What Does Carbon Monoxide Smell Like?
You wake up with a headache and notice a faint chemical smell near the furnace. Could that be carbon monoxide? The short answer, and the important one, is that carbon monoxide (CO) has no smell at all. It is completely odorless, colorless, and tasteless. The smell you noticed is likely something else: exhaust fumes, combustion byproducts, or the sulfur-like odorant added to natural gas. Those odors can signal a problem that also produces CO, but they are not CO itself. This page explains what people are smelling when they think they smell CO, how to tell a gas leak from a CO risk, and what to rely on instead of your nose.
This is general safety information, not medical advice. If you suspect CO exposure or a CO alarm sounds, move to fresh air and contact emergency services.
Key Takeaways
- CO is odorless, you cannot detect it by smell, taste, or sight.
- Smells like exhaust, smoke, burning plastic, or rotten eggs are not CO, but they can indicate combustion or venting problems that also produce CO.
- A "rotten eggs" smell usually means a natural gas or propane leak (odorized fuel), not carbon monoxide.
- The only reliable way to know CO is present is a working CO alarm or professional testing with a calibrated meter.
Three Gases People Confuse with Carbon Monoxide
If you are on this page, the answer to the title is no and the question underneath it is different: something smells, and you want to know what. It is almost always one of these three, and they need three different responses. None of them is carbon monoxide.
| Gas | Smell | Where it comes from | Action |
|---|---|---|---|
| Natural gas (methane + mercaptan) | Rotten eggs, sulfurous, slightly skunky. The methane itself is odorless; the smell is the added mercaptan, detectable below 1 part per billion. | Leak from a gas line, water heater, range, dryer, or furnace gas connection. | Leave, call the gas utility from outside (most US providers have a 24-hour leak line), do not flip switches or use a phone inside. |
| Hydrogen sulfide (H2S) | Stronger rotten-eggs smell, sometimes described as "decomposing." Detection threshold around 0.0005 to 0.002 ppm in clean air. High concentrations paradoxically reduce the smell (olfactory fatigue). | Sewer gas (broken trap), septic system venting indoors, bacterial breakdown in standing water or biological waste, some industrial or geothermal sources. | Ventilate, check drain traps for evaporation, call a plumber for a venting investigation. |
| Methane (pure, no additive) | Nothing. The molecule is odorless on its own, the same way CO is. Only the mercaptan that utilities add gives natural gas its smell. | Biological decay (landfills, manure pits, septic tanks, rotting organic matter) or sources upstream of utility odorization (well sites, some industrial pipelines). | Treat as a gas-leak situation if the source is unclear; methane is a fire and asphyxiation hazard in confined spaces. |
The takeaway: most "rotten eggs" smells indoors are natural gas with its mercaptan additive, which is a fire hazard rather than a CO event. None of the three smells above is carbon monoxide. CO occupies its own category: no smell at any concentration, and undetectable without an alarm.
Why People Think CO Has a Smell
When fuels burn incompletely, they produce CO along with soot, smoke, and unburned hydrocarbons that do have odors. People notice those smells and assume they are detecting CO. Another common mix-up: natural gas and propane are odorized with a sulfur-like additive ("rotten eggs") that signals a fuel leak, a different hazard from CO poisoning. The additive is mercaptan, and the reason it exists is the New London, Texas, school explosion of March 18, 1937, in which an undetected methane leak under the school killed about 300 students and teachers. Texas responded with a law mandating, in NOAA's summary of it, "that malodorants be added to all natural gas for commercial and industrial use, a practice that is now an industry standard". Other states followed. The smell that warns you of a gas leak today is the legacy of that event.
Why Nobody Ever Did the Same for Carbon Monoxide
The obvious next question is why the same trick was never applied to CO, and the answer is not that anybody forgot. Odorizing natural gas works because the gas is distributed: it is made somewhere else, travels through a system a utility controls, and passes a point where a measured amount of mercaptan can be injected into it. Every molecule that reaches your meter has been through that point.
Carbon monoxide has no such point. It is not delivered to the building; it is manufactured inside it, a few seconds before it reaches you, by a flame that is burning badly in your own furnace or engine. There is no pipeline to dose and no supplier to regulate, because the supplier is the appliance in the basement. That is the whole reason the alarm on the ceiling exists. Odorization put a warning into the fuel supply, and for a gas with no supply chain the warning has to be a device in the room instead.
Odors That Can Signal a Combustion or Venting Problem
Smells cannot confirm CO, but they can indicate conditions that make CO more likely. If you notice any of the following near a furnace, water heater, fireplace, or garage, investigate promptly.
| Smell | Likely Cause | CO Risk? |
|---|---|---|
| Exhaust / "car fumes" | Engine running near home or garage infiltration | High, exhaust contains CO |
| Smoke or soot | Incomplete combustion, blocked flue, backdrafting | Possible, same conditions produce CO |
| Burning dust (HVAC) | Seasonal furnace startup (usually brief) | Low if brief; investigate if persistent |
| Chemical / melting plastic | Electrical overheating, damaged wiring | Fire risk; may accompany venting issues |
| Rotten eggs | Gas leak (odorized natural gas/propane) | Not CO itself, but leave immediately |
CO vs Gas Leak: What's the Difference?
