The practical aspects of radiation safety

Ref: doctorspiller.com

Some people do not want x-rays because they have heard that the radiation is dangerous.  In fact, dental x-rays pose very little danger.  There are currently two methods of measuring exposure to radiation.  

The first, oldest and most frequently used unit of measure is called a rem.  A rem is a large unit, so exposure to medical radiation is generally measured in millirems (mrem).  (It takes a thousand millirems to make a rem.)  The average dental x-ray delivers about 2 mrem.  Thus a full mouth series of  dental x rays (18 intraoral films) delivers about 36 mrem.  (Note: These figures are based on the use of D and E-speed film.  Kodak InSight, an F-Speed film dental film, lets you reduce radiation exposure by up to 60 percent as compared to Kodak Ultra-Speed dental film, a D-speed product.)  A panorex film delivers about 4 mrem.  By comparison, the average person in the US is exposed to about 360 mrem per year just from naturally occurring background sources.  By this measure, it would take approximately 10 full series of dental radiographs to equal the background radiation that the average citizen is exposed to on a yearly basis.  Note that most dentists take a new full series every three to five years on average.  The Washington State Department of Health has set the maximum safe occupational whole body radiation exposure to 5000 mrem per year.  By this reckoning, it would take over 138 full mouth series of dental x-rays to equal one years maximum safe radiation level.  It would take 1,250 panorex films to get to this limit. 

Background radiation comes from outer space, the earth, natural materials (including natural foods), and even other people.   For example, flying cross country exposes a person to about 5 mrem over and above the normal radiation he receives from outer space while simply walking outdoors for the same length of time.   Cooking with natural gas exposes us to about an additional 10 mrem per year because of the naturally occurring radon gas the cooking gas contains.  Living in a brick building adds an additional 10 mrem per year over and above the radiation you would receive from living in a wooden structure.  Simply sleeping next to another person exposes each bed partner to an extra 2 mrem per year.

The second, newer measure of radiation is the millisievert (mSV) which is a unit of measure that allows for a more meaningful comparison between radiation sources that expose the entire body (such as natural background radiation) and those that only expose a portion of the body (such as dental and medical radiographs). The table below is lifted from the website of the American Dental Association and is quite helpful in comparing the amount of radiation received from dental x-rays to other medical and natural sources.  As you can see, by this more realistic measure, it would take 20 full series of x rays (taken with E-speed film) to equal the amount of radiation the average citizen picks up from naturally occurring background sources each year---that means 360 intraoral films:

What about danger to the x-ray technician?

The x-radiation figures mentioned above pertain to the patient who is in the direct line of fire from the x-ray tube.  The radiation received by the person taking the x-ray comes exclusively from scatter, which is most easily understood by thinking about a flashlight aimed at a wall in a completely darkened room.  The spot on the wall where the flashlight is aimed is the brightest because it is in the direct line of fire, however, the rest of the room is also dimly illuminated by the light that scatters off the wall.  This scatter is what concerns us since nothing but the patient's face and jaws is directly in the line of fire of the beam.  The flashlight analogy is inexact since x-ray beams are better collimated (they form a tighter beam), and much less x-radiation is scattered from the target than light from the wall because of the nature of the x-radiation itself.  But the analogy still helps you to understand the concept of scatter versus direct illumination.  Furthermore, the strength of the radiation (or light) hitting any unit area falls off geometrically depending on the distance from the source of scatter.  Think of  the flashlight analogy again.  In a very large, dark room the area of the wall two feet from the bright spot is much brighter than an area 20 feet away.  The "brightness" of the scatter illumination falls off as the square of the distance.  A person standing 6 feet away from the target receives one ninth (1/9) as much scatter radiation as a person standing two feet away from the target (6 feet is 3 times further away than 2 feet, and 3 squared is 9).  A person standing 10 feet away (5 times further away) from the target receives one twenty-fifth (1/25).