First of all, it is important to understand that an exposure to any radiation can be dangerous. X-rays are electromagnetic radiations. They can affect our health if they are used in an abusive practice.
We are surrounded by radiations: sun's radiations, TV's radiations, cosmic's radiations, radio waves, terrestrial's radiations, tobacco...
Radiations are measured in Sievert (S) and most of the time we talk about micro Sievert.
The graph below indicates, that a daily radiation exposure is around 0,01mS. A dental Xray can be between 0,001-0,007 mS! It is 10 times less than our daily exposure!!
Therefore, improvements of X-ray devices makes them less dangerous. X-ray equipment, are then sending very light levels of radiations. Make sure that your dentist uses a collimated X-ray machine with digital X-ray, which means less radiations and less risks of radiation scattering.
We consider that the best amount of radiography for our patient is around 1 panoramic X-ray every 3 years and 2 bitewings X-rays every year or 2 years.