A smoke alarm, also known as a smoke detector, is a device that detects smoke and issues an audible sound and/or a visual signal to alert residents to a potential fire.

Facts and Figures

According to the Consumer Product Safety Commission:

• Although most newer homes have smoke alarms, about one-third of all deaths in house fires in 2011 occurred in homes that lacked working smoke alarms. 
• Older homes are more likely to lack an adequate number of smoke alarms because they were built before requirements increased.
• In 23% of home fire deaths, smoke alarms were present but did not sound. Sixty percent of these failures were caused by the power supplies having been deliberately removed due to false alarms. 
• Every year in the United States, about 3,000 people lose their lives in residential fires. Most of these deaths are caused by smoke inhalation, rather than as a result of burns. 

Smoke Alarm Types

Ionization and photo-electric are the two main designs of smoke detectors. Both types must pass the same tests to be certified to the voluntary standard for smoke alarms, but they perform differently in different types of fires. Detectors may be equipped with one or both types of sensors — known as dual-sensor smoke alarms — and possibly a heat detector, as well. 

These sensors are described as follows:

• Ionization smoke sensors are the most common and economical design and are available at most hardware stores. They house a chamber sided by small metal plates that irradiate the air so that it conducts electricity. When smoke enters the chamber, the current flow becomes interrupted, which triggers an alarm to sound. These sensors will quickly detect flaming-type fires but may be slower to react to smoldering fires. 
• Photo-electric smoke sensors use a light-sensitive photocell to detect smoke inside the detector. They shine a beam of light that will be reflected by smoke toward the photocell, triggering the alarm. These sensor types work best on smoldering fires but react more slowly to flaming fires. They often must be hard-wired into the house’s electrical system, so some models can be installed only in particular locations.

While heat detectors are not technically classified as smoke detectors, they are useful in certain situations where smoke alarms are likely to sound false alarms. Dirty, dusty industrial environments, as well as the area surrounding cooking appliances, are a few places where false alarms are more likely and where heat detectors may be more useful.

Location

Individual authorities having jurisdiction (AHJs) may have their own requirements for smoke-alarm placement, so homeowners can check with their local building codes if they need specific instructions. The following guidelines, however, can be helpful.

Smoke alarms should be installed in the following locations:

• on the ceiling or wall outside of each bedroom; 
• in the basement, preferably on the ceiling near the basement stairs; 
• in the garage, due to all the combustible materials commonly stored there; 
• on the ceiling or on the wall, with the top of the detector between 6 to 12 inches from the ceiling; and/or 
• in each story within a building, including basements and cellars, but not crawlspaces or uninhabited attics.

Smoke alarms should not be installed in the following locations:

• near heating or air-conditioning supply and return vents; 
• near a kitchen appliance; 
• near windows, ceiling fans or bathrooms equipped with a shower or tub; 
• where ambient conditions, including humidity and temperature, are outside the limits specified by the manufacturer’s instructions; 
• within an unfinished attic or garage, or in other spaces where temperatures can rise or fall beyond the limits set by the manufacturer; 
• where the mounting surface could become considerably warmer or cooler than the rest of the room, such as an inadequately insulated ceiling below an unfinished attic; or 
• in dead-air spots, such as the top of a peaked roof or a ceiling-to-wall corner.

Power and Interconnection

Power for the smoke alarms may be hard-wired directly into the building’s electrical system, or it may come from just a battery. Hard-wired smoke detectors are more reliable because the power source cannot be removed or drained, although they will not function in a power outage unless they also have batteries for backup. Battery-operated units often fail because the battery can be easily removed, dislodged or drained, although these units can be installed almost anywhere. Older buildings might be restricted to battery-powered designs, while newer homes generally offer more options for power sources. If possible, homeowners should install smoke alarms that are hard-wired with a battery backup, especially during a renovation or remodeling project.

Smoke alarms may also be interconnected so that if one becomes triggered, they all sound in unison. Interconnected smoke alarms are typically connected with a wire, but new technology allows them to be interconnected wirelessly. The National Fire Protection Agency requires that smoke alarms be protected by arc-fault circuit interrupters (AFCIs).

More Tips:

• Parents should stage periodic night-time fire drills to assess whether their children will awaken from the alarm and respond appropriately. 
• Never disable a smoke alarm. Use the alarm’s silencing feature to stop nuisance or false alarms triggered by cooking smoke or fireplaces. 
• Test smoke alarms monthly, and replace their batteries at least twice per year. Change the batteries when you change your clocks for Daylight Saving Time.  Most models emit a chirping noise when the batteries are low to alert the homeowner that they need replacement. 
• Smoke alarms should be replaced when they fail to respond to testing, or every 10 years, whichever is sooner. The radioactive element in ionization smoke alarms will decay beyond usability within 10 years.
• Smoke detectors should be replaced if they become damaged or wet, are accidentally painted over, are exposed to fire or grease, or are triggered without apparent cause. 
• Note the sound of the alarm. It should be distinct from other sounds in the house, such as the telephone, doorbell and pool alarm.