Carbon Monoxide Poisoning: Dangers, Detection, Response, and Poisoning (AEN-193)

ISU Extension Pub # AEN-193
Author: Thomas H. Greiner, Extension Agricultural Engineer
Department of Agricultural and Biosystems Engineering, Iowa State University.
April 1997

Carbon Monoxide: Dangers, Detection, Response and Poisoning
Thomas H. Greiner, PhD., P.E, Iowa State University Edward Krenzelok Pharm.D., Pittsburgh
Poison Center, William P. Spohn, P. it., Bacharach, Inc.


Carbon monoxide poisoning has haunted mankind since the discovery of fire and remains the most common cause of death due to poisoning. As evidence, hundreds of papers in the contemporary medical literature describe the effects of carbon monoxide poisoning and debate the methods of treatment. Additionally, further debate centers around the usefulness of the consumer carbon monoxide alarms, appropriate response protocols and problem remediation. Not under debate. however, are the dramatic and tragic effects that we open read in the headlines – “Family of 10 Perish from Carbon Monoxide Poisoning.”

What Is Carbon Monoxide?

Carbon monoxide (CO) is a colorless, odorless, tasteless, non-irritating, and poisonous gas produced when carbon-based fuels burn incompletely. Complete combustion of carbon and oxygen produces carbon dioxide (C02), a non-toxic gas. Incomplete combustion occurs when there is insufficient combustion air, insufficient time for complete combustion, incomplete mixing of air and fuel, or when the temperature drops below combustion temperature. CO is slightly lighter than air (0.97) and easily moves through small cracks throughout an entire house.

Sources Of Carbon Monoxide

Incomplete combustion occurs in all fires and even in the most efficient appliances and furnaces. All fossil fuels (e.g. coal, fuel oil, kerosene, gasoline, natural gas) contain carbon, as do other natural fuels (wood and charcoal). When these fuels burn (or oxidize), CO may be emitted as one of the gaseous by-products. We are usually surrounded by potential sources, since so many home gas and oil appliances (furnaces, refrigerators, clothes dryers, ranges, water heaters, space heaters), fireplaces, charcoal grills, and wood burning stoves use fossil fuels as their source of energy. Fumes from automobiles and gas-powered lawn tools also contain carbon monoxide. Tobacco smoke produces low levels of CO in the smoker; however, the long term effects are not clear and are overshadowed by other detrimental effects associated with smoking.

Additionally, the inhalation of methylene chloride (CH2CI), a popular industrial solvent found in home products such as paint and varnish strippers, may result in CO poisoning via its conversion in the liver to carbon monoxide. Contrary to popular belief, the inhalation of unburned gaseous fuel (e.g. natural gas and propane) cannot produce CO poisoning; the fuel must first be burned.

Properly adjusted gas burners in residential heating appliances produce little CO, typically less than 50 parts per million (ppm). incorrectly operating burners can produce CO in extremely high concentrations, with units in excess of 4,500 ppm found. Reasons for excess CO production from heating appliance burners may include: insufficient air to burner, rust and dirt on burners, air blowing across burners, excess gas pressure, and incorrectly adjusted air shutters. Some sources always produce high concentrations of CO, such as wood burning in an open fireplace. smoldering embers, charcoal, and most small gasoline engines. Release of combustion products from any of these sources into enclosed areas is always extremely dangerous and must be avoided. All gasoline engines, even those with catalytic converters, produce high concentrations of carbon monoxide when first started. 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.

Dangers Of Carbon Monoxide

Carbon monoxide is rapidly absorbed by the lungs and quickly passes to the blood. The affinity of CO and the red blood cells, hemoglobin, is 2;0 to 270 times greater than the affinity of oxygen and hemoglobin. Hemoglobin carrying CO (carboxyhemoglobin), is incapable of releasing oxygen to the tissues. Even small amounts of carbon monoxide in the air breathed will quickly increase the percentage of carboxyhemoglobin. For instance, breathing air with 0.0 I % ( 100 ppm)carbon monoxide for two hours has been shown to increase blood carboxyhemoglobin concentrations to 16.0%, a concentration that can cause CO poisoning symptoms.

