These levels are far below what it would take to cause immediate illness, but the risk of long-term exposure to low-level radiation is unclear. Studies of radiotherapy patients and others indicate that there is a significant increase in cancer risk if lifetime exposure exceeds 100,000 microsieverts, according to the World Health Organization. A person exposed daily to radiation at the high end of the levels now seen at Miyakoji would reach that lifetime exposure level in fewer than 23 years.