REVIEW

Nuclear Disaster after the Earthquake and Tsunami of March 11
Naoyuki Shigematsu, Junichi Fukada, Toshio Ohashi, Osamu Kawaguchi and Tetsuya Kawata

Department of Radiology, School of Medicine, Keio University, Tokyo, Japan(Received for publication on September 17, 2011)(Revised for publication on December 10, 2011)(Accepted for publication on January 10, 2012)

We would like to explain the effects of radiation on human health and discuss the actual effects ofthe contamination with radioactive material present in Tokyo. Currently, external exposure doses arewithin the allowable range in Tokyo and will have no adverse health effects on adults or children. As
for internal exposure doses, there will likely be no problems as regards our ordinary dietary intakes.
However, hot spots of Cs-134, Sr-90 and others should be monitored further. (Keio J Med 61 (1) : 28–34,
March 2012)

Keywords: nuclear disaster, earthquake, tsunami, Japan

Introduction

measure of how much the radiation emitted from a radioactive material ionizes the air. The SI unit for exposure
dose is coulomb per kilogram of air (C/kg). The absorbed
dose is the amount of energy absorbed by an object fromionizing radiation. The absorbed dose unit is the Gray
(Gy), and 1 Gy is equivalent to 1 Joule of absorbed energy per kg material (J/kg). The effective dose reflects thetotal overall effect on various tissues and organs throughout the human body, and the SI unit is the sievert (Sv).
Commonly reported units are Bq and Sv in relation to
the effects of the Fukushima nuclear disaster. The effective dose coefficient is a conversion factor1 that is used
to determine the biological effects of radiation based on
the amount of radioactivity taken up by the human body.
For example, assume that we eat 100 g of spinach every
day for one year and that 10,000 Bq of I-131 is detected
in 1 kg of the spinach. The exposure dose is 1000 Bq per
100 g of spinach and the effective dose coefficient is 2.2
× 10−8 Sv/Bq. Thus, the following calculation can be carried out: 1000 × 2.2 × 10−8 × 365 = 0.00803 Sv =8.03 mSv,
and approximately 8 mSv of I-131 will be taken up in the
body in a year through consumption of such spinach. As
I discuss later, however, the effective dose coefficient is
known to vary depending on age, as well as radiation
quality, and to be approximately 10 times higher in children than in adults.1

First, we offer our deep condolences to the victims of
the Great East Japan Earthquake of March 11, 2011. It
is our hope that all afflicted areas will steadily recoverfrom this devastating event. As for the health effects of
the Fukushima nuclear disaster, many people are now
giving their opinions from a variety of viewpoints. Media
reports may sometimes be exaggerated, thereby arousing excessive anxiety in the Japanese public. Herein, we
would like to explain the effects of radiation on humanhealth and discuss the actual effects of the contamination
with radioactive material present in Tokyo.
Basic Issues
Units used for measuring radiation
There are four aspects to consider when measuring radiation: radioactivity, exposure dose, absorbed dose andeffective dose (Fig. 1). Radioactivity is the spontaneous emission of ionizing radiation as a consequence ofa nuclear reaction or directly from the breakdown of an
unstable nucleus. The standard unit of radioactivity measurement is the becquerel (Bq), and 1 Bq is equivalent to
the disintegration of one nucleus per second. Radiation iscapable of ionizing substances, and the exposure dose is a

Reprint requests to: Naoyuki Shigematsu, MD, Department of Radiology, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku,
Tokyo 160-8582, Japan, E-mail: shige@rad.med.keio.ac.jp
Copyright © 2012 by The Keio Journal of Medicine
28Keio J Med 2012; 61 (1): 28–34

29Fig. 2 Radiation types.
Fig. 1 Radiation units.

Fig. 3 Meaning of the half-life of radioactive materials.
Half-life refers to the amount of time needed for the radioactive emission of a radionuclide to decrease to half its original level. I-131 has a half-life of 8 days. Thus, emissions are reduced to 1/16 of
the original level 32 days (one month) later. Meanwhile, because Cs-137 has a half-life of 30 years,
emissions are reduced only to 1/4 even after 60 years have elapsed.

