牙医诊所的工作人员常说牙科 X光剂量很小，和日常生活或坐飞机被放射线照射差不多。问题是日常生活中的射线散布全身，牙科 X光却是有方向性的。整篇文章很有意思。。。
http://www.cnn.com/2011/09/15/health/living-well/dental-x-rays/index.html
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It's impossible to tell, because there are no good studies showing the right number of X-rays to give someone who isn't having any particular dental problem. While some dentists do bitewing X-rays every six months on a healthy patient, others hardly ever do them, relying instead on a visual examination of the mouth with a sharp explorer and a mirror.
It's an important question, since dental X-rays are the only form of medical radiation received on a regular basis by large numbers of American men, women, and children.
"It's crazy," says Dr. Nicholas Dello Russo, an instructor in the department of oral and maxillofacial surgery at the Harvard University School of Dental Medicine. "We're doing an experiment on a vast number of people in this country and we have no idea how it will play out in the next 15 or 20 years."
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