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 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. 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. 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. 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 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". 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, applicable in the