The official U.S. position had been that the growth in the strength of atomic bombs was not accompanied by an equivalent growth in radiation released. Japanese scientists who had collected data from the fishing vessel disagreed with this. Sir Joseph Rotblat, working at St Bartholomew's Hospital, London, demonstrated that the contamination caused by the fallout from the test was far greater than that stated officially. Rotblat was able to deduce that the bomb had three stages and showed that the fission phase at the end of the explosion increased the amount of radioactivity a thousandfold. Rotblat's paper was taken up by the media, and the outcry in Japan reached such a level that diplomatic relations became strained and the incident was even dubbed by some as "a second Hiroshima".
The sky in the west lit up like a sunrise. Eight minutes later the sound of the explosion arrived, with fallout several hours later. The fallout, fine white flaky dust of calcinated Bikini Island coral, had absorbed highly radioactive fission products, and fell on the ship for three hours. The fishermen scooped it into bags with their bare hands. With one fisherman, Matashichi Oishi, reporting that he "took a lick" of the dust that fell on his ship, describing it as gritty but with no taste. The dust stuck to surfaces, bodies and hair; after the radiation sickness symptoms appeared, the fishermen called it shi no hai (死の灰, death ash).
The US government refused to disclose the fallout's composition due to "national security", as the isotopic ratios, namely a percentage of uranium-237, could, through a radio-chemical analysis of the fallout, reveal the nature of the device to the Soviet Union, which had, as of 1954 not been successful with thermonuclear staging. Lewis Strauss, the head of the Atomic Energy Commission (AEC), issued a series of denials; he also hypothesized that the lesions on the fishermen's bodies were not caused by radiation but by the chemical action of the caustic burnt lime that is produced when coral is calcined, that they were inside the danger zone (while they were 40 miles away), and told President Eisenhower's press secretary that the Lucky Dragon #5 may have been a "Red spy outfit", commanded by a Soviet agent intentionally exposing the ship's crew and catch to embarrass the USA and gain intelligence on the tests device. He also denied the extent of the claimed contamination of the fish caught by Daigo Fukuryu Maru and other ships. The FDA however later imposed rigid restrictions on tuna imports. The United States dispatched two medical scientists to Japan to study the effects of fallout on the ship's crew and to assist their doctors.
In 1985, North Korea released Pulgasari, a kaiju movie similar to Godzilla. To make the film, North Korea used kidnapped South Korean director Shin Sang-ok. The special effects department from Toho was hired to produce the special effects. Kenpachiro Satsuma, the stunt performer who played Godzilla from 1984 to 1995, portrayed Pulgasari.