Low-Level Carbon Monoxide in Homes: A Safety Guide

Safety First

If a carbon-monoxide alarm activates or anyone experiences headache, dizziness, nausea, confusion, weakness, chest pain or difficulty breathing, move everyone into fresh air and call 911 or the fire department. Do not remain inside to troubleshoot the equipment.

Do not treat an alarm as a nuisance or assume it is false. Have the alarm and every fuel-burning appliance evaluated before the equipment is used again.

Carbon monoxide is colorless, odorless and produced whenever fuel burns incompletely. In a home the usual sources are furnaces, water heaters, gas ranges, gas dryers, fireplaces, space heaters, attached-garage vehicle exhaust and any fuel-powered equipment used too close to the building.

This guide explains what CO alarms are designed to do, why "low-level" is a genuinely difficult topic to write about honestly, and what to do at each level of concern. It does not diagnose anything, and it does not tell you to distrust the alarms your home is required to have.

Where CO Comes From

• Heating equipment. Furnaces and boilers with burner, heat-exchanger, venting or combustion-air problems.
• Water heaters. Atmospheric models with a draft hood run year-round and are the appliance most likely to spill when the house is depressurized by exhaust fans.
• Gas ranges and ovens. Normal cooking produces combustion products; a hood that exhausts outdoors is the control. Never use a range or oven to heat a room.
• Fireplaces and space heaters. Vented units depend on the chimney and damper. Vent-free units discharge combustion products into the room and are restricted in some jurisdictions and room types.
• Attached garages. Vehicle and small-engine exhaust migrating through the garage-to-house boundary. Never run an engine in an attached garage, even with the door open.
• Generators and fuel-powered tools. Keep them well away from the building and away from windows, doors and vents — never in a garage, crawlspace or breezeway.

CDC's carbon monoxide information covers sources, symptoms and response, and the U.S. Consumer Product Safety Commission's carbon monoxide information center covers alarm selection, placement and appliance safety.

Symptoms Are a Reason to Leave, Not a Diagnostic Tool

Reported CO poisoning symptoms include headache, dizziness, nausea, weakness, confusion, chest pain and difficulty breathing. Two things follow from that list, and they point in opposite directions:

• Those symptoms overlap with a great many ordinary illnesses and conditions. Symptoms alone cannot identify carbon monoxide, and it is not possible to conclude from how someone feels that CO is or is not present.
• Because CO cannot be seen or smelled, symptoms are sometimes the first thing anyone notices — which is exactly why the correct response to them is to get into fresh air and call 911 rather than to reason about it indoors.

We will not publish a table matching concentrations to symptoms or to "safe" and "dangerous" labels. Individual response varies with concentration, duration, activity level, age, pregnancy, cardiovascular and respiratory health, and other factors, and how a person responds to a given exposure is a medical question. If you are concerned about an exposure, contact a physician or poison control; a home measurement is not a clinical assessment.

Understanding Numbers You May Have Seen

Several figures circulate in homeowner discussions, usually detached from what they actually govern.

9 ppm. This is part of EPA's National Ambient Air Quality Standard for carbon monoxide, which addresses outdoor ambient air as a public-health standard over an averaging period. It is not a residential indoor limit, not an action level for your house, and not a number your CO alarm is designed around.

Alarm thresholds. Residential CO alarms listed to UL 2034 are life-safety devices, and their requirements are written as concentration-and-time combinations rather than a single trip point: higher concentrations must alarm faster, lower concentrations after longer exposure, with limits designed to avoid nuisance alarms from brief, ordinary sources. It is not accurate to say a listed alarm "never alarms below 70 ppm" — the requirements involve several concentration/time pairs, and behavior also varies by model. UL publishes background on carbon monoxide alarm standards and requirements; the specific behavior of your device is in its own manual.

"Low-level" monitors. Devices that display a numeric reading below listed-alarm thresholds exist and can provide useful information. What they are not is a substitute. A supplemental display device does not replace code-required listed CO alarms, and we do not recommend removing, disabling or ignoring listed alarms in favor of one. If you want additional visibility, add it alongside the required alarms — not instead of them. Because product availability, listings and models change, we do not name specific products here; verify current listing and suitability before buying anything, and check the manual for what its reading means.

Alarms: Getting the Basics Right

The required listed alarms are the part of this that measurably saves lives, so they deserve the attention:

• Install them where your local code and the manufacturer specify — typically on every level and outside sleeping areas — following the instructions for that model regarding height and distance from appliances.
• Test them on the manufacturer's schedule using the test button, and replace batteries as directed.
• Replace the alarm itself at its end of life. CO sensors age; the unit is marked with a replacement date, and an expired alarm is not protection.
• Never remove or unplug an alarm that keeps sounding. Get out, call for help, and have the cause found.
• Combination smoke/CO alarms are fine if listed for both, installed per instructions and within date.