After breathing carbon monoxide three to four hours of breathing fresh air eliminates only half the CO from the blood (i.e. a three to four hour half-life). Carbon monoxide is an extremely dangerous poison because it can not be seen. smelled. or tasted. Early symptoms are similar to the flu. Because CO reduces oxygen delivery to the brain, persons with elevated levels of CO in their blood do not think; clearly, and might not recognize the warning signs.

Extent Of Problem

Carbon monoxide is the leading cause of poisoning deaths in the U.S. Annually 3,500 to 4.000 die. and an estimated 10,000 people lose a day’s work or seek medical attention. Fires cause approximately two-thirds of known fatalities, with automobile exhaust and faulty heating equipment causing the remaining one-third. The true incidence of carbon monoxide exposures is unknown and may be greatly underestimated. It can be easily misdiagnosed by medical personnel as highlighted by a 1995 incident in Cleveland. Ohio. Five persons went to the local hospital with flu-like symptoms. Surviving relatives report the doctor was asked three times if it was carbon monoxide poisoning and that he said no, it was the flu. The family was sent home and three days later all five died from carbon monoxide poisoning. A Kentucky study found 23.6% of persons presenting to a hospital during February 1985 had elevated carboxyhemoglobin concentrations. None were initially diagnosed as suffering from carbon monoxide poisoning.

Symptoms Of CO Poisoning

The classic symptoms associated with CO poisoning are directly attributable to its ability to produce tissue hypoxia (suffocation) and be directly poisonous to cells. Therefore, organs which are the most sensitive to tissue suffocation will be affected most significantly – the nervous (brain) and cardio-vascular systems (heart).

CO poisoning may mimic flu-like symptoms or even food poisoning. Common symptoms associated with CO may include but not be limited to headache, nausea, vomiting, drowsiness, weakness and difficulty breathing with even minor activity. Furthermore the presence of CO may worsen underlying heart disease. These symptoms are consistent with flu-like syndrome and are often dismissed as the flu. High levels of CO may result in coma, convulsions and death.

Carboxyhemoglobin concentrations in the blood are often used to determine the level of CO poisoning. However, these levels do not often correlate well with clinical effects. Relying only on carboxyhemoglobin levels may result in treatment errors because of wide variations in patient susceptibility.

Delayed Neurological Effects

In treated patients with minimal exposures, the symptoms resolve after several hours. Even in severe cases that have been successfully treated. there is general resolution of the symptoms and apparent recovery. Unfortunately, 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. Long-term follow up in one series of patients revealed a 75% recovery rate after two years, but another study reported that I I % displayed gross neuropsychiatric damage, 33% had personality deterioration, and 43% suffered memory loss when they were re-evaluated three years after the poisoning incident.

It is not possible to predict who will develop delayed effects. Although problems may develop in patients who have had only mild to moderate toxic levels in their systems, there is a tendency for it to occur with greaterfrequency in thosepatients who have had prolonged episodes of unconsciousness. A Parkinson’s Disease-like syndrome has also been associated with CO poisoning. These persistent effects of CO are not generally appreciated and present a significant personal and financial dilemma for those affected.

Who Is At Risk?

Children are at greater risk to experience nervous system symptoms at lower levels due to their inherently higher metabolism. The incidence of delayed neurologic sequelae has been reported to be as high as 11% in children. Adults with preexisting heart disease may develop associated problems, such as angina, at significantly lower levels than healthy adults, placing them at greater risk for a heart attack. Smoke inhalation victims may become comatose at seemingly mild toxic levels of carbon monoxide, due to the inhalation of other toxic by-products of combustion such as cyanide that produces similar cellular suffocation. Pregnant females who are exposed to CO pose a significant risk to the fetus as the recovery half-life is significantly increased due to fetal to maternal blood transport complications.


