Kidde smoke alarms sold in Canada have an expiration date printed onto the side of the case. Smoke alarms by First Alert sold in the same store don't have that. First Alert suggests to note down the installation date and replace it 10 years later.
A google search finds the following reasons why smoke detectors have a maximum life time of 10 years:
There are two type of smoke detectors. One is called photoelectric smoke detector and the other one is an ionization chamber smoke detector. Ionization chamber smoke detectors are the older type. They contain a quite dangerous man made radio active element known as americium-241. It's an element that does not exist naturally and it's a strong alpha-emitter. The americium-241 isotope is made from plutonium-239. Alpha radiation does not get very far but it can cause enormous damage if it hits a living cell. Americium-241 emits also gamma rays, a kind of invisible light that is very hard to shield. Americium-241 is extremely dangerous if ingested or inhaled. It's not a good idea to throw americium-241 into landfills where it could one day show up in the ground water. It's not a good idea to throw it into a trash incinerator either because americium-241 vapors are equally dangerous. In many places it is however allowed and even recommended to just throw the americium-241 detectors every 10 years into the household garbage.
The photoelectric smoke detectors are very easy to understand. It's basically a light beam shining into a dark chamber. If the air is clear and clean then the beam is invisible. If dust or smoke is
in the air then the light beam become visible and a detector can see it.
Ionization chamber smoke detectors ionize the air in a small chamber by bombarding it with alpha radiation. This makes the air electrically conductive. Two metal plates with a voltage across will allow current to flow. If smoke get's into this air gap then less air molecules will be ionized since the
smoke particles which are rather large compared to air molecules will absorb most of the alpha radiation. The detector electronics will register a drop current flow between the metal plates and the alarm will go off.
Manufactures have built a test button into the alarm and they tell you to press it in order to test the smoke detector. What can such a button actually test? It will test the horn and maybe parts of the electronics. It can never test the actual sensor (aka smoke detector). This button does not simulate real smoke.
The only way to actually test a smoke alarm is to use real smoke. Here is how to do such a test:
While investigating the "smoke alarm expiration mystery" I came across a number of articles that recommend regular cleaning of the alarm with a vacuum cleaner.
A normal household vacuum cleaner can't clean the photoelectric chamber or the ionization chamber. It removes dust from the outside. Some people have done a tear-down of the smoke alarm and you can see how difficult it would be for a vacuum cleaner to get any dirt out of the ionization chamber:
There is no reason why a smoke alarm can not last a life time except maybe for those models that have a built-in battery that can not be changed. The 10 year expiration is totally arbitrary. It's to ensure a constant flow a income for the manufacturer and they have lobbied governments to publish nonsense guidelines in the name of safety. Fear and safety provide a good business model. How much dust and dirt accumulates has nothing to do with those 10 years. It's totally dependent on the environment around the alarm. It could already malfunction after a few years or it could last much longer. Cleaning of the alarm with compressed air and then testing it with real smoke can ensure that it actually works.
There is no reason why a fire alarm that is maintained and kept clean needs to be replaced after a specific number of years. I have a fire alarm that is fully functional and exactly as sensitive as on the first day but it's already 15 years old.
Guidelines for those alarms claim that the americium-241 isotope
is well encased and shielded inside the alarm. Outside radiation is supposed to be undetectable compared to background radiation. Is that really true?
The ionization chamber has openings on the side and the top of the chamber is made of solid shield metal about 0.5mm thick. This chamber sits inside the bigger plastic case of the smoke alarm.
The expiration time on the smoke alarms is a scam. Only regular maintenance and cleaning with compressed air can ensure that it really works. If you check it and clean it then it will last much longer than 10 years (or it might fail much earlier if you got a lemon). I would avoid smoke alarms with a built-in 10year battery because those have to be replaced when the battery runs out.
Americium-241 is a quite dangerous man made isotope that can contaminate the environment and millions of smoke alarms end up every year in landfills. Americium-241 has a half life of 432 years.
Keep a physical distance to smoke alarms with ionization technology. I would not install them in a room where you spend a lot of time. It may be OK in the staircase or the corridor where you are just passing by. This is to limit the radiation exposure time. Install it such that you will under normal circumstances not get closer to it than 3 feet. To be less than 1 foot away from the alarm for longer periods of time is in my opinion unsafe.
Most of the radio active radiation in living spaces comes from Radon gas. Dosimeter readings above 6 uSv/h due to Radon gas are considered unsafe for prolonged habitation. The radiation coming out of the smoke detector is a factor 14 smaller but it's not because of Radon gas. The smoke detector emits gamma rays and I don't know what dosimeter limits should be applied to gamma rays.