Why is carbon monoxide most dangerous
when you can't notice it?
No colour. No smell. No taste. No smoke. Without an alarm, human senses cannot detect it at all. Worse, this gas is the first thing to strip away your ability to think "I should escape." It's not that you fail to notice — you are made unable to notice.
A cold night. You've got an oil heater running in the 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. "Guess I'm tired," "maybe I'm catching a cold," you think. In fact, the symptoms really do look like a cold. Nobody suspects anything is wrong.
You start to drift off. Drowsiness is one of the symptoms too — but your ability to judge that is already slipping.
The whole time, the room has been filling with carbon monoxide. Your eyes and nose feel nothing, the whole way through.
Colourless, odourless, tasteless. Town gas and propane have a smell deliberately added, but carbon monoxide has none. Humans have no way to detect it — only an alarm can.
The first symptoms are headache, fatigue, drowsiness. In other words, the very ability to think "something's wrong, I should get out" is the first thing dulled. You lose the capacity to judge before you collapse.
Because these two combine, carbon monoxide poisoning happens in a way where "by the time you notice, you can't move." Let's go through it step by step.
Reason 1: it hijacks the body's "oxygen carrier"
We can breathe because red blood cells in our blood carry oxygen around the body. The job of carrying is done by a protein inside red blood cells called haemoglobin.
Haemoglobin has a seat where oxygen sits. It picks up oxygen in the lungs and gently drops it off wherever needed. This property of "binding loosely, releasing easily" is exactly what makes it work as a carrier.
But carbon monoxide clings to that very same seat far more strongly than oxygen does. A commonly cited figure puts that strength at over 200 times that of oxygen.
The result: even a tiny amount of carbon monoxide in the air occupies seat after seat. And once it binds, it barely lets go. The lungs work fine, breathing feels no harder, yet oxygen simply stops reaching the body.
We feel "short of breath" mainly when carbon dioxide builds up in the blood. In carbon monoxide poisoning, carbon dioxide keeps being expelled as normal.
In other words, the body is starved of oxygen, but no alarm of "it's hard to breathe" ever sounds. You never get that intense urge to "come up for air" you'd feel underwater. So you just drift off to sleep.
Reason 2: you breathe it in the same everywhere in the room
Chlorine gas, covered in an earlier article, is heavier than air and pools on the floor. Carbon monoxide is the opposite — it weighs almost exactly the same as air.
That means it doesn't settle high or low. It mixes evenly throughout the whole room. Tricks like "I'll be fine standing up" or "escape to somewhere higher" don't work. It also flows straight through gaps under doors into the next room.
Not a single clue.
Where does it come from
Carbon monoxide is produced whenever something burns without enough oxygen. This is called incomplete combustion. Put another way, it can happen with any fuel, in any sealed room where a flame is used.
- Oil or gas heaters in a sealed room. Especially dangerous while sleeping, when escape isn't possible
- Charcoal, portable gas stoves, or gas burners in a tent or car during an overnight stay. Confined spaces are the most dangerous. "It's fine, I left it slightly open" does not hold
- A car with its exhaust pipe buried in snow. Exhaust flows back into the cabin. If stuck, dig around the exhaust pipe first
- Running a generator indoors, in a garage, or under eaves. Accidents during power outages keep recurring. Generators must always be run outdoors in the open
- Incomplete combustion in water heaters. Happens when the exhaust vent is blocked or air supply is insufficient
- Charcoal cooking or a shichirin grill without the extractor fan running
One of the few clues you can catch: the colour of the flame. If a gas flame flickers orange or reddish instead of blue, if soot appears, or if glass or walls turn black, suspect incomplete combustion.
What should you do
- If you feel a headache, nausea, or fatigue, get outside firstThis is exactly the moment you think "maybe it's a cold." If everyone in the room feels unwell at the same time, and even the pets seem off, suspect it strongly. Breathe outside air before you try to find the cause.
- Open windows and doors to ventilate, and turn off the flameBut your own escape comes first. Don't rush into a room where someone has collapsed. Hold your breath, open a window, and get outside immediately. Rescuers who go in have collapsed one after another.
- Call emergency services. See a doctor even for mild symptomsSee a doctor even if "I felt better once I got outside." Memory and movement problems are said to sometimes appear days to weeks after an apparent recovery. When transported, tell responders what was burning, and how much.
Since human senses simply cannot detect it, a machine is the only option. Carbon monoxide (CO) alarms cost only a few thousand yen.
