Carbon monoxide (CO) is a colorless, odorless, and tasteless flammable gas that is slightly less dense than air.

So begins Wikipedia's article on carbon monoxide (CO). To the best of my internet researching ability, animals cannot 'smell' or otherwise detect CO.

How could a creature, like one that exists on Earth, "smell" carbon monoxide?

The creature must be in all other ways like an animal that exists on Earth (mammal, insect, etc). It must be terrestrial (i.e. I don't care about smelling in water, if that is even possible), and it must be able to detect carbon monoxide at concentrations lower than the toxic limit for humans (which is around 35 ppm).

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jul 16, 2019 at 14:34
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    $\begingroup$ While animals may not be able to smell it, it turns out that our own nervous system actually produces and uses CO (in small amounts) as a neuromodulator! So we already have the genes that would let us smell CO; they're just present deep in our brain instead of in our olfactory tissue. See here for some neat info. $\endgroup$
    – forest
    Jul 17, 2019 at 7:05
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    $\begingroup$ That's actually pretty trivial. Blood-sucking insects can smell CO2 just fine, they use that to find hosts. All it would take is a very slight modification of the G-protein coupled receptor on the glycosile end. I bet there actually exist singular mutants of that kind right now as I'm typing this which are capable of smelling CO (though their sub-species' fate remains doubtful...) $\endgroup$
    – Damon
    Jul 17, 2019 at 14:45

9 Answers 9


You just need to have the appropriate chemireceptor which is triggered by CO molecules.

Sensitivity is less of an issue: our nose can detect certain molecules at very low concentrations, like H2S, the molecule responsible for the smell of rotten eggs, which can be smelled at 0,0047 ppm.

Since CO reacts better with hemoglobin than oxygen and CO2, a suitable receptor can be based on a modified version of hemoglobin, with the reaction triggering the nervous pulse to the brain.

The affinity between hemoglobin and carbon monoxide is approximately 230 times stronger than the affinity between hemoglobin and oxygen so hemoglobin binds to carbon monoxide in preference to oxygen

Once the brain has its impulse, it will smell the gas.

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    $\begingroup$ Won't a hemoglobin receptor be cross-triggered by oxygen the same way that carbon monoxide cross-binds to hemoglobin and, in large doses, causes suffocation? $\endgroup$
    – Ash
    Jul 15, 2019 at 12:51
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    $\begingroup$ Carbon Monoxide does unbind, if it didn't all oxygen breathing life would slowly suffocate as it built up in their system, it just can't follow the same metabolic pathway as Oxygen. $\endgroup$
    – Ash
    Jul 15, 2019 at 12:56
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    $\begingroup$ It's actually really "easy" to create a receptor for carbon monoxide,it's a strong reducer. It's just that it's so rare in modern day world that we have evolved to not detect it. In a world where carbon monoxide is common (as common as rotten eggs) any animal who uses smell would detect it. $\endgroup$
    – paul23
    Jul 16, 2019 at 1:13
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    $\begingroup$ I'm pretty sure that if, during the many generations humans spent as hunter-gatherers, CO had been common enough to enact an evolutionary pressure, we would be able to smell it. $\endgroup$
    – vsz
    Jul 16, 2019 at 4:11
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    $\begingroup$ @Yakk why does the receptor need to make that differential? The evolutionary advantage to not die in high carbon monoxide environments is unlikely to impact the odds of surviving in high oxygen ones... in the same way that humans are unable to differentiate a range of other smells. $\endgroup$
    – UKMonkey
    Jul 16, 2019 at 14:51

Are you open to symbiosis? There are microorganisms that actively metabolise Carbon Monoxide. A creature that harbours colonies of such bacteria may not be able to directly smell atmospheric Carbon Monoxide but they could get feedback on it's local concentration based on the activity level of those colonies. This would be by way of sensing metabolic byproducts from those bacteria.

  • $\begingroup$ Possible issue with this approach is that you'd need a symbiote that can metabolize carbon monoxide and survive without it. Otherwise your creatures would have a hard time maintaining their colony. $\endgroup$ Jul 18, 2019 at 14:47
  • $\begingroup$ @JohnDvorak As long as it is a facultative anaerobe that shouldn't be an issue, the host will pick up on the stress hormones it produces when working with CO instead of Oxygen. $\endgroup$
    – Ash
    Jul 18, 2019 at 14:51

They can just smell it like anything else.

