National Institute of Radiological Sciences
as of April 22,2011
as of August 24,2011

Basic Facts- No.6
１．

The Japanese Government designated the “deliberate evacuation area” onApril 11. Will you explain what 20 mSv given as the reference level means?

The International Commission on Radiation Protection (ICRP) is an international academic
society of experts that makes recommendations about radiation protection. It has beenreported that the reference level is set, based on the ICRP’s recommendations and withadvice from the Nuclear Safety Commission of Japan.
The ICRP’s 2007 recommendations state that in emergencies radiation control standards
different from the ones for normal times should be used. Moreover, it is suggested thatemergency situations should be classified into two phases: an emergency phase and arehabilitation phase, to take the following protection measures as a guideline.
1) Normal time: the dose should be limited to 1 mSv or less in a year.
2) Emergency phase: the exposure dose attributable to an accident should not exceed
20-100 mSv.
3) Rehabilitation phase after the accident settles down: the dose should not exceed 1-20mSv in a year.
The

current

situationoftheFukushima

DaiichiNuclearPlant

appliestotheabove-mentioned 2) Emergency Phase.

The Japanese government‘s decision has adopted the lower limit of the range between 20
to 100 mSv for the emergency phase. This means, while responding to the situation tocontrol the total exposure dose of the residents in the plant’s surrounding area under 100mSv, it takes measures to lower the annual dose back to the level of 1 mSv per year in thefuture.

2. Some parts in the specified evacuation areas were observed to have a higher air
dose than areas closer to the plant.

Why did this happen?

As confirmed in the Chernobyl nuclear plant accident, it is not always the casethat contamination levels decline in a gradual linear way as the geographicaldistance gets farther from the accident site, and the levels in some spots arefound high.
If an emergency situation arises at a nuclear facility and radioactive materials
leak in the form of gas or in particles, they could travel together with the air likeclouds. This is called a radioactive plume. When a radioactive plume ispassing high up in the sky, the air dose rate temporarily rises in this spot. Also,
the plume may descend, being affected by geographical features such as a landdepressions, direction of the wind, rainfall and snowfall.
If radioactive
substances deposit on the ground surface, the air dose will rise. It is believedthat such phenomena cause some places far away from the plant to be observed
as having a high dose rate.

3. The air dose rate in Tokyo is declining, but it is still continuing to be
above normal. About how much milli Sv is the accumulated dose (for abouta month from March 14 to April 11) of a resident in Tokyo?
This section explains both the external radiation exposure and internal exposure,i.e. ingestion of radioactive substances in the body. The following calculated
values represent typical radiation doses, and the dose varies depending on
individual’ s behaviors and eating habits.
The dose of external radiation exposure from radioactive substances in the air isfound to be about 16 micro Sv for a month (assuming that the person stays
outdoors for 8 hours a day) since March 14, by adding up the data announced by
the MEXT and then subtracting the average dose at normal times.
Radiation exposure doses should take into consideration water, food andbreathing.
Suppose you drink 1.65 liter of tap water a day. The exposure from tap water
will be an estimated 10 micro Sv or thereabouts using the data from the Tokyo
Metropolitan Government (Please refer to Question 4).
Radiation exposure from food is far more difficult to estimate because it
significantly varies from person to person affected by his or her eating habits
and the actual amount of food intake. Suppose you drink milk containing
20Bq/kg iodine-131, 1 Bq/kg Cesium-137 and 1 Bq/kg Cesium-134, eat fish
likewise containing 2 Bq/kg, 1 Bq/kg, 1 Bq/kg, respectively, and also eat
vegetables containing 150, 10, 10 Bq/kg, respectively, every day for one month.
Your exposure dose is estimated to be about 69 micro Sv.

The exposure from inhaling radioactive materials in the air is estimated to be about 21
micro Sv based on the Tokyo Metropolitan Government’s data about radioactive
substances in the dust, in the case of breathing 22.2 cubic meter of air.
With all the above figures combined, the total is about 120 micro Sv for a month. This islower than the upper level* of radiation exposure in a round trip by airplane between Tokyoand New York and not hazardous to your health.

You, however, need to pay close

attention to announcements by the governmental offices and follow instructions andguidance when they are issued.
*Concerning calculation of radiation exposure dose associated in case of travel by
airplane, please refer to the JISCARD on the NIRS’s website.
JISCARD（ Japanese Internet System for Aviation Rout Doses）

4. How can I find my accumulated dose in the area where I live?
You can calculate it to a certain degree, though it is slightly complex.
First, you can find an amount of radiation received from radioactive substances in the air
by adding all the figures announced by the MEXT for a period from the day of the accident
to the present. The following is the calculation of the radiation dose in Tokyo mentioned inQuestion 3.
Example: Calculation of radiation exposure in reference to the announced air doses
The air dose was 0.0927 micro Sv/h on average according to the MEXT’s data (for 29 daysfrom March 14 - April 11)
0.0927 x 24 hours x 29 days ＝ 64.5 micro Sv ----- ①
The average air dose in Tokyo in normal times is 0.028-0.079 micro Sv/h, so the medianvalue 0.0535 is used as follows.
0.0535 × 24 hours × 29 days = 37.2 micro Sv ------ ②① － ② ＝ 27.3----- ③

If you go out for 8 hours and stay indoors for 16 hours, the reduction coefficient is:
1x 8/24＋ 0.4×16/24 ＝ 0.6---- ④
The dose of radiation received is:③ ｘ ④ = 27.3 x 0.6 = 16.38 ≒ 16 micro Sv.