- In Japan, residential fire alarms are legally required, but CO alarms are not. Smoke detectors don't detect carbon monoxide, so a separate unit is needed
- Install one in the room with the combustion appliance and in bedrooms. Since carbon monoxide weighs almost the same as air, height doesn't matter much (follow the product's instructions)
- Check the battery and lifespan (often 5–10 years). An expired alarm is as good as no alarm
- Small portable units are also useful for camping, sleeping in a car, or running a generator during a power outage
Something you can check in the kitchen (nothing dangerous)
- Light the gas stove and look at the flame colour. Normally it's a clear blue
- Check whether the bottom of the pan has soot on it. Blackening is a sign of incomplete combustion
- Also check for a clogged burner or dust in the air intake
A blue flame is a sign that the fuel is mixing fully with oxygen and burning completely. An orange flame signals that oxygen is running short — a state prone to producing carbon monoxide. Candles and campfires glow orange for the same reason (unburned carbon particles glowing), which is also why ventilation matters indoors.
Summary
What makes carbon monoxide so dangerous isn't just that it's exceptionally toxic. It's the structure of ①being undetectable by the senses, ②never causing breathlessness, ③being breathed in equally wherever you stand in the room, and ④stripping away your judgement to flee first — a threat with no alarm at all.
Human senses cannot catch it.
So put up one alarm.
For those who want to know more ― terms, formulas, and links to the textbookFrom junior-high science to topics still under research, each level is clearly labelled
- JHScovered in junior-high school science
- High Schoolcovered in high-school "Basic Chemistry" / "Basic Biology"
- High School+high-school "Chemistry" / "Biology," or advanced/column content in textbooks
- Universitynot taught in high school — university-level specialist content (toxicology, biochemistry)
- Researchnot even settled as "established fact" at university — what researchers are 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 partly oxidised, producing carbon monoxide (CO). 2C + O₂ → 2CO
- Carbon monoxide (CO): a colourless, odourless, tasteless gas. Its molecular weight of 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 has iron at its centre, which is where oxygen binds.
- Carboxyhaemoglobin (CO-Hb): haemoglobin bound to carbon monoxide. The percentage of blood it occupies is used as a gauge of how severe the poisoning is.
Note that town gas and propane have a smell deliberately added (an odorant) so leaks can be noticed. Carbon monoxide is a gas generated on the spot as a result of combustion, so no smell can be added to it in advance. That difference is, in itself, the difference in danger.
High SchoolWorking it out: why does just 0.02% threaten life?
What makes carbon monoxide so frightening is that it takes effect even at very low concentration. We can work out just how low, with a calculation.
Seat competition ＝ ratio of amount ÷ ratio of binding strength
| Ratio of amount | How much more of one gas there is in the air than the other |
| Ratio of binding strength | Carbon monoxide is said to be about 200–250 times that of oxygen |
| Seat | The binding site 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 the two, it becomes a division.
This is where the "250 times" matters. It means that even at 1/250th the amount, carbon monoxide can fight to a draw.
| Oxygen in air | about 21% ＝ 210000 ppm |
| Carbon monoxide | taken as 200 ppm (0.02%) |
| Amount ratio (how many times more O₂) | 210000 ÷ 200 ＝ 1050 times |
| Divide by binding strength | 1050 ÷ 250 ＝ 4.2 |
| Seat ratio (O₂ to CO) | 4.2 to 1 |
| Total | 4.2 ＋ 1 ＝ 5.2 |
| Share of seats taken by CO | 1 ÷ 5.2 ≒ 0.19 (about 19%) |
Something present at just 0.02% of the air takes away about a fifth of the blood's carrying capacity. At a concentration of 2 in 10,000.
※ This is a rough figure once things have settled after enough time. In practice 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 being similar to losing 20% of your blood. You might manage while resting, but moving would leave you short.
And on top of this comes the most dangerous property of this accident.
- No colour, no smell, no taste. The only clue is a change in how you feel
- The first symptoms are headache, fatigue, drowsiness. You think, "I'm tired, let's rest a bit"
- Lying down to rest means you stop moving — and keep breathing it in the whole time
- Judgement itself fails first. The very thought of fleeing is lost
The calculation shows "a 20% drop," but what actually happens is "a 20% drop you never notice." An alarm is needed precisely because human senses cannot detect this threat at all.
Carbon monoxide appears when something burns without enough oxygen. We can calculate how much oxygen is actually needed.
| Burning the main component of town gas | needs 2 units of oxygen per 1 unit of fuel |
| Oxygen's share of air | about 21% ＝ 0.21 |
| Air required | 2 ÷ 0.21 ≒ 9.5 times |
About 10 times the fuel's volume in air is needed. Burning something means consuming that much air.
