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Smoke Alarm Facts & Questions

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Smoke Alarm Facts 

Over 93 percent of homes in the U. S. now have at least one smoke alarm. As the number of smoke alarms in homes increased over the years, the residential fire death rate has dropped nearly 50 percent. This success story is tempered by the fact that the number of inoperable smoke alarms is growing. Experts warn that the trend to fewer fire deaths will reverse if we do not solve the problem of inoperable smoke alarms. They estimate that one in three homes have an inoperable smoke alarm. Some fire departments have found units in one half of homes to be disabled.

The two most common reasons for inoperable smoke alarms are:

  • Disabling them to prevent nuisance alarms.
  • Neglecting to replace units that are past their 10-year service lives.

Both problems are easily solved, but the sad truth is that fire departments have not made a concerted effort to attack the problem. Information on how to reduce nuisance alarms gives several tips on how to keep your smoke alarms working.  

Smoke Alarm Questions

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. See information below on ionization and photoelectric smoke alarms.

Just like any electrical appliance, the 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 increases 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?

All but the smallest 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. The objective of having a smoke alarm outside the bedroom area is to alert sleeping occupants of a fire that starts outside of the bedrooms. For this reason, if the bedrooms in a home are located in different areas, then each area should have its own smoke alarm. 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.

The requirements for existing homes are not as stringent. In 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, and we recommend that existing homes be equipped with at least the same number of 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. The manufacturers instructions will include the temperature range that the unit is designed for. Smoke alarms should not be used in garages for two reasons. First, garages are usually not heated or cooled, and thus are sometimes above or below the temperature range that the unit was designed for. Second, the smoke from engine exhaust fumes will cause nuisance alarms and clog the smoke alarm.

Another area of concern is the kitchen. A smoke alarm that is installed too close to cooking appliances may result in nuisance alarms. NFPA 72 addresses this problem by requiring that when a smoke alarm is installed within 20 feet of cooking, it should either be photoelectric or have a silencing button. A section below describes the two types of smoke alarms for residences.

What about heat detectors?

Heat detectors are now referred to as heat alarms. Smoke alarms consistently respond much faster to typical residential fires than heat alarms. The nationally recognized standard on fire alarms is Standard No. 72, published by the National Fire Protection Association (NFPA). Many fire officials use the chapter on Household Fire Warning Systems to set local 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, and refers to heat alarms there. 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 generally not recommended for kitchens because the cooking may cause 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 NFPA 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 designed for homes. One type is called an ionization alarm because it uses "ions," or electrically charged particles, to detect smoke in the air. Smoke particles entering the sensing chamber change the electrical balance of the air. The greater the amount of smoke, the higher the electrical imbalance. The horn will sound when the electrical imbalance 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. The sensing chamber is designed so that the light beam does not strike the sensor, but smoke particles entering the chamber deflect the light onto the sensor. The greater the amount of smoke entering the chamber, the more light will be deflected onto the sensor. The alarm sounds when the amount of light hitting the sensor reaches a preset level.

Is one type better than the other?

Both types can meet the 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 researchers have learned that a fire that generates 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, but these types of fires will make the room untenable to life more quickly, so time is of the essence. 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, but these types of fires take longer to make the room untenable to life. If you want the advantages of both, you can install one of each everywhere that a smoke alarm is required or recommended, or you can buy "combination" units that have both sensors.

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. If you choose this option, be aware of how air is flowing through the space. You don't want to move it away only to find out that the air current is carrying the cooking gases right to the new location.
  2. Install a photoelectric smoke alarm. You lose some warning time in a fire that generates smaller smoke particles, but you gain by removing the one nuisance that results in a lot of smoke alarms being disabled.
  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. The farther that smoke alarms are spaced, the farther they may be from a potential fire. The farther that smoke particles have to travel to reach a smoke alarm, the longer it will take for the alarm to sound. Installing more 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 recommended locations.

Another factor in deciding on the number and type of smoke alarms is the issue of "smoke aging." Some researchers have reported that the smoke from a fire can change as it travels away from a fire, and the change can impact on the type of smoke alarm that is installed. "Smoke aging" refers to how smoke particles can grow larger as they move away from the fire and the heat dissipates, although this may not be as prevalent in very hot fires. As smoke cools, the smoke particles join and form 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 can 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 while still giving earlier warning to flaming fires.

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 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. You can buy 10-year batteries that fit 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 in a year 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 some "experts" working for competitors have been known to make assertions about the so-called radiation hazard. We have found that the claims 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.