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Electronic carbon monoxide detector

Carbon monoxide detectors

Animals are pretty good at defending themselves. Take humans, for example. Our basic senses (eyes, ears, smell, taste, and touch) have all evolved, at least in part, to protect us from threats of various kinds. Back in the days of the cavemen, our ancestors could see and hear attacking animals approaching and either prepare themselves to fight or hurriedly get out of the way. In modern times, the same equipment is just as useful for keeping us safe when we cross the road. But some threats are simply too subtle for our bodies to detect. One of the most dangerous is poisoning by carbon monoxide, a gas produced when things like gas boilers are not properly ventilated. Evolution hasn't managed to adapt to alert us to carbon monoxide, but human ingenuity has stepped forward with clever gadgets that can do the same thing. Let's find out how they work!

Photo: An inexpensive, electronic carbon monoxide detector. This one lets you know there's a problem with a red LED indicator light and a piercing alarm. The detector part is hidden behind the bottom half of the unit; the batteries fill the top half. Check that the alarm is working correctly by pressing the "Test" button once a week or so. A green indicator light flashes every minute or so to reassure you that the battery is connected and working okay.

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  1. What is carbon monoxide?
  2. Why is carbon monoxide harmful?
  3. How do carbon monoxide detectors work?
  4. Have you got enough detectors?
  5. Find out more

What is carbon monoxide?

If you've been following all the news about global warming, you'll know that when we burn materials like coal, oil, and gas (collectively called fossil fuels) in engines or heating systems, they give off energy, but they also make a colorless, odorless, generally harmless gas called carbon dioxide. The trouble with carbon dioxide is that it's building up in the atmosphere and slowly smothering our planet like a blanket, gradually making it hotter and changing the climate. However, that's another story.

To make fuels burn properly, you need plenty of oxygen. If there isn't any oxygen available, things won't burn at all—and that's one of the secrets fire-fighters use. Devices like fire extinguishers, fire blankets, and sprinklers all try to remove the oxygen (or the heat or fuel) from a fire in order to make it go out more quickly. But what happens if you have only a small amount of oxygen—enough to make a fuel burn but not enough for it to burn properly? Instead of the fuel burning cleanly in oxygen to give energy, water, and carbon dioxide (CO2), you get the fuel burning incompletely, and giving off a poisonous gas called carbon monoxide (CO—because there's not enough oxygen to make CO2).

The chemical structure of carbon monoxide (CO) and carbon dioxide (CO2) compared.

Artwork: A molecule of carbon dioxide has two oxygen atoms (blue) attached to one carbon atom (red), where a molecule of carbon monoxide has only one.

Why is carbon monoxide harmful?

CDC campaign carbon monoxide can be stopped.

Artwork: Understand the risk from carbon monoxide. Find out from the US CDC how carbon monoxide poisoning can be stopped.

If you have a camp fire in the open air, there will always be enough oxygen to make the fuel burn completely, so you never have to worry about carbon monoxide. But suppose you have a gas central heating boiler in your home and it gets all its oxygen from the room in which you're sitting. You're getting all the oxygen you need from the room as well. Now if there's plenty of ventilation in your home, there's no problem: you can breathe and so can your fire. But what if there isn't? As the boiler burns, it will rapidly suck all the oxygen out from the room. Sooner or later, there won't be enough oxygen for the fuel to burn efficiently and, instead of combusting cleanly, it will start to make carbon monoxide. If the room's sealed up, the carbon monoxide will gradually build up.

That's when things become dangerous.

Carbon monoxide is harmful because it very readily attaches to haemoglobin, the protein in your red blood cells that ferries oxygen around your body. In fact, carbon monoxide is over 200 times better at attaching to oxygen. When you breathe in carbon monoxide, it effectively snatches up the haemoglobin, so stopping it from carrying oxygen to your brain and the rest of your body. [1]

Carbon monoxide is odorless and colorless so you don't notice it accumulating. It makes you feel drowsy, then it puts you to sleep, and finally it kills you. It's usually a painless death for the victim, but an entirely unnecessary one. Over 400 people die each year in the United States alone from CO poisoning, 14,000 are hospitalized, and 100,000 need emergency treatment. [2]

If you have a detector fitted in your home and your heating system starts to produce carbon monoxide, you'll hear an alarm sounding the minute the gas levels become dangerous. At that point, you can switch off your heating, open all the windows, evacuate your home, and call in a repairman.

It's not just faulty home heating systems that can produce carbon monoxide. It can be a major hazard in the workplace where people use engines, furnaces, forges, or any other equipment that uses oxygen to produce energy through combustion.

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How do carbon monoxide detectors work?

