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09 February 2012

Answer to Question #2296 Submitted to "Ask the Experts"

Category: Radiation Effects — Effects on Tissues and Organs

The following question was answered by an expert in the appropriate field:

I work at a veterinary office and am currently the radiologist, you might say. We have many items we use for protection such as lead gloves, apron, and thyroid protection. I have been trying to learn more about how radiation affects the eyes and was trying to get some protective glasses for x rays at work. Basically, I need more information on (1) Why are x-ray glasses important? (2) What are the effects of scatter radiation on the eyes? (3) How much protection do x-ray glasses really provide? Is it even worth purchasing them?

The only known effect of radiation on the eye is cataracts. Our fear of this effect came from misinterpretation of some early mouse data which claimed that there was an elevated risk of cataracts from doses as small as 1 rad (10 mGy). There were two major problems with the data:

The 1 rad had to be neutrons, which have a relative biological effectiveness of more than 10 for cataract induction. Thus, the 1 rad of neutrons was equivalent to more than 10 rad of x rays for this effect.
The cataracts at this dose were detectable only microscopically.

Available human data indicate that a cataract is a deterministic effect, with a threshold dose of at least 500 mGy (50 rad or 50,000 mrad) acute dose, and several times that if the dose is protracted or fractionated over time, such as occupational exposure. Current data indicate that such exposure in veterinary practice is generally very low. Latest estimates of typical doses are about 1 mSv (~100 mrad) per year in the United States, and about 0.6 mSv worldwide. In most states, this is below the level that requires personal monitoring devices to measure occupational exposure. So the typical radiation worker in a veterinary facility would have to work several lifetimes to accumulate enough exposure to exceed the threshold.

Conclusion: Lead eyeglasses are prudent but not really necessary for persons occupationally exposed to x radiation.

S. Julian Gibbs, DDS, PhD

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