Answer to Question #12205 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
We have two decorative tile shelves in our home made of many small, broken 1928 Malibu tile pieces—very pretty. However, we've just discovered they emit radiation! You have to be right on top of the tile to get a "high" reading of 1.63 microsieverts per hour (µSv h-1). If you are 0.3 meters (m) away, you get a normal reading of 0.23 µSv h-1, and when you are 1 m away, the reading is even less, 0.13–0.18 µSv h-1.
My question is whether this stuff is safe to have in our home. We are almost always a "safe" distance away, as the shelves are used to house only decorative items, not our china. But does the radioactivity leak out into the room somehow? As I said, the readings around the tiles are normal; only when the Geiger counter is set directly on top of the shelves do you get a high reading.
Anyway, we really love these beautiful tiles, but if they aren't safe we will take them down. I hope you can be of some help here; it is becoming a source of great argument between me and my husband, who thinks they are totally safe.
You have raised concerns about radiation from your tile pieces, which most likely contain small amounts of naturally occurring radioactive material (NORM). Everything that comes from the ground contains small amounts of NORM. This includes brick, stone, concrete, clay, and ceramic materials.
The primary form of radiation from NORM is beta particles, which do not travel very far through the air and may be blocked by clothing or the outer layers of tissue. Thus, beta particles do not penetrate into our bodies or cause damage to sensitive organs. While beta particles may deposit radiation energy in our skin, skin is not very sensitive to damage by radiation. Therefore, regulations allow 10 times higher dose to the skin than to the whole body.
Also, your Geiger counter readings may be overresponding to a signal from beta particles. Radiation from NORM is typically 90–95% from beta particles and only about 5% from gamma rays, which are measurable in units of µSv. Geiger counters are calibrated with a gamma radiation source for readings in µSv, excluding beta particles. Therefore, if your Geiger counter is measuring beta particles, then your readings in µSv could be too high by a factor of 20. You can determine if your Geiger counter is measuring beta particles by covering your tiles with about 50 millimeters (mm) of plastic. If this blocks the signal, this means the signal is primarily from beta particles which are easily blocked by a thin layer of plastic.
As you have noted, the signal on your Geiger counter goes down rapidly with distance away from the tile. Thus, exposure to NORM radiation from your tile is related to how close you are to the tile and how long you would be there. The regulatory limits for the general public for a whole-body dose from gamma radiation from NORM in the United States are 20 µSv in an hour or 1,000 µSv in a year. To reach this annual limit for radiation from your tile would require 600 h or more of direct contact. This is assuming that your reading of 1.63 µSv h-1 is entirely due to gamma radiation (when it may really be mostly beta). Also, these tiles do not "leak" radioactivity into your room.
Consequently, even though your tile shelves emit a radiation signal, there is essentially no way to get a significant radiation dose from these shelves. This means that your husband is right, although he may not know of the technical reasons given above. You can continue to enjoy the beauty of your tiles and not be concerned about radiation.
Ray Johnson, MS, PSE, PE, DAAHP, FHPS, CHP