Detectors come in two basic kinds: inexpensive detector strips (sometimes described as biomimetic detectors, because they supposedly mimic the way our bodies respond to carbon monoxide) that you stick on your wall and more expensive electronic alarms that run off a power outlet or battery supply. They have some things in common but work in different ways, so let's tackle them separately.

Chemical "blob" detectors

These simple detectors are pieces of plastic with a small beige-colored "blob" in the middle. If there's a high-level of carbon monoxide in the room, the blob changes color from beige to black. How does that happen? Chemistry! Look closely at the detector blob and you'll see it's gritty and sand-like. It's actually silica gel impregnated with a catalyst (something that accelerates a chemical reaction) made from chemicals that include palladium and molybdenum salts (such as palladium sulfate, palladium chloride, and ammonium molybdenate). [3]

When carbon monoxide touches the detector, it's oxidized by (steals some oxygen atoms from) the chemical salts on the strip and turns into carbon dioxide. The chemicals on the strip are simultaneously reduced (have some oxygen atoms stolen from them) and change color to black. The strip also contains a chemical salt made from a transition metal such as iron, nickel, or copper. Once the carbon monoxide's removed, this metal salt steals some oxygen from the air and changes the catalyst back to its original chemical form—so the detector spot changes color back to beige again. In other words, the catalyst regenerates itself in the air. [4]

Left: Carbon monoxide blob detector. Right: Closeup of the chemical detector area in the center.

Photo: An inexpensive, blob-type carbon monoxide detector. You can find these in hardware and grocery stores, or online, for just a few dollars. They're better than nothing, but electronic detectors give you better protection and work out considerably cheaper in the long run. Blob detectors have a beige spot in the center that goes black if dangerous levels of carbon monoxide are present. Look closely (right) and you can see the gritty, catalyst-impregnated blob that does all the work. You'll notice that detectors like this have a space on top where you're supposed to write the date when you opened them. That's because they stop working properly after more than a few months' exposure to air.

The advantage of simple detectors like this is that they cost only a few dollars (pounds) each, so they're a good way to give yourself basic protection if you can't afford anything more sophisticated. The disadvantages are multiple. First, these detectors don't sound an alarm: you have to keep looking at them to notice that the color has changed. That's fine if you're observant, or if there's only a small, slow build-up of gas, but it's much less satisfactory if there's a sudden, major problem with your heating system and the carbon monoxide is being produced very quickly: you might not notice the blackening detector blob until it's too late. Another problem is that strip detectors have to be replaced every 3–6 months or so, which means that, after a few years, you've spent almost as much as if you'd bought an electronic alarm in the first place.

How blob detectors work

Animation showing how carbon monoxide blob detector works

What makes a blob detector change color when it "sniffs" out carbon monoxide?

  1. Carbon monoxide molecules move toward the blob detector. Carbon is shown as red, oxygen as blue.
  2. Carbon monoxide is very oxygen hungry and readily "steals" oxygen from the chemical salts on the blob, oxidizing itself (gaining oxygen) to make carbon dioxide and reducing (removing oxygen from) the blob chemicals at the same time. This makes the blob chemicals turn black.
  3. Eventually, when the carbon monoxide has cleared and there is plenty of oxygen around, a transition-metal salt on the blob steals oxygen back again, reverting the blob to its original chemical form.

But can you really be sure that blob still looks okay? There will always be some doubt in your mind if you rely on a basic detector like this. Electronic detectors, regularly checked to ensure they're working, offer a higher level of protection.

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Electronic carbon monoxide alarms

Although electronic carbon monoxide detectors all look very similar, they work in a variety of different ways. Colorimetric detectors have a chemical blob inside similar to the ones we've just described. A light beam shines onto the blob and an "electronic eye" (also known as a photoelectric cell or photocell) measures the light reflected back. If there's no carbon monoxide around, the strip is beige and lots of light is reflected. With carbon monoxide present, the strip turns black, the light shining onto it is absorbed and little or none is reflected back. The electronic eye detects the lack of reflected light and sounds a shrill, piercing alarm. (Detectors like this are called colorimetric because they detect and measure a color change in the chemical blob.)

An animation showing how a colorimetric electronic carbon monoxide detector works

Photo: How an electronic, colorimetric carbon monoxide detector works. An LED shines through a colored chemical detector onto a photocell. When carbon monoxide is present, the detector changes color, the beam is interrupted, and the photocell no longer picks up light. This triggers the circuit, which sounds the alarm. I've shown carbon monoxide in blue here, but it's very important to remember that it's invisible: you cannot see or smell it!

