...Examinees are required to fill out detailed 24-hour records for the first two weeks, and the number of hours they spent outdoors each day for the rest of the four-month period.

If Fukushima Prefecture residents develop cancer of the thyroid gland in the future, authorities will determine the likelihood of the disease being linked to the nuclear disaster. They will base the decision on comprehensive assessments of individual doses and any increase in the cancer rate in the wider community.

Radiation damage to the thyroid gland depends largely on the body's internal dose, but knowing external doses helps to inform the general trend in internal conditions, experts said.

When dose data is unavailable, there is no way to tell whether an abnormality in the thyroid gland has anything to do with the nuclear accident.

"Dose estimates are essential for evaluating a causal relationship between disease and radiation in those cases where people unfortunately fall ill and consider applying for compensation," said Saburo Murata, deputy director of the Hannan Chuo Hospital in Osaka Prefecture.

In the past, Murata has helped atomic bomb survivors and nuclear plant workers apply for health compensation.

"I advise people to keep records of any changes in their health conditions and their whereabouts, including from now on," he said.