Why is carbon monoxide
so dangerous that you can't even tell it's there?
No colour. No smell. No taste. No smoke either. Without an alarm, there's no way for human senses to detect it at all. Worse, this gas disables your ability to think "I should get out" before anything else. It's not just that you don't notice — it stops you from noticing.
A cold night. You've got a kerosene heater running in the living room, watching TV. It's windy outside, so the windows are shut tight. The extractor fan isn't on. A few hours pass.
You start to get a headache, somehow. A bit sluggish. "Maybe I'm tired," "maybe I'm coming down with something," you think. The symptoms really do look like a cold. Nobody suspects anything is wrong.
Then you start to doze off. Drowsiness is one of the symptoms too — but the part of you that would judge that is already failing.
The whole time, the room is filling with carbon monoxide. Your eyes and nose feel nothing at all.
Colourless, odourless, tasteless. City gas and propane have a smell deliberately added — carbon monoxide has none. Humans have no way to detect it; an alarm is the only option.
The first symptoms are headache, fatigue, drowsiness — in other words, the ability to think "something's wrong, I need to get out" is the very first thing to dull. You lose the power to decide before you ever collapse.
These two things combine so that carbon monoxide poisoning happens as "by the time you notice, you can't move." Let's look at each in turn.
Reason 1: it hijacks the body's "oxygen courier"
We can breathe because red blood cells in our blood carry oxygen throughout the body. The job of carrying is done by a protein inside red blood cells called haemoglobin.
Haemoglobin has a seat for oxygen to sit in. It picks up oxygen in the lungs and gently sets it down wherever it's needed. This property — binding loosely enough to let go easily — is exactly what makes it work as a courier.
But carbon monoxide clings to that very same seat far more tightly than oxygen does. A commonly cited figure puts the strength at over 200 times that of oxygen.
The result: even a tiny amount of carbon monoxide in the air steadily takes over the seats one by one. And once attached, it barely lets go. Your lungs work fine and breathing doesn't feel hard, yet oxygen simply stops reaching the body.
The feeling of "I can't breathe" comes mainly from a build-up of carbon dioxide in the blood. In carbon monoxide poisoning, carbon dioxide keeps being exhaled as normal.
In other words, the body is short of oxygen, but no alarm bell of "it hurts to breathe" ever rings. You never get that intense "I need to come up for air" feeling you'd get underwater. So you just drift off to sleep.
Reason 2: you breathe the same amount wherever you are in the room
Chlorine gas, covered in an earlier article, is heavier than air and pools near the floor. Carbon monoxide is the opposite — it weighs almost exactly the same as air.
That means it doesn't settle up or down. It mixes evenly through the whole room. Tricks like "I'll be fine standing up" or "climb somewhere higher" don't work. It flows straight through gaps under doors into neighbouring rooms too.
There isn't a single clue to go on.
Where it happens
Carbon monoxide is produced whenever something burns without enough oxygen — what's called incomplete combustion. Put another way, burning any fuel in a sealed-up room can cause it.
- A kerosene or gas heater in a sealed-up room. There's no escaping it, especially while you're asleep
- Charcoal grills, portable gas stoves, or gas burners in a tent or car while camping. A small, enclosed space is the most dangerous of all. "I've left it a little open, so it's fine" doesn't hold
- A car with its exhaust pipe buried in snow. Exhaust flows back into the cabin. If you get stuck, dig out around the exhaust pipe first
- Running a generator indoors, in a garage, or under eaves. Accidents during power outages happen again and again. Always run generators outdoors in an open space
- Incomplete combustion in a water heater. This happens when the exhaust vent is blocked or there isn't enough air intake
- Cooking over charcoal or a traditional brazier without running the extractor fan
One of the few clues you can actually catch: flame colour. If a gas flame is flickering orange or reddish instead of blue, if it's producing soot, or if glass or walls are turning black, suspect incomplete combustion.
What to do
- At the first sign of headache, nausea, or fatigue, get outsideThis is exactly the moment to act, even if you think "maybe it's just a cold." If everyone in the room feels unwell at once, or even the pets seem off, take that seriously. Get outside into fresh air before you try to figure out the cause.
