Answer to Question #1638 Submitted to "Ask the Experts"

Q

I would like some information on the radiation in ionization smoke detectors. I know they use americium, and I have done some research on all this. But I would like to know, as I have one about a meter and a half to two meters from where I sleep with my child, how much radiation we might be receiving from it, both alpha and gamma? Is it safe or not?

A

You are correct, ionization-type smoke detectors contain americium-241 (²⁴¹Am). The activity of ²⁴¹Am in a typical home smoke detector is approximately 1 microcurie (1 µCi) which equates to 2.2 million disintegrations per minute. Each of these decays results in the emission of one alpha particle. In addition, a low-energy (59.5 keV) gamma ray is produced in approximately 36% of these decays. The only potential radiation exposure to your family is due to these gamma rays—alpha particles cannot penetrate a single sheet of paper, let alone the metal chamber in which they are located and the plastic housing of the detector. Indeed, the radioactive material is encapsulated in such a fashion that even if the device were opened up and the source removed and swallowed, the radioactive material could not be absorbed by the body.

Permit me to quote a relevant section from the Nuclear Regulatory Commission's publication NUREG 1717 "Systematic Radiological Assessment of Exemptions for Source and Byproduct Material":

"A resident who purchases, installs, and maintains two smoke detectors in the home, sleeps 8 h/day, and spends 4 h/day at other activities in the home would receive an annual EDE of 1 x 10^-5 mSv (0.001 mrem). Essentially all of the dose is from the single detector located in the master bedroom."

We can also do our own calculation assuming a distance of 1.5 meters:

The exposure rate at 1 meter from a bare one millicurie ²⁴¹Am source is listed as 0.015 mrem/hr. As such, the exposure rate at one meter from a one microcurie source would be 0.000015 mrem/hr. However, this completely ignores the significant attenuation of the low-energy gamma rays in the source encapsulation, the detector assembly, and the air. Factoring in the attenuation, the exposure rate at one meter has been estimated at 0.0000039 mrem/hr (this is consistent with measured values). At 1.5 meters, the exposure rate would be lower, approximately 0.0000017 mrem/hr. Sleeping eight hours per day equates to an exposure time of approximately 3,000 hours per year. As such, the annual exposure should be close to 0.005 mrem (0.0000017 mrem/h x 3,000 hrs). The lower value calculated in NUREG 1717 was undoubtedly due to a greater assumed distance between the smoke detector and the bed—exposure rate drops off rapidly with distance.

Is this exposure safe? Compare our estimated annual exposure from your smoke detector, 0.005 mrem, to the 10 mrem from a typical chest x ray or the 100 to 200 mrem per year the average homeowner receives from radon.

How about rephrasing the question: "Is not having a smoke detector safe?"

As I remember, and I must admit my memory is not what it used to be, some two dozen or more nursing home residents died in fires at the time of the accident at Three Mile Island because the homes lacked smoke detectors.

Paul Frame, PhD, CHP