NHK has learned details of an incident in which two Japan Coast Guard vessels were exposed to nuclear fallout from a US hydrogen bomb test in 1958. One crewmember died of leukemia the following year.

The incident followed the US atomic bombings of Hiroshima and Nagasaki in 1945 and a Japanese fishing boat's exposure to radiation from a US hydrogen bomb test at Bikini Atoll in the Pacific Ocean in 1954.

NHK has found that a report written by US military doctors said some crewmembers of the Japanese coast guard ships suffered significant declines in numbers of white blood cells.

In July 1958, the vessels Takuyo and Satsuma were sailing in the Pacific when they were exposed to nuclear fallout from the test at Bikini Atoll.

The next month, a panel of Japan's then health ministry said there was no indication of radiation damage.

In August 1959, Nagano Hirokichi, chief engineer on the Takuyo, died of acute myeloid leukemia.

The panel later concluded that the amount of radiation he was exposed to was so minimal that it was difficult to connect it to his death.

NHK worked with Dr. Saito Osamu, an expert on radiation medicine, to analyze the report by US military doctors who examined the crewmembers soon after the incident.

The report said blood tests were conducted on 24 of the 113 crew members 12 days after the exposure.

The results showed that two-thirds, or 16, of them had suffered declines in white blood cell counts, with some of their conditions considered severe.

The report also said serious abnormalities in ratios of white blood cell components were observed in half, or 12, of them.

The report noted that these conditions could be associated with radiation damage caused by exposure to 500 millisieverts or more.

But it concluded that there were no health effects, saying the estimated exposure on the vessels was low.

Saito called it problematic that the report failed to acknowledge the effects of radiation.