In the late 1930s, a smoker inadvertently made a discovery for detecting smoke. The Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. His device failed: small concentrations of gas had no effect on the sensor’s conductivity. Frustrated, Jaeger lit a cigarette—and noticed that a meter on the instrument registered a drop in the current. Smoke particles had apparently done what poison gas could not. Jaeger’s experiment was one of the advances that paved the way for the modern smoke detector.
Here’s something else surprising: Smoke detectors work because of radiation. They are an example of the beneficial uses of radiation and radioactive materials.
The first significant installations of commercial smoke detectors started in the US around 1969. Since then, the installation of smoke detectors has saved thousands of lives, numerous injuries, and millions of dollars. It has been reported that smoke detectors are installed in 93 percent of US residences. However, it is estimated that more than 30 percent of these alarms don’t work, as users remove the batteries or forget to replace them in a timely manner.
In the US, while smoke detector manufacturers and distributors are subject to NRC regulation, end users of smoke detectors (consumers) are typically not because of the small amount of radioactive material used in each detector.
The most common type of smoke detector consists of an ionization chamber, electronic circuitry, a power source that is usually a battery, an alarm mechanism, and an outer case. The ionization chamber is the main component. It consists of a source of ionizing radiation, usually Americium (Am-241) positioned between two oppositely-charged electrodes. The radiation source is a very small metallic foil disc about 3 to 5 millimeters in diameter.
To give you an idea of the small amount of radiation that is emitted by this disc, a person flying coast-to-coast gets more radiation from cosmic sources in one trip than a person sitting in the close proximity of an ionization smoke detector gets in a whole year.
Here is how the device works: Particles emitted during radioactive decay of the Am-241 interact with neutral air molecules flowing through the chamber and convert them to positive ions by removal of electrons. The removed electrons then form negative ions by attachment to other neutral molecules. The resulting positive and negative ions are attracted toward the electrodes, causing a small, reasonably steady current between the electrodes. The electronic circuitry monitors this current and, if the current drops below a preset level, which it will if the air entering the chamber contains enough smoke, it triggers an audible alarm.
If you are interested in the technical evaluations the NRC has done on smoke detectors and other consumer products containing radioactive material please see NUREG-1717 “Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials” .
Ujagar Bhachu Mechanical Engineer
U.S. NRC Blog 
you can’t “seal” gamma radiation. it will even go through lead. i have heard the tiny amount of radiation is not harmful unless you remove the tiny metallic disc and swallow it or touch it with an open wound (example a tiny paper cut) basically if it gets inside you it’s not a good thing. i don’t risk having them in my house for three reasons. the photoelectric detectors don’t have the radiation and will detect a smoldering fire like fabric or wood better than the ionization detectors plus they don’t go off as easily when you burn the toast…..really!
The amount of radiation is very minimal. There is no need to worry about health issues.
The sealed source in a smoke detector does not emit enough radioactivity to cause a burn, even if the smoke detector remains sealed in its original packaging for years.
Maureen Conley
Does radiation accumulate in the plastic packaging of smoke detectors? I just opened a package of unused detectors I bought years ago and my face feels sunburned. I wondered if I was exposed to accumulated radiation?
Terrific Article
Awfully interesting critique
It’s way more than just alarms. Microwaves, post-its, potato chips, and play-doh were also all invented by accident
Wow it’s weird to think smoke alarms, a device in just about every home in the US, came about accidently. Where would be if we didn’t make mistakes every once in a while?
There are no restrictions as to the number of smoke detectors containing Am-241 that can be disposed of at any one time by an end user because the potential radiation dose to both landfill workers as well as other members of the public is very small on a per detector basis. Rather, when the NRC examined the potential radiation dose from disposal of such smoke detectors the review was done on a cumulative dose basis assuming 10 million such detectors were disposed of annually across the country. See NUREG 1717 Subsection 2.15.4.3 beginning on page 2-221 of the NUREG (http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1717/)
Ujagar Bhachu
Whilst this is an interesting article, it should have included some basic safety information relating to the use and disposal of ionisation smoke detectors. Where I live, it is permissible to dispose of them in domestic trash (max 1 per bin, I believe), but I don’t know if this is universally the case.
I think New Zealand uses a lot of solar power. I admire that – Solar Power has a lot of potential.
an amazing story of how necessity breeds invention!
I disagree Matthew, your country’s anti nuclear status places it on a pedestal in terms of examples to follow. It has actually been one of the best forms of indirect marketing on the world stage, which actually makes it a capitalistic endeavour. Multi B$ Tourism sector or a few bucks off power? No brainier. With today’s renewable energy, your argument is even more moot.
What a fabulous invention to stumble upon. A great story.
With regard to the David Hahn, he did in fact extract the radioactive sources from smoke detectors and accumulate varying amounts of other radioactive materials from other commercially available sources. However, he never successfully built any type of device with those materials while causing a very localized contamination problem that needed to be professionally cleaned up.
Ujagar Bhachu
The primary radiation from a smoke detector is atomic particle called an alpha particle, which can travel only a short distance but has the ability to charge the air molecules in the ionization chamber inside the smoke detector thereby creating the current necessary to operate the detector. A microwave oven creates electromagnetic waves, which in the frequency spectrum from 300Mhz to 300Ghz, are called microwaves.
Ujagar Bhachu
While it may be time consuming and expensive, it is not impossible. Look up the story of the “Radioactive Boy Scout”. http://en.wikipedia.org/wiki/David_Hahn
He used Americium from smoke detectors.
Huh never knew there was raidiation in a smoke detector, thats kinda weird. Anyways I imagine I get more radioactive exposure from a freshly microwaved meal then from a smoke detector can put out anyday.
Yes, there are optical smoke detectors that do not rely on radioactive material. Those smoke detectors are more effective at quickly detecting smoldering/smoky fires while the ionizing smoke detectors (those containing radioactive material) are more effective at quickly detecting fires that proceed quickly to flame without as much smoke. Because each type of detector is more effective in certain situations, many fire experts recommend that they be used in combination with each other along with carbon monoxide detectors.
With regard to concern about aggregation, each smoke detector contains a very small amount of radioactive material, it therefore would be both time consuming and expensive to collect even a modest amount of material, and even if a large quantity of detectors was accumulated the form of the material inside each detector makes dispersal of the material difficult.
Ujagar Bhachu
Very interesting article, I too wonder about the cost to our bodies to the little radiation, would love to hear more on the subject.
This is a very interesting article and pertains to a subject that I have thought about for years. I do understand the small amount of radiation that a person is exposed to in this situation. However, what happens when you contemplate all of the different exposures we face today? There have been some great technological advances which have made life more convenient. I wonder at what cost to our bodies. You know, cell phone radiation, air port screenings, microwaves, xrays and MRIs and the list goes on and on. I believe the rate of cancer was much lower in the pre-50s era. I often times wonder if we are killing ourselves.
Is there any kind of risk of the radioactive substance becoming a danger? I worry that with so many smoke detectors around the country, someone could get a large amount of them and use them together, are there types of smoke detectors without a source of radiation?
Pity that my home country (New Zealand), is against any and all forms of Nuclear Power. I would love to enjoy the cost savings of having a nuclear power plant supplying the entire country, but sadly we are a tiny country of left-winged radicals.