There are two types of smoke alarms on the market.
- Ionizing Smoke alarms
- Photoelectric Smoke alarms.
How do they work?
Ionization smoke alarms are generally the most responsive to flaming fires. They have a small amount of radioactive material (americium-241) between two electrically charged plates which ionizes the air and causes current to flow between the plates. When smoke enters the chamber, the current flow becomes interrupted thus triggering the alarm.
Photoelectric smoke alarms are generally more responsive to fires that begin with a long period of smouldering (smouldering fires). They contain a light sensitive photocell and a beam of light. When smoke enters the chamber, the light is reflected toward the photocell which then triggers the alarm.
Which type should I use in my home?
Most homes in Canada that have smoke alarms installed use Ionizing alarms. They are less expensive than Photoelectric alarms and unfortunately, they don't preform as well in real world situations. Saving a few dollars on a less expensive alarm may be a costly mistake. Studies have shown that, when all factors are taken into account (i.e.- how often each alarm gets disabled due to nuisance tripping, how they respond across the full spectrum of fires, etc.) photoelectric alarms have a clear advantage over ionizing alarms. In smouldering fires, ionization alarms respond an average of 15 to 50 minutes slower than photoelectric alarms. Several studies indicate that they will outright fail to activate up to 20-25% of the time. The vast majority of residential fire fatalities are due to smoke inhalation, not from the actual flames and almost two-thirds of fire fatalities occur at night while we sleep.
Ionization alarms are also notorious for nuisance tripping (going off when you cook, burn toast, shower, etc). When alarms nuisance trip, people become frustrated and intentionally disable the alarms leaving your home and family without any protection. Ionization alarms account for the vast majority of disabled alarms.
The International Association of Firefighters (IAFF) is the largest firefighters union in the US and Canada with nearly 300,000 members. In 2008, the IAFF adopted an official position recommending that only photoelectric smoke alarms be installed. The IAFF position also commits the organization to working for changes in the law and model codes to require photoelectric technology alarms. Further, the IAFF position specifically states that combination type alarms are not acceptable.
What about Combination Alarms?
These units have the same issues as ionization only detectors. In some cases – they may be worse. A US Consumer Product Safety Commission (CPSC) study shows that they may be even more prone to nuisance tripping than ionization alarms when in close proximity to cooking sources. Combination alarm use what is called a Gated Logic technology.
In one type, either sensor tripping will sound the alarm. In these units, the photo portion will pick up the smouldering fires so the ionization sensor does not become a factor. However, the ion portion is still susceptible to nuisance tripping. The manufacturers do not want the customer to disable the alarm, so to combat nuisance tripping, they often desensitize the smoke sensitivity/response of ionization portion of these units. In effect, this type of combination alarm performs similarly to a photoelectric only alarm.
Another combination type alarm requires both sensors to trigger before the alarm will sound. In these units, the photoelectric portion will pick up the smouldering fires first, but will not sound until the ionization sensor triggers. Since a smouldering fire usually pose the greatest danger, this is a problem.
Current smoke alarm standards are essentially the same as those developed in the 1970’s. They use two test scenarios. One is The Fast Flame scenario, the other is The Smouldering Fire scenario. A fast flame fire is a fire that is based on accelerants, such as gasoline, cooking oils, grease, and paper fire. A smouldering fire is the early stages before open flames develop and is characterized as slow moving with significant smoke. The smouldering fire tests standards were developed when most home furnishings were natural materials, cotton, wool, etc. Today, virtually all furnishings and a large percentage of the building materials are synthetic and engineered materials. The behaviour of natural and synthetic materials in a fire is radically different. Yet the test standards have not been adjusted to account for this shift.
Ionization alarms will typically respond about 30 to 90 seconds faster to fast-flame fires than photoelectric smoke alarms. However, in smouldering fires, ionization alarms respond an average of 15 to 50 minutes slower than photoelectric alarms. Several studies indicate that they will outright fail to activate up to 20-25% of the time. The vast majority of residential fire fatalities occur at night while we sleep and are the result of smoke inhalation- not from the actual flames.
There is no alarm that can save all people in every type of situation. New technologies are being developed that project promising improvements. With everything we know, all the facts tell us that photoelectric alarms provide superior protection in real-world fatal fires over ionization alarms. They are affordable and available today. This year, don't just replace your smoke alarm batteries – replace your alarms with photoelectric alarms and recommend that everyone you know do the same!
Where should I locate My Smoke Alarms?
Smoke alarms should be installed in the following locations:
- In each bedroom. Most fires occur at night while people are sleeping.
- On the ceiling or wall outside each separated sleeping area.
- On the basement ceiling near the stairs.
- In the garage.
- In each story within a building including basements and cellars.
Smoke alarms should not be installed in the following locations:
- Near heating or air-conditioning supply and return vents.
- Near kitchen appliances.
- Near windows, ceiling fans or bathrooms equipped with a shower or tub.
- Within unfinished attics or garages, or in other spaces where temperatures can rise or fall beyond the limits set by the manufacturer
- In dead-air spots, such as the top of a peaked roof or a ceiling-to-wall corner.
Power and Interconnection
Power for the smoke alarms may be hard-wired directly into the building’s electrical system, or it may come from a battery. Hard-wired smoke detectors are more reliable because the power source cannot be easily removed or drained, although they will not function in a power outage. Battery-operated units often fail because the battery can be easily removed, dislodged or drained, although these units can be installed almost anywhere. If possible, homeowners should install smoke alarms that are hard-wired with a battery backup, especially during a renovation or remodelling project.
Smoke alarms may also be interconnected so that if one becomes triggered, they all sound in unison. Interconnected smoke alarms are typically connected with a wire, but new technology allows them to be interconnected wirelessly. The National Fire Protection Agency requires that smoke alarms be AFCI-protected.
Recommendations and Tips
- Consider replacing your ionizing smoke alarms with photoelectric smoke alarms.
- Parents should stage periodic night-time fire drills to assess whether their children will awaken from the alarm and respond appropriately.
- Never disable a smoke alarm. Use the alarm’s silencing feature to stop nuisance or false alarms triggered by cooking smoke or fireplaces.
- Test smoke alarms monthly, and replace their batteries at least twice per year. Change the batteries when you change your clocks for Daylight Saving Time. Most models emit a chirping noise when the batteries are low to alert the homeowner that they need replacement.
- Smoke alarms should be replaced when they fail to respond to testing, or every 10 years, whichever is sooner. The radioactive element in ionization smoke alarms will decay beyond usability within 10 years.
- Smoke alarms should be replaced if they become damaged or wet, are accidentally painted over, are exposed to fire or grease, or are triggered without apparent cause.