While almost everyone accepts that a significant amount of radiation is harmful, what amount qualifies as significant?
Radiation exposure associated with dentistry represents a minor contribution to the total exposure from all sources. The National Council on Radiation Protection and Measurements (NCRP)[1] has estimated that the mean effective radiation dose from all sources in the United States is 6.2 millisieverts (mSv) per year, with about 3.1 mSv of this dose from natural sources and about 3.1 mSv from man-made sources. About half of the man-made radiation exposure is related to CT scanning.
— ada.org/en/member-center/oral-health-topics/x-rays
Assuming some amount of radiation from dental x-rays is negligible and helpful for diagnostic purposes, how often should one have x-rays done?
X-rays are usually recommended by dentists every six to 24 months depending on the person, their history of tooth decay, age and the condition of their mouth.
People who fall into the high-risk category who may need X-rays taken more frequently include smokers, those with gum disease and oral health conditions such as dry mouth.
— webmd.boots.com/oral-health/guide/dental-x-rays-when-get-them
However, there are some that would say any amount of radiation is unwarranted unless a professional analysis first reveals definitive cause for problems that need radiation for a clearer diagnostic picture. These people claim links to cancer and thyroid problems from any amount of dental x-rays, much less a lifetime of them.