Our office use Extremely Low Dose Digital Radiography (ELDDR), which uses one tenth of the radiation dosage of a tranditional x-ray film. Below are some radiation exposure comparison tables. You can see the radioation exposure for 4 bitewing dental x-ray is even smaller than the average of daily environmental exposure, read more from Deardoctor.com.

Cited from Deardoctor.com.

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Cited from Localdentist. "A millisievert is a measure of radiation which accounts for the fact that The millisievert dose also allows for the different effects of different types of radiation, x rays, gamma rays, neutrons, alpha particles and beta particles. Dental X-ray".

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Typical values of dose for various medical and dental x-ray procedures
Cited from Moderndentist.