Answer to Question #1356 Submitted to "Ask the Experts"

There is a partial answer to your question in the "Historical" category of "Ask the Expert."

Perhaps the best single source of information is the Nuclear Regulatory Commission's NUREG-1717, "Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials." When the NRC Web site is back up and running, this report should be available at http://www.nrc.gov/reading-rm/doc-collections/nuregs/. Refer to pages 3-285.

Some general points:

The radioactive component of the lenses is ²³²Th (and its decay products). The thorium was added to increase the index of refraction of the lenses. The fact that thorium is radioactive was irrelevant. The first use of thoriated lenses seems to date from the late 1930s or WW II. The production of thoriated camera lenses seems to have ceased in the late 1980s. Lenses used by the military are particularly likely to be radioactive. Almost everything is radioactive to some extent. I have seen ordinary glass double the counts on a Geiger Muller detector because of the potassium content of the glass (most glass will not do this however).

One tissue of concern might be the lens of the eye—the potential consequence being cataracts. The development of cataracts as a result of a radiation exposure is referred to as a "nonstochastic" or "deterministic" effect. This means that in order for cataracts to occur, an effective threshold dose must be exceeded. The minimum dose at which cataracts can occur is a few hundred rad (the threshold depends to some degree on the type of radiation).

Thorium emits alpha, beta, and gamma radiation. For cameras employing thoriated lenses, only the gamma rays result in a dose to the lens of the eye (or any other part of the body for that matter).

The major determinant of the dose to the lens of the eye is the length of time the photographer is holding the camera up to the head. As an example, the dose rate near the viewfinder of an old Pentax camera of mine (Super Takumar lens) is roughly 100 microrad per hour—approximately ten times background. In other words, looking through the viewfinder of this camera for one minute results in the same dose that I receive every ten minutes when not using it. I would have to hold this camera up to my eye for several million hours to exceed the threshold dose for cataracts. For what it's worth, I have never had any qualms about using the camera and to my knowledge, there have been no documented cases of individuals developing cataracts as a result of cameras employing thoriated lenses.

Other parts of the body might also receive an exposure, in particular the abdomen due to the camera hanging from the neck. NUREG 1717 reported an "effective dose equivalent" of 0.7 mrem per year to users of cameras employing thoriated lenses. This is the dose to the whole body that would carry the same risk as a specified dose to a single tissue or tissues (for example, the dose to the abdomen and eye). This 0.7 mrem is substantially less than 1% of the 300 mrem that we receive each year due to our exposure to background radiation.

Of significantly more concern is the possibility that the eyepiece itself employs a thoriated lens. This has been the case in some military devices and some unauthorized commercial devices. This is a much greater problem because of the proximity of the eyepiece to the eye and the fact that alpha and beta emissions now contribute to the dose. If the eyepiece is radioactive, the germinal cells of the cornea become the tissue of concern and the potential annual dose becomes quite high. NUREG 1717 discusses this in some detail. Fortunately, thoriated eyepieces are comparatively rare.

As an additional point of interest, it was not unusual many years ago (1940s) for photographers to employ a uranium-containing toner to tint photographic prints (Kodak made such a toner). These prints would be measurably radioactive.

References

• Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources. National Council on Radiation Protection and Measurements Report 95
   
• Frame P, Kolb W. Living with Radiation—the First Hundred Years  
    
• Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials. NUREG-1717

Paul Frame, CHP, PhD