Baby girls in the region have the greatest relative risk increase – 70 percent for thyroid cancer – the agency said in a 168-page health risk assessment.
But the agency cautioned that's on top a small baseline lifetime risk of 0.75 percent, so that the absolute increase in cases of thyroid cancer is expected to be small.
Read this story on www.medpagetoday.com.
The assessment also says that male infants exposed at the highest level – between 12 and 25 millisieverts – have about a 7 percent relative risk increase in the lifetime risk of leukemia and that female infants have about a 6 percent increase in the lifetime risk of breast cancer.
For all solid tumors combined, infants in the hardest-hit region have about a 4 percent increase in the lifetime relative risk, the agency reported.
The magnitude 9 earthquake on March 11, 2011 – followed by a tsunami -- killed nearly 19,000 people and severely damaged the Fukushima Daiichi nuclear plant, causing radiation leaks and forcing about 160,000 people from their homes.
The WHO assessment said exposures to radiation varied. In two areas of Fukushima prefecture, doses ranged from 12 to 25 millisieverts, while in much of the rest of the area, they were between 3 and 5.
On the other hand, some parts of the prefecture – and the rest of Japan -- faced levels of only about 1 millisievert, while neighboring countries and the rest of the world were well below that level.
The data, broken down by age, sex, and proximity to the nuclear plant, "show a higher cancer risk for those located in the most contaminated parts," according to Dr. Maria Neira, director of the agency's department of public health and the environment.
But outside those regions, she said in a statement, "no observable increases in cancer incidence are expected."
A journal report in 2011 suggested the U.S. had already suffered an excess of 14,000 deaths, possibly linked to the damaged Daichi plant.
About one in three emergency workers who were involved in the wake of the disaster are also thought to have increased cancer risks, the WHO report said.
"The risk among emergency workers would be increased for thyroid cancer particularly, and some circulatory disorders," Neira told reporters.
The agency report said direct effects of radiation – the so-called deterministic effects, which include cataracts and infertility – would not be expected at the estimated radiation levels.
The report was not able to say how many people were exposed in the hardest-hit areas. But data collected by emergency medical teams suggested that few people had high radiation levels after the explosions and leaks at the power plant.