Two Conditions For a Carbon Monoxide Hazard

ISU Extension Pub # none: electronic file only
Author: Thomas H. Greiner, Extension Agricultural Engineer
Department of Agricultural and Biosystems Engineering, Iowa State University.
September, 1997


1. Carbon monoxide must be produced
2. Combustion gases must be released into the structure.

If either one or two is present, 50% of the hazard exists. One factor of safety is gone.

1. Complete combustion does NOT produce carbon monoxide

Check for complete combustion

1. Measure gas flow
2. Measure air/fuel/dilution
3. Measure CO in combustion products
4. Check venting

2. Combustion gases must be released into the structure.

Common causes:

Vent failure
Unvented appliances
Internal combustion engines


Carbon monoxide is a deadly, lethal poison. It is called the “Quiet Killer” and “The Great Imitator.”

Diagnosing carbon monoxide problems requires careful investigative work.

Symptoms and Effects of Carbon Monoxide.

* Headaches
* Dizziness
* Weakness
* Nausea
* Trouble thinking
* Shortness of breath
* Visual problems
* Loss of consciousness
* Increased temperature
* Milder hypertension
* Diarrhea
* Cherry-red skin (uncommon)
* Bullous lesions
* Sweat-gland necrosis
* Rhabdomyolysis
* Renal failure from myoglobinuria
* Pulmonary problems
* Reduced exercise tolerance for patients with chronic obstructive lung disease
* Low levels have deleterious effects on cell structure in terminal airways
* Decrease in mid- and end- expiratory flow rates
* Noncardiogenic pulmonary edema
* Cardia performance affected
* Lack of compensatory coronary dilation in presence of CO
* Earlier onset of exercise-induced angina in coronary atherosclerosis
* Ischemic ST-segment depression
* Aggravation of angina pectoris
* Intermittent claudication occurs earlier
* Causes polycytermia
* Level of intraerythrocytic 2,3-diphophoglycerate decreases
* Impaired oxygen transport capability
* Cardiac arrhythmia (main cause of CO poisoning)
* Threshold for ventricular fibrillation is reduced
* Disability or death from effects on the central nervous system
* Lethal cerebral edema
* Cell death from hypoxia and interference with cellular respiration
* Symmetric, destructive frontal and posterior parietal leukoencephalopathy
* Degenerative changes in the basal ganglia, especially the globus pallidus
* Parenacentral scotomas
* homonymous hemianopia
* Temporary of permanent blindness
* Flame-shaped superficial retinal hemorrhages
* Vestibular dysfunction
* Hearing loss
* Delayed central nervous systems disorders:
Affective incontinence, including:
Increased irritability
Mood changes
Verbal aggressiveness
Personality changes
Cognitive abnormalities
Neurological abnormalities
* Memory impairment
* 3-yr follow-up of coma found 11% still suffered neuropsychiatric disturbances
* Urinary or fecal incontinence, or both
* Gait disturbances
* Mutism
* Tremor
* Speech disturbances
* Epilepsy
* Abdominal cramps
* Mimics flu-like viral illness
* Paraesthesiae
* Parkinsonism
* Akinetic mutism
* Acidosis of cerebrospinal fluid
* Increase in left ventricular ejection time
* Symptoms are often vague
* Physical examination may be unremarkable
* Measurement of carboxyhemoglobin may not correlate well with symptoms

Carbon Monoxide: Detection and Response Case Studies

1. Nearly all carbon monoxide detector alarm activations are caused by carbon monoxide.

Follow-up investigations by Iowa State University (ISU) found sources of carbon monoxide in nearly all cases of alarm activation, including instances where previous
investigators found no carbon monoxide. Sources included kitchen ranges, furnaces, water heaters, automobiles, and a snowblower.

2. The public, and many professionals, do not understand how CO detectors operate.

Differing technologies are used in CO detectors. Response time, concentrations that cause alarm activation, and the time to clear vary widely. Several homeowners
were mistakenly told they had a defective alarm by investigators who did not understand the detector characteristics.

3. Determining the cause of carbon monoxide in a dwelling requires proper equipment and training.

Carbon monoxide is colorless, odorless, tasteless, and non-irritating. Equipment to detect CO in the ambient air and the flue is absolutely vital for any investigation. Other useful equipment includes: smoke pencil, blower door (both to determine house leakage and to depressurize the house), micromanometer, combustion analyzer (gives additional information concerning adequacy of combustion, and gives air free CO measurements), draft gauge, duct leakage equipment, manometer (to measure gas pressure), stop-watch (to clock the gas meter), velocity probe, flow hood, recording carbon monoxide detector, and self-contained breathing apparatus (to protect the safety of the investigators). Finding a source and determining the cause of elevated carbon monoxide concentrations requires a thorough understanding of combustion processes and air movement. The house must be considered as a system.