- Open the windows and doors to ventilate, and stop the fireBut your own escape comes first. Don't rush straight into a room where someone has collapsed. Hold your breath, open a window, and get outside immediately. Rescuers who go in and collapse themselves are a recurring type of accident.
- Call emergency services. Get checked even for mild symptomsGet checked even if you feel better once outside. Memory problems or movement difficulties are said to sometimes appear days to weeks after you seem to have recovered. When transported, tell them what was burning and roughly how much.
Since human senses simply can't detect it, a machine is the only option. A carbon monoxide (CO) alarm costs only a few thousand yen.
- In Japan, home fire alarms are mandatory, but CO alarms are not. Smoke detectors don't sense carbon monoxide, so you need one separately
- Install one in any room with a combustion appliance, and in bedrooms. Since carbon monoxide weighs almost the same as air, height placement isn't a major concern (follow the product's instructions)
- Check the battery and lifespan (often 5–10 years). An expired alarm is as good as none at all
- A portable, small unit is also useful for camping, sleeping in a car, or running a generator during a power cut
Something you can check in the kitchen (nothing dangerous)
- Light the gas stove and look at the colour of the flame. If it's normal, it will be a clear blue
- Check whether soot has built up on the bottom of the pan. Blackening is a sign of incomplete combustion
- Also check the burner for clogging and dust at the air intake
A blue flame is a sign that the fuel is mixing well with oxygen and burning completely. An orange flame is a sign that oxygen is in short supply — a state prone to producing carbon monoxide. The orange glow of a candle or campfire comes from the same cause (unburned carbon particles glowing), which is also why ventilation matters indoors.
Summary
Carbon monoxide isn't frightening just because it's especially toxic. It's dangerous because of a structure in which no alarm ever goes off at all: ①your senses can't detect it, ②it never feels like suffocating, ③you breathe the same amount wherever you are in the room, and ④it strips away your judgement to flee before anything else.
Your senses can't catch it.
So place one alarm.
Want to know more? ― Terms, chemical formulas, and links to the textbookFrom junior-high science to topics still being researched, each level is clearly labelled
- JHSCovered in junior-high school science
- High SchoolCovered in high-school "Basic Chemistry" / "Basic Biology"
- High School+Covered in high-school "Chemistry" / "Biology", or treated as advanced/column material in textbooks
- UniversityNot covered in high school — specialist university subjects (toxicology, biochemistry)
- ResearchNot yet settled even as "established fact" at university — what researchers are actively investigating right now
JHSTerms, and why it forms
- Complete combustion: when there's enough oxygen. Carbon becomes carbon dioxide (CO2). C + O₂ → CO₂
- Incomplete combustion: when oxygen is insufficient. Carbon is only partially oxidised, producing carbon monoxide (CO). 2C + O₂ → 2CO
- Carbon monoxide (CO): a colourless, odourless, tasteless gas. Its molecular weight, 28, is almost identical to air's average (about 29), so it spreads evenly through a room.
- Haemoglobin: a protein inside red blood cells that carries oxygen. It contains iron at its centre, where oxygen binds.
- Carboxyhaemoglobin (CO-Hb): haemoglobin with carbon monoxide bound to it. The percentage of blood it occupies is used as a rough measure of how severe the poisoning is.
Note that city gas and propane have a smell deliberately added (an odorant) so leaks can be noticed. Carbon monoxide, on the other hand, is generated on the spot as a product of combustion, so there's no way to add a smell to it. That difference alone accounts for much of the difference in danger.
High SchoolWorking it out with numbers: why does a mere 0.02% matter to your life?
What makes carbon monoxide frightening is that it takes effect even at very low concentrations. We can work out exactly how low, with a calculation.
Share of seats taken = amount ratio ÷ binding-strength ratio
| Amount ratio | How much more of one gas there is in the air than the other |
| Binding-strength ratio | Carbon monoxide is said to bind about 200–250 times more strongly than oxygen |
| Seats | The binding sites on haemoglobin in the blood |
Oxygen and carbon monoxide compete for the same site on haemoglobin. Having more of you helps, but binding more strongly also helps. Because it's a tug-of-war between those two factors, it comes down to a division.
