Everybody knows that x-rays are dangerous. They change the basic molecular structure of cells, including the DNA instructions for specific cell growth. X-rays can produce severe visible symptoms as well as doing all kinds of unseen damage. X-rays penetrate your tissues throughout, even to the point of changing the water in your body cells to hydrogen peroxide.
Some people are more sensitive to x-rays than others.
Think about what happens as the dentist exposes your face from all sides to x-rays. It is your tongue. The tongue is exposed from all sides. The surface of the tongue is composed of some of the most rapidly reproducing tissue in your body. This type of tissue is most sensitive to radiation.