How effective 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.

When do 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 smoke is present. But it is working every minute, constantly monitoring the air 24 hours a day. For example, an ionization smoke alarm goes through 3.5 million monitoring cycles in 10 years. In a photoelectric smoke alarm, a light operates 24 hours a day to check for smoke particles in the air.

Just like any electrical appliance, the working components of smoke alarms wear out over time. When a smoke alarm reaches 10 years of use, the potential of failing to detect a fire has increased substantially. Replacing them after 10 years reduces the likelihood of failure.

My smoke alarms are wired into my electrical system. Do 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?

An 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 following recommendations are based on a standard published by the National Fire Protection Association (NFPA 72), which contains a chapter on household fire alarms.

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 a home is large, it is better to use more than one on each level. The closer the smoke 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 the additional smoke alarms that are required 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. These recommendations are located in Appendix A of the standard.

Are there places where I should not put a smoke alarm?

Smoke alarms are not designed to work in extreme heat or cold, or in areas where smoke and dust are common. Thus, they are not recommended for unheated attics or similar spaces. They should not be used in garages for two reasons. First, garages are usually not heated or cooled. Second, the smoke from engine exhaust fumes will cause nuisance alarms and clog the smoke alarm.

Another area of concern is the kitchen. If a smoke alarm is installed too close to cooking and baking, this may result in nuisance alarms. The latest edition of NFPA (to be published in late 1999) addresses this problem by requiring that smoke alarms installed within 20 feet of cooking either be photoelectric or have a silencing button. A section below describes the two types of smoke alarms for residences.

Is there more than one type of smoke alarm, and how do they differ?

There are two kinds of sensors available, an ionization-type and a photoelectric-type.

The ionization sensor gets its name because it measures the electrical balance in the air between positive and negative charges, or ions. Inside the sensor, a tiny piece of radioactive material creates a small electrical current in the air that flows through the sensor chamber. A computer chip on the circuit board monitors the electrical current. When smoke particles enter the sensor chamber they upset the balance between the positive and negative electrical charge. This changes the current flow. As the smoke gets more dense the imbalance increases. When the imbalance reaches a certain threshold, the horn will sound to alert you.

The photoelectric sensor is named so because it uses a light source to measure smoke density. The light source is constantly on. A light sensor is also in the sensing chamber but not in the direct path of the light beam. When smoke enters the sensor, the particles break up the light beam, scattering some of the light toward the receiver. As the smoke density increases, more light is scattered toward the receiver. When the amount of deflected light reaches a certain threshold, the horn will sound to alert you.

Both types of sensors pass the tests required by Underwriters Laboratories for residential smoke alarms, but they do differ in their sensitivity to certain kinds of smoke.

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 large particles.

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.

My smoke alarm goes off when I cook. How can I stop this?

Ionization-type smoke alarms are more sensitive to cooking smoke and gases than the photoelectric type. Although you cannot see any smoke, the combustion from the flame is causing "products of combustion" to be released. When these particles are under one micron in size, they cannot be seen with the naked eye. When they are larger than one micron, these "products of combustion become visible smoke.

Although the small particles are too small to be seen, they will change the electrical balance in the ionization sensing chamber, thus making the smoke alarm operate. 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 particles a chance to dissipate; The fewer particles per volume of air, the less likely they are to cause the imbalance in the sensing chamber. If the smoke alarm is ceiling-mounted, moving it to a wall can also reduce nuisance alarms. However, this will also make it a little slower to respond to a real fire. If you do this, make sure to follow the manufacturers instructions for the correct height. If you misplaced your instructions, stop in at a hardware store and review the instructions in one of the smoke alarm boxes.

Another option is to replace the ionization-type alarm with a photoelectric alarm. This alarm is less sensitive to the smaller (sub-micron) 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 will respond 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 my alarm?

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 or affect the connections, which can make the alarm inoperable.

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 batteries?

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 are normally sealed to prevent the replacement of the battery. This is 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.

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.

** This Article provided courtesy of the Wake County Fire Marshals Office.

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