Health minister Yoko Komiyama announced Friday that the government will lower the allowable amount of radiation in food products from 5 millisieverts per year to 1, but some experts are puzzled.
Permanent limits for various categories of food will be set based on recommendations submitted Thursday by the government's food panel.
The current limit of 500 becquerels per kilogram of radiation for meat, fish and vegetables is also expected to be lowered by about one-fifth in April.
Citing findings from various studies, the food safety panel concluded Thursday that a cumulative dose of 100 millisieverts or more throughout one's lifetime poses significant health risks.
But experts question the focus solely on internal exposure from food and drink, while ignoring external exposure from radioactive materials, such as fallout on the ground, roofs and in ditches.
"I can't think of a reason why they decided to omit external exposure as a factor in the proposal this time," said Dr. Eisuke Matsui, who heads the Gifu Environmental Medicine Research Institute.
The radiology expert noted that while consuming food contaminated with radiation is a far bigger risk to human health than being exposed to radiation from the environment, it does not mean it can be disregarded.
"Think of the children in the cities of Fukushima or Minamisoma, where there is a relatively high level of radiation in the environment," Matsui said. "Any guideline on radiation should consider the total exposure and not only the limit of contaminated food one can consume."
According to the International Commission on Radiological Protection, a cumulative dose of 100 millisieverts increases the risk of dying from cancer by 0.5 percent. An organization of scientists, the ICRP's recommendations serve as the basis for radiation regulations of many developed countries, including Japan.
The current limits for food and drink were set on a provisional basis soon after the nuclear crisis broke out in March at the troubled Fukushima nuclear power plant.
In July the same panel proposed in a preparatory report that 100 millisieverts be the combined limit of both internal and external exposure to radiation.
Questioned by reporters about the decision to drop external exposure from consideration, panel Chairwoman Naoko Koizumi offered no clear explanation.
Members of the panel, consisting of independent experts, asserted fallout across eastern parts of the country from the March meltdowns at the Fukushima No. 1 nuclear plant did not dramatically increase the risk from external exposure.
Koizumi also noted that other branches of the government should conduct studies on the matter, not just the food safety panel.
There are few studies of the effects of low-level radiation for extended periods, a key subject of debate among experts for years.
One study cited by the panel to explain the 100-millisievert regulation looked at cancer rates among survivors of one-time exposure to high levels of radiation, such as in the nuclear bombings of Hiroshima and Nagasaki in 1945.
Radiology expert Matsui argued that the government should make certain of the facts before advancing policies.
"Personally, I think the cumulative 100-millisievert limit is too high, whether it is only for internal exposure to radiation or not. For children, I think it should be at least one-tenth of that," he said.