Since the underlying process of CO poisoning is the accumulation of CO and the suffocation of the cells, the main treatment steps include removing the victim from further exposure to CO and the use of oxygen. The use of high concentrations of oxygen will enhance the elimination of the CO from the blood and provide oxygen to the tissues that have become oxygen-starved. In more serious CO poisoning cases, hyperbaric (higher than atmospheric pressures) oxygen is administered to the patient as a way to dissolve large amounts of oxygen in the blood, which reverses the suffocation and enhances more rapid elimination of the CO.

CO Alarm Technology

There are two predominant sensor technologiesin use in the CO alarms currently being sold at the consumer level: selective chemical detection (or “big-mimetic”) and semiconductor. However, electrochemical and non-dispersive infrared sensor technologies are available in a few consumer models.

Biomimetic sensors rely upon the selective transport of CO into a chemical tablet which then darkens in proportion to the amount of CO received. The unique feature of this patented technology is that the sensor can be designed to release half of the CO absorbed in three to four hours, allowing it to mimic the human response to CO (see above).

Semiconductor sensors employ heated beads which are chemically-doped to selectively burn CO on their conductive surfaces. The combustion of CO on the sensor’s surface will change the resistance of the sensor and this change is proportional to the concentration of CO near the sensor bead.

Electrochemical sensors used in consumer style CO alarms bear traits common to the electrochemical sensors used in field meters. described below. However, parts per million precision is not required since the sensor is being used for consumer alarms.

Non-dispersive infrared sensors rely upon the selective absorption of specific wavelengths of light by the CO molecule. This light absorption may be readily correlated with a specific concentration of CO gas. This absorption wavelength is chosen to be distinct from other compounds that may be present, thus providing selectivity of the sensor.

Detection Of CO In The Field

Flame color is not an accurate measure of CO concentrations. What is normally accepted as a safe blue flame may still be producing excessive concentrations of carbon monoxide. Measurement of CO in the flue gases is the only reliable method to determine if complete combustion is occurring.

For many years the best available method for measuring CO in the field was stain-length tubes. This affordable method has limitations in accuracy, interpretation of reading and heat interference. All these limitations have been overcome with the application of electrochemical CO sensors in field meters.

Electrochemical sensors generate an electrical current proportional to the CO molecules that interact with a chemical solution that is behind a permeable membrane. Some sensors employ sophisticated constructions and filters allowing for the specific determination of CO concentration, even when directly measured in the flue gas. Flue gas measurement capability is the preferred method of measurement. as correction of the problem often involves adjustment of the offending combustion appliance or source.

Response To CO Alarms

A large number of homeowners report their carbon monoxide detectors are alarming. The causes for many detector alarms remain unresolved. with possible reasons including: detector failure (a “false” alarm), indoor concentrations elevated due to elevated outdoor CO concentrations. or indoor concentrations elevated due to local sources (heating or cooking appliances, cars in garages, or smoking). There is no agreement on:

Reliability of detectors,
Competency of investigators,
Number of persons exposed to carbon monoxide at low or high concentrations,
Sources and reasons for exposure to carbon monoxide,
Acceptable or expected concentrations of carbon monoxide,
Risks of exposure to low concentrations of carbon monoxide,
Medical treatment.
Design criteria for heating appliances.

There are wide differences of opinion concerning carbon monoxide detector risks and carbon monoxide detectors. The American Gas Association (AGA) is concerned about the reliability and sensitivity of residential CO detectors. and does not recommend their use. An affiliate of the AGA, International Approval Services (IAS) recently proposed a standard (IAS 6-96) to supplement the current UL standard for residential CO detectors (UL-2034). In part. this standard raises the CO level at which the alarm resists to 30 ppm from 15 ppm in the UL-2034 standard. The U.S. Consumer Product Safety Commission (CPSC) recommends every home have at least one detector. Individuals and groups responding to detector alarms report widely differing results. The American Gas Association believes the majority of alarms are “false” or “nuisance” alarms with the detector responding inappropriately to non-existent or low levels of CO. Detector manufacturers believe most “false” alarms represent appropriate response of the detector to low CO levels or failures of the investigators to locate the sources of carbon monoxide.

