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What the Fire Service Community Should Know About Smoke Alarms

Firefighters and inspectors often are asked questions about smoke alarms for single family dwellings.

• Are they really effective?
• How many do I need?
• Which sensor type is best?
• Where should I put them?
• Will my family and I be able to hear them in an emergency?
• Are my current smoke alarms the best for my situation?

This information from the Consumer Product Safety Commission (CPSC) may help you answer some of those questions.

Fire Experience with Smoke Alarms

The value of having functional smoke alarms to protect one's life cannot be overestimated. United States fire loss statistics show that more than half of the approximately 3,932 annual fire deaths occurred in homes with no smoke alarms.

• Half of the deaths in homes equipped with smoke alarms resulted from fires in which the smoke alarm did not sound.
• Almost three quarters of the fire deaths occurred in homes without alarms or with non-functional smoke alarms.
• The death rate for homes with smoke alarms (operational or not) is 40-50% lower than the rate for homes without alarms.

Smoke Alarms Sensing Technology

Currently, there are two main types of residential smoke alarm sensing technology that make up most of the consumer market: ionization and photoelectric detection.

Ionization sensing units employ a microscopic amount of radioactive-emitting material that senses changes in the air due to smoke particles attaching themselves to the radioactive ions. Ionization detectors are best suited for sensing fast building, flaming types of fires.

Ionization sensing detectors tend to be the most popular for home installation because they cost less than other devices. One of their less attractive features is that they can be extremely sensitive to small environmental irritants such as steam, burned food, or insects. Frustration with the frequency of unwanted alarms caused by these non-fire sources often encourages homeowners to remove or disable their smoke alarms.

Photoelectric units employ a light beam in their sensing chambers to detect the presence of smoke. They are more effective on slow building, smoldering fires, and are less susceptible to nuisance alarms. Comparatively, they also cost more than ionization type alarms.

Smoke alarms that employ both photoelectric and ionization sensing technology also exist, but tend to be more expensive for the consumer market.

Improving Detection

Current research in the smoke alarm manufacturing industry is aimed at improving sensor detection to minimize the likelihood of unwanted or nuisance alarms. This is being studied to see if sensing chambers can do a better job at discriminating between normal environmental background particles and products of combustion. Research also is being conducted to determine if a lower concentration of combustion products can be detected without creating overly sensitive smoke alarms. The goal is to prevent tenants from removing or disabling devices.

Additional studies are evaluating improved sensor locations. National Fire Protection Association 72, National Fire Alarm Code, currently recommends the placement of smoke alarms outside sleeping areas, inside sleeping areas, and on each floor level within a dwelling including basements. The standard also encourages interconnection of all the alarms so if one senses smoke, the alarm devices will sound simultaneously.

There is additional research being conducted to see if there are performance improvements by increasing the number of sensors in a given area or locating sensors near heat producing equipment such as furnaces, fireplaces, boilers, and similar appliances.

Audibility Challenges

There have been numerous television news programs regarding audibility issues with home smoke alarms, especially with concerns about waking sleeping children and adolescents. In addition to these groups, there is equal concern about older adults or other persons with hearing loss, and intoxicated occupants whose sensory responses might be compromised.

One of the problems with audibility is that the noise emitted from typical home smoke alarm drops off dramatically as one moves away from the device. Sound pressure levels are measured in decibels (dBA), and are subject to acoustical behavior such that each time a unit of distance doubles away from the sound source, the sound pressure level drops about 6 dB. In a real room, reverberation will minimize some of this effect.

