Carbon monoxide (CO) in the home is almost always caused by something burning fuel and not venting properly, which lets this invisible gas build up indoors instead of going safely outside.

What actually produces carbon monoxide?

Any time something burns fuel (gas, oil, wood, charcoal, gasoline, kerosene), it can produce CO if combustion is incomplete.

Main in‑home sources include:

  • Gas boilers and furnaces (natural gas, oil, propane)
  • Gas water heaters
  • Gas stoves and ovens
  • Gas or paraffin/kerosene space heaters
  • Fireplaces and wood‑burning stoves
  • Wood, coal, or pellet fires
  • Portable generators running near or inside the house
  • Attached garages with idling cars or gas‑powered tools (lawnmowers, snowblowers, etc.)
  • Charcoal grills used indoors, in garages, or on enclosed porches
  • Tobacco smoke (usually lower levels but still a source)

Why these sources become dangerous

They usually turn dangerous when:

  1. Incomplete combustion
    • Burner not adjusted correctly, dirty or damaged parts, low oxygen around the flame.
    • This leads to CO instead of mostly carbon dioxide.
  1. Poor or blocked ventilation
    • Blocked chimneys or flues from soot, creosote, bird nests, leaves, or snow.
    • Vents for furnaces, water heaters, or dryers covered or damaged.
    • Using grills, generators, or heaters in enclosed or semi‑enclosed spaces (garages, tents, basements, sheds).
  1. Faulty or aging appliances
    • Cracked heat exchangers in furnaces, worn‑out burners, damaged flue pipes.
    • Old appliances that haven’t been inspected or maintained regularly.
  1. Improper installation or DIY work
    • Gas appliances installed without a qualified professional.
    • Renovations that change airflow or vent routes so exhaust no longer drafts correctly.
  1. Human error and “just this once” decisions
    • Running a car or generator in the garage “with the door open.”
    • Bringing a barbecue, camping stove, or charcoal grill indoors because of bad weather.
    • Forgetting to open the fireplace damper before lighting a fire.

Quick safety checklist (what to actually do)

These are the key prevention steps experts emphasize:

  • Install CO alarms on every level of the home and near sleeping areas.
  • Have fuel‑burning appliances (furnace, boiler, water heater, fireplace, gas stove) inspected yearly by a qualified technician.
  • Keep chimneys and flues cleaned and clear of nests, debris, or heavy soot.
  • Never run cars, motorcycles, or gas tools in an attached garage, even with the door open.
  • Never use charcoal grills, camp stoves, or portable gas/kerosene heaters indoors or in enclosed porches/tents.
  • Make sure appliance and dryer vents outside are not blocked by snow, ice, or plants.

If you suspect CO right now

Health services warn that symptoms like headache, dizziness, nausea, confusion, chest pain, or sudden extreme sleepiness in multiple people at once can be signs of CO poisoning. If that might be happening:

  • Get everyone outside into fresh air immediately.
  • Call emergency services from outside or from a neighbor’s home.
  • Do not re‑enter the home until professionals say it is safe.

Mini “forum-style” take

“Most surprise CO incidents people talk about come down to one of three things: an old furnace that nobody serviced, a generator in the garage during an outage, or someone bringing a grill or heater inside because it’s cold or raining.”

CO issues don’t come from “bad luck” so much as small, fixable habits—regular maintenance, good ventilation, and proper alarms dramatically cut the risk.

TL;DR: What causes carbon monoxide in the home? Fuel‑burning appliances, heaters, fireplaces, generators, and vehicles that aren’t burning cleanly or venting properly, especially when combined with blocked vents, poor installation, lack of maintenance, or using outdoor equipment indoors.

Information gathered from public forums or data available on the internet and portrayed here.