are smoke alarms?
Residential fire deaths
have decreased steadily as the number of homes with smoke
alarms increased. Reports from the National Fire Protection
Association show that people have nearly a 50 percent better
chance of surviving a fire if their home has the recommended
number of smoke alarms.
I need to replace my smoke alarm?
Smoke alarms that are
10 years old are near the end of their service life and
should be replaced. Some people think that their smoke alarm
sits idle until there is smoke present. Actually, it is
working every minute, constantly monitoring the air 24 hours
a day. An ionization smoke alarm, for example, goes through
3.5 million monitoring cycles in 10 years. In a photoelectric
alarm, a light is on constantly to look for smoke coming
into the sensing chamber.
Just like any electrical
appliance, the working components of smoke alarms wear out
over time. When the smoke alarm reaches 10 years of use,
the potential of failing to detect a fire increases. Replacing
them after 10 years reduces the likelihood of failure.
alarms are wired into my electrical system
I need to replace them as often as battery-operated alarms?
Yes. Both the hard-wired and battery-operated
alarms are equally affected by age.
How many smoke
alarms should I have?
recommendations are based on standards published by the
National Fire Protection Association (NFPA). The average-sized
home or apartment needs more than one. The exact number
depends on two things, the number of levels in the home
and the number of bedrooms. The NFPA publishes model requirements
that state or local authorities can adopt as law. The standards
are published in NFPA 72. The chapter on household fire
alarms contains separate requirements for new and existing
homes. For new homes the standard requires a smoke alarm
in each bedroom, one outside the bedroom area that is close
enough to be heard through closed doors, and a minimum of
one on each level of the home.
If the home is
large, use more than one on each level. The closer the alarm
is to the fire source, the faster it will work, so extra
units give you more safety. Some homes may have more than
one bedroom area, so they should have extra smoke alarms.
For existing homes,
the NFPA only requires a smoke alarm outside the bedroom
area and one on each level of the home. However, it also
recommends that homeowners install additional smoke alarms
like you would find in new homes. It makes sense to install
a smoke alarm in each bedroom. A good number of fires start
in bedrooms, and the closer the smoke alarm is to the fire,
the faster it will alert you. The recommendations are located
in Appendix A of the document.
places where I should not put a smoke alarm?
are not designed to work in extreme heat or cold, or in
areas where smoke and dust are common. They are not recommended
for unheated attics or similar spaces. They should not be
used in garages because they are usually not heated or cooled.
Also, the smoke from engine exhaust fumes will cause nuisance
alarms and clog the smoke alarm.
of concern is the kitchen. If a smoke alarm is installed
too close to cooking and baking, this may result in nuisance
alarms. The NFPA is addressing this problem and will soon
require that smoke alarms installed within 20 feet of cooking
be photoelectric or have a silencing button. We have a section
below on the two types of smoke alarms for residences.
What about heat
Heat detectors are
now referred to as heat alarms. Smoke alarms consistently
respond much faster than heat alarms. The nationally recognized
standard on fire alarms is Standard No. 72, published by
the National Fire Protection Association. Many fire officials
refer to the chapter on Household Fire Warning Systems for
determining their requirements for new and existing homes.
NFPA 72 does not require heat alarms in either new or existing
homes. It does, however, contain recommendations for additional
protection in Appendix A. The primary recommendations in
the appendix are for more smoke alarms, but heat alarms
are also addressed.
The reference in the
appendix talks about heat alarms for areas in the home where
smoke alarms are not recommended. The examples include garages,
attics, unheated crawl spaces and kitchens. The air in the
spaces like garages can become too hot or cold for smoke
alarms to operate properly. Smoke alarms are not recommended
for kitchens if the cooking causes nuisance alarms. On the
other hand, if the smoke alarm does not cause nuisance alarms,
then its location is fine.For all other spaces like living
rooms, dens, dining rooms and bedrooms, the standard recommends
smoke alarms. It cautions you against depending primarily
on heat alarms. The Consumer Product Safety Commission takes
a similar position.
Is there more than
one type of smoke alarm, and how do they differ?
There are two types of smoke alarms currently
available for homes. One type is called an ionization alarm
because it monitors "ions," or electrically charged particles
in the air. Smoke particles entering the sensing chamber
change the electrical balance of the air. The alarm's horn
will sound when the change in electrical balance reaches
a preset level.
The other type
of alarm is called photoelectric because its sensing chamber
uses a beam of light and a light sensor. Smoke particles
entering the chamber change the amount of light that reaches
the light sensor. The alarm sounds when the smoke density
reaches a preset level.
Is one type better
than the other?
Both types meet the
same test standards of Underwriters Laboratories, but each
has its own advantages. The ionization alarm responds faster
to small smoke particles, while the photoelectric responds
faster to large smoke particles. Flaming fires produce more
small smoke particles and smoldering fires produce more
Fire research shows
that a fire with a lot of small smoke particles will cause
an ionization smoke alarm to sound sooner than a photoelectric.
The time delay between the two is relatively small. On the
other hand, a fire with a lot of large smoke particles will
cause a photoelectric smoke alarm to sound sooner than an
ionization. In this case the time delay between the two
can be relatively long.
Because the ionization
type alarm is sensitive to small smoke particles, it will
respond more quickly to cooking. If you experience this
problem, you have several options:
1) Move the unit farther away from the
cooking area. The cooking gases will be more diluted in
the air when they reach the smoke alarm.
