X-rays do their harm at any dose; there is no threshold at which the harm begins.

X-ray damage is not limited to the area being investigated. Deflected x-rays affect adjacent areas, and toxins produced by irradiated cells travel in the bloodstream, causing systemic effects. Dental x-rays cause thyroid cancer and eye cancer.

Recent experiments have shown that low doses of radiation cause delayed death of brain cells. The action of x-rays produces tissue inflammation, and diseases as different as Alzheimer’s disease and heart disease result from prolonged inflammatory processes.

I have never known a physician who knew, or cared, what dose of radiation his patients were receiving. I have never known a patient who could get that information from their doctors.

The radiation exposure used to measure bone density may be higher (especially when the thigh and hip are x-rayed) than the exposure in dental x-rays, but dental x-rays are known to increase the incidence of cancer. Often, dentists have their receptionists do the x-rays, which probably doesn’t matter, since the dentist is usually no more concerned than the receptionist about understanding, and minimizing, the dose. Even radiological specialists seldom are interested in the doses they use diagnostically.

It was only after a multitude of dentists had a finger amputated that it became standard practice to ask the patient to hold the film, while the dentist stood safely back away from the rays.

Just after the beginning of the century, Thomas Edison was helping to popularize x-rays, but the horrible death of his chief technician turned Edison into an enemy of the technology.
By the 1940s, the dangers of radiation were coming to be understood by the general public, and it was only the intervention of the US government, to popularize atomic bombs and nuclear power, that was able to reverse the trend.