Answer to Question #4738 Submitted to "Ask the Experts"

Q

I have a question about the americium oxide contained within smoke detectors.
In the reading that I've done, americium oxide has been described as "insoluble," which I assume refers to its solubility in water. I've also read that someone could swallow the source without dissolving it, so I assume that this means it is insoluble in hydrochloric acid.

In my house, things splash, splatter, and spill all the time. Once, before I knew that smoke detectors contained americium, hair tonic from a broken plastic bottle spilled on an unused smoke detector that was lying on the counter. What kinds of chemicals would dissolve americium oxide, and, in a worst-case scenario, what would be the risk if such a chemical were spilled on the americium source inside of the smoke detector or accumulated there because of vapors? Is it encapsulated well enough that nothing could get through to it anyway?

A

First of all, the americium in a smoke detector is not encapsulated because the alpha particles it emits into the air inside the detector are essential for proper functioning and cannot penetrate encapsulating material. The americium oxide is bonded to a substrate and exposed directly to the air inside the smoke detector.

The U.S. Environmental Protection Agency describes how americium smoke detectors are constructed and how they work. Another fascinating Web site, "How Stuff Works," also provides information.

A question about water seeping through an americium smoke detector has been previously asked and answered by our expert in Question 4198.

One helpful Web site by the Uranium Information Center Ltd, has this to say (with some minor editing):

Americium is a silvery metal, which tarnishes slowly in air and is soluble in acid. Its atomic number is 95. Its most stable isotope, americium-243 (²⁴³Am), has a half-life of over 7,500 years, although ²⁴¹Am, with a half-life of 432 years, was the first isotope to be isolated. Americium oxide (AmO₂) was first offered for sale by the U.S. Atomic Energy Commission in 1962 and the price of $1,500 per gram has remained virtually unchanged since. One gram of americium oxide provides enough active material for more than 5,000 household smoke detectors.

The radiation dose to the occupants of a house from a domestic smoke detector is essentially zero, and in any case very much less than that from natural background radiation. The small amount of radioactive material that is used in these detectors is not a health hazard. On the other hand, the ability of domestic smoke detectors to save life and property has been demonstrated in many house fires.

Even swallowing the radioactive material from a smoke detector would not lead to significant internal absorption of ²⁴¹Am, since the dioxide is insoluble. It will pass through the digestive tract, without delivering a significant radiation dose. (Americium-241 is however a potentially dangerous isotope if it is taken into the body in soluble form in sufficient amounts. It decays by both alpha activity and gamma emissions and it would concentrate in the skeleton.)

The Agency for Toxic Substances and Disease Registry of the U.S. Centers for Disease Control and Prevention has published a toxicology profile for americium in which it states that AmO₂ is "soluble in acid." The constituents in hair tonic are not acidic enough, if they are acidic at all, to dissolve a significant amount of AmO₂. If they were, the hair tonic would be painful to use; it is also unlikely that such a strong solvent capable of dissolving AmO₂ could be ingested in significant quantities without causing immediate harm to the victim that is much greater than that which the dissolved AmO₂ might produce. If enough solvent (acid) is present to dissolve a large part of the AmO₂ in a smoke detector, the solvent is the danger, not the americium.

Robert Cherry, CHP, PhD