The study authors also calculated that these girls are now 70% more likely to develop thyroid cancer, but they emphasized that the absolute risk was still very small, rising from 0.75% to 1.25%.
Although emergency workers had some of the highest levels of radiation exposure, they had yet to demonstrate acute radiation effects, the scientists found. The only effects that are expected in this group are "possible thyroid disorders in those few workers who inhaled significant quantities of radioactive iodine," the authors wrote.
Six Fukushima plant workers died during or soon after the March 2011 disaster. A United Nations report last year determined that none of them perished due to the effects of radiation and attributed their deaths and injuries to physical trauma, cardiovascular stress and heat stress. One reported leukemia death could not be attributed to the meltdown because of the short time between radiation exposure and death, the U.N. said.
The report said exposure levels were insufficient to cause an increase in miscarriages, stillbirths or birth defects. The report did not assess potential psychosocial or mental health impacts from the disaster.
No discernible increase in health risks was expected outside of Japan, the scientists concluded.
Much of the data that were used to develop the risk forecast model were taken from survivors of the Hiroshima and Nagasaki atomic bomb blasts and from the Chernobyl nuclear plant disaster.
The study authors said they took pains not to underestimate potential health risks from the Fukushima disaster. As such, they assumed that people living nearby the power plant took longer to evacuate than they actually did, and that they ate only food produced in the area.
Edwin Lyman, a nuclear physicist with the Union of Concerned Scientists, a watchdog group, said the WHO report's focus on the increased risk to each person "tends to dilute the impact" of the disaster.
Lyman pointed to another study done last year by Stanford University scientists who estimated that the meltdown would cause about 310 cases of cancer, including about 130 deaths. That study was published in the journal Energy & Environmental Science.
Times staff writer Emily Alpert contributed to this report.
