reduce gamma dose by half.^[98] Its half-value thickness.

The effective gamma ray energy of the cloudshine is not especially high, 200 KeV.^[99]

Unlike the relatively low-yield, or low explosive energy "A-bombs" dropped on Hiroshima and Nagasaki, which did result in a sizable proportion of injuries from prompt radiation, higher yield "hydrogen bombs" (thermonuclear weapons) are not expected to result in very many such injuries – as the range at which the ionizing radiation from higher yield devices is of primary concern, is already well inside the hyper-lethal blast and flash burn areas.^{[100][36][40]}

Delayed nuclear radiation, "fallout"

Apart from the intrinsic "prompt effects" of nuclear detonations, that of thermal flash, blast and initial radiation releases, if any part of the fireball of the nuclear detonation contacts the ground, in what is known as a surface burst, another, comparatively slowly increasing, radiation hazard will also begin to form in the immediate area.^[101][102]

Putting aside the possibility of the detonation occurring during an already established heavy rain-storm, the formation of this life-threatening "delayed nuclear radiation" manifests only when the altitude, or "height of burst" of the explosion, is such that both the fireball and the buoyant updrafts it creates, sufficiently heats and lifts the soil that was below it into the core of the mushroom cloud. Once there, the very hot radioactive isotope products of the nuclear reactions that produced the explosion, begin to coalesce with the cooler and denser soil. Upon cooling, this mixture begins to locally fall-out or precipitate-out of the mushroom cloud, falling back to the surface of the earth, near to the point of detonation, over the next few minutes and hours.^[103]

While the duck and cover countermeasure, in its most basic form, offers a small to negligible protection against fallout, the technique assumes that after the effects of the blast and initial radiation subside, with the latter of which being no longer a threat after about "twenty seconds" to 1 minute post detonation,^[91][97] a person who ducks and covers will realize when it is wise to cease ducking and covering (after the blast and initial radiation danger has passed) and to then seek out a more sheltered area, like an established or improvised fallout shelter to protect themselves from the ensuing potential local fallout danger, as depicted in the film.

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 or fallout—may be looming, which then require movement to high ground and radiation protection, respectively.

However, if such a shelter is unavailable, the person should then be advised to follow the Shelter in Place protocol, or if given, emergency 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.^[104] Taking upper atmospheric winds into account, surface winds alone are not to be depended upon as indicative of the direction of fallout movement.^[105] "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.^[106][107]

In the era the advice was originally given, the most common nuclear weapons were weapons comparable to the US Fat Man and Soviet Joe-1 in yield. The most far-reaching dangers that initially come from the nuclear explosion of this, and higher, yield weapons as airbursts, are the initial flash/heat and blast effects and not from fallout. This is due to the fact that when nuclear weapons are detonated to maximize the range of building destruction, that is, maximize the range of surface blast damage, an airburst is the preferred nuclear fuzing height,^[108] as it exploits the mach stem phenomenon. This phenomenon of a blast wave occurs when the blast reaches the ground and is reflected. Below a certain reflection angle the reflected wave and the incident wave merge and form a reinforced horizontal wave; this is known as the 'Mach stem' (named after Ernst Mach) and is a form of constructive interference and consequently extends the range of high pressure.^{[109][110][111][112]} Air-burst fuzing also increases the range that people's skin will have a line-of-sight with the nuclear fireball. However, as a result of the high altitude of the explosion, most of the radioactive bomb debris is dispersed into the stratosphere, with a great column of air therefore placed between the vast majority of the bomb debris/fission reaction products and people on the ground for a number of crucial days before it falls out of the atmosphere in a comparatively dilute fashion. This "delayed fallout" is henceforth not an immediate concern to those near the blast. On the other hand, the only time that fallout is rapidly concentrated in a potentially lethal fashion in the local/regional area around the explosion is when the nuclear fireball makes contact with the ground surface, with an explosion that does so, being aptly termed a surface burst.^[113] For example, in the Operation Crossroads tests of 1946 on Bikini Atoll, using two explosive devices of the same design and yield, the first, Test Able (an air burst) had little local fallout, but the infamous Test Baker (a near surface shallow underwater burst) left the local test targets badly contaminated with radioactive fallout.