"Odorless" just means "human noses didn't happen to evolve the ability to smell this particular chemical."

If humans or other animals evolved in an environment where being about to smell carbon monoxide was beneficial, they'd just be able to smell it.

People are talking about particular mechanisms, which is fine and all, but I suspect you don't care about the exact biochemical pathway activated when you smell, say, a cheeseburger.

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    $\begingroup$ I am so happy to have evoled in a world of cheeseburgers ;) $\endgroup$ Jul 17, 2019 at 9:52
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    $\begingroup$ It's rather cheeseburgers have evolved around our sense of smell. $\endgroup$ Jul 17, 2019 at 18:20

It's not the same as "smelling" it, and as far as I can find, there are no studies testing this ability, but there are countless stories about cats (and sometimes dogs) who saved their human families (and dogs) from carbon monoxide poisoning.

In these cases, the CO levels were high enough to cause severe symptoms in humans, but they were asleep and didn't notice. The cat noticed and woke at least one person up.

...The time around 1 a.m. when everyone was sound asleep.

“All of the sudden Gracie, I heard she was pounding, knocking, knocking, knocking at the door,” Shanahan said. “And so I got out of bed and to stop her from pounding at the door, and I looked to my left and Annette was there in the chair.”

“I was hanging onto the arm of the chair, and I thought I was dying,” his wife Annette said.

“And she called 911, and all she could say was ‘can’t breath.'”

When firefighters arrived, they discovered lethal levels of carbon monoxide in the home. It was caused by a malfunction in the hot water heater.

Annette and Kevin were taken to the hospital, where they spent the night.

Had it not been for Gracie — well, they’d rather not think about that.

There are many more stories. In several the family has a working CO alarm, which sounds after the cat has already woken them.

We humans can detect CO quite well. But we've been conditioned by a couple centuries of living indoors with combustion to ignore mild symptoms. Even manufacturers don't make detectors for low-level exposures that may not be immediately life-threatening, but certainly do affect your health.

Most studies are about acute exposures to high levels of CO, at least 30-50 ppm over a few hours (and all the charts about "safe" exposure levels say "for healthy adults"). But health effects can occur with chronic exposure to low levels, like 10 ppm, especially in children and people with pre-existing health conditions.

Evidence that exposure to low concentrations of carbon monoxide can affect a number of organ systems is accumulating. It is, perhaps, easiest to explain effects on the heart in subjects with incipient myocardial ischaemia. Less easy to explain are effects on the central nervous system; that these effects may not be accurately predicted on the grounds of blood COHb concentration does, however, seem increasingly clear. Whether long term exposure to low concentrations of carbon monoxide can produce long lasting effects on the brain does not yet seem to be settled. If such effects do occur, the impact on public health may be large: many homes are heated with gas appliances and a significant number by solid fuel; failures are inevitable and known to be common, and thus a significant number of people must be being exposed to levels of carbon monoxide in excess of those found in ambient air. Even if only a modest proportion of those exposed sustained effects, the impact on public health may be significant. These findings may have implications for the setting of occupational exposure limits. The Health and Safety Executive recommend a limit of 30 ppm, which can cause COHb levels to rise above 2.5% in less than one hour. However, it should be noted that the evidence for low level effects of carbon monoxide does not arise from occupational exposure studies. The patterns of exposure of people exposed in their homes may be quite different from those exposed occupationally.

We humans may not recognize CO as such, but we do usually know something is wrong with our bodies. Most people ignore that or get belittled by doctors when they try to get help. Or they might get a diagnosis that doesn't acknowledge the source, or even attempt to test for it. If tested, it's dismissed as an issue, because the prevailing wisdom is that CO below 30-50 ppm isn't dangerous (in some cases, the thresholds are even higher).

Animals can also be trained not to "bother" humans with complaints. But generally they know something is wrong and don't care what humans think about it. Many will go to great lengths to alert their humans as well.

In your creature design, take animals that have intelligence levels similar to cats and dogs and add in something specific that CO can do. If it's a world with lots of CO around in pockets, this could be something an animal evolved to detect easily.