Types of radiation
I-131, which is a major radionuclide of current concern
in relation to the Fukushima nuclear disaster, emits beta
particles. Cesium, another radionuclide of concern, emits
strong gamma rays; these gamma rays are generated by
the production of Ba-137m via the decay of Cs-137 and by
the production of Ba-134 via the decay of Cs-134. Strontium-90, which was recently detected in Fukushima city,although at a low level, emits beta particles. To date, fortunately, neither neutrons created by the decay of U-235,
the essential component of nuclear fuel in the Fukushima
nuclear power plants, nor alpha particles generated by the
production of Am-241 via the decay of Pu-241, found inradioactive wastes from nuclear power plants, have been
detected. Beta particles are essentially electrons and they
travel from several millimeters to several centimeters
within the human body. In contrast, gamma rays are photons (electromagnetic waves), similar to x-rays, and therange of gamma rays in the body is in terms of meters.
The biological effects of both beta particles and gamma
rays can be determined employing a similar calculation.
When beta particles (such as those from I-131) travelfrom several millimeters to several centimeters and aretaken up in the body, internal exposure becomes an issue
in terms of health hazards. Meanwhile, because gamma
rays (such as those from Cs-137) have a range of meters,
not only internal but also external exposure poses a threat
to human health (Fig. 2). Radionuclides deposited internally through respiration and food, i.e., internal exposure,are of concern in regions far from Fukushima groundzero. For areas surrounding the crippled nuclear power
plants, not only internal but also external exposure is amajor issue.
Half-life
Physical half-life refers to the amount of time neededShigematsu N, et al: Nuclear Disaster in Japan, 201130

Table 1 Radionuclides have physical and biological half-lives

131I
137Cs
90Sr

Physical half-life

Biological half-life

8 days
30 years
29 years

120 days
70 days
49 years

Biological half-life: how long the radionuclide is retained in the body.

for radioactivity to fall to half its original level. For example, the half-life of I-131 is 8 days, i.e., the amountof I-131 is reduced to 1/2 after 8 days have elapsed andto 1/4 after 16 days (Fig. 3). It has almost disappeared(1/16 of its original level) approximately one month later.
In contrast, the physical half-life of Cs-137 is as long as
30 years. This means that it takes 60 years to reduce theamount to 1/4 of the original level. For Sr-90, the physicalhalf-life is 29 years. Radionuclides also have a biologicalhalf-life, which refers to the elimination of internal radioactivity via urine, feces and perspiration. I-131 is retainedin the thyroid for a long time, and its biological half-life
is 120 days. Cs-137 is widely distributed in the body witha biological half-life of approximately 70 days. Sr-90 istaken up in bone, and its biological half-life is very longat 49 years. The radionuclides of concern after the present nuclear disaster are I-131 and Cs-137. I-131 has a shortphysical half-life, although it is retained in the thyroid fora long time. Meanwhile, although Cs-137 is widely distributed in the body and its physical half-life is long, it iseliminated in a relatively short period, with a biologicalhalf-life of approximately 70 days. Sr-90 is taken up inbone and is hazardous because of its very long physicaland biological half-lives, although it has been minimallydetected, to date, since the Fukushima nuclear disaster(Table 1).
Radiation effects on DNA
The biological effects of radiation start with DNA damage (breaks in DNA strands). Humans have 46 chromosomes, which are essentially made of double-stranded
DNA helices (Fig. 4). When radiation is absorbed by
the body, it results in ionization, thereby generating ahighly energetic electron. This electron may directly
damage DNA itself, which is referred to as a direct action, or it may generate hydroxyl radicals (active oxygen)
via water molecules and thereby damage DNA, which isreferred to as indirect action (Fig. 5).2 When one DNA
strand is broken, it is called single-stranded DNA damage; when two DNA strands are broken, it is called double-stranded DNA damage. When DNA is separated bydouble-stranded damage at the same position, it is calleda double-stranded DNA break (Fig. 6). DNA breaks canresult in the binding of different another different sites,

Fig. 4 Chromosome structure.
Humans have 46 chromosomes, made up essentially of double-stranded DNA helices.

Keio J Med 2012; 61 (1): 28–34

e

31

Direct Action-

PHOTON

OH .

e

-

H2O

Indirect Action

Fig. 5 Mechanism of DNA damage by radiation.
When radiation is absorbed by the body, it produces ionization,
thereby generating an energetic electron. This electron can directly damage DNA itself (direct action) or it can generate hydroxyl
radicals via water molecules, which then damage DNA (indirect
action).

thereby forming abnormal DNA (chromosomes), which
in turn causes cell death. If many cells die, tissue death
results, ultimately leading to somatic death. Early radiation effects are associated with the above DNA damage
processes. DNA damage is often repaired when it is limited to one of the two strands (a single-stranded break) or
two broken strands that are far from each other. However,
if DNA repair is not achieved (or DNA is miss-repaired),secondary carcinogenesis or genetic effects of radiation
can occur. Radiation effects in humans fall into two categories: effects on individuals exposed to radiation (somatic effects) and effects on offspring of exposed individuals who were not themselves exposed (hereditaryeffects). Somatic effects are further divided into early andchronic effects: early effects manifest as early complications (e.g., burns, immunosuppression, gastrointestinal
ulcers) within a few months after exposure, and chronic
effects manifest as delayed complications (e.g., cancer,
cataracts) from a few months onwards. Hereditary effects and carcinogenic effects resulting from minor DNA
damage are called stochastic effects, and others are called
deterministic effects. Stochastic (carcinogenic and hereditary) effects sometimes manifest spontaneously andcan also be accelerated with small amounts of radiation.
Meanwhile, deterministic effects have a certain exposure
dose (threshold) that leads to cell death (Fig. 7).2

Fig. 6 Radiation-induced DNA damage.
When one DNA strand is broken, it is called single-stranded
DNA damage; when two DNA strands are broken, it is called
double-stranded DNA damage. When DNA is separated by double-stranded damage at the same position, it is called a doublestranded DNA break.

