The plague of radioactive water at the wrecked Fukushima No. 1 power plant has renewed fears both in Japan and abroad over the contamination of seafood and the habitat it comes from.

The government is trying to reassure consumers that all fish that find their way to market are safe, pointing to marine life sampling data and regulations regarding shipment of contaminated marine products.

But how safe — or dangerous — could fish caught in the Pacific be? On what data and safety standards is the government basing its claims of safety?

Let’s hear from some experts and examine in more detail the radiation data the government is currently using.

Among 64 radioactive materials released from the melted nuclear fuel at the wrecked reactors, two are now considered the main threats as far as fish contamination is concerned: cesium, which can cause cancer through internal exposure, and strontium, which can accumulate in bones and lead to bone cancer.

Today, many mainstream experts agree the contamination from radioactive cesium has steadily decreased since the March 2011 meltdown crisis started, and now poses little danger to consumers.

However, fewer studies have been conducted on strontium due to the high cost of carrying out such tests, prompting experts to call for more wide-ranging surveys out across the ocean.

Still, compared with cesium, far less strontium is believed to have been released into the environment, and sampling data of fish and ocean water have yet to show any alarming signs, experts say.

“Contamination levels of fish now coming to the market are well below the government safety threshold. We consider them safe to eat,” said Jun Misonoo, a consulting researcher at the Marine Ecology Research Institute, a government-linked nonprofit research body based in Tokyo.

Tokyo Electric Power Co. has turned up high strontium readings from groundwater samples taken from test wells. Tepco also believes that about 400 tons of radioactive groundwater is flowing into the Pacific every day.

But so far, it appears this flow has had only a minor impact on fish.

According to a Tepco estimate, up to 1.8 quadrillion becquerels of cesium-137 were released into the sea in a span of just six days in April 2011 shortly after the meltdown crisis began.

From May 2011 until today, however, between about 1 trillion to 20 trillion becquerels of cesium-137 have reached the Pacific, according to Tepco’s estimates.

“Compared with the release of radioactive materials in the initial stage of the crisis, the amount of material now is overwhelmingly small,” Jota Kanda, a professor at Tokyo University of Marine Science and Technology and an expert on maritime movement of radioactive substances, said in a recent interview with The Japan Times.

“This is not something that has a big impact on fish in the sea,” he said.

The Fisheries Agency has compiled and published data on a total of 14,773 samples of fishery products caught in and near Fukushima Prefecture and 24,360 samples from Pacific coastal areas in other parts of Japan from March 2011 through last month.

According to the data from the Fukushima coast, some 53 percent of the samples caught from March through June 2011 contained radioactive cesium exceeding the government safety threshold of 100 becquerels per kilogram. However, the ratio has consistently decreased, falling to 2.2 percent for the July-September period this year, despite the continuing flow of contaminated water from the plant.

The corresponding ratio for outside Fukushima Prefecture has similarly kept falling, from 6.5 percent early in the crisis to 0.4 percent in the last quarter.

Commercial fishing off Fukushima Prefecture has been suspended except for trial operations. It has been limited to 18 types of marine life, including “shirasu” whitebait, various shellfish species, squid and octopus, that have been found to contain little or no radioactive cesium in recent samples.

If any fish are found contaminated with cesium exceeding the safety threshold, shipment of that species from the areawould be stopped, according to the government.

Consumers may fear the possibility of eating tainted seafood that escapes through the net of sampling surveys and shipment regulations.

Misonoo argued this sort of fear is misplaced, as there is little chance one person would be so unlucky as to eat enough highly radioactive fish to suffer a health problem.

He pointed out that even if someone were to ingest 1 kg of fish contaminated with cesium of 100 becquerels per kilogram every day for a year, that person would receive an internal radiation exposure of 0.47 millisievert.

According to the International Commission on Radiological Protection, exposure of 100 millisieverts in total would increase the risk of death by cancer by 0.5 percent over the course of a lifetime.

Thus, exposure of 0.47 millisievert would pose only a minor health risk to a consumer, Misonoo said.

“And you would never keep eating 1 kg of fish every day. It’s quite unrealistic,” he said.

As for strontium, the Fisheries Agency itself tested only 40 samples from April 2011 to last December due to the high costs involved.

Yet the agency has claimed that as long as the level of radioactive cesium is below the acceptable maximum, the amount of strontium should likewise be below the safety threshold.

According to the now-defunct Nuclear and Industrial Safety Agency, the amount of cesium-137 released into the environment was more than 10 times that of strontium-90.

But the government set the safety thresholds for cesium and strontium on the assumption that a person would receive the same internal radiation exposure from the two isotopes — the reason the government claimed that the regulatory level has a high margin of safety and checking only for cesium levels is sufficient.

Misonoo also pointed out that strontium accumulates mainly in fish bones, and little goes into and stays in the meat.

According to the International Atomic Energy Agency, when fish take in radioactive cesium, the density of cesium in their meat is condensed to between 50 and 100 times more than when it is in seawater.

But for strontium, this rate is believed to be only between one and three times, meaning far less strontium is accumulated in fish meat than cesium.

“And you won’t eat much in the way of fish bones at any rate,” Misonoo added.

For people who are still worried about cesium and strontium in their seafood, reducing the risk is possible by avoiding certain types of fish.

Currently, most of the ocean fish exceeding the safety threshold of cesium are bottom fish such as “hirame” and “karei” flounder, “ainame” greenlings and “mebaru” rockfish.

On the other hand, squid and octopus are known to accumulate little cesium and strontium. This is one of the reasons fishermen in Fukushima first test-fished “mizu-dako” octopus and shipped their catches to markets on a trial basis last month.

On the other hand, more care should perhaps be taken with freshwater fish from certain lakes and rivers, though public attention has been pinned on fish from the Pacific.

Of the 5,706 fish samples taken from July to Oct. 2, 64 were found to contain cesium exceeding 100 becquerels per kilogram, according to data published by the Fisheries Agency.

Of the 64 fish, 29 were from rivers and lakes, including 17 samples from Fukushima, six from Gunma Prefecture, four from Lake Teganuma in Chiba Prefecture and one from Lake Chuzenjiko in Nikko, Tochigi Prefecture.

According to Hideo Yamazaki of Kinki University, freshwater fish tend to take in and retain more cesium than sea fish, and lakes are a much more closed environment than an ocean.

That has apparently pushed up the cesium density in fish in areas hit by radioactive plumes soon after the 2011 meltdown crisis started, Yamazaki said.

He warned that Japan will need to keep monitoring various radioactive materials from Fukushima No. 1 for 100 years — and possibly even longer — as work to scrap the four damaged reactors stretches on for decades.