If you search on the Internet for health risks as a result of radiation of X-rays, particularly dental X-rays since that's the most common source of X-rays we're exposed to, you'll find hundreds of articles. The most trustworthy ones are from well-respected web sites, such as Annual X-rays May Expose Patients to Unnecessary Risk on WebMD, New Dental X-ray Guidelines Spell Out Radiation Reduction on Medscape. Many articles including these two mention the 2012 study Dental x-rays and risk of meningioma that links brain tumor to frequent dental X-rays. On the other hand, dentists and some articles argue that dental radiation is insignificant compared to background radiation. According to Wikipedia, "A dental x-ray delivers a dose of 5 to 10 µSv" (or 0.005 to 0.01 mSv). "The average American receives about 3 mSv of diagnostic medical dose per year" (including chest, dental and other types of medical X-rays), while "Average annual human exposure to ionizing radiation" for an American is 3.10 mSv, mostly from inhalation of air, which naturally contains radon. If medical X-rays give us about the same amount of radiation as the background, how do we reconcile the apparent discrepancy between this fact and the 2012 study? Some say the study relies on people's memory of past medical check-ups, which is unreliable. But I'm thinking of another explanation.
In April 2011, I blogged Sun UV to trigger Vitamin D and skin cancer: intensity matters?, where I propose that given the same total exposure of sunlight for the purpose of producing Vitamin D, it's better to scatter the light around your body than to focus it on a small part. Now I believe the same theory can explain the risk of medical X-rays as opposed to the relative safety of continuous background radiation. An X-ray exam must have sufficient intensity to develop the film. Intensity is exposure per unit time per unit area. In contrast, background exposure has much lower intensity, especially on the time dimension. The reason why intensity rather than total accumulated exposure matters is that a biological body has self-correction mechanism. A cell can repair itself if it's not seriously damaged, as in the case of long time low intensity exposure to the background radiation. This is as simple as warming up your whole body in a warm water shower for a few minutes as compared to scalding your finger in boiling water for a few seconds, both receiving the same amount of heat energy.
I'm not aware of scientific study proving this theory. Total radiation exposure continues to be the standard by which health risks are assessed. I hope discrepancies in conflicting reports will be explained by radiation intensity as one of the contributing factors.
So, is there too much radiation from X-ray exams? The question can be rewritten as, Is there a limit of radiation below which radiation is safe? I see two ways to answer this question. (1) If the human body reaches the limit of repairing radiation-damaged cells, that's the limit of safe radiation. (2) If the intensity of X-ray radiation is comparable to that of background radiation, it's safe. It's difficult to provide Answer (1) and it's of course dependent on the person's health, age, etc. As to Answer (2), it is completely impractical to meet the requirement of such low radiation intensity and be able to develop the film. In short, treat all X-rays as risky for now, even if the equipment is the state-of-the-arts using the fastest film and is digital. As to frequency of dental X-rays, unfortunately, there's no better guideline than this.