CO is odorless and produced by combustion, a CO alarm detects it. Natural gas and propane are intentionally odorized for leak detection, a standard CO alarm does not detect fuel leaks unless it is a combined device labeled for both. If you smell rotten eggs, treat it as a gas leak; if a CO alarm sounds, treat it as CO. Both require immediate action, but they are different hazards.
What to Rely On Instead of Your Nose
Since CO is invisible and odorless, your best defenses are working CO alarms on every level, regular maintenance of fuel-burning appliances, and awareness of symptoms that improve when you leave the building. UL 2034, the US performance standard for residential CO alarms, sets time-to-alarm thresholds (the alarm must trigger at 70 ppm between 60 and 240 minutes, faster at higher concentrations) that no human sense can match. The history of CO incidents is full of cases where families woke up to a working alarm and survived; the Plymouth, Massachusetts, home where seven-year-old Nicole Garofalo died in January 2005 had no CO alarm and a furnace vent blocked by snowdrifts. Massachusetts passed Nicole's Law within the year mandating alarms in residences with fuel-burning equipment.
The Standard Alarm Is Deliberately Silent Where Your Nose Is Useless
Read the UL 2034 thresholds above once more and notice what they are not. They are life-safety thresholds, written to catch a concentration that will hurt somebody quickly, and a conventional alarm is built to stay quiet through long exposure well below them. That is a design decision rather than a defect, because a device that sounded at every low reading would be pulled off the wall inside a month. It does mean the two things you might rely on, your nose and a standard alarm, are both silent in the same band: the one where a slow venting fault is filling a house overnight.
One jurisdiction in the fifty-one legislated its way out of that gap, and it did it by asking for an instrument instead of an alarm. Santa Fe, New Mexico is the only place in our survey of 93 municipal codes that requires a device to detect carbon monoxide at a low concentration and to show the reading. Section 11.901.12 of its Residential Green Building Code, adopted as Exhibit A to Chapter VII and applied to new single-family and attached dwellings, puts the alarm in the usual place, outside each separate sleeping area in the immediate vicinity of the bedrooms, hard-wired with battery back-up where existing electrical lines are accessible, third-party certified to CSA 6.19 or UL 2034. Then comes the sentence with no counterpart anywhere else in the corpus: at least one of the alarms must be capable of detecting concentrations as low as 9 parts per million and must include a visible display.
Nine parts per million is well under the level a UL 2034 alarm is required to react to at all, so what Santa Fe has asked for is a monitor rather than a warning device: something that answers "is there any" instead of "is there enough yet". If you have ever wanted a number rather than a silence, that is the specification to shop against, and you do not need to live in Santa Fe to buy one. Two cautions from reading the code itself. The section carries a mandatory exception rather than a permission, excusing the alarms only where no attached garage, no gas-fired appliances within the conditioned space, and a third stated condition are all true, which reads as a requirement with a narrow escape. And the chapter runs an incentives layer alongside it at 7-4.2, so confirm with the city which measures it treats as mandatory before relying on the point.
What to Do If You Smell Fumes or Suspect CO
If you notice unusual combustion odors or feel unwell indoors, leave with everyone (including pets) and call emergency services from outside. Do not ignore a CO alarm activation, treat it as real until proven otherwise.
Full emergency response: What to do if you suspect a carbon monoxide leak.
Frequently Asked Questions
Do standard CO alarms detect natural gas?
No. Standard CO alarms use electrochemical sensors tuned only to CO. Gas-leak detection requires a dedicated combustible-gas detector or a combination alarm specifically labeled for both CO and combustible gas. Two separate hazards, two separate sensors.
Can air fresheners or scented candles hide a CO problem?
CO has no odor, so fragrances cannot mask it directly. The risk is indirect: pleasant smells may make you less likely to notice combustion byproducts (soot, exhaust, mercaptan) that would otherwise prompt you to investigate. A working CO alarm is the only reliable detection layer.
Sources & References
- EPA: Carbon Monoxide's Impact on Indoor Air Quality
- CDC: Carbon Monoxide Poisoning Prevention
- CPSC: Carbon Monoxide Information Center
- NFPA: Carbon Monoxide
- NOAA, 1937 Texas Tragedy and Lessons for Today's Responders (mercaptan additive history)
- Gloucester MA, Nicole's Law (2005 Plymouth incident reference)
- UL 2034, Single and Multiple Station Carbon Monoxide Alarms (alarm threshold spec)
- Santa Fe City Code, Residential Green Building Code Section 11.901.12, adopted as Exhibit A to Chapter VII under Subsection 7-4.2, for the 9 ppm detection and visible-display requirement, read 11 August 2026 for our municipal survey
Last updated: August 12, 2026