Example: Calculation of exposure dose from radioactive substances occurring infood and water
The exposure dose from radioactive substances in water or food (including potential
exposure from radioactive substances ingested into the body) can be estimated by thefollowing formula.
Dose of radiation received (micro Sv) = Effective dose coefficient (the values given in thechart below) x radioactive concentration (Bq/kg) x food intake (kg)

Effective dose coefficients ※ （ micro Sv/Bq）

New-born baby
(3 months)
Infant(Age 1-2）
Child(Age 3-7）
AdultIodine-131
0.18

Cesium-137
0.020

Cesium-134
0.026

0.18
0.10
0.022

0.012
0.0096
0.013

0.016
0.013
0.019

※ (Oral ingestion; NIRS compiled the data based on “ ICRP Database of Dose
Coefficients: Workers and Members of the public, CD-ROM, 1998”)
Suppose 1kg of water contains 8.59Bq iodine-131, 0.45Bq Cesium-137 and 0.28Bq
Cesium-134 ※ .

If an adult drinks 1.65 liter of water a day for 29 days, the radiation

exposure doses will be:
Iodine-131： 0.022 x 8.59 x 1.65 x 29 = 9.0 micro Sv.---①
Cesium-137： 0.013 x 0.45 x 1.65 x 29 = 0.28 micro Sv----②
Cesium-134： 0.019 x 0.28 x 1.65 x 29 = 0.25 micro Sv----③
Exposure dose ＝ ① ＋ ② ＋ ③ ＝ 9.53＝ about 10 micro Sv.
※ The average of the data announced by the Tokyo Metropolitan Government from March
18 to April 11.
The estimate given above in (3) uses values close to the average of the results of radiation
tests on distributed foods (11 samples of milk, 241 samples of vegetables and 5 samplesof fish) presented by the Ministry of Health from March 19 to April 11.

When a total ofCesium isotopes 134 and 137 combined is given, the ratio of the two substances isassumed to be 1 to 1.

Example: Calculation of dose of radiation received from radioactive substances
existing in the air
To estimate radiation dose from inhaling radioactive substances in the air, you need aconcentration of radioactive substances in the air. Such data, however, are rarelyannounced.

In question (3) mentioned above, the data presented by the Bureau of Laborand Industrial Affairs (http://www.sangyo-rodo.metro.tokyo.jp/) are used to calculate as
follows.
Dose of radiation received (micro Sv) = Effective dose coefficient (the values in the chartbelow) x radioactive concentration (Bq/m 3 ) x respiratory coefficient (22.2m 3 a day here) x
number of days

Iodine-131

Iodine-132

Cesium-137

Cesium-134

New-born baby
(3 months)
Infant(Age 1-2）
Child(Age 3-7）
Adult

0.072

0.0011

0.11

0.070

0.072
0.037
0.00740.000960.000450.000094

0.010
0.070
0.039

0.063
0.041
0.021

※ (Oral ingestion of particles (Type F); NIRS compiled the data based on “ ICRP
Database of Dose Coefficients: Workers and Members of the public, CD-ROM,1998”)
The highest levels of radioactive concentration in the dust in the air in Tokyo wererecorded from 10:00 to 11:00 on March 15; the levels of Iodine-131, Iodine-132,Cesium-137 and Cesium-134 for this one hour were 241, 281, 60 and 64 Bq/m 3 ,respectively.

It is said that the adult inhales an average of 22.2m 3 of air in a day. Thus,

the amount of radiation you would potentially receive in the future from the radioactive
substances taken in this one hour would be roughly estimated as follows.
Iodine-131:

0.0074 x 241 x 22.2 x 1/24 = 1.05 micro Sv. ---①

Iodine-132:

0.000094 x 281 x 22.2 x 1/24 = 0.0244 micro Sv. ----②

Cesium-137:

0.039 x 60 x 22.2 x 1/24 = 2.16 micro Sv. ----➂

Cesium-134: 0.021 x 64 x 22.2 x 1/24 = 1.24 micro Sv. ---- ➃Radiation dose in this hour = ① +② +➂ +➃= 5.07 micro Sv.
Repeat this calculation for all the hours and add them up. The above question (3) refers to
the total of all values recorded from March 14 to April 11. Iodine vapor is not included.
With respect to children’s respiratory coefficients per day, the ICRP suggests (inpublication 71) 2.86 m 3 /day for a new born baby (3 months), 5.16 m 3 /day for an infant (age1), 8.72 m 3 /day for a child (age 5), 15.3 m 3 /day for a child (age 10) and 20.1 m 3 for a child(age 15).
You can estimate your accumulated dose, to some extent, by combining theradiationdose by external exposure to your bodies as a result of the accident, the dose frombreathing in air containing radioactive substances and the radiation dose from the intake
of food or water containing radioactive substances.