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Radiation and Children Section 3: A Correct Understanding of the Permitted Radiation Level

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Keywords: DNA, iodine 131, health, nuclear, children, radiation, reactive oxygen, thyroid, Toshiya Inaba, genes

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Section 2 addressed the possible effects on health of irradiation and the increased probability of cancer, which is particularly evident when the instantaneous value of exposure exceeds 100 millisieverts.

What is instantaneous value of exposure?

The instantaneous value of exposure (not a technical term, but one that is used here to facilitate understanding) recalls that instant when the doctor tells the patient to exhale, inhale and then takes the x-ray. This level of radiation is the instantaneous value of exposure. By contrast, if the total amount of exposure is the same, but occurs over a prolonged period of days, years, or over a number of times, this is referred to as the cumulative value of radiation.

As previously mentioned, living creatures are accustomed to DNA strand breakage caused by radiation, and once broken, the damage is apprehended and repaired in a matter of hours. Once exposed to radiation, DNA will sometimes break in several places, and with this increased activity, errors occur in replication which can lead to cancer in the future.

However, when irradiation takes place over a period of time, DNA does not break at once, but can manage to keep functioning. Errors are less likely to occur. In other words, even if the total dose of the radiation is the same, the one-time exposure (instant value) means a higher probability of cancer while the prolonged exposure (cumulative value) means that it is less likely. The difference is evident in animal experiments.

How do we know that 100 millisieverts (mSv) is dangerous?

We know this from the tragic experience of the atomic bomb. With the cooperation of atomic bomb victims, the Radiation Effects Research Foundation in both Hiroshima and Nagasaki has conducted research for over half a century, which still continues today. Their research indicates that exposure of 100mSv increases the incidence of cancer by a small percentage of less than 5%. Above this dosage, there is a greater increase in the number of people who will get cancer.

Does that mean that exposure below 100mSv will result a lower incidence of cancer? There is no answer to this important question. However, some specialists, including myself, think that cancer does not increase below this level. In particular, the cumulative value (prolonged exposure) of less than 100mSv is not considered likely to cause an increase in cancer. And even if the incidence increases slightly, the percentage is not very significant when the intake of other harmful substances in daily life is considered.

Cigarettes are most typical of these ?other harmful substances.? With regard to lung cancer, the likelihood of lung cancer among heavy smokers can be compared to high-level radiation exposure among those near ground zero in Hiroshima and Nagasaki. As mentioned in Section 2, this is comparable to someone being irradiated at such a high level of radiation that it adversely affects health. Of course, cigarette smoking is a voluntary act and cannot considered on par with exposure to radiation whether one likes it or not. Cigarettes are far more harmful than a small amount of radiation.

How to determine the ?permitted level?

Excuse the digression, but these days, we hear about the so-called ?permitted level? of radiation in tap water, vegetables, meat, and fish, everywhere, in the newspaper and on TV. Should we refrain if the amount of radiation is near the permitted level? Is half of the permitted level safe then?

In the first place, how is this permitted level determined? That is a difficult question, so let?s consider it in terms of the expiration date on food. If a certain perishable food begins to spoil in 24 hours, then how do you determine the period of time during which it can be consumed?

Of course, you would not set it at 24 hours. A small difference in ingredients and temperature can mean that the food will spoil before the 24-hour period ends, so this calculation does not allow for this consideration. Perishable food begins to spoil soon after it is prepared, so eating it right away may be optimal from the perspective of food hygiene but it is not practical. In other words, how much time do we have before it spoils?

In the case of the permitted level of radiation, an instantaneous value of 100mSv is the limit. (This would be equivalent to 24 hours above.) This does not give us much leeway. As mentioned in Section 1, we are exposed to a cumulative value of 0.01mSv each day in the course of our everyday activities, so this figure would be comparable to the perishable food just after it is made. Another standard we can use is the radiation exposure from x-rays (instantaneous value of 0.6mSv). These factors are all taken into consideration when deciding the permitted value.

Is the permitted level correct?

If the permitted value is too high or too broadly defined, safety cannot be ensured. If, however, it is set too low or too strictly defined, it is not very helpful either. Practically speaking, it will cause problems.

If the permitted value is too low, we will have to distribute bottled mineral water instead of drinking tap water, destroy large quantities of farm produce, and take other measures. As rumors spread, we will suffer widespread economic damage and other high social costs, which will become a financial burden to all citizens. Unless this cost to us produces real effects, it will just be a waste of an enormous amount of money that could be better used.

There are various opinions and views. As a pediatrician who has studied the effects of radiation on the human body, I don?t think that the current permitted level is too high, though it might be too low. It is comparable to setting expiration at two to three hours in the example of perishable food mentioned above. Consequently, we can consider ourselves safe if the radiation level is lower than the permitted level.

The original report was posted on the CRN Japanese site in April 2011.

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Toshiya Inaba
Professor and Vice Director of Hiroshima University Research Institute for Radiation Biology and Medicine, Doctor of Medicine.
Graduated from Tokyo University Medical School. After employment at Saitama Children’s Medical Center, St. Jude Children’s Research Hospital, Jichi Medical University School of Medicine, appointed Professor, Hiroshima University Research Institute for Radiation Biology and Medicine in 2001, and Vice-Director since 2009. Specializes in hematology (mechanism of the onset of leukemia, pediatric hematology), microbiology, and radiation biology.
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