A chimney's job is to put combustion gases outside. When it stops doing that, nothing about the failure is visible or audible — which is what makes it the hazard worth understanding.
Every other chimney problem announces itself eventually. A leak stains a ceiling. Spalling brick drops grit in the yard. A blocked flue pushes smoke back into the room and you notice within a minute.
Carbon monoxide does none of that. It has no colour, no smell and no taste, and the early symptoms — headache, dizziness, nausea, fatigue — are the symptoms of about forty ordinary things. That's the entire reason it's worth a page of its own.
~430
Deaths a year in the U.S. from unintentional, non-fire-related carbon monoxide poisoning
Centers for Disease Control and Prevention
~102,000
Emergency department visits a year for the same cause
Centers for Disease Control and Prevention
Those are all-cause figures — vehicles, generators, grills and heating systems together. The narrower number, and the one relevant to a chimney, comes from the Consumer Product Safety Commission: over 200 people a year are known to die from carbon monoxide poisoning caused specifically by problems venting the gases their heating system produces.
Risk is not evenly spread. CDC data puts the death rate for men at more than three times the rate for women, and the highest rates of all in people aged 65 and over.
What actually goes wrong
A chimney is a one-job appliance: it moves combustion gases from inside your house to outside it. Carbon monoxide problems are that job failing. The Chimney Safety Institute of America lists the usual mechanisms, and every one of them is something an inspection finds:
- A deteriorating or cracked flue liner, which lets gases leak into the structure instead of carrying them up and out.
- Debris blocking the flue — leaves, twigs, fallen mortar, chunks of old liner.
- Animal and bird nests. A chimney is a warm vertical shaft with a hole at the top, and it is exactly what a nesting bird is shopping for.
- Soot buildup narrowing the passage, which is a particular issue on oil-fired appliances.
- Terracotta liner deterioration from years of corrosive flue conditions.
- A chimney improperly sized for the appliance it serves, which never drafts properly in the first place.
Note what most of that list has in common: it applies whether or not you have ever lit a fire. A furnace or water heater vented through a chimney depends on that flue exactly as much as a fireplace does. A nest doesn't care what the flue is connected to.
The other thing coming out of a fireplace
Separate from carbon monoxide, wood smoke itself is a documented health issue. The EPA's concern is fine particulate matter — PM2.5 — small enough to get deep into the lungs and into the bloodstream. Documented effects include worsened asthma and triggered attacks, lung inflammation, reduced immune response, and heart attacks, stroke, irregular heart rhythms and heart failure in people already at risk.
The scale is bigger than most people expect. EPA figures put residential wood combustion above on-road and non-road vehicles combined as a source of PM2.5, and five times higher than petroleum refineries, cement plants and pulp and paper mills combined. Nationally it is the second largest contributor to wintertime PM2.5.
None of that is an argument against having a fire. It's an argument for the smoke going up the flue rather than into the room, and for burning seasoned wood in an appliance that drafts properly.
What we'd actually tell you to do
Buy the detectors first, then call us
We are a chimney company, and we are telling you that the highest-value thing you can do this week costs about thirty dollars at a hardware store. Put a carbon monoxide alarm on every level of the house and outside every sleeping area, and test them. No inspection — ours or anyone's — is a substitute for a working CO alarm, because an inspection is a snapshot and a nest can be built in a fortnight.
- Get the chimney inspected once a year. This is CSIA's recommendation, the EPA's, and what NFPA 211 requires — and it applies to the flue serving your furnace and water heater, not only the fireplace.
- Take a CO alarm seriously every single time it sounds. Get everyone outside first and call from outside.
- If anyone in the house has unexplained headaches or nausea that improve when they leave and come back when they return, that pattern is worth mentioning to a doctor and worth a call to us.
If you think there is carbon monoxide in your house right now, stop reading, get everyone out, and call 911. Our emergency page leads with that number rather than ours, on purpose.