At least four North Korean defectors might have been exposed to dangerous radiation, South Korea announced Wednesday. The four people are among a group of defectors from a North Korean county that contains a nuclear test site. South Korea has tested 30 of those people for evidence of radiation exposure. The four people had been exposed to radiation between May 2009 and January 2013, during which time North Korea ran multiple nuclear tests. All defected from North Korea before the country’s most recent test. South Korean researchers said they could not confirm whether the people had been exposed to radiation as a result of North Korea’s weapons program, as there are multiple ways to become exposed to radiation.