In the 1950s, it was decided, absent data, that low dose radiation was as bad as high dose and so no dose would be tolerated. This was called the Linear No-Threshold (LNT) dose hypothesis. Today this is being overturned in medicine and eventually in dentistry.

https://www.forbes.com/sites/jamesconca/2020/01/28/ditch-that-lead-apron-in-the-x-ray-room/#227a1e413bd5

https://www.nytimes.com/2020/01/14/well/live/radiation-exposure-x-rays-scans-lead-apron.html

https://www.sciencedirect.com/science/article/abs/pii/S0764446999800444