Dr. Sidney M. Fireman, Dr. Susanne E. Perschbacher
Toronto, CBCT, St. Clair, Consult, Implants


It is well known that nuclear and therapeutic sources of radiation can cause cancer. However, it has not been demonstrated scientifically that the very low doses received in dental x-ray examinations can be harmful. We respect, but do not share patient concerns for unverified risks. We attempt to keep exposures as low as reasonably achievable and take only those views which are necessary.


In order to better understand the biological significance of diagnostic imaging, it is important to have a perspective on the radiation we are exposed to every moment of our lives. Background radiation is the greatest source of radiation to the population today.



Background Radiation


Despite the widespread use of x-rays in medicine and dentistry, the diagnostic and therapeutic uses of radioisotopes, nuclear power generators and atomic detonations, natural background radiation remains by far, the greatest source of radiation to the population today.


Background radiation reaches us externally via cosmic rays and terrestrial  radiation, and internally through ingestion and inhalation.


The annual per capita background dose is highly variable, but the average  exposure is estimated to be in the order of 200 millirem (mrem). Cosmic rays  contribute 28 mrem and terrestrial sources contribute 32 mrem. Internally deposited radioactive materials account for the remainder of the radiation.


Cosmic rays emanate from the entire galaxy, including our sun. They consist  of high-energy particles, primarily protons (90%) and helium ions (10%). As  these rays approach the earth, they are affected by the earth's magnetic field,  such that radiation levels are highest at the poles and lowest at the equator.  Because of the shielding effect of the atmosphere on cosmic rays, man's exposure  varies as a function of altitude. The dose at 1800 metres is double that at sea  level.


Terrestrial radiation is emitted in the form of alpha & beta particles,  electrons, and gamma rays. This radiation varies with the composition of the  earth's crust, and with the amount of water present. Moist soil will reduce  radon gas escape by up to 20%. 20cm of snow can reduce the gamma component by up  to 50%.


Internal radiation enters and can remain in the body from the food we eat and  the air we breathe. The lung is subject to the greatest amount of radiation:  local doses range from 4-400 mrem annually.


Depending upon geographical location, climatic conditions, food and shelter,  average annual background levels can vary from 75 to many hundred mrem. The  annual terrestrial exposure to a person living in Colorado is 60 mrem more than  a person living in Ontario by virtue of the higher altitude of that mountainous  state. Depending upon brick type, people living in different houses can have  background level differences of 10-20% or more. These differences can be  substantial if the bricks are made from soil near uranium mining sites. People  who fly to only one vacation site can easily increase their annual background  levels by up to 10%, depending upon the length and altitude of the flight.  Sharing beds with one's mate will increase background levels a few percentage  points because of internal radiation levels.


Background radiation contributes 99.985% to the average annual thyroid dose,  while dental xrays contribute only 0.015%