The following table provides examples of some comparative decibel ratings:

Category	dBA Range	Examples
Extremely loud	110	Rock music, model airplane
	106	Timpani and bass drum rolls
	100	Snowmobile, chain saw, pneumatic drill
	90	Lawnmower, shop tools, truck traffic, subway
Very loud	85	Smoke alarm
	80	Alarm clock, busy street
	70	Busy traffic, vacuum cleaner
	60	Conversation, dishwasher
Moderate	50	Moderate rainfall
	40	Quiet room
Faint	30	Whisper

Here are a few facts from residential smoke alarm research:

Smoke Alarm Audibility - Children and Older Adults

• Children under the age of 16 have longer periods of deep sleep compared to adults.
• Current smoke alarms may not reliably wake children under the age of 16.
• Current smoke alarms are effective in waking most adults not under the influence of drugs, alcohol, and sleep deprivation.
• Current smoke alarms may not reliably notify or alert seniors who are hearing impaired or have high frequency hearing loss.

In one test, 28 children aged 6-15 were exposed to an alarm received at 89 dB in the entire room. One-half of the test subjects slept through one or both tests.

• Of the 6-10 year olds, 71% did not reliably awaken.
• Of the 11-15 year olds, 28.5% did not reliably awaken.

In another test, 17 of 20 children aged 6-17 slept through one or both tests with alarms at 60 dB at their pillow. All their parents woke in the tests.

Children Under 5 - Higher Risk

• Children between the ages of 6 and 19 have death rates lower than the all-age average.
• Children 5 and under and adults 65 and over have death rates at least twice the all-age average.
• Present analysis of fatality data in home fires does not confirm the existence of a distinct problem with alarm effectiveness in rousing children in the 6 to 10 and 11 to 15 age groups.

Older Adults - Higher Risk

In older adults, the risk of dying from a fire increases with age. In 2002, adults 65 years and older had a fire death rate 2.5 times that of the general population, and those 75 years and older had a fire death rate 3 times the national average.

The increased risk for older adults may arise in part from decreased mobility, hearing loss, and resistance to new technologies including smoke alarms and residential sprinklers. One in three people older than 60 and half of those older than 85 have hearing loss.

Intoxicated - Higher Risk

Nearly half of all adult victims of home fires are impaired by alcohol when fatally injured, based on special studies that had access to blood alcohol results of victims. Alcohol and other drugs - whether prescription, non-prescription, or illegal - can make users harder to wake.

Smoke Alarm Audibility - Sound Loss

In 2005, the CPSC staff conducted sound loss measurements in three different test homes using single station smoke alarms. The homes were built between 1960 and 1989, and ranged from approximately 1,000 square feet to 3,300 square feet.

In the test measurements, CPSC discovered that the complexity of the path that the sound must travel determines the amount of attenuation (reduction) in the alarm signal. Closing a door lessens a smoke alarm signal 10 to 20 dBA. Each (home) level that sound travels attenuates a smoke alarm signal about 20 dBA.

The CPSC tests showed that in a small, single-level home one single station smoke alarm may be sufficient to alert occupants, even if the bedroom doors are closed. On two-level homes, sound levels from a single detector may not be sufficient to alert occupants in all areas of the home, causing a delay for some individuals to respond immediately. Sound levels may drop as low as 50 dBA, roughly equivalent to the sound level of a moderate rainfall.

In three-level homes the results were even more dramatic. A single smoke alarm may not be sufficient to alert occupants in all areas of a three-level home. Sound levels as low as 30 dBA (whisper) were reported.

Interconnecting multiple smoke alarms can improve audibility results.

What's (Maybe) in The Future?

Here are just a few of the changes that might appear in single station smoke alarms in the near future:

• Smarter smoke alarms that can discriminate between environmental nuisances and products of combustion.
• Multiple sensors within single alarms that operate on both ionization and photoelectric principles.
• New sensors that use yet undeveloped technology to sense products of combustion.
• Faster detection sensors that operate at lower smoke thresholds.
• New notification methods that offer between audibility, sound discrimination, and alerting.
• Better guidance on placement and usage for different types of smoke alarms for different areas in the home, including changes to NFPA 72, National Fire Alarm Code.

The Message to Take With You and Share?

Other than residential sprinklers, having an adequate number and placement of operating smoke alarms, with regularly practiced home exits drills, is your best defense against dying in a fire while asleep.

For More Information:

• Building and Fire Research Laboratory
• Home Smoke Alarm Tests