Because x-rays are invisible and silent, it is easy to conceive of them as harmless – despite the fact that they contain ionizing radiation (also found in a nuclear blast). Of course, the amount of radiation you receive from an x-ray is miniscule compared to what you get from a nuclear bomb.

So what do x-rays mean when it comes to our health? Using REM (Roentgen Emission Man) – the traditional units to measure the amount of radiation to which we are exposed – the amount of radiation you receive from background cosmic radiation is about 350 millirem, whereas the amount you receive from a single dental x-ray is about 3 millirem (the same as a cross country plane flight). This isn’t very much. By comparison, a mammogram provides about 500 millirem per breast, the same amount of radiation you receive from an x-ray of the lumbar spine. This still isn’t very much, thus I order mammograms on my patients routinely, although I am conscious of the cumulative effects.

By comparison, the amount of radiation you receive from a dental x-ray these days is very, very small. But it wasn’t always this way. A new study from Yale just published in the journal Cancer looked at more than 1400 people who developed a mostly benign brain tumor known as a meningioma (a tumor in the lining tissue of the brain) between 2006 and 2011. The study found that those patients with meningiomas were more than twice as likely to report having had frequent focused dental x-rays known as bitewings. Panoramic x-rays of the whole mouth done at a young age raised the risk of up to five times.

Though surveys are a weak method of science because they are often inaccurate, the results are still striking and convincing. Even the study’s authors point out the association between dental x-rays and brain tumors is softened by the fact that most of the exposure to x-rays in the study took place in the 1960s – when doses were much, much higher than they are today.

Still, this study is an important reminder that x-rays aren’t completely benign. I recommend with all x-rays that patients ask their doctors for an explanation of why they are receiving them. X-rays aid a doctor in diagnosing diseases, but a doctor (or dentist) must always have a stated purpose in doing them. It would be helpful if this study raises that public awareness, but a shame if it generates a public fear against a useful test that is generally safe.

Marc Siegel MD is an associate professor of medicine and Medical Director of Doctor Radio at NYU Langone Medical Center. He is a member of the Fox News Medical A Team and author of The Inner Pulse; Unlocking the Secret Code of Sickness and Health.