The "250 times" is what does the damage here. This number means that even with 250 times less of it by amount, it can still come out even.
| Oxygen in air | About 21% = 210,000 ppm |
| Carbon monoxide | Assume 200 ppm (0.02%) |
| Amount ratio (how much more oxygen) | 210,000 ÷ 200 = 1,050 times |
| Divide by binding strength | 1,050 ÷ 250 = 4.2 |
| Seat ratio (oxygen : CO) | 4.2 : 1 |
| Total | 4.2 + 1 = 5.2 |
| Share of seats taken by CO | 1 ÷ 5.2 ≒ 0.19 (about 19%) |
Something present at only 0.02% of the air takes away about a fifth of your blood's carrying capacity. At 2 parts in 10,000.
※ This is a rough figure once things have settled over enough time. In reality it varies greatly with exposure time, breathing depth and individual differences, and the binding-strength ratio itself has a range.
A 20% drop in carrying capacity is easiest to picture as something like losing 20% of your blood. You might manage while sitting still, but moving around quickly becomes too much.
And this is where the most dangerous feature of this kind of accident stacks on top.
- No colour, no smell, no taste. The only possible clue is a change in how you feel
- The first symptoms are headache, fatigue, drowsiness. You end up thinking "I'm tired, let me rest a bit"
- Lying down to rest means you stop moving, so you just keep breathing it in
- Your judgement itself fails first. The ability to even think of fleeing is lost
The calculation shows "a 20% drop" — but what actually happens is "a 20% drop you never notice." The reason an alarm is necessary is that human senses cannot detect this opponent at all.
Carbon monoxide is produced when something burns without enough oxygen. We can calculate just how much oxygen is actually needed.
| Burning the main component of city gas | Needs 2 units of oxygen per 1 unit of fuel |
| Proportion of oxygen in air | About 21% = 0.21 |
| Amount of air required | 2 ÷ 0.21 ≒ 9.5 times |
About 10 times the fuel's own volume in air is needed. Burning something means consuming exactly that much air.
A sealed-up room, an exhaust vent blocked by snow, a tent, a garage. When the air supply can't keep up, combustion doesn't simply stop — it continues incompletely. That's where carbon monoxide is born.
If the flame goes out, it stops — but the most dangerous case is when it keeps "burning incompletely" without going out. It stays warm, the light stays on, nothing looks wrong, and all the while the calculation in ② keeps running.
| Tens of ppm | May cause headaches over a long exposure |
| Around 200 ppm | Headache and nausea within a few hours |
| Hundreds to 1,000 ppm | Risk of impaired consciousness in a short time |
| Several thousand ppm or more | Life-threatening within a short time |
※ The effect depends on the combination of concentration and exposure time, and individual differences are also said to be large. Pregnant people, those with heart or lung conditions, and infants are said to be affected at lower concentrations.
High SchoolHigh School+Another, lesser-known type of harm
Carbon monoxide's harm isn't just "stealing seats." Haemoglobin has four binding sites, and when it releases oxygen, those sites cooperate with one another (which is why the oxygen dissociation curve is S-shaped).
When carbon monoxide occupies some of those sites, the oxygen bound to the remaining sites becomes harder to release where it's needed. In biological terms, the oxygen dissociation curve shifts to the left. In other words, not only is less oxygen carried, but even what is delivered can't be handed over. A double blow.
That's why measuring only "the amount of oxygen in the blood" can miss the danger. The fact that a fingertip pulse oximeter can show a near-normal reading in carbon monoxide poisoning is related to this.
UniversityIt interferes at the cellular level too
Carbon monoxide is known to bind not only to haemoglobin but also to an enzyme in the mitochondrial respiratory chain of cells (cytochrome c oxidase). In other words, even when oxygen does arrive, the ability to use it can also be blocked.
It also binds to myoglobin, which stores oxygen in muscle. This is thought to be why damage to the heart muscle becomes a concern.
