When scientist Junko Nakanishi stepped into radiation-contaminated towns and villages in Fukushima Prefecture 10 months after the nuclear power plant meltdowns of 2011, she realized how difficult the job of decontamination would be.

Surveying the thinly populated areas surrounded by hills and rice paddies, she wondered how much time and money it would take to reduce the radiation.

“I thought decontamination wouldn’t succeed without a concrete plan,” Nakanishi, 76, a leading expert on chemical risk assessment, said in a recent interview with The Japan Times.

Decontamination is the biggest issue when thinking of Fukushima’s future, because it determines when, or even if, residents will be able to return home, she said.

Three years on, however, the government is still stumbling toward a realistic decontamination goal, leaving thousands of evacuees in limbo, she said.

The central government is responsible for decontaminating evacuation zones in 11 municipalities where dosage readings exceed 20 millisieverts per year. But many areas remain untouched or in the midst of decontamination, with their 80,000 residents still displaced as of April 1, by government order.

“The longer their evacuation period gets, the more residents will miss the timing to return home,” said Nakanishi, a fellow at the National Institute of Advanced Industrial Science and Technology. “It’s about time to think of ways to live under a certain level of risk.”

The government did not set a specific radiation goal for decontamination but settled on a threshold instead. The main condition for lifting an evacuation order is that the annual radiation dose must be 20 millisieverts or less.

An annual dose of 1 millisievert has meanwhile been set as a “long-term goal” for decontamination, without a specific time frame.

Nakanishi said that the 20 millisievert threshold is too high for many residents to accept and that the 1 millisievert figure is unrealistic in heavily contaminated areas, given the limits and cost of decontamination technology. As an alternative, she proposes a maximum exposure level of 5 millisieverts per year as a target for decontaminating evacuation zones, based on her assessment of the various risk factors.

“Somebody has to find a common ground where people can return to their homes as early as possible. We need to set a goal for radiation. . . . But no politician, bureaucrat or expert seems to make such suggestions,” she said. As a scientist, Nakanishi said it’s her job to find that magic number.

Since the 1970s, Nakanishi has studied the environmental and health risks of such toxic chemicals as dioxin and mercury. At the core of her research is the concept of risk trade-off, which means reducing one risk while allowing another to rise.

For example, chlorinating the drinking water of a population would reduce its risk of contracting an infectious disease. But it would also increase the risk of cancer. Nakanishi searches for the best mix of acceptable risks when trying to reduce overall risk.

Nakanishi looked at health factors, technological limits, cost and time to assess the tainted areas in Fukushima and concluded that an annual radiation exposure of 5 millisieverts or less would be the best goal for repopulating them.

According to her calculations, a 5-millisievert goal would allow some 65,000 residents to return home in another one to two years and cost around ¥1.8 trillion to execute.

A resident would be exposed to around 38 millisieverts over 15 years, a risk that, when compared to the average risk of exposure to a chemical like dioxin, is not high, Nakanishi said. The dose would drop to less than 1 millisievert a year after 15 years due to natural radioactive decay and land erosion from rain and wind, she said.

According to the International Commission on Radiological Protection, an annual dose of 100 millisieverts increases the risk of dying by cancer by 0.5 percent over one’s lifetime. Below that, the risks are too small to distinguish from the effects of other cancer risks, such as smoking and an unbalanced diet.

“The risk is not zero, but we need to think about the amount we can tolerate,” Nakanishi said. “It’s difficult, but that’s the reality. It’s more honest to say that the risk is not zero rather than it is safe.”

For areas with annual radiation readings over 50 millisieverts, residents need to give up on returning and relocate, with financial support from the state, she said.

Nakanishi also emphasized that there is a need for the government to financially back those who want to relocate even if an annual radiation dose drops to less than 5 millisieverts a year. Given Japan’s experience with the atomic bombings of Hiroshima and Nagasaki, some find it extremely hard to prevent horrific images of the aftermath from entering their minds when they hear the word radiation, she said.

“For a long time, I’ve been analyzing the risk of chemicals, and through the research I have come to realize that Japan has never set a goal for regulating chemicals by itself. It was always based on decisions made by international organizations or other countries,” she said.

“(In Japan, people are) not used to finding a mutually acceptable common ground by considering different conditions and risks,” Nakanishi said. “(Fukushima) was a very unfortunate nuclear disaster. But I see it as a chance for Japan to learn to strike a balance of risks, and find risk levels that we can accept.”