Answer to Question #1593 Submitted to "Ask the Experts"

Thorium is often found in the tungsten rods used for electric arc welding. By weight, the rods are typically 2 percent thorium. An average rod therefore contains something like 30 microcuries of thorium. The thorium is added for several reasons, e.g., to increase the alternating current carrying capacity and to reduce electrode erosion.

The radiation dose is primarily due to the inhalation of the thorium and its radioactive decay products that become airborne during the welding operation. The dose due to external radiation is relatively unimportant.

Dose estimates (which must be very approximate) have been generated for three groups: heavy-use welders, occasional welders, and individuals who happen to be nearby the welding operations. Keep in mind that the dose would depend, among other things, on the thorium content of the rods, the ventilation system, the number of hours welding, the amperage used, and whether it is AC or DC welding (roughly 30 times higher doses for AC welding).

According to NCRP Report No. 95, the committed effective dose equivalents for one year of exposure would be:

•	Heavy users:	0.4 to 14 mrem
•	Occasional users:	0.1 to 3 mrem
•	People working with welders:	0.6 to 4 mrem

Somewhat higher numbers were estimated in NUREG-1717:

•	Heavy users (welding in shops):	20 mrem for DC welding and 500 mrem for AC welding
•	Occasional users (welding in homes):	3 mrem for DC welding and 100 mrem for AC welding

By "committed" dose, we mean the dose over the 50-year period following the one year of exposure (remember, some of the inhaled thorium will stay in the body long after the material is inhaled). By "effective" dose we mean the uniform whole-body exposure that carries the same risk as the nonuniform internal dose (in this case, the internal dose is primarily to the bone).

For comparison, a typical chest x ray would result in a 10 mrem dose.

Is there a health risk? There is no evidence that doses this low carry an excess risk. However, if we take the risks observed at higher doses and extrapolate them to the low-dose region, we would conclude that 1,000 mrem (much higher than the estimated doses described above) carries an excess risk of dying of cancer of approximately five in ten thousand. By excess risk, we mean the risk exclusively due to the radiation exposure, above the one in five risk we face due to other causes.

Paul Frame, CHP, PhD

References:

• National Council on Radiation Protection and Measurements Report No. 95. "Radiation Exposure of the U.S. Population from Consumer Products" and NUREG-1717 "Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials" 2001.
   
• Frame and Kolb, Living with Radiation: The First Hundred Years, 2000.