Carbon Monoxide Frequently Asked Questions


1. What is carbon monoxide?

Carbon monoxide (CO) is a colorless, odorless, tasteless gas that is poisonous in relatively low concentrations. It consists of one atom of carbon and one atom of oxygen.

2. Where does carbon monoxide come from?

Carbon monoxide (CO) is generated to some degree whenever anything burns. This includes:

Liquid Fuels – gasoline, diesel fuel, kerosene, lamp oil
Gasses – natural gas, LP gas, methane, any gas that burns
Solids – wood, paper, charcoal, cloth, etc.

For natural and LP gas, higher quantities of carbon monoxide are created when any of these things happen:

1. The burner flame “impinges” or hits metal while burning.
2. The flame burns with less oxygen than it needs.
3. The flame RE-burns air it has burned before.

An open burner on a residential gas stove produces almost NO carbon monoxide. As soon as you put a metal pot on the burner, CO output rises and can run anywhere from 20 to 100ppm (parts per million.) This is because the ends of the burner flames are contacting the metal pot.

As the pot heats up, the amount of CO being produced starts to drop, although it never gets back to 0ppm.

A gas oven can produce anywhere from 100 to 800ppm CO when it operates. These levels are within the requirements published in the AHRI standards. Knowing this, it is wise to crack a window open when using a gas oven for an extended period of time – like baking a turkey.

Re-burn is the most deadly situation and is typically what contributes to the few bonafide CO fatalities that do occur. Re-burn happens when the flue does not work properly, is blocked or disconnected, or is being back-drafted by some other force like an attic or exhaust fan, or an open window during a windy day.

Re-burn is the reason that ALL user manuals and operating notices caution against using a gas stove or oven for emergency heat.

During cold weather, occupants keep their homes closed up as tight as possible. This leaves little chance for outside air to enter the structure.

As the gas burners operate they consume oxygen and emit carbon dioxide and water vapor. After a period of time, the oxygen level in the area begins to drop and the flames consume more carbon dioxide and begin generating large quantities of carbon monoxide (CO). Once the flames begin emitting CO, the rate increases quickly to the point where the indoor air becomes leathal.

Re-burn is why any small non-vented gas heater becomes dangerous when operated for periods of time within a confined space. This includes open flame construction heaters, radiant heaters, overhead linear radiants and cook stoves.


If you don’t catch much else – understand this:

Carbon Monoxide Deaths are caused by RE-BURN and the lack of oxygen.

Deaths are not caused by faulty equipment. A correctly working radiant heater will kill you if you use it in a small camper or sealed tent during cold weather. (Unless you have a window or vent open.)

 3. What levels of carbon monoxide are safe?

Any level of carbon monoxide in the air you are breathing is cause for concern. The long-term effects of continuous low-level exposure has not been sufficiently investigated. There are no long term studies that have monitored the overall impact of living in a constant CO-laden environment.

Even safety organizations (government and private) have different conclusions concerning safe levels of exposure.

The chart below shows the general effects of various levels of exposure. Note that OSHA limits safe exposure at 70ppm for no more than four hours.

Exposure in(ppm) Carbon Monoxide Effects:
4000 ppm Death, within 5 min.
2000 – 2500 ppm Unconscious, within 10 min.
1000 – 2000 ppm Confusion, drowsiness & nausea
600 -1000 ppm Headache & discomfort
25 ppm SAFE ZONE?? Short term

Toxic gasses are measured in parts per million (ppm).
1% volume = 10,000 ppm

    • 200 ppm is like a drop of blue food coloring in a 5,000 gallon swimming pool. 
    • 21 ppm is like standing in the bleachers trying to find a green 4″x7″ postcard
      on an NFL football field. 

OSHA Exposure Limits

What are the OSHA standards for CO exposure?

The OSHA PEL is 50 parts per million (ppm). OSHA standards prohibit worker exposure to more than 50 parts of the gas per million parts of air averaged during an 8-hour time period.

The 8-hour PEL for CO in maritime operations is also 50 ppm. Maritime workers, however, must be removed from exposure if the CO concentration in the atmosphere exceeds 100 ppm.

