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Why NFPA recommends home smoke detectors be replaced after 10 years:

Smoke detectors are one of the most important safety features of your
home. Properly installed, working smoke detectors will give you the
early warning you need to safely escape from a fire. But how do you
make sure your detectors are working? One important way is to replace
them after 10 years.

As electronic devices, detectors are subject to random failures.
Product, installation, and maintenance standards are used to assure
products work as designed despite this. Part of the technical basis for
the first detector product standard was an assessment of expected
failure rate, estimated at four per million hours of operation or one
every 30 years. Early field studies of detector reliability, notably by
Canada's Ontario Housing Corporation, confirmed the essential accuracy
of this estimate, restated as a 3% failure rate per year. This means a
very small fraction of home smoke detectors will fail almost
immediately, and 3% will fail by the end of the first year. After 30
years, nearly all the detectors will have failed, most years earlier.

How soon should you replace your detector? This is a value judgment.
Only 3% of detectors are likely to fail in the first year, and annual
replacement would be very expensive, so that doesn't make sense. At 15
years, the chances are better than 50/50 that your detector has failed,
and that seems too big a risk to take. Manufacturers' warranties for
the early detectors typically ran out in 3-5 years. So, in ten years
there is roughly a 30% probability of failure before replacement. This
seemed to balance safety and cost in a way that made sense to the
responsible technical committees.

If a 30% failure probability still seems too high, remember that
replacement on a schedule is only a backup for replacement based on
testing. A national study found home smoke detectors, when they fail,
tend to fail totally, as opposed to hard-to-detect creeping failure,
such as a loss of sensitivity.1 Regular monthly testing will help
discover detector failure as well as a dead or missing battery. You can
replace your detector when it needs replacing.

The same study showed all the inoperable detectors tested in 1992 were
at least 5 years old and predated a 1987 change in product standards
that reduced sensitivity to reduce nuisance alarms. Changes in detector
chip design, among other improvements, make it likely that electronic
failure now occurs at a rate much less than 4 times per million hours
of operation.

Replacing detectors after 10 years protects against the accumulated
chance of failure, but monthly testing is still your first, best means
of making sure detectors work. Today's detectors are even less
vulnerable than the original detectors. Regular maintenance of the more
sophisticated systems used in larger buildings can keep them working
very reliably for many decades.

1 Julie I. Shapiro, Smoke Detector Operability Survey, Washington: U.S.
Consumer Product Safety Commission, October 1994 revised.