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 about a 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 is found 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 and breathing.

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 5).

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 is lower than the upper level* of radiation exposure in a round trip by airplane between Tokyo and New York and not hazardous to your health. You, however, need to pay close attention to announcements by the governmental offices and follow instructions and guidance 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)