A sealed room, a snow-blocked exhaust vent, a tent, a garage. When air can't be exchanged fast enough, combustion doesn't stop — it just keeps going incompletely. That's where carbon monoxide comes from.
If the flame goes out, it stops — but the most dangerous case is when it keeps "incompletely burning" without going out. Warm, lit, nothing looking wrong, while the calculation in ② keeps progressing.
| Tens of ppm | headaches etc. may appear over long exposure |
| Around 200 ppm | headache, 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 in a short time |
※ Effects are said to depend on the combination of concentration and time, and individual variation is 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 harm
Carbon monoxide's harm isn't just "taking the seat." Haemoglobin has four sites, and they cooperate with each other when releasing oxygen (which is why the oxygen dissociation curve traces an S-shape).
When carbon monoxide occupies some of the sites, oxygen bound to the remaining sites becomes harder to release where it's needed. In biological terms, the oxygen dissociation curve shifts left. In other words, less oxygen is carried, and what does get delivered can't be handed over — a double interference.
So measuring "the amount of oxygen in the blood" alone can miss the danger. The fact that a fingertip pulse oximeter can show a near-normal reading during carbon monoxide poisoning is related to this.
UniversityIt interferes even at the cellular level
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). That means even if oxygen does arrive, the machinery that uses it can be blocked too.
It also binds to myoglobin, which stores oxygen in muscle. This is thought to be why effects on the heart muscle become a concern.
Treatment involves having the patient breathe high-concentration oxygen to drive the carbon monoxide out. A more aggressive method, hyperbaric oxygen therapy, has the patient breathe pure oxygen in a pressurised environment, which is said to greatly shorten the time needed to clear it.
ResearchWhat's still unsettled
- Whether hyperbaric oxygen therapy really reduces long-term after-effects remains unresolved. Multiple randomised controlled trials have been run, but the results disagree. Some trials show benefit, others show no difference, and differences in patient selection and treatment timing mean they can't be simply compared. Debate continues over which patients should receive it, and when.
- Who will develop delayed neurological damage cannot be predicted. Days to weeks after an apparent recovery, memory impairment, apathy, or Parkinson's-like symptoms can sometimes appear. But there is no established way to tell in advance who will and won't develop them. It's also known that the blood carboxyhaemoglobin level doesn't reliably correspond to the severity of after-effects.
- The body actually produces carbon monoxide on its own. An enzyme that breaks down heme (heme oxygenase) constantly generates a tiny amount of carbon monoxide, and it's been found 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 underway, but none has reached practical use yet.
- The effects of long-term exposure to low concentrations are also 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 studied.
It may look like a "simple poison," but a great deal is still unknown. That's exactly why preventing it from forming and noticing it early matter far more than relying on treatment.
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, function of red blood cells |
| High School | Basic Chemistry: chemical equations / amount of substance | Equations for complete and incomplete combustion, comparing molecular weight 28 to air |
| High School | Basic Biology: blood and haemoglobin | Oxygen transport, comparing binding strength |
| High School+ | Biology: oxygen dissociation curve / Chemistry: chemical equilibrium | The leftward shift of the curve, binding strength and equilibrium |
| University | Biochemistry, toxicology, emergency medicine | Action on the mitochondrial respiratory chain, hyperbaric oxygen therapy |
| Research | Emergency medicine, molecular biology (unresolved) | Efficacy of hyperbaric oxygen therapy, predicting delayed neurological damage, carbon monoxide produced in the body |
| ― | Disaster prevention / safety education | Get out → ventilate → call emergency services, CO alarms, preventing secondary harm to rescuers |
- Fire and Disaster Management Agency (総務省消防庁) and local fire departments' public advisories on carbon monoxide poisoning (including guidance on heaters, generators, and cars stuck in snow).
- Japan Poison Information Center (公益財団法人 日本中毒情報センター), poisoning information on carbon monoxide.
- 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 of hyperbaric oxygen therapy (results are not consistent).
- 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 role).
- National Institute of Technology and Evaluation (NITE) (独立行政法人 製品評価技術基盤機構), accident cases and advisories on combustion appliances.
※ The ratio of binding strength, and the correspondence between concentration and symptoms, both vary with measurement conditions. This article presents commonly cited approximate figures.
※This article is a general-audience explanation aimed at preventing accidents. If you feel unwell, don't try to judge it yourself — leave the area, call emergency services or a medical facility, and follow the instructions of the fire department and medical staff. For inspecting or installing combustion appliances, always follow the instruction manual and the guidance of the retailer or installer. The figures given here are approximate values meant to help you understand the mechanism.