Answer to Question #2224 Submitted to "Ask the Experts"

Category: Consumer Products — Watches, Clocks, and other Glow-in-the-Dark

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

I was just reading somewhere that the tungsten filaments in typical house light bulbs sometimes contain thorium oxide. Is that true today or is this something no longer used (I have thought it was just tungsten only in the light bulb filament)? Also, I have a few relatively new (1994) dimmer wall switches for lights in my house that, when electricity is connected, light up with an orange color. I assume the filament in the "light bulb" here is also just tungsten, right?

You are correct, thorium has been used in the tungsten filaments of ordinary light bulbs. Such use might still continue but, if so, it is limited. For the most part, the thorium has been replaced by rhenium. While thoriated tungsten filaments are still manufactured, this seems to be for specialty lighting (e.g., flashbulbs for cameras and automobile lights) rather than for use in ordinary incandescent lights.

Even if thorium were still used, the potential doses would be very low. The U.S. Nuclear Regulatory Commission calculated an effective dose equivalent of less than 0.001 mrem per year for an individual spending eight hours per day at one meter from a lamp employing a thoriated tungsten filament and 20 hours per day at two meters from two such lights. Compare this with an annual average exposure of approximately 300 mrem due to natural background.

Interestingly, the rhenium used to replace the thorium is itself naturally radioactive—rhenium primarily consists of the radionuclide ¹⁸⁷Re, a weak beta emitter. Not to worry, the potential doses from rhenium are even lower than those possible with thorium.

While I am unfamiliar with the filaments used in dimmer switches, I would be very surprised if they employed thorium since thorium is primarily used where intense light is required—this is not the case with a dimmer switch.

Reference:

U.S. Nuclear Regulatory Commission. Systematic radiological assessment of exemptions for sources and byproduct materials. Washington, DC: U.S. Nuclear Regulatory Commission; NUREG-1717; 2001.

An electronic version of this document can be obtained at the Nuclear Regulatory Commission Web site.

Paul Frame, CHP, PhD

Answer posted on April 9, 2003. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.