Dose limit
According to the International Commission on Radiation Protection (ICRP) recommendation, the maximum
allowable radiation dose for the general public is 1 mSv
per year. For medical professionals, the maximum dose is
100 mSv/5 years (which is obtained by an average of 20
mSv for 5 years) or a maximum 50 mSv per year. Thus,
a dose of 20 mSv/year is considered to be the dose limit.3
This level was also considered to be allowable for thegeneral public at the time of the emergency after the Fukushima disaster; however, this level remains controversial. Exposure to this level raises the question of whether
it carries a risk of secondary carcinogenesis. Data fromindividuals exposed at Hiroshima and Nagasaki showedcancer incidences to have increased at 5–10 years after
exposure for leukemia, at 10–15 years for thyroid cancer,
at 15–20 years for breast and lung cancer, and from 20
years onwards for gastrointestinal cancer.4 The level of 1
Sv is a significant risk factor for increased cancer occurrence, and raises cancer occurrence by 50%. Given thatcancer occurrence is associated with stochastic effects ofradiation, when individuals are exposed to 100 mSv, 1/10
of 1 Sv, for a year, cancer occurrence is theoretically predicted to increase by 5% (Fig. 8).4 However, after actual
exposure to this level of radiation, it is still controversial
whether the incidences of cancers, including leukemia,
will increase (Fig. 9).5 This may be explained by the involvement of various other confounding factors. A survey conducted by the National Cancer Center revealed
the relative risk for cancer occurrence to be 1.05 for exposure to 100 mSv, 1.6 for smoking, 1.4 for excessive alcohol consumption, and 1.2–1.3 for leanness or obesity

Shigematsu N, et al: Nuclear Disaster in Japan, 201132

Fig. 7 Classification of radiation effects.
Carcinogenic effects resulting from minor DNA damage and hereditary effects of radiation are called stochastic effects, and others are called deterministic effects. Stochastic (carcinogenic andhereditary) effects sometimes manifest spontaneously and canalso be accelerated as a result of even a very small amount of radiation. Meanwhile, deterministic effects have a certain exposure
dose (threshold) that leads to cell death.

