Answer to Question #12581 Submitted to "Ask the Experts"

Oftentimes hobbyist (lapidaries) make jewelry or cabochons from petrified wood. Much of the wood harbors 80 parts per million (ppm) uranium. Does this pose a danger to the hobbyist, assuming that proper breathing protection is maintained? Would a pendant made from pet wood (80 ppm uranium) represent a risk to a person?

Excellent question, and a great chance to do some calculations. Let me blow the dust off my calculator and break out some references . . .

According to my trusty copy of Environmental Radioactivity (Eisenbud and Gessell 1997), 1 ppm of natural uranium is about 14 millibecquerels per gram (mBq g^-1). So, your petrified wood contains about 1,120 mBq g^-1; to keep the math easy, let's call if 1 Bq g^-1.

That's the easy part. But we need to know how much total uranium you've got. I'm assuming that the pendants are heavier than 1 g and hoping that they're lighter than 1 kilogram (kg). So I'm going to assume that they're about 100 g in weight—that's probably on the heavy side so it'll give us a worst case. That means that a 100 g pendant will contain about 100 Bq of uranium.

So now we need to know how much radiation this is going to give off. This is where it gets fun!

Uranium does not exist in isolation—uranium-238 (²³⁸U) and uranium-235 (²³⁵U) both decay to radioactive progeny nuclides, those progeny decay to further radioactive progeny, and so forth. Uranium-238 has over a dozen of these intermediate steps and ²³⁵U has nearly the same number. So the petrified wood in your pendant also contains radium, polonium, thorium, radioactive isotopes of lead and bismuth, and more. Some of these give off alpha radiation—this isn't a risk unless you expect your customers to be grinding up the pendant and eating or snorting it. Some emit beta radiation—this can penetrate into the skin, but much of the beta activity will be buried within the material of the pendant and won't have enough energy to escape to cause much exposure. But we'll take a look at this just to see. And then there are some gamma-emitters also—gamma radiation can penetrate into the body, and they have the energy to make their way out of the pendant as well. So we'll take a look at these also.

To do the skin dose calculations I need to know the physical size of the pendants. I'm going to assume a size of 25 square centimeters (cm²) for the sake of argument. Using a program called VARSKIN to calculate radiation dose to the skin and adding in all of the decay series nuclides from both ²³⁵U and ²³⁸U, wearing the pendant on direct contact with the skin for 1 hour (h) would give a radiation dose to the skin of about 15 microgray (µGy), and wearing it for an eight-hour day would give about 120 µGy to the skin. To put this in context, in one year a radiation worker is permitted to receive as much as 0.5 sievert (Sv) to the skin. So even wearing this pendant eight hours each day, five days a week, for 50 weeks annually (2,000 hours a year) will give a dose of about 20 mSv in a year. This is not a huge risk, especially considering that even a thin blouse will reduce radiation exposure a bit, as well as the fact that most people don't wear the same piece of jewelry all day, every day.

OK, so now let's look at gamma dose to the whole body. For this I decided to go to a useful online calculator that I've had good luck with in the past. Setting the appropriate parameters here, it looks as though radiation exposure from 100 Bq of natural uranium plus decay series nuclides is about 1 mSv annually if the pendant is being worn for 2,000 hours each year. By comparison, an American radiation worker is permitted to receive 50 times this much exposure in a year, and a member of the general public can receive this level of exposure. It's also comparable to the amount of radiation one might receive from a couple of x rays.

So here's the bottom line: someone who wears one of your pendants 2,000 hours annually (and again, that would be eighjt hours a day, five days a week, 50 weeks a year) will receive about 1 mSv of gamma radiation to the whole body and about 20 mSv of beta and gamma radiation to the skin directly beneath the pendant (if they wear the pendant on contact with the skin and not on the outside of their clothing). I should note that the calculated beta dose does not include self-shielding (that is, the pendant itself will absorb much of the beta radiation). If they wear the pendant only occasionally, the dose will be proportionally lower. But even the higher level of exposure is not dangerous. It's not nearly enough to cause skin burns, and the risk of developing cancer is so low that the Health Physics Society recommends against even trying to calculate a risk estimate. So your customers don't need to worry!

Finally, you also asked about respiratory protection. This is a good idea; not because of the risk from airborne uranium, but from the silicon that has permeated and petrified the wood—you don't want to develop silicosis.

P. Andrew Karam, PhD, CHP
 

Reference

Eisenbud M, Gesell T. Environmental radioactivity. New York: Academic Press/Elsevier, Inc.; 1997.