Four North Korean defectors have shown symptoms linked to radiation exposure, according to the Yonhap News Agency, citing the South Korean government. However, scientists were unable to determine whether the defectors' symptoms were the direct cause of nuclear tests.

The defectors are among 30 former residents of from Kilju county in North Korea, the location of a known nuclear test site. South Korean officials began testing defectors from that area for signs of radiation exposure in October, one month after North Korea claimed to detonate a hydrogen bomb.

U.S. officials said that explosion was almost 10 times larger than the blast from the bomb dropped over Hiroshima during World War II. The October blast registered as a magnitude 6.3 earthquake, and satellite images showed several landslides around the test area, prompting fears of radiation exposure.

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However, the four individuals who exhibited symptoms of exposure defected before the most recent test. A researcher at the Korea Atomic Energy Research Institute, which conducted the exams, told reporters that the defectors were exposed to radiation between 2009 and 2013, Reuters reported.

North Korea has conducted six nuclear tests at the Punggye-ri test site in Kilju county since 2006.

While the South Korean researchers noted changes to chromosomes in four of the defectors, they say that could have been caused by other factors aside from nuclear tests, including smoking.

More data is needed to determine the cause of the symptoms, including information about the defectors' lifestyles prior to leaving North Korea.

The South Korean government plans to conduct future radiation testing and is advising North Korean defectors to get regular check-ups.