4. Not all professionals have the necessary equipment and training.

ISU investigations into CO incidents found heating contractors, gas suppliers, fire departments and first responders with no carbon monoxide equipment responding to incidents of CO detector activation. In some instances they have mistakenly informed the occupants there was no CO problem. A survey of heating contractors indicates that most do not check firing rate or gas pressure when responding to CO complaints; most do not perform a “worst case” downdrafting test; and most have not attended classes or received training. Preliminary survey indicate many Iowa volunteer fire departments have no equipment to measure for CO.

5. The factors causing elevated carbon monoxide in a dwelling are numerous, and can interact in complex ways.

Carbon monoxide is produced when the carbon in a fossil fuel does not burn completely. Carbon monoxide enters a dwelling when the fuel burning equipment is not vented or when the vent system fails. Gasoline engines and charcoal grills are two examples of non-vented equipment that must NEVER be operated indoors. Natural draft vent systems operate with extremely small pressure differences which can easily be overcome, causing vent failure.

6. Flame color is not a reliable indicator. A BLUE FLAME CAN PRODUCE EXCESSIVE AMOUNTS OF CARBON MONOXIDE!

Numerous examples of furnaces with blue flames producing high concentrations of carbon monoxide in the combustion products have been found, with some over 4,500 parts per million. At Carbon Monoxide Conferences heating contractors are asked to determine if furnaces are burning “clean” based on flame color. They are shocked when shown the furnaces they deemed “clean” were producing extremely high concentrations of CO, and surprised when shown that a simple and quick reduction of gas pressure could reduce the CO concentrations to negligible amounts.

7. Venting system failure is common in dwellings with CO problems caused by vented heating appliances.

The number of homes experiencing venting system failure is still being determined, with various researchers reporting from 10 to 55%. In the ISU investigations of CO caused by heating appliances, all the vent systems failed. The failures were often intermittent and sporadic, making them difficult to replicate.

8. House depressurization is a major problem.

Homes in the ISU studies have had downdrafting problems caused by depressurization. Exhaust fans, recessed ceiling lights, roof-mounted attic fans, fireplaces, heating appliances, open windows on the lee side of the house, and leaks in return air ductwork can result in CO problems.

9. Attached garages can contribute to carbon monoxide in houses.

Starting a vehicle in an attached garage, even with the overhead door open, can contribute to CO in the house. Limited studies last winter revealed that CO tailpipe emissions from cars can be over 80,000 parts per million when started cold. This can fill the garage with CO in a short time even with the door open. Once the car is backed out and the door closed, large concentrations of gas still remain, where it can seep into the house for hours. In one case, a two-minute warm up in the garage left 575 parts per million CO in the garage. Concentrations in the house rose to 23 parts per million. Eight hours later CO concentrations in the house still remained above 9 parts per million.

10. Case studies during the presentation will graphically illustrate the problems of carbon monoxide in homes.


As state housing engineer with Iowa State University Extension at Ames, I recommend yearly checks for all heating appliances and the installation of alarm sounding carbon monoxide detectors. The detectors indicate when life threatening levels of CO are present. Finding out what causes CO incidents is not always clear cut.

Obvious reasons include cracked heat exchangers in the furnace or blocked or disconnected venting systems. But, many causes escape detection without the use of sophisticated equipment now available. This equipment includes a blower door to determine the tightness of the structure, a micromanometer which determines very small pressure differences, flow measuring equipment, carbon monoxide detectors, and combustion analyzers. Six case studies from my investigations follow.


An entire family found themselves in the emergency room being treated with oxygen for CO poisoning. A heating contractor investigated the problem. He installed a higher vent and larger screens to keep ice from forming around combustion air inlets. Carbon monoxide detectors were also installed.

The detectors went off, the contractor was called back and this time determined the fireplace needed a glass door and more combustion air was required by the furnace and the fireplace. Even with these improvements the detectors continued to intermittently sound.

At this point the home owner contacted my office for additional help. Investigation revealed a home with so many exhausting appliances (seven bathroom fans, a kitchen fan, fireplace, clothes dryer, water heater and two furnaces), that four combustion air openings and leaks in the house could not furnish adequate combustion air. Total exhaust capacity was 1,685 cubic feet per minute (cfm). Total inlet air provided through 9-inch, 6-inch, and 4-inch combustion air openings was only 334 cfm. My recommended solution required replacing the existing natural draft furnaces, which require air from inside the house, with sealed combustion units which provide their own air through sealed plastic pipes. A power vent was added to the water heater. The gas fireplace still spills, and is being monitored by the homeowner.


A family experiencing flu-like symptoms over several weeks. A week prior to the emergency call, the daughter suffered convulsions and seizures, often symptoms of CO poisoning. At 3 am the father awoke extremely ill. He tried arousing other members of the family without success. Completely disoriented and passing in and out of consciousness he searched for a phone. Two hours later he managed to dial 911 before again passing out. Emergency workers found him collapsed over the phone. The entire family received treatment at a local emergency room.

