(8) A visitor from abroad asks, "Is there still radiation from the bomb?"

The radiation level returned to normal one year after the bombing

61 years have passed since the atomic bomb was dropped on Hiroshima, which now has a population of 1,150,000. But do people still wonder about radiation in the city? I'll try to determine the facts.

First, I went to the A-bomb Dome with a device that measures the amount of radiation. It recorded 0.08 micro sievert (Sv) per hour.

This device measured an amount of 0.08 micro Sv of radiation per hour

But shouldn't it be zero? I was surprised.

Masaharu Hoshi, Professor of the Research Institute for Radiation Biology and Medicine at Hiroshima University, explained that my result measured the natural radiation on earth and from space. And so, "In Japan, we have the same amount of radiation everywhere."

If there is more than 500 mSv (500,000 micro sievert), the lymph nodes and white blood cells are then affected so the amount I measured is only about 1/10,000,000 of that dangerous level.

After the bombing, the rumor that "Nothing will grow for 70 or 75 years" spread quickly and widely. This false idea provoked great fear of radiation and, even today, many young people are probably familiar with that old rumor.

According to Professor Satoru Ubuki of Hiroshima Jogakuin University, this idea appeared in an interview with Dr. Harold Jacobsen who worked on the Manhattan Project. This interview was published in The Washington Post on August 8, 1945, just two days after the atomic bomb was dropped on Hiroshima.

The headline of the interview reads, "Area Struck by Atomic Bomb Is Saturated with Death for 70 years, Scientist Declares". And in the text itself, Dr. Jacobson remarks, "Tests have shown that the radiation in an area exposed to the force of an atomic bomb will not dissipate for approximately 70 years."

However, the American government immediately refuted this statement. Dr. Robert Oppenheimer, who led the Manhattan Project, stated, "There is no evidence to show measurable radiation remains on the ground in Hiroshima." Professor Ubuki thinks this assertion was meant to counter Dr. Jacobsen's remark and avert "criticism on humanitarian grounds".

What, then, are the facts regarding radiation?

"Radioactivity" refers to the power to produce "radiation". We can classify radiation from the atomic bomb into three types: (1) "initial radiation" which is produced when the bomb explodes; (2) "leading radiation" which makes the ground and buildings radioactive through neutron radiation; and (3) "fallout" which is radioactive dust.

The most harmful of these is "initial radiation". However, "leading radiation" and "fallout" are "radioactive residuals" and it isn't known how long they remained after the bombing.

Professor Kiyoshi Shizuma of the Faculty of Engineering at Hiroshima University comments, "The area around the hypocenter was affected by radiation for at least a year." Leading radiation consisted of about 20 types and its impact was strongest for 10--20 hours, with the effect continuing for about 100 hours. "After about a year," Professor Shizuma says, "the radiation level would recover to the same level prior to the bombing."

At the same time, some people who entered the city after the bombing-but when the radiation had supposedly decreased-still suffered serious harm to their health, such as loss of hair and bleeding gums. This point cannot clearly be explained.

Today, though, it is certain that the city of Hiroshima is no longer affected by radiation from the bombing. (Takashi Kenda, staff writer)