See also Where to Place Carbon Monoxide Detectors, Smoke Alarms, and Fire Extinguishers in Your Home

Carbon monoxide is the leading cause of poisoning deaths in the U.S. Annually 3,500 to 4.000 die.

Carbon monoxide, the product of incomplete combustiion (see below), inhibits the blood's capacity to carry oxygen. In out lungs, CO quickly passes into our bloodstream and attaches itself to hemoglobin (oxygen carrying pigment in red blood cells). Hemoglobin readily accepts carbon monoxide - even over the life giving oxygen atoms (as much as 200 - 270 times as readily as oxygen) forming a toxic compound known as carboxyhemoglobin (COHb).

By replacing oxygen with carbon monoxide in our blood, our bodies poison themselves by cutting off the needed oxygen to our organs and cells, causing various amounts of damage - depending on exposure.

Cognitive performance is impaired at COHb levels as low as 5%.
Headache, nausea and loss of consciousness occur at COHb levels of 25-40%.
This will occur after about 6-8 hrs. with an air concentration of 200 ppm (parts per million) or 0.02%. It will occur faster at higher concentrations.
Permanent brain damage and death follow if COHb levels exceed 45%, which will occur with concentrations of 500 ppm or higher.

The time to reach that level is dependent on the concentration, measured in parts per million (ppm) and level of exertion.
Breathing air with 100 ppm carbon monoxide for two hours has been shown to increase blood carboxyhemoglobin concentrations to 16%.
400 ppm in air may give you a headache in 35 minutes, and can cause death after two hours.

See chart for other concentrations from the CO Fact Sheet at Renewable Energy Policy Project (REPP) discussion on biomass cooking stoves.

The uptake rate of CO by blood when air containing CO is breathed increases from 3 to 6 times between rest and heavy work.

The half life of COHb in the blood varies from 2-7 hours with an average of 3-4 hours.
Breathing of 100% oxygen removes CO quickly.

Note: all individuals react to CO differently. This table is only a rough guide to assessing the effects of CO exposure.

CO (ppm)
COHb in
Blood (%)
Symptoms Time till symptom
70 10 Shortness of breath upon vigorous exertion; possible tightness across the forehead.  
120 20 Shortness of breath with moderate exertion; occasional headache with throbbing in the temples. 2 hrs.
220 30 Decided headache; irritability; easily fatigued; disturbed judgment; possible dizziness; dimness of vision. 2 hrs.
350-520 40-50 Headache; confusion; collapse; fainting upon exertion. Possible death if exposure is prolonged. 1-2 hrs.
800-1220 60-70 Unconsciousness; intermittent convulsions; respiratory failure; Probable death within several hours. 30-45 min.
1,950 80 Rapidly fatal. 15 min.
3,200  Death within 30 minutes 5-10 min.
12,800  Death within 1-3 minutes 1 min.
Source: Journal of the American Medical Assn., Sept. 27, 1976

Small animals are more sensitive than humans, so if your hamster keels over unexpectedly, you may want to open some windows and get a CO detector.

Delayed Neurological Effects:
Most patients recover within hours. However, some patients regress and develop delayed neurological sequelae. or delayed, subsequent effects. This delayed syndrome is reported in 2 - 12% of CO poisoned patients. The onset may rapidly occur (within two days of the apparent recovery) or may be delayed for four to six weeks.

The most prevalent symptoms include mental deterioration, fecal and/or urinary incontinence. and gait (ability to walk) disturbances. Common aspects of mental deterioration include persistent headaches. personality changes, confusion, memory loss, and irritability.

Carbon monoxide can escape from any fuel-burning appliance, furnace, water heater, fireplace, woodstove, or space heater.

Complete combustion of fuels yields only CO2 and water vapor, but when fuels are burned in non-ideal conditions, other compounds are emitted. These compounds are called products of incomplete combustion (PICs), and include carbon monoxide (CO), methane (CH4) and other volatile organic compounds (VOCs) as well as particulate matter (PM). CO is the most prevalent PIC.

Most newer homes are built very air-tight, thus cutting down on the supply of fresh air to your furnace - and creating an oxygen starved flame. Tight closing replacement windows and doors, as well as additional insulation can cause similar problems in older homes.

