What Is In A Smoke Detector?

Smoke alarms detect fires by sensing small particles in the air. Once they detect those particles above a certain level, they signal the alarm to sound so that you and your family can get to safety and call 911. When working smoke alarms are present in your home, the risk of dying in a home fire is cut by 60 percent, according to the latest NFPA research. Fire spreads fast—working smoke alarms give you early warning so you can get outside quickly.

How Ionization Smoke Alarms Work

Ionization detectors use a small bit of safely shielded radioactive material that electrically charges, or ionizes, the air molecules between two metal plates. This produces a small electric current flowing from one plate to the other in the air. When particles enter the chamber, they attract the ions and carry them away, reducing the current. When the number of particles entering the chamber is enough to reduce that current below a certain amount, the device will register those particles as smoke and the alarm will sound.

Most of the radiation from the radioactive material is blocked inside the device, and even then, the radiation levels in the device are much lower than the natural background radiation to which we are exposed every day. Ionization alarms are more responsive to flaming fires.

How Photoelectric Smoke Alarms Work

Photoelectric detection technology works by detecting light that is reflected off particles from a light beam inside the sensing chamber. When no particles are present in the sensing chamber, the light from the beam does not strike the light detector, indicating all clear. When there are particles present and the amount of light registered by the light detector reaches a certain threshold level, the alarm sounds.

Photoelectric alarms are more responsive to smoldering fires. Photoelectric smoke alarms are the best type of alarms to be installed near the kitchen and bathrooms to reduce nuisance alarms.

Dual Sensor and Multi-Criteria Alarms

• Dual sensor smoke alarms utilize both photoelectric and ionization technologies to ensure maximum coverage.
• Using a mix of photoelectric and ionization alarms — or dual sensor units — is recommended for broader fire detection coverage.
• New multi-criteria smoke alarm technology will state "helps reduce cooking nuisance alarms" on the packaging.
• Consider using new multi-criteria alarms when it is time to replace existing photoelectric or ionization alarms.

Types of Smoke Alarm Power Sources

10-Year Sealed Battery

10-year sealed battery alarms eliminate the need for battery replacements and late-night low battery chirps for the life of the device.

Hardwire Smoke Alarms

Most hardwire detectors include a hardwire connector plug that makes installation easy. For coverage during power outages, a back-up battery feature provides continuous detection.

Battery-Operated Smoke Alarms

Battery-operated detectors are easy to install throughout your home where needed and are simple to maintain. Depending on the model, they can be mounted onto walls, ceilings, or placed on desktops.

Combination and Interconnected Alarms

Combination smoke and carbon monoxide (CO) detectors provide 2-in-1 detection against both fire and CO in one convenient device. For the best protection, use combination smoke alarms that are interconnected throughout the home. These can be installed by a qualified electrician so that when one sounds, they all sound. This ensures you can hear the alarm no matter where in your home the alarm originates. Interconnected alarms link throughout your home, ensuring that when one alarm detects a threat, all other connected alarms sound.

For interconnected systems, alarms must be compatible — ideally from the same brand and series — to ensure they communicate properly. Mixing alarms from different manufacturers may prevent alarms from triggering together, even if the wiring is the same.

Where to Install Smoke Alarms

1. Install smoke alarms in every sleeping room and outside each separate sleeping area.
2. Install alarms on every level of the home, including the basement.
3. Do not put smoke alarms in your kitchen or bathrooms.
4. A smoke alarm should be on the ceiling or high on a wall.
5. Keep smoke alarms away from the kitchen to reduce nuisance, or false, alarms. They should be at least 10 feet (3 meters) from the stove.
6. A two-story home with three bedrooms typically needs at least five alarms. Large homes may require more based on layout and code requirements.
7. Only qualified electricians should install hardwired smoke alarms.

Smoke Alarm Sounds and What They Mean

Continued set of three loud beeps — beep, beep, beep

Means smoke or fire. Get out, call 9-1-1, and stay out.

A single chirp every 30 or 60 seconds

Means the battery is low and must be changed.

Chirping that continues after the battery has been replaced

Means the alarm is at the end of its life and the unit must be replaced.

Testing and Maintenance

• Test all smoke alarms at least once a month. Press the test button to be sure the alarm is working.
• If the alarm does not sound, change its battery or, if the unit is over 10 years old, replace the entire unit.
• Follow the manufacturer's instructions for cleaning to keep smoke alarms working properly.
• First alert smoke alarms should be replaced at least every 10 years. Check the manufacturer date on the back of the alarm — if it is 10 years old or older, the entire unit should be replaced, even if it still seems to work, as the smoke sensors can lose sensitivity over time.
• All smoke alarms must be replaced 10 years after their manufacturing date.

Alarms for People Who Are Deaf or Hard of Hearing

People who are deaf or hard of hearing may not be able to depend on a traditional smoke alarm to alert them to a fire. There are smoke alarms and alert devices designed for these individuals. These devices include strobe lights that flash to alert people when the smoke alarm sounds. Pillow or bed shakers designed to work with your smoke alarm can also be purchased and installed — these work by shaking the pillow or bed when the smoke alarm sounds. These products can be found online and in stores that sell smoke and CO alarms. Make sure to choose smoke alarms and accessories for people who are deaf or hard of hearing that are listed by a qualified testing laboratory. It is also good practice to sleep with your mobile phone and your hearing aids or implants close to your bed.

Difference Between a Smoke Detector and a Smoke Alarm

There is a difference between a smoke "detector" and a smoke "alarm." A smoke detector is just a sensor that monitors for smoke and is connected to a whole building fire alarm system. A smoke alarm has both the sensor to monitor for smoke and the speaker that makes the sound to notify the home occupants. When referring to the devices found in most homes, smoke alarms is the correct term to use.

In the event of fire, you may have as little as 2 minutes to escape safely, which is why smoke alarms need to be in every bedroom and sleeping area, outside of the sleeping areas such as a hallway, and on each level of the home including the basement. In two to three minutes, a typical living room or bedroom can become fully engulfed in flames. Working smoke alarms are an important part of a home fire escape plan, and make sure everyone in the home understands the sounds of the smoke and carbon monoxide alarms and knows how to respond.