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Carbon Monoxide Hazards in the Home

Q: I have heard that newer "energy efficient" homes are at higher risk for dangerous levels of carbon monoxide than older, less efficient homes. Is this true and can it be avoided?

A: The ability to build tighter and more energy efficient houses has made it necessary for homeowners to pay special attention to the air quality inside the home. Properly built, tight homes can achieve excellent indoor air quality when provisions are made to control air exchanges at a specified rate. However, if no steps are taken to force fresh air into a home, any home can be a health hazard when carbon monoxide is introduced with no means of escape. Carbon monoxide, or CO, is an odorless, colorless gas produced by combustion appliances, like gas water heaters, gas stoves, kerosene heaters, wood stoves, gas logs, gas fireplace inserts and wood burning fireplaces.

Studies show a trend that is cause for concern. Not only are people building tighter homes, but people are continuing to choose combustion appliances, which can introduce CO into the home. More houses are also built with attached garages, which can also be a source for CO from a car's engine. To illustrate the severity of the situation, the information below discusses how several common household scenarios can cause dangerous and possibly lethal amounts of CO to be introduced into a home.

Attached Garages

Tom Greiner and Charles V. Schwab relay the following information in their paper Carbon Monoxide Exposure from a Vehicle in a Garage, which was presented at the Thermal Performance of the Exterior Envelopes of Buildings conference, sponsored by the Department of Energy.

On a cold winter day in the Midwest, a homeowner opened the overhead door of his attached garage and started his car. The car, facing inward, was allowed to warm up for a few minutes before being driven. The owner backed out, closed the garage door and drove away. Several minutes later, carbon monoxide detectors throughout the house went off. Although technicians from the utility company and the local heating contractor verified the house had high concentrations of CO, they could not identify the cause. The natural-gas furnace and water heater were not back drafting and there was greater pressure in the mechanical area than in the chimney.

Upon further investigation, Greiner and Schwab discovered the stack effect in the house was causing enough negative pressure in the basement to draw air in from the garage, through the adjacent or common wall/floor joints. The forced-air furnace then circulated the contaminated air throughout the house, raising the amount of CO in the house to a dangerously high level.

Even though the catalytic converter was working properly on the car, Greiner and Schwab measured the car's tail pipe CO concentration at more than 87,000 parts per million (ppm) on cold star-up. This amount dropped to 43,500 ppm after five minutes, and only after the car was fully warmed up, the CO level measured 100 ppm. (Government air quality standards recommend that CO concentration be no more than 200 ppm for a one-time, acute exposure. Carbon monoxide detectors, which are designed to alert homeowners of a high concentration of carbon monoxide, are required to sound an alarm when concentrations are greater than 100 ppm. A lethal concentration of CO is 800 ppm.) Greiner and Schwab also discovered a fully open garage door does not guarantee adequate ventilation of the garage.

Ventless Gas Fireplaces

Ventless gas heater sales have skyrocketed in the past few years but building scientists working on indoor air quality have warned that these products can produce enough combustion by-products to make occupants sick. All heaters manufactured since 1980 have come equipped with an oxygen depletion sensor (ODS) and while there have been no documented deaths due to ODS equipped vent-free gas heating appliances, some scientists and homeowners are not convinced of their safety.

Sandy Wisner of Medford, Oregon, installed a ventless heater in 1996 and soon after developed flu-like symptoms. She installed a CO alarm, which sounded as soon as she used her unvented fireplace. She went to her doctor and found the levels of carbon monoxide in her blood were 30 times higher than normal concentrations. Building scientists are very critical of the gas industry's claims of the safety of their products. While every brochure, video, and Web site about unvented heaters relates their safety to the oxygen depletion sensor, Greg Traynor, formerly an indoor air quality researcher at Lawrence Berkely National Laboratory, says there is almost no correlation between oxygen depletion and increase pollutant concentration. (Source: Home Energy Magazine January/February 1998)

Backdrafting of Other Appliances

Even vented gas appliances are susceptible to back drafting. Recently a customer called the REMC stating that when she ran her clothes dryer, her CO detector always went off. The dryer was electric and she couldn't understand why running her dryer would cause the CO levels in her house to rise. Upon further investigation, it was apparent that when the dryer was running, it was drawing air from across the room and from upstairs where there was a gas fireplace insert with a pilot light on. The force of the air being drawn by theclothes dryer was enough to pull air down the flue, past the pilot light, bringing high levels of CO into the house. In this case, the combustion appliance was not even operating at the time, but the pilot light in combination with a forced air clothes dryer was enough to cause dangerous levels of CO to be produced. A similar scenario can occur any time negative pressure is caused in a home by turning on a bathroom exhaust fan, using a whole house vacuum or if the stack effect in the home is great enough to pull air from the flue of a combustion appliance back into the house. Combustion appliances include wood burning stoves, gas logs, gas water heaters, gas stoves or any appliance that burns a carbon-based fuel to produce heat.


Every home that has an attached garage or any type of combustion appliance should have a good carbon monoxide detector. If the detector goes off, get to a source of fresh air quickly, either outside or near an open window. Many times the occupants will not notice any environmental differences when CO levels are high. Common symptoms of carbon monoxide poisoning are nausea, headache, dizziness and fatigue. Continued exposure to carbon monoxide can cause permanent brain, nerve, or heart damage or possibly even death.

If you are installing new appliances in your home, consider electric appliances or if you prefer gas, consider sealed combustion chamber appliances. These combustion appliances are more expensive than open combustion types, but the combustion chamber is sealed to prevent CO from entering the surrounding environment. If you currently have open combustion appliances, consider investing in an air exchange system that brings fresh air into your home at a regulated rate.

For more information on diagnosing carbon monoxide hazards in your home, or choosing appliances to prevent carbon monoxide in your home, contact your local electric cooperative.


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