For example, have the lack of blood flow CO causes that turns gums red also produce heat. Uncomfortable levels of heat or a burning sensation. It doesn't matter what, as long as it's exclusive to CO exposure. This can be inside the mouth and nose. Or it could be on the paw pads or anus or someplace that other animals can see the change to bright red (make sure their eyes can detect that color change). The animals might also be able to touch noses to noses (or butts) and feel the heat.

These are all signs to raise the alarm, scruff the children, and get the hell out.

  • $\begingroup$ What is the mechanism for detection? This just makes it sounds like "I can't breathe" is the actual thing that is detected, which isn't a symptom specific to CO. How would you tell CO in the room from Argon in the room, then? $\endgroup$
    – kingledion
    Jul 15, 2019 at 15:34
  • $\begingroup$ @kingledion Sometimes you can't tell the difference between different poisons. This is true with smell as well. Our senses of smell tend to be pretty good at differentiating things, but we don't always get it right. CO causes a lot of broad symptoms, especially for low level exposures (which you're not asking about), but mostly they are symptoms related to poor blood oxygenation, which is a lot more than "I can't breathe." (more) $\endgroup$
    – Cyn
    Jul 15, 2019 at 15:39
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    $\begingroup$ I once had that (not from CO but from low O2) on an airplane (I'm more sensitive than most) and I can tell you that it felt very different from having an asthma attack (which certainly can be an "I can't breathe" scenario). And both feel different from having someone lie on top of you in a way that makes it hard to take a full breath (bad experience with a mosh pit in college). While the low oxygenation might not always be from CO, it's the most likely possibility outside of specific factories and labs. (more) $\endgroup$
    – Cyn
    Jul 15, 2019 at 15:42
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    $\begingroup$ I'd say being able to detect that specifically (and I suggested enhancements for your creature design to do just that) would be close enough to detecting CO itself. $\endgroup$
    – Cyn
    Jul 15, 2019 at 15:42
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    $\begingroup$ There are low-level CO detectors out there, it's just that they aren't within the scope of the UL2034 spec for CO alarms, which means that UL won't list them, which causes all sorts of ruckus downstream due to people who expect UL's imprimateur on devices of such a nature. (I personally think that UL should work on a UL2034A for Supplemental Low Level CO Detectors, but I don't have a whole lot of pull in the matter, either.) $\endgroup$
    – Shalvenay
    Jul 16, 2019 at 11:38

Have a look at the Wikipedia page on CO detectors: https://en.wikipedia.org/wiki/Carbon_monoxide_detector

Especially the sections about Biomimetic and Electrochemical sensors could be used here. Biomimetic sensors try to emulate hemoglobin which darkens in the presence of CO. To emulate that process they use Cyclodextrines which can totally be produced biologically.

The Electrochemical sensors work like a fuel cell that digests CO to produce minimal amounts of electricity. I don't see why a very simple version of that couldn't be grown in an animal.

The question is what kind of circumstances would be necessary for an animal to evolve something like that? Usually, CO is not an issue when you are outside of buildings or caves, both of which are places that wild animals don't live in.

  • $\begingroup$ An animal living near volcanic activity might be exposed to carbon monoxide on a regular basis. $\endgroup$
    – user4574
    Jul 15, 2019 at 20:38
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    $\begingroup$ Some animals live in caves... $\endgroup$
    – hkBst
    Jul 16, 2019 at 9:58

To the best of my internet researching ability, animals cannot 'smell' or otherwise detect CO.

They can't detect it? But they can! CO is even used as a neuromodulator and gasotransmitter.

Soluble guanylyl cyclase (sGC) is able to detect both nitric oxide and carbon monoxide, and that is an enzyme we all have. Although CO is a polar molecule and thus not effectively detected with membrane-bound receptors, an intracellular receptor could be made to work. It turns out that sGC is able to fully discriminate between NO and CO, so it would be an ideal chemoreceptor to "smell" carbon monoxide.

Guanylyl cyclase is one of the candidates for CO and NO's surprising behavior as a neuromodulator. While sGC is more sensitive to NO, it has mechanisms to detect which one it is bound to. Because sGC is a signalling molecule, it can communicate the fact that it has bound to CO to the rest of the cell, leading to a neuronal response. In an olfactory neuron, it could be used to selectively detect CO.