Widespread radioactive fallout itself was not recognized as a threat among the public at large before 1954, until the widely publicized story of the 15-megaton surface burst of the experimental test shot Castle Bravo on the Marshall Islands. The explosive yield of the Castle Bravo device the Shrimp was unexpectedly high and therefore correspondingly higher amounts of local fallout were produced. When this arrived at their location carried by the wind, this caused the 23 crew members on a Japanese fishing boat known as the Lucky Dragon to come down with acute radiation sickness with varying degrees of seriousness^[114] and due to complications in the treatment of the ship's radio operator months after the exposure, resulted in his death.

It is, however, unlikely that a well-funded belligerent with nuclear weapons would waste their weapons with fuzing to explode below or on the surface, as both test shot Baker and Castle Bravo were respectively. Instead, to maximize the range of city blast destruction and immediate death, an air burst is preferred,^[113] as the ≈500 meter explosion heights of the only nuclear weapons used on cities, Little Boy and Fat Man also attest to.^[115] Moreover, with air bursts the total amount of radiation contained in the fallout, in units of activity/becquerel, is somewhat less than the total that would be released from a surface or subsurface burst, as in comparison, depending on the height of burst, little to no neutron activation or neutron induced gamma activity of soil occurs from air bursts.^[116] Therefore, the initial danger from concentrated local/'early' fallout (which takes on the color of the soil around the fireball, commonly with a dusty pumice or ash-like appearance, as experienced by the crew of the Lucky Dragon) remains low in a global nuclear war scenario. Instead the fallout most likely to be encountered by most survivors in this scenario is expected to be the less dangerous but widely spread global/'late' fallout.^[117] An air burst at optimum height will produce a negligible amount of early fallout.^[118]

A notable comparison to underline this is found when one compares the 50 megaton air-burst 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, was implicated in the death of 1 of 23 crew on the Lucky Dragon and made the entire Bikini Atoll unfit for further nuclear testing until enough time elapsed and the intensity of the radiation field had decayed to acceptable levels.^{[lower-alpha 3]}

An Operation Doorstep mother and daughter mannequin pair in an improvised basement lean-to shelter prior to testing in Upshot-Knothole Annie. To shelter-in-place in such an area would offer, in a number of outdoor dose rates, an adequate fallout radiation protection factor (PF) or "dose reduction factor" of 20 or more.^[42] More effective basement spaces did/do exist however, of "10 million" homes assessed in 1968, 500,000 US basements were found to have a PF-40.^[97][120]

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 one's chance of absorbing a hazardous dose of radiation.^[121] 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 rate levels safe enough for an evacuation to be considered.^[122][123] 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.^[122]

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.^{[124][97][125][126]}

A protection factor of at least 40 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".^[42]

In units of rads, this is a simplistic model (Gaussian) of a wind-blown fallout map,^[64] which depicts the unshielded ground level fallout gamma ray dose and dose rate contours expected to follow a 2 megaton land surface burst detonation, with 1 megaton of the yield coming from fission reactions. Because of radioactive decay, the dose rate contours (on the right) contract after fallout has arrived, but the total absorbed dose contours (left) continue to grow. A similar color version of the right panel, dose rate contour plot, for a 0.1 to 10 kiloton surface burst is also available.^[127] Acutely dangerous regions of fallout are accompanied by fallout particles which are large enough to be detected by eye during its falling out/deposition, that is, they are equal to or larger than the size of dust.^[64][97] All of the ARS deaths following the Chernobyl accident were due to external beta burns. The beta particle to gamma energy ratio is usually greater than 3 in the timeframe of fallout with the greatest threat to life. Protection from physical contact with the dust therefore greatly reduces radiation exposure.^[128] The beta decaying uranium-237 and neptunium-239, that are generated from the neutron capture of U-235 and Pu-239 respectively, are regarded as the leading hazardous radioisotopes in the first hour-to-week period following nuclear fallout, with Np-239 dominating "the spectrum for several days".^[129][130]

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.^[104] An accurate rule of thumb, applicable in the time-period of days to a few weeks post-detonation which approximates the radioactive dose rate generated by the decay of the myriad of isotopes present in nuclear fallout, is the "7/10 rule".^[131][103] The rule states that for each 7-fold increase in time the dose rate drops by a factor of 10.^[132] For example, assuming the fallout process has ended 24 hours post detonation and the dose rate would be lethal if a few hours of exposure occurred, 50 roentgens per hour, then 7 days after detonation the dose rate will be 5 R/hr and 49 days after detonation (7×7 days) the dose rate will be 0.5 R/hr at which point no special precautions would need to be taken and venturing outside into that dose rate for an hour or two would pose a close to negligible health hazard,^[131][133] thus permitting an evacuation to be done with acceptable safety to a known contamination free zone. Following a surface-burst nuclear detonation, approximately 80 percent of the fallout would be deposited on the ground during the first 24 hours.^[43]

