Quincy, MA, October 23, 2001-Replacing batteries in home smoke alarms will be a common ritual this weekend for many people as daylight savings time ends. But if smoke alarms in your home are more than 10 years old, NFPA (National Fire Protection Association) recommends replacing them, as well.

Why? According to NFPA, aging smoke alarms don’t operate as efficiently and often are the source for nuisance alarms. Older smoke alarms are estimated to have a 30% probability of failure within the first 10 years. Newer smoke alarms do better, but should be replaced after 10 years. Unless you know that the smoke alarms are new, replacing them when moving into a new residence is also recommended by NFPA.

Smoke alarms, when properly installed, give an early audible warning needed to safely escape from fire. That’s critical because 85% of all fire deaths occur in the home, and the majority occur at night when most people are sleeping. Last year, NFPA documented 3,420 home fire deaths.

Fully 94% of U.S. homes had at least one smoke alarm as of 1997, according to NFPA, but as of 1998, 40% of the home fires reported to U.S. fire departments and 52% of home fire deaths still occurred in the small share of homes with no smoke alarms. Half of the deaths from fires in homes equipped with smoke alarms resulted from fires in which the smoke alarm did not sound–usually when batteries were dead, disconnected or missing.

“Simple steps like maintaining smoke alarms and replacing older ones help diminish the possibility of fire deaths in the home,” says John R. Hall, Jr., NFPA’s assistant vice president for fire analysis and research. “Smoke alarms in the home are largely responsible for the decreasing number of home fire deaths over the last decades.”

NFPA offers the following smoke alarm safety tips:

Install new batteries in all alarms once a year or when the alarm chirps to warn that the battery is dying.

Test units at least monthly. Test the units using the test button or an approved smoke substitute.

Clean the units, in accordance with the manufacturers’ instructions.

Do not use an open-flame device for testing because of the danger the flame poses.

Smoke alarms should be placed outside each sleeping area and on each level of the home, including the basement.

In new homes, smoke alarms are required in all sleeping rooms, according to the National Fire Alarm Code.

Alarms should be mounted on the wall 4-12 inches from the ceiling; ceiling-mounted alarms should be positioned 4 inches away from the nearest wall. On a vaulted ceiling, be sure to mount the alarm at the highest point of the ceiling.

As electronic devices, alarms 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 alarm 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 alarm 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 alarms will fail almost immediately, and 3% will fail by the end of the first year. After 30 years, nearly all the alarms will have failed, most years earlier.

How soon should you replace your alarm? This is a value judgment. Only 3% of alarms 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 alarm has failed, and that seems too big a risk to take. Manufacturers’ warranties for the early alarms 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 alarms, 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 alarm failure as well as a dead or missing battery. You can replace your alarm when it needs replacing.

The same study showed all the inoperable alarms 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 alarm 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 alarms after 10 years protects against the accumulated chance of failure, but monthly testing is still your first, best means of making sure alarms work. Today’s alarms are even less vulnerable than the original alarms. 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. NFPA (National Fire Protection Association)

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