(CNN) -- Every six months like clockwork, Melissa O'Brien, a freelance writer in Kennesaw, Georgia, takes her two children, Alexandra and Evan, to the dentist for a cleaning. They're nearly perfect patients -- Alexandra, 13, has no cavities, and Evan, 10, has had only one -- but O'Brien suspects she may not be the dentist's favorite mom, because when the dentist suggests X-rays, O'Brien sometimes says no, and she can tell that doesn't sit well.
"I usually get a hard time from the dentist, like I'm a bad mother or something," she says. "Maybe I'm being silly as a parent. Maybe it's such a minute amount of radiation, but I think, my kid is going to get how many X-rays in his life, and if I can prevent some of them why not do it?"
Maybe she is being silly -- but maybe not. 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."
Worries about children, thyroid cancer
Like many medical procedures, dental X-rays have an upside and a downside. The upside is that an X-ray allows your dentist to see bones, tissue, and hidden surfaces of your teeth that he or she can't see with the naked eye. The downside is that X-rays expose you to radiation. Four bitewing X-rays, which is what many people get in a routine exam, give about .005 millisieverts of radiation, according to the American College of Radiology. That's about the same amount of radiation you get in a normal day from the sun and other sources. A panoramic dental X-ray, which goes around your head, has about twice that amount of radiation.
While those are small amounts of radiation, there's no such thing as a completely safe exposure, and radiation is cumulative over your lifetime. Children are particularly vulnerable, since they're small and their cells are dividing rapidly. The thyroid gland, which gets zapped in a dental X-ray unless it's shielded, is also quite vulnerable to radiation.
"My brother-in-law is a radiologist, and he's told me that radiation is radiation, and if it's not necessary, don't do it," O'Brien says.
Many dentists not following guidelines
The question, of course, is what's necessary? According to guidelines written by the American Dental Association and the Food and Drug Administration, dentists should evaluate each patient. Some people with a lot of problems or who are at high risk for cavities may need frequent X-rays.
But for patients who are not having problems, the need is less frequent. The guidelines say children who are not at a high risk for cavities should get X-rays once every one to two years; teens who are not at high risk should get them every year and a half to three years; and adults who aren't at high risk should get them every two to three years.
The American Dental Association has never studied whether dentists are actually following these guidelines, according to Dr. Matthew Messina, a spokesman for the ADA and a practicing dentist in Cleveland.
But many dentists say too many of their colleagues aren't following them, and are giving X-rays far too frequently, exposing patients to unnecessary radiation, not to mention increasing health care costs.
"Maybe dentists aren't aware of these guidelines, but I can't understand why, since they've been out since 1987," says Dr. Stephen Matteson, a research professor at the dental school at the University of Texas Health Science Center in San Antonio. "It's really frustrating."
Do dentists have a financial incentive to X-ray your mouth?
Matteson has an idea why dentists might be doing X-rays more frequently than necessary: money.
"Bitewings cost about $50 or $60, and a lot of dental insurance plans pay for them once a year," he says. "So I hate to say it, but there's probably a financial reason for dentists to do them once a year."
O'Brien says this crosses her mind as she's tried to decide how often she and her children should get dental X-rays.
"A lot of dentists tout their new panoramic X-ray. They're so proud to have it, and in the back of my mind I'm thinking, am I paying for this machine every time you ask for an X-ray?"
Deciding how often to get X-rays
If you have a lot of decay or other dental problems, you'll likely need more X-rays than other people. But if you have a healthy mouth and aren't at high risk for decay, you don't necessarily need X-rays every year.
Stuart White, a professor emeritus at the UCLA School of Dentistry, says the key is that each patient should be assessed by the dentist before getting an X-ray.
"I would be wary of situations where the dental assistant makes the X-rays before the dentist has seen the patient," he says.
The second step is to ask why X-rays are needed, particularly if you've had them done recently.
If the answer is that your insurance pays for them -- the answer O'Brien received -- that's not sufficient.
"Be wary also of an answer along the lines of 'We make bitewings on everybody every six months,'" White says.
Third, make sure your dentist uses a thyroid shield to cover your neck. Sometimes it's a part of the lead apron that covers your chest, and other times it's a separate piece.
"When I take my grandchildren to the dentist and they're getting X-rays and there's no thyroid collar, I'm jumping up and down and saying 'Stop!'" Matteson says.