The culprit in this case was the furnace which, during a remodeling project 6 years before, was enclosed in a small room with no combustion air openings. The problem was compounded by a disconnected cold air return which depressurized the furnace room, drawing cold air back into the house through the chimney and spilling all the combustion products into the home. Although the furnace operated several years under these conditions, as the burners built up dirt and burned less efficiently, CO levels increased to the point of severe poisoning. The carpenter had been unaware of the dangers of the disconnected cold air return. After the emergency room treatment the owner hired a heating contractor, who added a 5 foot vertical roof extension to the vent. When I arrived the furnace and water heater were still spilling combustion products into the basement.


A couple failed to respond to calls from family and friends. Upon entering the dwelling both were found dead from carbon monoxide. In this case a multitude of problems contributed to the deaths, including a high capacity boiler and water heater located in a small space without sufficient combustion air. The undersized vent system did not have sufficient rise or height and was rusted through. Cupolas on the roof depressurized the attic. Lack of soffit vents and a loose ceiling allowed the depressurized attic to depressurize the living spaces. The basement door had been removed, allowing the depressurization to continue into the basement. Carbon monoxide was exhausted into the home. A heating contractor had added a spill switch to the boiler, which had not saved the couple.


Soot in the house and hot flue gases escaping at the draft diverters of the furnace and water heater indicated a severe backdrafting problem. The heating contractor failed to identify the exact cause but did correctly install a new sealed combustion furnace to replace an old and outdated model. Follow up testing revealed the basement was depressurized to 0.06 inch water column (15 Pascals) by an attic vent fan which depressurized the attic and pulled air from the house. This in turn depressurized the basement, pulling air DOWN the water heater vent. An additional factor was new siding placed over the attic gable vents and restricting air flow into the attic. Obviously the siding contractor did not realize the danger in which he placed the homeowner. The heating contractor did not check for depressurization, which occurred only when the fan was operated. Until the depressurization problem is corrected the homeowner will not use the fan.


This case illustrates the lack of training and education of some heating contractors now being asked to locate and deal with an increasing number of reported CO episodes. The family purchased a battery operated CO detector, which alarmed. The contractor arrived and found no problems with the furnace or the water heater. The family assumed the detector faulty and returned it to the manufacturer who replaced it. The furnace and water heater were again inspected by the heating contractor at the beginning of this year’s heating season. No problems were found. With the onset of cold weather the detector went off. Not trusting the single detector, the family purchased two more. Now all three detectors intermittently indicated the presence of carbon monoxide.

My office placed monitoring devices in the home and by 6:30 pm recorded readings of 8 parts per million in the home and 3,000 parts per million in the furnace, much higher than the 20 ppm typically found. A call was made to the heating contractor, who obviously had no idea how dangerous the readings were. He convinced the homeowner to wait until morning for a service call. When the service man finally arrived the next morning he visually checked the flame, which he pronounced to be okay. He had no plans to check for over-fueling and when asked, did so with a defective gauge. When convinced to double check using my gauge the results indicated a dangerous over-fueling condition, the result of a defective gas control. The over-fueling caused incomplete combustion and extremely high levels of CO production. His solution was to reduce the firing rate of the furnace by partially closing the furnace gas shut-off valve, which produced another dangerous situation. A defective gas control requires the furnace be shut down immediately. A sealed combustion furnace was installed. Had the homeowners depended on the heating contractor, they would have continued to be at risk of CO poisoning.


The couple received a carbon monoxide detector as a gift and the detector went off. The couple called 911. The fire department arrived, found readings of 28 parts per million and determined dirty pilots on the kitchen stove were the cause of the problem. To prevent further problems, the couple purchased a new gas kitchen stove with electronic ignition and sealed burners.


Carbon monoxide cases often require a great deal of detective work. When simple answers are not available it leaves the homeowner concerned and confused. Training and education in CO detection is not always required or available to heating contractors. In many locations there are no mechanical codes, nor are contractors required to be licensed. Fire departments and utilities often are checking for the presence of CO and do not check venting and appliances for downdrafting. They are often called after windows and doors are opened and the carbon monoxide dispersed, resulting in no readings at all. Some medical personal are not aware of the possible extent of the problem and may misdiagnose what on the surface looks like a simple case of the flu.

Carbon monoxide symptoms mimic other medical conditions such as the flu, heart attack, and muscular sclerosis. Carbon monoxide kills by replacing oxygen in the blood. High levels kill in a short period. Moderate levels, because CO is a cumulative poison, can cause misdiagnosed health problems or even death. Low levels over a long period can cause health problems, especially for patients at risk. Exposure can result in long lasting or permanent effects including brain damage. After exposure, merely removing the victims from the area is not enough. Some form of medical attention is required. The most effective treatment uses a hyperbaric chamber where, under pressure, the CO is quickly replaced with oxygen.

I believe the number of CO poisonings could be reduced by the following:

1. Installation of U-L listed carbon monoxide detectors in every home.

2. Annual inspection of heating appliances by a QUALIFIED heating contractor using the latest equipment and knowledge to fully check for vent failures and complete

3. Continuing education for professionals.

4. Public awareness campaigns.

5. Installation of sealed-combustion direct-vent heating appliances or electric heating appliances.