See also NO and CO differentially activate soluble guanylyl cyclase via a heme pivot-bend mechanism.

  • From Wikipedia, s.v. Sentinel species:

    The idea of placing a canary or other warm blooded animal in a mine to detect carbon monoxide was first proposed by John Scott Haldane, in 1913 or later. Well into the 20th century, coal miners brought canaries into coal mines as an early-warning signal for toxic gases, primarily carbon monoxide. The birds, being more sensitive, would become sick before the miners, who would then have a chance to escape or put on protective respirators.

    Canaries were iconically used in coal mines to detect the presence of carbon monoxide. The bird's rapid breathing rate, small size, and high metabolism, compared to the miners, led birds in dangerous mines to succumb before the miners, thereby giving them time to take action." (Wikipedia)

  • From the Canadian Institute of Mining, Metallurgy and Petroleum (CIM):

    Small animals like cana­ries proved useful for detecting poisonous gases because of their rapid breath­ing rate and high metabolism, making them more sensitive to the effects of poisonous gas. When exposed to low levels of CO, a canary has difficulty breathing and becomes quite visibly agitated and unsteady. Miners knew there was poisonous gas around when the bird began to sway on its perch or ­collapse.

    Regular miners rarely brought canaries with them into the mines on their shifts. The birds were, instead, primarily used by rescue crews following explosions resulting from regular detonations during mining, sparks from mining equipment, or the open flames of the miners’ carbide lamps. Combustion in the mines produced CO, which could kill miners through asphyxiation.

    Canaries were brought underground in cages about the size of a lunchbox, made out of a durable, transparent material known as Perspex. The handles doubled as a small oxygen canister, and if a canary collapsed from exposure to CO, the miner could cover the ventilation holes and open the oxygen canister to revive the bird. It was not in anyone’s interest to let the birds die, and miners were known to grow fond of them and treat them like pets, whistling to them as they worked. Some even carried extra oxygen bottles with them especially for the canaries just in case they needed a refill. (Correy Baldwin, "Who brought the canary into the coal mine?", in CIM Magazine, November 01, 2014)

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    $\begingroup$ I mean, sure, you can detect CO by dying from it, but that is in no way "smell"-ing in the sense that I used the word... $\endgroup$
    – kingledion
    Jul 15, 2019 at 12:52
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    $\begingroup$ @kingledion: If you read the excerpt from CIM Magazine, the miners too care that the cananaries didn't die from it. $\endgroup$
    – AlexP
    Jul 15, 2019 at 12:53
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    $\begingroup$ I acknowledge that, but the mechanism of detection is still dying, whether they end up dying all the way or not. $\endgroup$
    – kingledion
    Jul 15, 2019 at 12:55

The creature cannot smell carbon monoxide directly, but is extremely sensitive to the other gasses that usually accompany it.

This is, after all how we humans generally manage to avoid breathing too much CO most of the time. We can't smell CO directly, but we tend to avoid breathing anything that has a lot of the bad-smelling combustion products that are normally emitted along with the CO. Automobile exhaust fumes (for example) are deadly because of the CO, but they also smell pretty bad because of all the other combustion products in there. If you are standing in a garage that has a dangerous concentration of exhaust fumes, you will probably notice the smell and get out of there quickly.

However, some sources of CO produce much less odor than others, so there will be some cases where human noses are not good enough to notice the smell in time. In a situation where CO poisoning is a major risk, it would make perfect sense to train some animal to detect the subtle scent that accompanies dangerous levels of CO. Some animals, like dogs, pigs, and rats, have a vastly better sense of smell than us, and could be trained to recognize the danger signs long before humans could.


Mosquitos can detect carbon dioxide from a distance, and follow the "scent". Shouldn't be that much different for an animal and carbon monoxide.

In the real world, the main difference is that carbon dioxide occurs naturally, while carbon monoxide pretty much doesn't, so animals haven't evolved the ability to detect it. (Also, carbon dioxide sinks while carbon monoxide rises, so that would make it less likely for a ground-based or near-the-ground animal to detect carbon monoxide in general.)


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