Detector manufacturers indicate that approximately 10% of households have detectors, vet already the number of CO calls in some communities has increased to beyond what can be handled by local fire departments. It appears the prevalence of carbon monoxide is much larger than previously estimated. The number of reported CO exposuresis likely to increase as more detectors are installed, and the number of CO sources identified is likely to increase as professionals become better trained and equipped.

Nationwide uniform data on the number of emergency carbon monoxide calls or the results of those calls do not exist. In many communities, the fire departments respond to emergency 9-1- I calls. Fire department data report forms do not include a separate category for carbon monoxide, making it difficult to collect data. A survey of fire departments is now taking place in Iowa. Analysis of data from the Pittsburgh CO response efforts is presently underway. Preliminary indications are that fire departments and utility companies are responding to thousands of requests, with varying results.

A survey of Iowa heating contractors indicate they are responding to hundreds of carbon monoxide-related calls. Some contractors report they found carbon monoxide in all cases after a CO detector alarmed. Others report never finding CO after an alarm. The disparity raises serious questions.

Iowa indoor air quality studies in 65 Iowa homes found 29 homes with excessive concentrations of carbon monoxide produced by the furnace or water heater. The response of professionals contacted by homeowners for help was deemed inadequate; ten professionals said they had fixed the problem – BUT HAD NOT; six professionals reported no carbon monoxide problem and told the homeowners the problem was the CO detector – BUT THERE WAS A CO PROBLEM; three professionals indicated there was a problem but failed to located it; two professionals correctly diagnosed and corrected the problem(s); one professional said the problem was caused by a freak occurrence of weather, and would not likely reoccur – BUT THERE WAS A CO PROBLEM THAT WOULD LIKELY REOCCUR.

A California study of 277 California homes suggested that for 5 to 10% of California residences, indoor concentrations of CO exceed the federal air standards for outdoor air. The authors noted “60 to 70% of the homes have indoor carbon monoxide not measurably greater than the outdoor concentrations. Conversely, 30 to 40% of the homes have measurable increases of CO, thus implying the existence of indoor carbon monoxide emissions.” One-hundred forty seven potential carbon monoxide, heating system, or garage problems were identified.

The individual has little direct or immediate control over the outdoor exposure in the macroenvironment, such as exposure from power plants during air inversions. Other outdoor exposures at the micro-environment level can be modified. For instance, a jogger can avoid jogging next to busy streets and people can use electric lawn mowers rather than gas.

Typical Causes Of CO Alarms

Carbon monoxide problems can be caused by: engines run in attached garages; unvented appliances; problems with heating system design, poor installation, improper modifications, poor condition, poor maintenance; inadequate combustion air; house depressurization; and blocked or failed chimneys.

The amount of indoor carbon monoxide resulting from carbon monoxide entering the home can be difficult and expensive to reduce. Outdoor CO can be a significant reason for elevated indoor levels. Indoor carbon monoxidefrom sources such as from engines (cars) run in attached garages can be reduced by opening the outside door and limiting the time the engine is run before the engine (car) is removed from the garage. Carbon monoxide exposure from gas kitchen stoves can be reduced by propermaintenance and operation of a vented range hood.

Exposure to CO caused by intermittent spillage from vented appliances is often difficult to detect, but has serious consequences. Accidental exposures can be sporadic and widely isolated. Small-scale short-term monitoring of ambient carbon monoxide concentrations in homes is unlikely to discover these exposures. The previously quoted California study of 277 homes stated, “…the samplers were placed in the homes for only 48 hours. This data represents a ‘two day snap shot’ in each home during one winter period. While the distribution of concentrations was well defined, it is not possible to project the proportion of homes that have hazardous conditions leading to life- threatening concentrations of carbon monoxide. A large-scale residential survey with considerably more homes would be necessary to identify enough homes with high CO concentrations in order to uncover patterns and common causes of the high concentration outliers.”