Other electronic detectors work in different ways. Metal-oxide detectors have open chambers containing sensors made of metal (tin or platinum) oxide. When there's carbon monoxide around, the metal oxide reacts with it: the carbon monoxide "steals" oxygen from the metal oxide, converting itself into carbon dioxide, turning the metal oxide into pure metal, and producing heat at the same time. An electronic circuit monitors the temperature inside the chamber and sounds the alarm if too much heat is produced too quickly. In some detectors, the circuit measures the resistance of the sensor element and infers the presence of carbon monoxide from that. Detectors like this are often battery operated, but they can also be powered from your main electricity outlet. [5]

A third type of detector, known as an electrolytic detector, works a bit like a battery. It has terminals called electrodes, made from platinum metal, dipped into a chemical solution called an electrolyte. When carbon monoxide is present, the electrolyte conducts electricity more readily, making a current flow in a detector circuit, and triggering an alarm. Electrolytic detectors are usually the most sensitive and accurate—and therefore also the most expensive. They also need powering from an electricity outlet, instead of batteries, which means they may not be suitable if you don't have an outlet nearby.

Two electronic carbon monoxide detectors by Kidde and Fire Angel

Photo: Most electronic alarms have warning/indicator lamps (left), a test/reset button, and a chamber where gas enters. In both these models, the detection chamber is on the right (behind the holes in the Kidde and behind the louvres in the Fire Angel). These two alarms sit side-by-side in my kitchen.

Electronic detectors vary considerably in sophistication. The simplest ones are either "on" or "off": if carbon monoxide is present in dangerous levels, they trigger an alarm and maybe flash a warning light at the same time. You don't know there's anything wrong until the alarm sounds. More sophisticated detectors have a digital display showing the amount of carbon monoxide present as a reading in parts per million (PPM). Detectors like this can alert you to a gradually worsening problem with carbon monoxide by showing a progressively increasing PPM reading. Any reading over about 35PPM (the maximum exposure level permitted in workplaces for any eight-hour period by National Institute for Occupational Safety and Health, NIOSH) is a cause for concern, but you might want to check out your appliances or your room ventilation if you get any significant PPM reading at all. [6]

Have you got enough detectors?

Electronic carbon monoxide detector

Photo: This battery-powered, wall-mounted carbon monoxide detector has an LCD display showing the CO concentration in parts per million (PPM). At the moment, we're completely safe because the display reads "0 PPM".

It's not enough to have only one carbon monoxide detector in a building; you need one near each appliance, fire, furnace, fuel-burning engine, or other device that could potentially produce CO gas if it malfunctions. So if you have a gas boiler in one room of your house and separate, standalone gas fires (or coal fires) in other rooms, more than about 10m (30ft) away from your detector, or on other floors of the same building, you need one detector next to each of them.

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Find out more

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Government-advice websites

Here are some helpful, reliable sources of information from government agencies:




  1.    K. McClatchey (ed). Clinical Laboratory Medicine, Lippincott Williams & Wilkins, 2002, p806.
  2.     Carbon Monoxide Poisoning: FAQ, US Centers for Disease Control and Prevention (CDC), March 27, 2023 and the earlier (archived) version Carbon Monoxide Poisoning (CO), US Centers for Disease Control and Prevention (CDC), March 17, 2021 [Archived via the Wayback Machine].
  3.    For technical details of how these detectors are made, and how they work, see US Patent 2,738, 257: Composition, method, and device for detecting carbon monoxide by William K. Darby, March 13, 1956. Earlier detectors based on chemical reactions involving palladium salts are described in Carbon-monoxide detector by Chester Gordon et al, AT&T, October 4, 1927
  4.    The specific detector shown in my picture uses a palladium (II) dichloride dihydrate–copper (II) chloride cocatalyst (two catalysts working together), and chemistry similar to the Wacker process (also known as Wacker oxidation). When carbon monoxide is present, it reduces the palladium dichloride to palladium metal, producing carbon dioxide, and turning the detector black. With the carbon monoxide removed and oxygen present once more, the copper (II) chloride changes the palladium metal back to palladium chloride, changing into copper (I) chloride in the process. This is then oxidised back to copper (II) chloride by oxygen from the air. See "A Detector Calls" by Rob Kingston, Chemistry in Britain, April 1999, reprinted in Health, Safety and Risk: Looking After Each Other at School and in the World of Work by Dorothy Warren, Royal Society of Chemistry, 2001, p.46. This explains the chemistry of an EI1200 detector made by EI Electronics of Shannon, Ireland and quotes the exact oxidation-reduction reactions.
  5.    For a technical overview, see Metal-oxide semiconductors for carbon monoxide (CO) gas sensing: A review by Sunil Mahajan and Shweta Jagta, Applied Materials Today, Volume 18, March 2020 and Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring by George F. Fine et al, Sensors (Basel). 2010; 10(6): 5469–5502.
  6.    Carbon monoxide, National Institute for Occupational Safety and Health (NIOSH), reviewed September 28, 2011.

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