The peak CO level for employees engaged in Ro-Ro operations (roll-on roll-off operations during cargo loading and unloading) is 200 ppm.

 4. Is it safe to use my oven to heat my house?

The short answer is – NO!!

Don’t use your oven for heat under any circumstances. (It is best to crack a window open when you use the oven for cooking for more than an hour.) You will eventually run out of oxygen in the area because the flames in the oven will consume it.

Since there is no exhaust, as the oven burners operate they will begin to burn oxygen-depleted air that has already been burned before. When that happens, the oven will quickly begin producing high quantities of carbon monoxide.

The same thing applies to stove-top burners. Eventually, they will burn oxygen-depleted air and emit high levels of carbon monoxide.

Never, ever, use your gas stove or oven to heat your home.

 5. Why is carbon monoxide toxic?

Carbon monoxide is called “the silent killer” for good reason. You could be standing in a room full of it and not know it.

In high concentrations (around 2,000 ppm) you’ll become unconscious in a few minutes. If you are not moved to fresh air or given oxygen, you’ll be dead in a few hours.

CO interferes with the blood’s ability to carry oxygen throughout the body. Carbon monoxide attaches to the iron molecules in the blood and prevents oxygen molecules from being absorbed. In effect, the body suffocates.

6. Does Everyone React to Carbon Monoxide?

NO, people are affected differently. The harmful effects of CO depend on exposure time, concentration of gas, age, health, body size, lung characteristics and probably a hundred other criteria.

A room full of people exposed to carbon monoxide will have a variety of reactions. Most will get headaches, some will be nauseous, a few will become dizzy and some folks will become visibly weak and confused. Some folks will show no effects at all.

7. What is a flue?

A gas furnace flue is a metal pipe (or sometimes a brick chimney) that carries the products of combustion (exhaust) out of the house. The nasty gasses from the flames rise up the pipe (warm are rises – that’s what make a flue operate.)

Most metal flues are called “double-walled” flues and are actually a pipe inside an outer pipe with an air space between them. For gas appliance applications, they are designated as “Class B Flues“. The purpose of a double-walled flue pipe is to prevent fire and protect the structure the pipe attaches to. It also contains the heat within the pipe which helps evaportate moisture.

A brick chimney that is used for gas appliances is normally lined with clay tiles or sleeved with a metal pipe that contains the flue gasses until they exit the top of the chimney. The clay tiles and metal sleeve or flexible liners protect the chimney mortar from moisture.

If the moisture from the burning flame is absorbed by the mortar a couple of things may happen. First, the moisture may freeze and cause the mortar to pop out between the bricks. Freeze-thaw cycles during the winter can cause a chimney to deteriorate to the point where it has to be repaired or dismantled. Otherwise, high winds may cause it to fall down. Second, the moisture has a slight acid content that attacks the mortar and will turn it back into powder. When that happens long enough, chimney repair is needed.

8. What is the difference between a carbon monoxide alarm and a carbon monoxide detector?

Both terms mean the same thing to the general public. The term carbon monoxide detector is also sometimes used to describe a test meter or piece of electronics that controls other devices like exhaust fans or motorized shutters and/or a central alarm panel.

9. What is a “gravity furnace”?

grav-furn-side-layoutGravity furnaces were one of the first forms of central heating in a home. They were located in a basement and had big, pot-bellied fire boxes that were surround by a steel shell. Large round ducts were connected between the steel shell and floor or low sidewall registers located on the upper floors.

These furnaces were originally fired with wood or coal. They had integral grates and shakers to move ashes to ash pans in the bottoms of the furnaces. Next came oil-fired burners and refractory brick lined heat exchangers. In later years, as natural gas came to Glenwood-coal-furnneighborhoods, the furnaces could be converted to natural gas. The systems originally had no blower and depended on rising warm air moving through big ducts to heat the home. The system were quiet and reliable.

This is also where the first problems with carbon monoxide from a heat exchanger originated. Cracks or splits in these heat exchangers would allow CO gasses and soot to rise up into the home with the warm air.

Here is a Minnesota home with a gravity system.