Five workers at a Japanese nuclear facility were exposed to high levels of radiation after a bag containing plutonium broke during an equipment inspection on Tuesday, reported AP. The Japan Atomic Energy Agency said on Wednesday that the contamination was found inside the nostrils of the five men.

All five were also found to have radioactive material on their limbs after removing protective gear and taking a shower. One of the men had high levels of plutonium exposure in his lungs. He had opened the lid of the bag containing 300 grams of plutonium and uranium. The incident took place at JAEA’s Oarai Research and Development Centre in Ibaraki prefecture just north of Tokyo.

Scientists believe internal exposure to plutonium raises the risk of cancer. The man whose lungs were affected had inhaled 22,000 Becquerels of plutonium, which is not life threatening but could increase his risk over time, Makoto Akashi, a doctor at the National Institute of Radiological Sciences said. The workers are now being treated at the institute.

The cause of the incident is under investigation, said JAEA, but Shunichi Tanaka, the chairman of the Nuclear Regulation Authority, said complacency in the work routine could be a possible cause. Possible violations of safety standards are also being investigated.

Japan has large plutonium stockpiles from the country’s spent-fuel recycling program. To reduce the stockpiles, the country plans to burn plutonium in the form of Mox fuel – a mixture of plutonium and uranium - in conventional reactors.

But Japan has been slow to restart its nuclear reactors owing to increased anti-nuclear public sentiment following the 2011 Fukushima disaster caused by a major earthquake and tsunami. On Tuesday, the Japan Times, the country’s largest English-language daily, had reported that seven more Fukushima Prefecture residents who were aged 18 or under at the time of the 2011 nuclear accident have been found to have thyroid cancer, taking the total number of such patients up to 152.