If dental x-rays are so safe; why do they run out of the room whenever they take the x rays?

Your exposure to radiation is to capture an image for diagnostic purposes. Anyone else being exposed by the scatter receives no benefit. The radiation is minimal for each exposure and will cause no harm, but being exposed to 1000's of needless doses that provide no benefit is not prudent. That is why the staff of any facility leaves the room or moves behind a lead barrier. It's really that simple.

Especially in these days of computer enhanced x-rays, the amount not radiation is negligible. You receive more radiation spending a day in the sun, or taking an airplane flight. Yes, all radiation is cumulative, that's why Dentistry has made such great strides in minimizing your exposure. Staff leaves the room because they take films all day long every day. At 89 have they ever caused you harm?