By continuously monitoring for CO, residential carbon monoxide detectors are one means of preventing CO accidents. Utility companies report thousands of alarms sound each year. Minnegasco (a Minnesota natural gas utility) responded to 13,000 carbon monoxide calls in one year. Data from field investigations initiated because of residential detector alarms raises the following questions: how many CO detectors are in place, how many CO detectors alarmed, are residential detectors reliable, what were the CO concentrations causing the alarm, what was the extent of the exposure, was medical treatment sought, was adequate medical diagnosis performed, what was the source of CO, were the investigators trained and equipped to discover CO sources, how adequate was the investigation, and how many times do investigators fail to find a source that was present?

Accidental carbon monoxide exposure in a home can be difficult to reproduce and document. A team performed follow-up CO investigations in 50 Minnesota homes. The occupants of these homes called Minnegasco Utilitytwo or more times for carbon monoxide checks. Negligible concentrations were found. The follow-up investigations found sources of carbon monoxide in 49 out of 50 cases. The home with no sourcefound did not have a CO detector.

As houses become “tighter”, providing proper combustion air and avoiding excessive depressurization is critical. Vented appliances and appliances that exhaust air must have adequate combustion and make-up air. Houses must be considered as a “system”. 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; size, shape, and location of air leaks; attic ventilation; vented gas appliances; wood-burning fireplaces; operation of furnace blower; and furnace return and supply ducts location and leakage. The interactions of these factors combine in complex ways to cause vent failure in natural draft appliances that depend on extremely small pressure differences to draft correctly. For example, in some cases simply adding an exhaust fan or recessed ceiling lights, sealing basement windows, or closing a supply register can upset the balance, causing combustion products to spill into the home. It appears that in many instances no one is taking responsibility for ensuring a house functions correctly, either during design, construction, maintenance, or modifications. There are few organizations or individuals with the training or equipment to either design or troubleshoot such systems.

In many U.S. locations response to carbon monoxide calls are hampered because heating contractors, utility companies, and fire departments do not have adequate training or equipment to respond. improving the response is hampered by lack of funding, incentives, licensing or regulatory requirements.

Preventative Measures – Short Term

The following actions reduce consumer risks of CO poisoning in their home:

Installation of detectors meeting UL-2034 requirements;

Yearly service of heating appliances by a qualified service technician;

Immediate action to protect all building occupants when a detector alarms.

Correction of problems after a carbon monoxide detector alarms.

Because many professionals lack equipment and training to adequately diagnose and correct carbon monoxide problems, those exposed to carbon monoxide often must be persistent to get the problems diagnosed and corrected.

Preventative Measures — Long-Term

 Some things that can reduce the risks of CO poisoning include:

Education of the public, heating contractors,utility contractors, medical personnel, manufacturers of heating and ventilating appliances, homebuilders. home remodelers, government agencies, policy-makers, architects, and engineers

Development of guidelines for carbon monoxide response and home investigations.

Assignment of responsibilities for: proper design, installation, and maintenance of heating appliances andhomes; response to emergency and non-emergency instances of carbon monoxide in homes.

Mandatory licensing, minimum competencyand continuing education requirements for professionals performing work that affects health and safety.

Research and data collection to determine: the number of medical-related CO poisonings; the number of CO emergency calls; the number of “false” alarms and the number of CO detector failures; the number of “undisclosed” source alarms; the number of medically misdiagnosed cases; the number of times responders inspecting the home do not correctly locate the source; the frequency, duration, and magnitude of carbon monoxide exposures in the home, workplace, transportation, and recreational activities.

Accident investigations to determine the causes of CO exposures.


Carbon monoxide poisoning causes severe and permanent health problems and death. It is preventable. Action is needed to reduce the needless costs, pain, and suffering it causes.