Carbon monoxide can spill from vent connections in poorly maintained or blocked chimneys. If the flue liner is cracked or deteriorated, CO can seep through the liner and into the house - slowly creeping up to dangerous levels. If a nest or other materials restrict or block the flue, CO will mostly spill back into the house.

Improperly sized flues connected to new high-efficiency furnaces and water heaters can also contribute to CO spillage. (Many new furnaces and water heaters are installed using the existing chimneys which may be the wrong size to allow the furnace to vent properly.)

Everything that changes air flow can adversely affect the safe operation of heating appliances including: weather; house orientation, shape and size; exterior obstructions; vent location, height, size, and type; bathroom exhaust fans; vented kitchen range hoods; clothes dryers.

Warming up vehicles in an attached garage, even with the garage door opened, can allow concentrated amounts of CO to enter your home through the car port door or near-by windows. During a cold start, tailpipe concentrations can exceed 90,000 ppm Catalytic converters, after warm up, reduce CO concentrations to only a few parts per million.

DO NOT BURN charcoal inside a home, cabin, recreational vehicle, or tent.

In the last 15 years CO has killed at least 93 people while they were boating. CO comes from the exhaust emissions of the engine driving the boat, the engine powering a generator, or a cook stove or heater. Swimming or leaning over the rear near the exhaust can cause a problem.

Scuba diving with a tank with CO from faulty or badly sited diving air compressors.

Cigarette smokers generally have blood levels of 2 to 10% CO-hemoglobin.

Install CO detectors in your home (see placement below) Test CO detectors at least once a month, following the manufacturer's instructions. See 10 Steps to Carbon Monoxide Safety at Have a second alarm around as a backup (see below) Check your alarm owners manual.

Signs That You May Have A Carbon Monoxide Problem

  • Streaks of carbon or soot around the service door of your fuel-burning appliances
  • The absence of a draft in your chimney (indicating blockage)
  • Excessive rusting on flue pipes or appliance jackets
  • Moisture collecting on windows and walls of furnace rooms
  • Fallen soot from the fireplace
  • small amounts of water leaking from the base of the chimney, vent or flue pipe
  • Damaged or discolored bricks at the top of your chimney
  • Rust on the portion of the vent pipe visible from outside your home

Some alarms (at least our 2004 model) may take several days to reset or not reset at all. I would keep a second, plug-in, unit or a newer unit which will reset around in a plastic bag, so you can test after you fix what you think may be a potential problem. My nephew closed the damper on the fireplace while there were still coals burning and the detector went off at 1AM in February when it was 10° outside, after opening the damper and opening some windows the alarm would not go off. He should have left the house and called 911. However, after the fire department determined your home safe, you would be without an alarm until yours was reset or you replaced the detector unit.

American Sensors

Look for a digital display. These show relatively precise CO levels in parts per million, rather than simply beeping. Some also show the peak level since they were reset, warning you of any spikes that occurred while you were away.

Alarm times:
Underwriters Laboratories Inc. (UL) requires alarms to sound at the following levels:
level time till alarm
70 ppm 1-2 hrs.
150 ppm 10-50 min.
400 ppm 4-15 min.

First Alert® suggests mounting the detector on the ceiling. This also puts the detector out of the way of potential interference, such as pets or curious children.

What do I do if my carbon monoxide alarm goes off?:
If any member of the household is feeling sick, leave the building immediately and call 911 or the fire department. Do a head count to be sure everyone is accounted for. Do not re-enter the building until the fire department says it is okay to do so. Have the problem corrected at once.

If no one is feeling ill, press the test/silence button on the alarm. Turn off all appliances or other sources of combustion at once. Open doors or windows to get fresh air into the house. Call a qualified technician to correct the problem.
Source: First Alert

Some alarms (at least our 2004 First Alert model) may take several days to reset or not reset at all.

Carbon Monoxide at
Carbon Monoxide Poisoning - Dangers, Detection, Response and Poisoning at Iowa State
What You Need To Know About Carbon Monoxide at Terri's Consumer Blog
Consumer Products Safety Commission (CPSC)
Underwriters Laboratories Inc. (UL)
Carbon Monoxide Poisoning at WikipediA

last updated 12 Feb 2007