An annual maximum occupational exposure for radiation workers in the United States is 5,000 millirems (measure for radiation). When you spread low doses out over a period of time, it is not as destructive to the body because it has time to recover. To help put this in perspective to dental x-rays, you would need to have 2,000 dental x-rays to equal the radiation in 1 mammogram.
In order to reach that maximum safety dose, you would need to have to take approximately 10,000 dental x-rays. On average people receive 3 dental bite-wings worth of radiation a day just from being outside in the sun or around concrete buildings and roads.