2) Install a photoelectric smoke alarm.
3) Install a combination ionization/photoelectric
alarm. When the two sensors are combined in one unit, the
ionization sensor is set at a slightly less sensitive level.
Thus the unit is less prone to nuisance alarms and you have
a unit with greater sensitivity to both fast-flaming and
slow smoldering fires.
4) Install an ionization smoke alarm
that has a hush button. This allows you to silence the unit
for 15 minutes while you remove the source of the nuisance
alarm (e.g., burnt toast) and clear the air. This is not
as convenient as options 2 or 3, but it will work as long
as you can reach the button and don't get aggravated enough
to remove the battery.
What is more important,
the type of alarm or the number?
The number of smoke
alarms is important. Smoke alarms that are spaced farther
apart may be farther from the fire, lengthening the time
it will take for smoke particles to reach it. Installing
several smoke alarms reduces the potential distance from
the fire source and provides better coverage, so the most
important thing is to install enough alarms in the proper
The distance from a
fire can also change the smoke, and this is where the difference
between the ionization and photoelectric smoke alarms becomes
important. What can take place is called "smoke aging."
As smoke particles move away from the fire, the heat dissipates
and the smoke cools. As smoke gets cooler, the smoke particles
and coagulate into larger particles. The farther the distance
from the fire to the smoke alarm, the cooler the smoke and
the larger the smoke particles. Ionization smoke alarms
are better at detecting smaller particles, while the photoelectric
respond better to larger particles. In these situations,
photoelectric smoke alarms have an advantage.
Happily, you no longer
have to choose between an ionization or photoelectric smoke
alarm. Units with both types of sensors are also available.
Because the unit has both types of sensors, the manufacturers
do not have to make the ionization sensor as sensitive,
thus making it less prone to nuisance alarms from cooking.
My smoke alarm
goes off when I cook. How can I stop this?
alarms are more sensitive to cooking smoke and gases than
the photoelectric type. There are several options for handling
this problem. One way is to replace the ionization alarm
with one that has a "silence button" that quiets it for
a few minutes. Another option is to move the alarm farther
away from the cooking area, giving the smoke a chance to
dissipate. Moving a ceiling-mounted alarm to a wall can
also reduce nuisance alarms. However, this will also make
it a little slower to respond to a real fire.
Another option is to
replace the ionization-type alarm with a photoelectric alarm.
This alarm is less sensitive to smaller smoke particles
and thus is less affected by cooking smoke.
You can also select
a combination ionization/photoelectric alarm (both sensors
in one unit). It will be less sensitive to cooking and responds
faster to smoldering fires than ionization units. The reason
that it is less sensitive to cooking is because the manufacturer
can make the ionization sensor less sensitive when a photoelectric
sensor is also present.
How can I test
Every smoke alarm comes
with a test button. We recommend that people test their
alarms regularly, at least once a month.
Should I use real
smoke to test my alarms?
This is not recommended
because the burning objects used to create the smoke might
cause a fire. Some stores sell pressurized cans of simulated
smoke for this purpose. When using this product, follow
the operating instructions and do not get the can too close
to the alarm. If held too close, the simulated smoke may
coat the alarm's sensing chamber, which can make the alarm
How important is
it to clean my alarms?
Periodic cleaning is
very important. Smoke alarms have small screens around the
sensing chamber to keep small bugs and dust particles out.
But dust can accumulate on the screen and slow air movement
through it. Cleaning is easy. Just vacuum around the outside
of the alarm at least once a year.
What about changing
Smoke alarm batteries
should last at least one year under normal conditions. The
biggest reason that smoke alarms don't work is because people
remove the batteries - either to stop the low battery signal
or a nuisance alarm - and forget to replace them. When a
battery reaches the end of its service life, the alarm will
give a short beep every minute or so. It is easy to remove
the battery and then forget to replace it. The best way
to prevent this is to replace batteries at the same time
each year before the low battery signal begins.
Are 10-year batteries
a good idea?
Smoke alarms sold with
10-year batteries should be sealed to prevent the replacement
of the battery because the smoke alarm itself should be
replaced after 10 years. The low-battery signal will begin
to "chirp" (a brief beep about once every minute), meaning
that it is time to discard the whole unit and replace it
with a new smoke alarm. You can buy 10-year batteries that
can be placed in other smoke alarms, but you should never
put one in an older new smoke alarm. The battery will outlast
the working life of the smoke alarm.
NOTE: Ten-year batteries will
not last for their stated service life in photoelectric
smoke alarms because this type of smoke alarm uses more
power than an ionization type. However, there is nothing
wrong with installing a long-life battery in them as long
as you remember to replace the smoke alarm itself when it
is ten years old.
Is the radioactive
element in ionization units a hazard?
No. The bit of Americium
241 that is used will not cause harm. It has less power
than the radium that is used in watch dials. To give you
an idea of its safety, a person flying coast to coast gets
more radiation in the sky than you do with a long-term exposure
to an ionization smoke alarm. In fact, you can discard the
smoke alarms in the trash without any special precautions.
When you read claims
that ionization smoke alarms are hazardous, be aware that
competitors or experts working for competitors have been
known to make assertions about the so-called radiation hazard.
We have found that they have no basis in fact. When ionization
smoke alarms were first manufactured, the federal government
monitored their use to assure that they would not pose a
risk to consumers. After several years of monitoring, the
government was confident that they were safe and halted
its monitoring program.