Dr. Tsubokura's Radiation Lecture Vol.135

Author: Masaharu Tsubokura PhD.

Editors: Akihiko Ozaki M.D.,PhD., Yuki Senoo

269.. Challenges reflected in a health survey of workers

Occupational exposure refers to exposure to ionizing radiation during the performance of one’s duties. Occupational groups that are routinely exposed to radiation include coal miners, who usually work in high-natural-radiation areas, airline crews, workers at nuclear power plants, physicians, nurses, and radiology technicians.

Naturally, the radiation exposure of the workers at the Fukushima Daiichi Nuclear Power Plant, who were required to engage in radiation work immediately after the 2011 accident, was also considered to be occupational exposure, regardless of its urgency.

According to a report that the United Nations published in 2013, the average exposure dose over a period of 19 months after the Fukushima nuclear accident among workers at the Fukushima Daiichi Nuclear Power Plant was approximately 10 millisieverts. Hoverer, it was also reported that 0.7% (173) of the workers had exposure doses exceeding 100 millisieverts. Of these workers, the maximum exposure dose was 679 millisieverts. A large gap exists between the average exposure dose of workers at the nuclear power plant after the nuclear accident and that of the general population. The report also states that the exposure doses of the workers estimated in the report were based on measurements performed during the early phase of a post-nuclear accident in a time of turmoil, and therefore, further investigation is required before confirming the conclusion.

Routine health checkups have been conducted for workers at the nuclear power plant since the 2011 Fukushima nuclear accident. However, several issues surround these routine health checks; these include, for example, eligibility for health checkups and problems regarding long-term follow-up and adherence to them.

270. The upper limits of occupational radiation exposure dosage

The government sets the regulatory dosage limit for exposure to ionizing radiation during the performance of job duties. Occupational groups that are routinely exposed to radiation include coal miners, who usually work in areas of high natural radiation; airline crews; workers at nuclear power plants; physicians; nurses; and radiology technicians. Currently, the effective dose limit for occupational exposure is 50 millisieverts/year or 100 millisieverts averaged over five years.

Initially, the occupational radiation exposure dose limits originated from the health recommendations on medical radiation exposure for medical professionals issued 100 years ago in countries other than Japan. These health recommendations addressed several measures to prevent radiation damage to the skin, internal organs, and blood, such as taking long vacations, shortening working hours, minimizing direct exposure to X-rays, and keeping as much distance as possible from the source of radiation.

However, at the time, the designated occupational radiation exposure dosage limit was much higher than the current one. In 1934, the recommended annual occupational radiation exposure dosage limit was approximately 500 millisieverts, which is ten times higher than that of current Japanese regulations. Furthermore, immediately after World War II, a revision was made to lower the annual occupational radiation dosage limit to approximately 150 millisieverts.

As further research gradually revealed the health effects of radiation, occupational radiation exposure dosage limits were revised several times to lower the limit, finally arriving at the current recommendations.

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 15th and 22th March 2020 was reproduced for MRIC Global under the author's permission.