Radiation Measurement by Tokyo Metropolitan Government

Measurement of ambient radiation levels by the Tokyo Metropolitan Government was conducted within Inagi City. The measurement results announced by Tokyo Metropolitan Government are as follows.

Measurement Results

Measurement Location

Central Park General Ground

Measurement Date

June 15, 2011

Height (cm)	Dose Rate μSv/h (micro sieverts/hour)	Dose Rate Annual Equivalent Value mSv/y (millisieverts/year) (Note)
5	0.05	0.26
100	0.05	0.26

• Note: Based on the measured values, the estimated annual equivalent value was below 1 mSv per year, which is the indicator for normal radiation exposure for the general public recommended by the International Commission on Radiological Protection (ICRP), excluding natural radiation levels.
• Note: The conversion values include natural radiation levels, etc.

Measurement Overview

Measurement Date

June 15, 2011 (Wednesday)

Locations

Central Park General Ground (Surface Soil)

Measurement Height

Height of 5 centimeters and 100 centimeters from the ground surface

Measurer

Tokyo Metropolitan Government Employee (Health and Safety Research Center)

Measuring Equipment

Scintillation Survey Meter Hitachi Aloka Medical TCS166

Measurement Items

Ambient radiation level "0.05μGy/h (microgray/hour)"
Note: 1 gray of radiation in the atmosphere can be converted to 1 sievert.

Tokyo Metropolitan Institute of Public Health Website

• Related to the Great East Japan Earthquake Reporting (Includes related tests conducted by the Health and Safety Research Center) (External Link)

Estimation of Annual Equivalent Dose of Ambient Radiation

Regarding the estimation of the annual equivalent dose of ambient radiation in the city, a calculation was made under the condition that "the measurement results of ambient radiation in the city by the Tokyo Metropolitan Government (0.05μSv/h: June 15, 2011)" continued for one year.

Annual Conversion Value Estimation Method (This is a simple calculation)

Annual conversion value mSv/y = (Measured value - Natural radiation level) × (8/24【Time spent outdoors】＋16/24【Time spent indoors】×0.4【Radiation reduction rate indoors】) × 365 days ÷ 1000【Conversion to millisieverts】

⇒0.05μSv/h × (8/24 + 16/24 × 0.4) × 365 days ÷ 1000 = 0.2628 mSv/y

• Note: This conversion value includes natural radiation levels.
  [Reference] The average natural radiation level in the country is said to be 0.05 μSv/h
• Note: Conditions
  • Assuming 8 hours outdoors and 16 hours inside a wooden house
  • Reduction effect (0.4) during 16 hours of stay inside a wooden house
• Note: The estimation assumes that the same values continued for one year from June 15, when measurements were taken within the city.

Glossary

• Sv (sievert): A unit representing the biological effects of radiation
• mSv (millisievert) is a unit of one-thousandth of a sievert
• μSv (microsievert): A unit equal to 1/1,000,000 of a sievert (1/1,000 of a millisievert)

ICRP (International Commission on Radiological Protection) standards

• Emergency: 20 to 100 mSv per year
• Recovery period after an emergency accident: 1 to 20 mSv per year
• Normal times: less than 1mSv per year

Note: The above ICRP standards do not include radiation doses received from natural background radiation or medical procedures.