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Smoke Alarms Are Not Created Equal

They may all look the same, but smoke alarms mainly use one of two ways to detect smoke. Some argue one kind is clearly superior to the other.

Earlier this week, a fire broke out in a rowhouse in Philadelphia, killing at least 13 people. The deputy fire commissioner stated during a news conference that there were four smoke alarms but none of them apparently worked during the fire.

Closer to home, firefighters knocking door to door in a suburb of Montreal in 2019 realized that 40% of the apartments they visited did not have a working smoke alarm. Either the batteries were dead or there was no alarm present.

We often think of smoke alarms as those annoying devices that loudly remind us that the dinner we put in the oven will inadvertently be served Cajun style tonight.

But inside their plastic casing, there are fascinating technologies, making use of light sources and radioactive molecules, to alert us to the presence of smoke.

Since their invention in the 1960s, smoke alarms have become integral parts of our homes, often mandated by law. There are two main types of residential smoke alarms, an important fact that is often not mentioned. Moreover, some have argued that one of these kinds of alarms is both unreliable and slow to be triggered by the type of fire likely to occur while we’re sleeping.

That kind of smoke alarm is by far the most common one found in homes.

A tiny bit of radioactivity

In 1944, scientists detected a new chemical element for the first time as part of the Manhattan Project, the infamous research and development effort that led to humanity’s entry into the atomic age. If we want to think of atoms as little solar systems, this newly discovered element had, in its most common state, 95 protons and 148 neutrons acting as a massive sun and 95 electrons orbiting around it like so many planets. This element is typically created when plutonium, used in nuclear reactors, is bombarded by neutrons, and it once existed on Earth in what are referred to as natural nuclear reactors in now-ancient uranium deposits. But these sites ceased to function a billion years ago, and this important element decayed over time, with none surviving to this day. Until, that is, humans began to smash atoms together.

The element is called americium, after America where the main sites of the Manhattan Project were located and as a play on europium, a similar element previously discovered and named after Europe. If you read Hank Green’s fantastic novel, An Absolutely Remarkable Thing, you know that americium, this child of the nuclear age, is present in many homes. It is at the heart of a popular type of smoke alarm, the ionization smoke alarm.

In an ionization smoke alarm, there is a tiny bit of americium, about 0.29 microgram, which is roughly one thousandth the weight of a grain of salt. We are talking an itty-bitty amount, but enough that it functions as part of a detector.

Over time, this americium decays. It loses two protons and two neutrons as a bundle called an alpha particle. This is a type of radioactivity.

Inside the smoke alarm, these lost alpha particles slam into molecules that are present in the ambient air, namely oxygen and nitrogen, and those molecules lose electrons in the process. What happens when an electrically neutral molecule, like oxygen, loses electrons? The negatively charged electrons go one way and the rest of the molecule becomes positively charged. We end up with ions.

The electricity feeding the smoke alarm (either from a battery or from being plugged into the electrical system of the house) keeps two plates electrically charged. The ions in the air are thus attracted to one of these plates and start to move in one direction. We now have a tiny current inside the smoke alarm.

When smoke enters the smoke alarm, these nitrogen and oxygen ions created by the americium’s alpha radiation attach themselves to the smoke particles. The ions slow down and get carried away. And the tiny current that was flowing flows no more. This is what triggers the alarm.

Before you begin eyeing your smoke alarm with anxiety, thinking about the radioactive heart it contains, do know that these alpha particles, unlike the gamma particles that turned Bruce Banner into the Hulk, are mostly harmless in this context. They are typically blocked by a piece of paper and they turn into stable atoms after travelling a few centimeters in the air. The plastic of the smoke alarm traps them in and the amount of radiation is insignificant. Some of the food we eat, like bananas, is also naturally radioactive, but it’s the dose that makes the poison.

Speaking of food, ionization smoke alarms are easily triggered by cooking smoke, so it is often recommended to install them away from the kitchen area to prevent nuisance alarms, which can drive a person mad enough to remove the battery or unplug the alarm, thus rendering it useless.

While ionization smoke alarms are extremely common—many sources cite 90% of homes as having them, though I suspect the number varies from country to country—some jurisdictions have effectively banned them, requiring homes to use a different type of smoke alarm.

The reason? Ionization smoke alarms may not be as reliable as we keep being told.

An alarming silence

In a 2014 episode of Australia’s 60 Minutes television programme, a journalist is seen accompanying the assistant fire commissioner for the country’s Northern Territory inside of a fire simulation, consisting of a small house with a soldering iron left on a couch to simulate a smouldering cigarette. Wisps of smoke rapidly appear until, thirteen minutes into the experiment, they are told they have to leave, as they would be dead without their breathing apparatus. A smoke alarm went off seven and a half minutes into the test, but not the ionization one. That one stayed mum the whole time. The one that did get triggered is a photoelectric smoke alarm.

