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Along the same lines, you shouldn’t buy your average smoke detector online, either. The radioisotopes used in the detector mechanism necessarily have a half-life, and it behooves you to check the manufactured/expiration date on the packaging before purchase.

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Cheap smoke detectors are pretty much all optical these days, and even the unbranded £2 Chinese ones have pretty sophisticated self-tests: https://www.youtube.com/watch?v=uzKAZCKjpU8

To be honest, I'm not sure whether there's even any point having expiration dates on modern photoelectric detectors, except as a way to sell more smoke alarms.

There is an expiration date on the battery which should be listed.

Smoke detectors use Americium-241 which has a half life of 432 years. Unless that expiration date is a few centuries in the future, it's not because of radioactive decay.

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