Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Health physics implications of the ionisation smoke detector.

with 5 comments

It’s still occasionally heard from some sources, even after the technology has been in widespread use for many, many years, that the common household ionisation smoke detectors, which contain a very small radioactive source, present some kind of health hazard.
They don’t.

Such devices usually contain a sealed source, of 0.9 to 1.0 μCi of Americium-241. The source itself is a tiny little thing, about three millimeters in diameter – it’s a point source. *

241Am principally decays by emission of an alpha particle at 5.49 MeV, but this is not of any significance, since the α-particle cannot escape the device at all. However, 241Am also emits some low-energy gamma photons as it decays, principally a gamma ray of 59 keV, and it is this γ-radiation that can pass through the device and conceivably result in some dose to the householder, possibly.

The specific gamma-ray dose constant (“Γ factor”) for 241Am is 0.314 rem m2 Ci-1 h-1, or 3.14 x 10-9 Sv m2 μCi-1 h-1

(Here’s my source, a useful little reference table for this type of health physics information for several common, important, industrial, scientific and technical radionuclides.)

The dose rate, then, to the whole body from an external exposure to a point radioactive source which is emitting photon radiation is just the Γ-factor for the particular nuclide, multiplied by the activity of the source, multiplied by the familiar old inverse-squared distance term.

Suppose you’ve got such a detector on the ceiling, right next to your bed, which would place you about , say, 3 m away from the source. Suppose, additionally, that you spend your entire life in that bed. Of course, here I’m taking the most conservative scenario possible, to set an upper bound on the plausible dose.

3.14 x 10-9 Sv m2 μCi-1 h-1 x 1 μCi x 8765.8 hours x (3 m)-2 = 3.06 microsieverts per year.

Of course, the average worldwide dose from natural background radiation is almost 1000 times that – around 2.4 to 3 millisieverts per year, and in some places, far, far higher.

However, there is a more surprising and interesting context that we can put such a dose rate in. If you sleep in bed next to your partner every night, then the ionising radiation dose that you receive, due to the radioactivity of your partner’s body, from 40K and things like that, is about five microsieverts per year. [Source]

That’s assuming a realistic amount of sleep each day, of course – if you were actually in bed 24 hours per day, the dose from this source would be three times as much; 15 μSv per year.

Thus – the dose from sleeping next to your partner is 4.9 times what it is from the smoke detector. Surprising, isn’t it?

Obviously there is no reason to expect any significant health physics implications of any kind at such extremely low doses. Heck, such low doses are probably even too small to have potential significance with regards to radiation hormoesis. But we do know they’re proven to be extremely effective at protecting against the threat of fire destroying your home.

* Just as an aside, although technically illegal in the US and possibly in other localities as well, people (usually physicists and people that do know what they’re doing) do often remove these sources, and use them for educational demonstrations of radioactivity, of Geiger-Marsden style scattering, charged particle absorption, and to test and calibrate alpha and X-ray detectors – it’s quite tempting, since these devices are far, far less expensive than purchasing exactly the same sort of tiny sealed radioactive sources through “proper” channels, and no more dangerous.

Written by Luke Weston

October 23, 2008 at 10:33 am

5 Responses

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  1. Hi Luke. Thanks for linking to that reference table. I’ll have all kinds of fun with that.

    Why don’t you drop by my blog to check it out? Admittedly it’s nowhere near the quality of yours, and so far only has two posts, but I’ll get around to filling it out sometime soon.



    October 24, 2008 at 4:00 am

  2. Of course, Finrod. The discussion about complexity is interesting stuff. Of course, being able to move entropy around because we have access to plenty of energy does not just have interesting implications for chemical or thermodynamic systems – like life – but it also has profound implications in the context of information theory. Being able to move around information, and store enormous amounts of organised information is profoundly important, not only in the context of our brains, but also in our technology and our civilisation with all its prevalent information technology.

    By the way, I’ve always seen you around on DailyKos and various other websites, and I never did realise you were Australian.

    Luke Weston

    October 24, 2008 at 6:31 am

  3. I just wanted to say thankyou. You have very comprehensively provided a clear and concise description of the safe use of a radiation source in a domestic setting. I hope your’ blog has a lot of visits from people with a sufficiently open mind to realise the truth. I also hope that one day people will realise the cure for fearing the unkown is to find out more about it, preferably from a reputable source!

    Andrew Clarke

    December 15, 2008 at 2:35 pm

  4. Hi:

    I’ve been tuning into just being a lurker for a while now.
    I figured that I need to get involved and communicate with the other users here.
    I’m hoping to connect with a lot of wonderful people and discover some wonderful stuff.

    I really hope this message didn’t end up in a bad area. I am sorry if “” is incorrect.


    Farmers and Ranchers


    December 29, 2010 at 1:45 pm

  5. Luke

    I’ve been thinking about ionising smoke alarms for a while now, so finding this was interesting. Have you considered a multi-level building and the proximity to the body that might cause? For example, we live in a tall and skinny apartment which has four levels with an alarm on the ceiling of each. Obviously, that directly translates to an alarm less than 50cm from my head under the bed and an alarm about the same distance under a couch on a higher level. To add further complications, in each instance an alarm is on the ceiling about 3m away as well, so there are two sources (one at 50cm and one at 3m) in the bedroom and living area, not to mention two other alarms at greater distances with nothing much above them but floor space..I assume we would walk over them regularly.

    I guess what I am hinting at is that normal exposure might be higher than your calculations, given multiple levels (and thus proximity) and multiple devices.

    What are your thoughts? (and is there any way of measuring my bedroom scenario using your method above? – I’d be fascinated to know)

    Thanks and regards



    June 18, 2013 at 8:03 am

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