It is impossible to tell the two models apart just by quickly looking at them, and both are commonly sold in the same stores. Often, though, a little “I” will appear somewhere on the alarm or its packaging, sometimes at the beginning of the model number, to indicate the ionization technology. The letter “P” signals a photoelectric alarm. Any mention on the back of the device of radioactivity or americium means it is ionization.

Photoelectric smoke alarms detect smoke in a different way. No bit of americium decaying, no ionizing of the air. Photoelectric alarms are all about light. Imagine the letter T, shaped like a tree. There is a light source in the left branch that shoots light in a straight line to the end of the right branch. But when smoke is present, it scatters some of that light down the trunk of the T, where a sensor is placed. This sensor suddenly detects light and the alarm is triggered.

When the existence of these two technologies is brought up, it is often framed in this way: photoelectric alarms are really good at detecting visible smoke particles from slow, smouldering fires (think cigarette left on the couch), whereas ionization alarms are really good at detecting the smaller particles of fast fires (think an oil fire in the kitchen), so homes need both. Underlying this argument is the assumption that ionization alarms are not good at picking up slower fires and that photoelectric alarms are equally bad at warning us of faster fires. But when some fire safety advocates look at the numbers, they call this argument misleading.

In many tests of both types of smoke alarms, ionization alarms are indeed faster at warning us of a fast fire than photoelectric alarms… but faster by seconds. Meanwhile, photoelectric alarms often pick up on slower fires many minutes before their ionization siblings, if these latter alarms even go off at all. As summarized by Vyto Babrauskas, Ph.D., for a 2008 edition of the Fire Safety & Technology Bulletin, different experiments have reported that photoelectric alarms gave 31, 59, 68, or 113 minutes extra warning in the case of a slow fire compared to ionization alarms. In the middle of the night and as the house fills with smoke, minutes of extra warning can make an important difference, especially since most fire deaths are not caused by burns but by breathing in the toxic smoke. (For a thorough look at the literature by one advocate of photoelectric alarms, I recommend Joseph Fleming’s review here.)

Yet, the advice that the two technologies are actually complementary, with each being better for a particular type of fire, is everywhere. It is the position of the U.S. Fire Administration (with a tiny caveat that some tests show considerable differences in response time). It is the position of the global, non-profit National Fire Protection Association. It is the position of the International Association of Fire Chiefs. A 2008 report from the National Institute of Standards and Technology on the performance of home smoke alarms is particularly odd: the authors recognize that ionization is “somewhat better” than photoelectric for fast fires and that photoelectric’s response to slow fires is “often considerably faster,” yet they conclude that either type of alarm consistently provides time for people to escape from most home fires.

Meanwhile, on the Government of Canada webpage for fire safety in your home, on the website of the Canadian Association of Fire Chiefs, and the City of Montreal, there is no mention of the different technologies. The Quebec government website mentions ionization and photoelectric alarms, but only to note that the former is best in bedrooms and hallways and the latter, in bathrooms and kitchens.

Some sources recommend a dual or combination sensor alarm, which is a single smoke alarm that contains both ionization and photoelectric technology. It may seem like the best of both worlds, but a word of caution. I have seen the claim here and there that, because the ionizing technology they contain is easily triggered by cooking smoke, either the dual alarm is desensitized during the manufacturing process or both the ionization and the photoelectric detectors must be triggered for the alarm to ring in some models. I have so far been unable to confirm this information.

Meanwhile, some jurisdictions are now mandating for new residential smoke alarms to be photoelectric only. This is the case of New Zealand, Australia’s Northern Territory and Queensland, the American state of Vermont, and the City of Albany, California.

Not everyone agrees on the unreliability of ionization smoke alarms, but most see eye to eye on the following safety principles. Smoke alarms that are only powered by a disposable battery are a no-no. They have to either be hardwired into the home’s electrical system (with a removable battery as a back-up in case of a blackout) or contain a non-replaceable lithium battery that lasts ten years. (This is now mandatory in Montreal.) Backup batteries should be changed twice a year, and smoke alarms should be replaced after ten years. To maximize your chances of hearing an alarm triggered in a distant room, all alarms in a house should be interconnected, so that when one of them rings, they all ring.

Of course, if the alarm is disconnected because nuisance tripping became a constant irritation, that plastic puck stuck to the ceiling will be useless when you need it the most.

Take-home message:
-Ionization smoke alarms use a tiny bit of radioactive material to electrically charge molecules in the air inside the alarm, which creates a current that is disrupted by smoke
-Photoelectric smoke alarms use a beam of light that is scattered toward a detector when smoke is present
-Many experiments show that, while photoelectric alarms are practically as quick at detecting fast fires as ionization alarms, ionization alarms often don’t detect slow, smouldering fires as quickly as photoelectric alarms, if they detect them at all


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