Some regions of the Marshall Islands in the central Pacific Ocean have far higher levels of radiation than Chernobyl or Fukushima, new research says.

Measuring soil samples, ocean sediment and a variety of fruit, a Columbia University research team found that the concentration of nuclear isotopes on parts of the island nation exceeds the legal exposure limit agreed upon by the United States and the Republic of the Marshall Islands.

The agreement came following nuclear testing conducted by the U.S. during the Cold War, according to the three studies published this week in the Proceedings of the National Academy of Sciences.

Between 1946 and 1958, the U.S. detonated 67 nuclear bombs on the chain of atolls (a ring-shaped reef, island or chain of islands formed of coral) located halfway between Australia and Hawaii. The largest of these hydrogen bombs was known as "Castle Bravo" and was detonated in 1954 at Bikini Island.

The bomb was 1,000 times more powerful than either of the bombs dropped on the Japanese cities of Hiroshima and Nagasaki during World War II.

Joe Lofaso, 86, of Commack, New York, was stationed there in his early 20s and recalls the detonation of Castle Bravo in the middle of the night.

"It was pitch black outside, and when it detonated, the nighttime turned to the eeriest, brightest day you could ever imagine ... way past normal daylight," he told ABC News. "And then it slowly went back to normal darkness."

Widespread radiation resulted from years of testing, with the highest concentrations found on 11 islands located on the atolls of Bikini, Enewetak, Rongelap and Utirik. The radiation forced residents of these tiny islands to flee to the country's two main islands, which have since become overcrowded. In total, 75,000 people call the island nation home.

According to the studies, the researchers found concentrations of americium-241, cesium-137, plutonium-238 and plutonium-239,240. The highest levels of radiation were found on the island of Naen in the Rongelap atoll. One theory for the high levels on Naen is that the island was used as a dumping ground for waste cleanup on Rongelap, the researchers said.

Rising sea levels and resulting runoff could contaminate other islands, the researchers warned, noting that the islands must be cleaned up, and soon, so that residents can return to their home islands and so no further contamination ensues.

"Based upon our results, we conclude that to ensure safe relocation to Bikini and Rongelap Atolls, further environmental remediation ... appears to be necessary to avoid potentially harmful exposure to radiation," wrote the study authors, including Ivana Nikolic Hughes, associate professor of chemistry at Columbia.

The U.S. Environmental Protection Agency notes that exposure to high levels of radiation can lead to skin burns and acute radiation syndrome, or radiation sickness, which can result in long-term illnesses such as cancer and cardiovascular disease.