New research shows a tiny island halfway between Australia and Hawaii has concentrations of nuclear material up to 1000 times higher than at Chernobyl or Fukushima.

Research carried out by Columbia University and published this week shows deadly plutonium levels are far higher than previously thought on the Marshall Islands. The group of 29 atolls was subject to 67 US nuclear tests between 1946 and 1958, with locals forced to flee as the country dropped bomb after bomb in paradise.

The United States entombed nuclear waste under a dome on the island of Runit that some believe is leaking into the Pacific Ocean. However the real impact of the contamination is only now being realised.

Researchers wrote that two atolls, Bikini and Enewetak, "were used as ground zero" and took the brunt of the impact.

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On Enewetak, the first-ever hydrogen bomb was tested. But Bikini was the site of the world's largest-ever hydrogen bomb test — known as Castle Bravo.

Operation Castle, American series of high-energy nuclear tests at Bikini Atoll in the Marshall Islands. Photo / Getty Images
Operation Castle, American series of high-energy nuclear tests at Bikini Atoll in the Marshall Islands. Photo / Getty Images

The tests, researchers say, "caused unprecedented environmental contamination and, for the indigenous peoples of the islands, long-term adverse health effects".

Researchers tested levels of radioactive isotopes in soil and food sources and found "a real concern" on Runit where the huge dome was designed to contain radiation but is not working.

"The presence of radioactive isotopes on the Runit Island is a real concern, and residents should be warned against any use of the island," researchers said.

"Moreover, wash-off of existing isotopes off the islands into the ocean from weathering and continued sea level rise continues to threaten, further contaminating the lagoon and the ocean at large."

On Bikini, researchers found concentrations of particular radioactive material "were up to 15-1000 times higher than in samples from areas affected by the Chernobyl and Fukushima disasters".

Though residents were banished from the Marshall Islands during the height of the Cold War, many have returned. The Los Angeles Times reports more than 600 people call Enewetak, Runit and Enjebi home.

Explosion of nuclear device 'Seminole' on Eniwetok in Marshall Islands. Photo / Getty Images
Explosion of nuclear device 'Seminole' on Eniwetok in Marshall Islands. Photo / Getty Images

Jan Beyea, a retired radiation physicist, told the newspaper: "Implicitly, I think these results might caution efforts to return because of the readings found."

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News.com.au previously reported rising sea levels were degrading the concrete dome at Runit, and the US Department of Energy concluded the "burial site" was leaking highly toxic waste.

Locals refer to it as "the poison" and have already been complaining of birth defects and high cancer rates.

After Castle Bravo, islanders more than 160km away mistook fallout for snow. It "caused skin burns, hair loss, nausea and eventually cancer" in many who were exposed, the Times reports.

The warnings from researchers clash with advice from the US Government, which signed a memorandum of understanding with the Republic of the Marshall Islands agreeing it was safe for those who wished to return home.

In the Marshall Islands, the most common cause of death is diabetes, which is related to a thyroid disorder. The second most common cause of death is cancer.

The population of the Marshall Islands is around 70,000 people, with local Marshallese people allowed to live and work in the US without a visa as part of the reparations for the nuclear testing that took place.

Over a third have already moved to the US. It is said when you leave the Marshall Islands, you buy a one-way ticket.