When You Suspect a Problem

Alarm sounding, or anyone feeling ill: everyone outside into fresh air, call 911 or the fire department, do not re-enter until responders clear the building, and have all fuel-burning appliances tested before use.

No alarm, no symptoms, but something looks or smells off — soot, a flame that looks different, a persistent odor during operation, heavy condensation near an appliance, an appliance that will not stay lit: schedule appliance testing by a qualified technician. Details are in Incomplete Combustion in Homes.

Testing involves calibrated instruments and several parameters read together — flue-gas CO, oxygen, draft, temperature rise, gas pressures — through a full operating cycle, plus ambient CO measurement in the appliance space and occupied areas, spillage and draft checks with the house in a worst-case depressurized condition, and supply-airstream measurement where applicable. Results are interpreted against the appliance manufacturer's published specifications and the applicable standards, not a generic chart. Our $89 Comfort Audit includes combustion safety checks on gas appliances, and furnace repair covers the corrective work.

Reducing the Odds

• Have fuel-burning appliances inspected and serviced on the manufacturer's schedule by a qualified technician.
• Keep vents, flues and terminations clear and undamaged.
• Use a range hood that exhausts outdoors while cooking on gas.
• Do not use ovens, ranges or outdoor equipment for space heating.
• Keep vehicles and fuel-powered equipment out of attached garages, and air-seal the garage-to-house boundary.
• Have combustion air and venting verified when you add exhaust equipment, tighten the envelope or change appliances — added exhaust capacity can depressurize a house enough to affect an atmospheric appliance's draft.
• Fix return-duct leakage in mechanical spaces, which can pull on the area around an appliance.

Repair or Replace

If testing identifies a fault, the decision belongs on evidence rather than on a formula. What actually matters: what specifically failed and whether it can be repaired to the manufacturer's specification, whether parts are available, whether the venting and combustion-air installation can be brought into compliance, what the repair costs relative to the value of the equipment, warranty status, and how the equipment has behaved over time.

Age alone does not decide it, and neither does a percentage-of-replacement-cost rule — those are rough screens, not conclusions. What settles it is whether the appliance can be restored to safe, verified operation. Any appliance that cannot be must be taken out of service until it is replaced. If you want the broader framing, see Repair vs Replace in Texas Heat.

Where to Read Next

• Home Comfort and Indoor Air Quality Guides — the full Home Comfort hub.
• Incomplete Combustion in Homes — warning signs and how appliances are tested.
• Indoor Air Quality Guide for North Texas Homes — how combustion fits with particles, moisture and gases.
• Home Office CO2 and Ventilation — why carbon dioxide is a different subject entirely.

Frequently Asked Questions

Is 9 ppm the indoor limit for carbon monoxide?

No. The 9 ppm figure comes from EPA's National Ambient Air Quality Standard for carbon monoxide, which applies to outdoor ambient air over an averaging period as a public-health standard. It is not a residential indoor limit and not the level your alarm is designed around. Treat any confirmed indoor CO as a reason to have appliances tested, and treat an alarm activation as an emergency.

Do CO alarms really ignore anything below 70 ppm?

That is a simplification of how listed alarms work. Alarms listed to UL 2034 use concentration-and-time requirements: higher concentrations must trigger faster and lower concentrations after longer exposure, with limits intended to prevent nuisance alarms from brief everyday sources. Behavior varies by model, so check your alarm's manual — and do not conclude from the myth that your alarms are inadequate.

Should I buy a low-level monitor instead of a regular CO alarm?

No. A device that displays low readings can add useful visibility, but it is not a replacement for code-required listed alarms and should never be installed in place of them. Keep the listed alarms installed, tested and within their replacement date, and treat any supplemental monitor as extra information rather than as your life-safety device.

Can symptoms tell me whether carbon monoxide is the problem?

No. Headache, dizziness, nausea, weakness and confusion have many possible causes and overlap with common illnesses, so symptoms cannot identify carbon monoxide. What they can do is prompt the right action: if anyone feels this way and CO is possible, get everyone into fresh air and call 911 or the fire department. Questions about an exposure or someone's health belong with a physician or poison control.

My alarm went off but everything seems fine now. Can I reset it and move on?

No. An alarm activation is a reason to leave, call for help, and have both the alarm and every fuel-burning appliance evaluated before the equipment is used again. Carbon monoxide levels can rise and fall with appliance cycles, wind, and exhaust-fan use, so an area that reads clear later does not mean nothing happened. Never disable or remove an alarm to stop the noise.

Does my furnace need replacing if it produced carbon monoxide?

Not necessarily. What matters is what specifically failed, whether it can be repaired to the manufacturer's specification with available parts, and whether the venting and combustion-air installation can be brought into compliance — then verified by measurement. Some faults are straightforward repairs; some cannot be corrected safely, and that equipment must stay out of service until replaced. Age or a percentage-of-cost rule is a rough screen, not the deciding factor.