Some agencies that promoted "evacuate immediately" guidance as a response to potentially lethal fallout arriving, advice which may have been influenced by these agencies assuming simplistic single wind-driven cigar/Gaussian shaped fallout contours would be representative of reality, have since retracted this advice. This can actually result in higher radiation exposures as it would put people outdoors and in harm's way when the radiation levels would be highest. The Modeling and Analysis Coordination Working Group (MACWG) – which was set up to resolve conflicting advice given by various agencies, has reaffirmed that the best blanket advice that would reduce the number of casualties by the greatest amount is: "Early, adequate sheltering followed by informed, delayed evacuation."^[64]

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,^[42] and 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.^[42] Therefore, sheltering for the first few hours can save lives.^[134] Indeed, death and injury from local fallout is regarded by experts as the most preventable of all the effects of a nuclear detonation, being simply dependent 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.^{[135][136][137][138]} Or even higher if the remaining occupants of the city are made aware of the contaminated areas, by emergency systems, within hours of the event's aftermath.^[137][139] In 2009 to 2013 a further iteration on sheltering-in-place was made to determine the optimal improvised fallout-shelter-residence-times following a nuclear detonation, with computer analysis, and including a summary of prior studies and guidance. It was found that individuals should quickly get into the best intact building at least under 5 minutes distant in travel time following the detonation and they should stay there for at least 30 minutes before venturing out to find a shelter with a higher protection factor but that is a greater travel time away than 10 minutes.^{[140][141][142][143]} However, although this would be effective in cases where the initial building protection factor is less than about 10, it requires a high degree of individual situational awareness that may be optimistic to assume following the shock of a nuclear detonation. If a building with a PF of 20 or more is nearby, such as the fallout shelters depicted in the film, in the vast majority of fallout circumstances, it would not be advisable to leave it until 3+ hours have elapsed following the initial arrival of the local fallout.^[137][138]

Following a single IND (improvised nuclear device) detonation in the US, the National Atmospheric Release Advisory Center (NARAC) would, within minutes to at most hours, after the detonation have a reliable prediction of the fallout plume size and direction. When armed with this prediction they would then begin attempting to corroborate this with readings from radiation survey meter equipment that would fly over close to the ground in the affected area by means of helicopter or drone (UAV) aircraft on material intelligence gathering missions,^[144] which would also follow within tens of minutes to at most hours after the detonation.^{[lower-alpha 4]}

Once a general outline and direction of the fallout is determined, disseminating this information to citizens sheltering-in-place would soon follow, by means of loudspeaker, radio, cell phone etc., with a "Fallout App" containing maps for smart phones being regarded as an area of interest so that survivors don't inadvertently evacuate downwind further into harm's way.^[145][146] A number of questions the affected public are likely to have after a nuclear detonation have been compiled and pre-answered to help communications in the immediate aftermath.^[147][148]

Nuclear electromagnetic pulse, non-lethal

In respect to the other non-lethal weapon effects from an IND detonated on or near the surface, the detonation's blast wave would likely produce a momentary electric grid blackout due to the loss of a large portion of a city's electrical equipment drawing power/electrical load, while the electromagnetic pulse (EMP) from a surface/ground-burst explosion would cause little damage outside the blast area, so cell phone towers that survive the blast should be capable of carrying communications.^[145] But if communications during the 9/11 attacks or after a major hurricane are anything to go by and the cell phone network towers survive, the service would be overloaded (a mass call event) and thereby made useless soon after; however, if prior arrangements between the cell network and emergency responders are made to give them priority and bar access to all other individuals, then it may be an effective service.

The Civil Defense (CD) shelters, as depicted in the film, were stocked for such an eventuality. They contained among other things, at least one ruggedized CDV-715 radiation survey meter and one CD emergency radio receiver which would respectively be used to facilitate a safe delayed evacuation, regardless of outside help though if communications continued, the radio receiver was to inform them of the outside situation as it developed.