That comes mainly from the works of Professor Emeritus EJ Sternglass, and Professor Emeritus JW Gofman with Dr AR Tamplin.
There were a number of studies of radiation - particularly by the famous Oxford Head of Science, Dr. Alice Stewart, furthered by Dr. MacMahon, which was the basis of ending of x-raying of babies in the womb, as that was shown to significantly increase the risk of leukaemia.
Radiation dosage is cumulative (it doesn't just disappear), and nowadays we are subject to routine dental x-rays, routine CRT scans (which consist of between 200 and 2,000 x-rays), we have mobile phones right next to our bodies emitting large annual doses of radiation, and we have mobile-phone towers nearby, and wi-fi everywhere.
And, of course, if you wonder why 1 in every 2 of us contracts cancer, you must be a member of the tin-foil hat brigade!
Professor Sternglass wrote a book, "Low Level Radiation" detailing his history of trying to get the US authorities to recognise the scientific facts about deaths from radiation, however the US and UK thought it was vital to plough ahead with nuclear weapons and nuclear energy, whatever the human cost, and put in place the dodgy work-arounds that allowed them to do that.
Nowadays we have industries which are the major funders of political parties and tell governments what they have to do, and these unfortunately have their appointees on the boards and committees which were originally meant to protect us from rampant corporate greed - the likes of Big Pharma, Big Comms, Big Chemical, and Big Oil all threaten human health with total impunity.