(Table 2). This reveals that the relative risk for developing cancer is far higher for smoking than for this level ofradiation exposure. Thus, real radiation effects are likely
to be masked by these confounding factors. Taking theconfounding factors into account may also raise the possibility that exposure to 100 mSv/year is not significantly
hazardous to human health.6
Actual Radiation Exposure
External exposure in Tokyo
A level of 0.8 μSv/h was detected in Tokyo on March
15. This was calculated to be 16 times the usual level(around 0.05 μSv/h). In addition, approximately 81,000
Bq/m2 of I-131 fallout was observed in Tokyo from 21
through 23 March. Thus, the entire Tokyo metropolitan
area should have been a radiation-controlled area forthe aforementioned week. However, we are constantly
exposed to environmental radiation from space and theground and are sometimes exposed to medical radiation in our daily lives. Japanese people are exposed to alevel of around 3.75 mSv (3750 μSv) per year. Assumingthat we continue to be exposed to the present level of 0.8
μSv/h for a year, the exposure dose will be 7 mSv in ayear. This level is two or three times higher than the actual annual exposure level of the general public, although
it is 16 times higher than that of environmental radiation.
Moreover, even if we are exposed to a level of 7 mSv ina year, no symptoms manifest nor are any abnormalities
detected by examinations. (This may be explained by thefact that the dose limit is 20 mSv/year even for the generalpublic, as stated previously.) In addition, radiation monitoring conducted on March 17 showed the level to havefallen to 0.05 μSv/h. I-131 fallout also rapidly fell and waszero in June and thereafter. Given the half-life (8 days) of
I-131, the level of I-131 is speculated to have fallen to an
acceptable level in April and thereafter. In total, approximately 7000 Bq/m2 of Cs-137 fallout was observed in
March, but this radionuclide was undetectable from May
15 onwards. Given its half-life of 30 years, the effect of
cesium is considered to persist (Fig. 10). In fact, however,no human health hazards have been reported as a resultof nuclear weapons testing in the latter half of the 20th
century, despite several thousand Bq/m2 of Cs-137 fallout
being detected in the atmosphere.
Exposure in Tokyo
The radionuclides raising concern after the Fukushima disaster are I-131 and Cs-137. At first, I-131 was the
major problem to be dealt with because it is depositedin the thyroid gland. According to news releases, I-131
exceeded the standard level, as indicated by cow’s milk
(1000 Bq/kg), vegetables including spinach (10,000 Bq/kg) and drinking water at Kanamachi water purification
plant (210 Bq/kg). Using the effective dose coefficient,the following exposure doses of I-131 are obtained whena person eats 100 g of spinach or drinks 500 mL of cow’s
milk or 2000 mL of tap water every day: 8 mSv/year for
spinach, 4 mSv/year for cow’s milk and 3.4 mSv/year for
tap water. These are below the allowable level of 20 mSv/
year. Considering that I-131 fallout has not been detected
since June and that tap water at the treatment plant had alevel below the standard the next day, it can be concluded
that the above levels are not a matter of concern for thegeneral public, including children.
Cs-137 has a long half-life and was also detected in tea
leaves and beef soon after the Fukushima disaster. Subsequently, cesium levels were monitored in various foods,
and the radioactivity of this radionuclide has become acommon topic of discussion. In a September 2011 news
release on cesium, the level was 30–50 Bq/kg for rice,
about 100 Bq/kg for peaches and grapes, and 1000–2000
Bq/kg for beef, fish and mushrooms. These levels havebecome an issue. However, assuming that we eat 2 measuring cups (about 360 mL) of rice every day, 500 g ofbeef twice a week, and 150 g of sashimi (raw fish) threetimes a week, we can calculate internal exposure doses
using the effective dose coefficient. The following doses
are thereby obtained: 0.05 mSv/year for rice, 0.5 mSv/
year for beef, and 0.2 mSv/year for sashimi. These levels
do not exceed the annual dose limit of 20 mSv and therefore pose no threat to human health.

Keio J Med 2012; 61 (1): 28–34

33

Fig. 8 Exposure doses and their associated carcinogenesis risks in Hiroshima and Nagasaki.
A level of 1 Sv represents a significant cancer risk factor, and raises cancer occurrence by 50%.
Given that cancer occurrence is associated with stochastic effects of radiation, when individuals
are exposed to 100 mSv, 1/10 of 1 Sv, over a year, cancer occurrence is theoretically predicted toincrease by 5%. (Reproduced from Fig.1, ref 4)

Table 2 Relative risk of cancer occurrence for each factor
FactorSmoking
Excessive alcohol consumption
Leanness (BMI < 19)
Obesity (BMI > 30)
High salt intake
Insufficient intake of vegetables
Radiation exposure (100 mSv)

Fig. 9 Exposure doses and their associated carcinogenesis risks
in Hiroshima and Nagasaki.5
Real exposure to 100 mSv did not significantly increase the incidences of leukemia and other types of cancer. Thus, levels exceeding 100 mSv are considered to be hazardous to human health.

Relative risk
1.6
1.4
1.29
1.22
1.13
1.06
1.05

Above all, we imagine that pregnant women and mothers of young children worry about radiation effects on human health after a nuclear disaster. It is true that fetuses
and infants are highly sensitive to radiation; however, we
do not have enough data to determine the radiation effects for them. This aspect of the effects of radiation isclearly a topic for further discussion.

Shigematsu N, et al: Nuclear Disaster in Japan, 201134

Fig. 10 Radiation doses of fallout (e.g., in rain) in the Shinjuku area of Tokyo.
Considerable amounts of radioactive materials poured into the atmosphere fromMarch 20 through approximately May 20, but have been undetectable since June.

Summary

Acknowledgments

At present, external exposure doses are within the allowable range and will have no adverse health effects on
the population of Tokyo, including children. As for internal exposure doses, the authorities have halted shipmentsof affected produce. Thus, there are no problems as regards our ordinary dietary intakes. However, hot spots of
Cs-134, Sr-90 and others, which have recently become anissue of concern, need to be further monitored.
Conclusion
Finally, we would like to express our deep admiration
for the courageous workers at the Fukushima nuclear
power plants and also to express our sincere condolences
to people in the evacuation zone. We had hoped to be able
to present an analysis of what is going on in the areas surrounding the Fukushima nuclear power plants, but thathas not proved possible. We believe that Tokyo, as a partof the Kanto district, is recovering and that people arethus living normal lives again. Our wish is that all Japanese people will act together to support reconstruction
after the Great East Japan Earthquake.

The authors gratefully acknowledge Mr. Takuo Takihana for his support and assistance during this study.
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