The only scientific evidence available about the impact of low level radiation derives, tragically, from Hiroshima and Nagasaki. To my knowledge, at least twice or three times over the last 50 years, the threshold level of radiation deemed to be harmless has had to be reduced as evidence accumulated that people exposed to what were supposed to be safe levels of radiation, post-Hiroshima and Nagasaki, began to show signs of damage to their physiology, manifested in the appearance of tumours and such like. Given this, the acceptable level of what was regarded as safe radiation dropped.