CBS Newsletter
Spring 1995
pg. 6

An Inexpensive CO Sensor

A schematic of the prototype CO passive sensor.

Carbon moNOxide is a colorless, odorless, toxic gas whose primary source indoor is the incomplete combustion of fossil fuels. This gas can be a potential problem in any house that uses combustion appliances for space or water heating, cooking, or idling an automobile in an attached garage. Although most appliances work correctly, a problem can exist in houses when the appliance is unventilated or its ventilation system does not properly eliminate exhaust gases from the house. Since Americans spend 90% of their time indoors and 65 to 70% in their residences, understanding how and when CO builds up indoors could save lives. We have very little systematic data on how CO hazards are distrubuted in the indoor environment, but mortality data from the Centers for Disease Control in Atlanta suggests that the lifetime risk of unintentional fatal CO poisoning indoors is about one in 3,000. This is 300 times greater than the risk at which the Environmental Protection Agency regulates toxic chemicals such as benzene.

Last year's death of tennis star Vitas Gerulaitis by CO poisoning from a faulty space heater focused public attention on the danger of carbon moNOxide poisoning. More than 12,000 carbon moNOxide poisonings were reported to the American Association of Poison Control Centers in 1993, but the Association believes this represents only a fraction of the actual number of events-often, nonfatal poisonings are misdiagnosed as flu or other afflictions.

Each year, about 1,500 deaths result from CO poisonings. Of these about 1000 are from CO emissions caused by malfunctioning, incorrectly installed, or misused combustion appliances such as furnaces and gas ranges, by the improper indoor use of outdoor appliances like barbecues, and by operating automobiles or generators in garages.

The possibility of hazardous CO exposure is greater in houses that have been sealed to improve their energy efficiency if precautions were not taken. CO from vented combustion appliances can enter a home through a cracked heat exchanger, a blocked vent, or by appliance "Backdrafting." Backdrafting is a reversal of the normal appliance ventilation flow; outside air is pulled through the appliance vent and hot combustion gases flow into the indoor environment. This potentially serious situation can occur when the vented gas appliance is located indoors and the house is severely depressurized through the operation of an unbalanced forced-air heating system, or if powerful exhaust fans such as range hoods or bathroom fans are used, or even when a large fire is built in a fireplace. Although not every house has a backdraft problem, it's best to call in a professional when one is suspected. The gas company or a heating contractor can test for CO and recommend such measures as changes to the ventilation system or appliance repairs to remove the hazard.

One reason for the lack of research on CO hazards in the United States is the absence of a simple, inexpensive way to measure its concentration in the field. Mike Apte and Greg Traynor, Indoor Environment Program researchers, are developing an inexpensive CO passive sampler designed for large-scale indoor surveys in cooperation with The Quantum Group of San Diego. The technology could also be adapted as an occupational hazard CO sampler or as a residential warning system that would turn off combustion appliances when CO reached dangerous indoor levels.

The prototype passive sampler consists of a flat disk with a chemical coating that darkens when exposed to CO. The disk is housed within a sealed vial filled with silica gel, a substance that removes moisture (which could interfere with accurate measurement) from the air. A narrow diffusion tube with known dimensions brings external air to the sensor disk. The sampler is exposed to air for one week or less and is then analyzed using a spectrophotometer to measure a "time-averaged" CO concentration. Researchers have conducted preliminary tests at an outdoor location, a parking garage, a toll both, a residence with a gas range, and in an environmental chamber. They compared the passive sampler to a standard analytical measurement technique and found that the prototype sampler was accurate to nearly 1 ppm at low concentrations (5 ppm) and to within 20% at higher levels. The sampler can eliminate the need to test each site using expensive equipment and trained technicians. For example, a public health department or gas utility conducting a hazard study could mail the sampler to customers, who could use it in their homes for a week and then mail it back for analysis.

The team's plans include further refining and field-testing, making the sensor widely available through commercialization, and using it for large- scale field studies of residential and occupational environments. Eventually researchers hope to conduct regional field surveys to characterize the distribution and magnitudes of CO hazards in homes.

—Allan Chen

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Mike Apte
(510) 486-4669
Greg Traynor
(510) 486-5729
Indoor Environment Program
(510) 486-6591 fax


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