Frequent dental X-rays, particularly in childhood, may increase the risk of developing a brain tumor. Researchers analyzed 1,433 patients who were diagnosed with meningiomas – the most commonly diagnosed brain tumor in adults in the U.S. and compared them with 1,350 people without tumors.

Adults who developed brain tumors were more than twice as likely to say that they had bitewing X-rays yearly, if not more frequently, according to the findings. Bitewing X-rays, which require patients to bite down on an X-ray film holder, show the crowns of the upper and lower teeth at the same time.

The researchers also found a link between the tumors and panorex dental exams, which use an X-ray outside the mouth to take a broad image of a patient’s full mouth and are often used to look at problems such as infections and fractures. The increased risk of meningioma was particularly pronounced among individuals who were younger than 10 when they received the exam.

The American Dental Association, the world’s largest national dental society, says that the number of times the average person undergoes X-rays depends on a slew of factors, including their age, risk for disease and overall oral health. Its guidelines for dentists, last revised in 2004, state that practitioners must weigh the benefits of taking X-rays against the risk of exposure — with the effects accumulating over time.

The new findings are important because dental X-rays are the most common source of exposure to ionizing radiation among residents of the United States, according to the study’s authors. And while they write that full-mouth and bitewing X-rays are associated with lower levels of exposure than other types of medical imaging, the No. 1 environmental — and generally modifiable — risk factor for meningioma is exposure to ionizing radiation.