Treatment involves having the patient breathe high-concentration oxygen to drive the carbon monoxide out. A more aggressive option is hyperbaric oxygen therapy, where pure oxygen is breathed under pressure, which is said to greatly shorten the time needed to clear it out.
ResearchWhat's still unresolved
- Whether hyperbaric oxygen therapy truly reduces after-effects is not settled. Multiple randomised controlled trials have been run, but the results don't agree. Some trials show a benefit and others show no difference, and because the patients selected and the timing of treatment differ, they can't simply be compared. Debate continues over which patients should receive it and when.
- There's no way to predict who will develop delayed neurological damage. After apparently recovering, memory problems, apathy, or Parkinson's-like symptoms can appear days to weeks later. But there's no established way to tell in advance who will develop them and who won't. It's also known that the carboxyhaemoglobin level in the blood doesn't correlate well with the severity of after-effects.
- In fact, the body produces carbon monoxide on its own. An enzyme that breaks down heme (heme oxygenase) constantly generates a tiny amount of carbon monoxide, and it has turned out to act as a signalling molecule that relaxes blood vessels and suppresses inflammation. A gas known as a poison turns out to be useful inside the body. Research into drugs that exploit this — compounds that release a small amount of carbon monoxide at a targeted site — is under way, though none has reached practical use yet.
- The effects of long-term exposure to low concentrations also remain unclear. How much chronic exposure from ongoing incomplete combustion in a home, or from living in a high-traffic area, affects the cardiovascular system and nervous system is still being researched.
What looks like a "simple poison" still has a great deal that remains unknown. That's precisely why, rather than hoping for treatment, preventing it from forming and noticing it early matter decisively more.
Links to the textbook (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| JHS | Science: combustion and oxygen / properties of gases / human respiration | Incomplete combustion, flame colour, the role of red blood cells |
| High School | Basic Chemistry: chemical equations / amount of substance | Equations for complete and incomplete combustion, comparing molecular weight 28 with air |
| High School | Basic Biology: blood and haemoglobin | Oxygen transport, comparing binding strength |
| High School+ | Biology: oxygen dissociation curve / Chemistry: chemical equilibrium | The curve shifting left, binding strength and equilibrium |
| University | Biochemistry, toxicology, emergency medicine | Action on the mitochondrial respiratory chain, hyperbaric oxygen therapy |
| Research | Emergency medicine, molecular biology (unresolved) | Effectiveness of hyperbaric oxygen therapy, predicting delayed neurological damage, carbon monoxide produced within the body |
| ― | Disaster prevention / safety education | Get outside → ventilate → call emergency services, CO alarms, preventing secondary harm to rescuers |
- Public safety advisories on carbon monoxide poisoning from Japan's Fire and Disaster Management Agency (消防庁) and local fire departments, covering heaters, generators, and what to do if a car becomes stuck.
- Carbon monoxide poisoning information from the Japan Poison Information Center (公益財団法人 日本中毒情報センター).
- U.S. Centers for Disease Control and Prevention (CDC), Carbon Monoxide Poisoning (symptoms, concentration guidelines, alarm recommendations).
- Weaver, L. K. et al., Hyperbaric oxygen for acute carbon monoxide poisoning, New England Journal of Medicine 347, 2002, and other clinical trials on hyperbaric oxygen therapy (results are not consistent across studies).
- Motterlini, R. & Otterbein, L. E., The therapeutic potential of carbon monoxide, Nature Reviews Drug Discovery 9, 728–743, 2010 (carbon monoxide produced in the body and its physiological roles).
- Accident cases and safety advisories concerning combustion appliances from Japan's National Institute of Technology and Evaluation (NITE, 製品評価技術基盤機構).
※ Binding-strength ratios and the relationship between concentration and symptoms vary with measurement conditions. This article presents commonly cited rough figures.
※This article is general-audience guidance intended to help prevent accidents. If you notice anything unusual with your health, don't make your own judgement — leave the area, call emergency services or a medical provider, and follow the instructions given by fire and medical personnel. For inspection and installation of combustion appliances, always follow the instruction manual and the guidance of the retailer or installer. The figures given here are rough guides intended to help you understand the underlying mechanism.