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 actually smell, 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.

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"), which is not CO — it signals a fuel leak, a different hazard from CO poisoning.

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.

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

What does CO smell like in a car?

CO has no smell. In cars, you may notice exhaust fumes from an exhaust leak or poor cabin sealing. If you suspect exposure, stop driving safely and have the vehicle inspected.

Does a rotten eggs smell mean carbon monoxide?

No. A rotten eggs smell indicates a natural gas or propane leak — these fuels are intentionally odorized. CO is completely odorless. Both require action, but they are different hazards.

Can you smell carbon monoxide from a furnace?

No — CO is odorless. A malfunctioning furnace may produce other smells (soot, hot metal) from incomplete combustion, which can also generate CO. Have persistent odors checked by a technician.

Do CO alarms detect natural gas?

Standard CO alarms detect CO only. Gas leak detection requires a dedicated gas detector or a combined alarm specifically labeled for both CO and combustible gas.

Can air fresheners or scented candles hide CO?

CO has no odor, so fragrances cannot mask it. However, pleasant smells may make you less likely to notice combustion byproducts that signal a problem. A CO alarm is the only reliable detection method.

Sources & References

Published: January 15, 2024