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Low Exposure Carbon Monoxide Poisoning

Low Exposure Carbon Monoxide Poisoning


Chronic Carbon Monoxide Poisoning

What is Chronic CO Poisoning?

Chronic CO poisoning usually involves lower levels of the gas in the air and lower blood CO (COHb) concentrations. Exposure usually continues for many days to months. The boundary limit between acute and chronic exposure is indistinct.

The word chronic should be reserved to describe the type of exposure, not the subsequent condition or effect! A damaging effect of CO poisoning, or in fact, any change which persists, should be referred to as a residual effect.

Chronic CO poisoning may not elicit the typical symptoms of (acute) CO poisoning such as headache, nausea, weakness, dizziness, etc. Mucous membranes of the body will almost never be cherry pink. Chronic CO poisoning is often misdiagnosed as chronic fatigue syndrome, a viral or bacterial pulmonary or gastrointestinal infection, a "run-down" condition, immune deficiency, etc. Patients may occasionally present with polycythemia, increased hematocrit, etc.

Chronic CO poisoning is, in fact, difficult to diagnose by those not skilled in its presentation. As stated above, it is often mistaken for chronic fatigue syndrome, viral or bacterial pulmonary or gastrointestinal infection, excessive heat, etc. Similar symptoms are seen simultaneously in more than one person, and which disappear upon removal from an environment are tip-offs that CO is involved. COHb is usually not excessively elevated. More often than not, by the time air CO or blood CO levels are measured, the presence of CO in the environment has been corrected, making measurement impossible. Computed tomography (CT) and magnetic resonance imaging (MRI) generally show no lesion, even when psychological/psychiatric and neurologic evaluations may detect functional deficits.

This is a subject about which many exciting new data have become available during the past 2 years. Summaries of some of these dates are seen on this website. A body of animal data is also available which is of some value in understanding and predicting human responses. See the very useful British study by CO Support and the other studies contained in the section called Chronic CO Poisoning.

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Definitions of Types of Co Exposure

Acute CO Poisoning - Exposure to CO occurs only once and lasts no longer than 24 hrs.

Chronic CO Poisoning -

  • Exposure to CO occurs more than once and lasts longer than 24 hrs.
  • Usually involves lower CO levels / lower COHb saturations
  • Exposure usually continues for many days to months
  • Boundary limit between acute and chronic exposure indistinct

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Definition of the Word 'Chronic

Chronic -

(Gk.) khronos = time | (Lat.) chronicus | (Fr.) chronique

  1. 1) Of long duration
  2. 2) Subject to a habit or disease for a lengthy period

Syn. continuing, lingering, persistent, prolonged, protracted

Webster's New College Dictionary, Houghton Mifflin Co., 1986.

The term chronic is sometimes used as in definition #2 - "A history of CO inhalation and an awareness of the typical distributions of lesions are important for recognition of the effects of CO poisoning, especially when patients are in the chronic stage." (Uchino et al., 1994, Neuroradiology, 36, 399-401)

Note: In this condition, ie. chronic CO poisoning, we are concerned with how long the insult (exposure) lasts, not how long the resulting effects last.

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A Paradox of CO Physiology:


  • It limits oxygen delivery, binds to the intracellular energy generating system, kills cells, causes damage to tissues and organs, and kills people.

Natural / Helpful:

  • It is generated by the human body as a by-product of hemoglobin metabolism
  • Along with NO (nitric oxide), it is an integral part of the vascular control mechanism.
  • Most blood vessels dilate as COHb increases, allowing more blood to flow through.

Elevated CO Concentrations are More Likely in:

  • Smaller multi-unit dwellings
  • Households using gas ranges for cooking
  • Dwellings heated by gas wall furnaces

Low(er) CO Concentrations are More Likely in:

  • Single-family dwellings
  • Homes with forced-air furnaces
  • Residences with electric cooking appliances

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Table of Indoor Air Pollutant Concentrations

PollutantConcentrationLocation / Condition
Carbon Dioxide 860 ppm Lecture Hall
Carbon Dioxide 600 - 2500 ppm Schoolroom
Carbon Dioxide 9000 ppm Nuclear submarines
Carbon Monoxide 2.04 +/- 2.55 ppm U.S. homes
Carbon Monoxide 2.5 - 28 ppm Offices, restaurants, bars, arenas
Carbon Monoxide 3.1 - 7.8 ppm Home kitchens with gas stoves
Carbon Monoxide 1 - 5 ppm median outdoor concentration in cities, 1979
Carbon Monoxide 0 - 3 -27 ppm Max. 1 hr. average outdoor concentration
Carbon Monoxide 0 - 3 - 22 ppm max. 1 hr. average indoor concentration
Carbon Monoxide 20 ppm Room polluted with cigarette smoke
Hydrogen Cyanide 56 ppb Room polluted with cigarette smoke
Nitric Oxide 1.05 ppm Room polluted with cigarette smoke
Nitrogen Dioxide 5 - 110 ppb U.S. homes with gas stoves
Nitrogen Dioxide 5 - 317 ppb English homes with gas cookers
Nitrogen Dioxide 20 - 66 ppb Median outdoor concentration in cities, 1979
Nitrogen Dioxide 25 - 177 ppb Homes, 48 hr. average
Nitrogen Dioxide 200 ppb Room polluted with cigarette smoke
Ozone 2 - 68 ppb Photocopying room
Ozone 2 - 18 ppb Homes with electrostatic air cleaners
Ozone 7 - 60 ppb Median outdoor concentration in cities, 1979
Ozone 0 - 700 ppb Using an electronic air cleaner
Sulfur Dioxide 8 - 37 ppb Yearly averages in Chicago & NY
Methane 2 ppm Atmospheric air

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Non-Fatal vs. Fatal CO Poisonings

All 4.6
Vehicular 3.0
Furnaces (non-vehicular) 19

Thus, for every CO death due to a malfunctioning furnace, there are 20 non-fatal CO poisonings.

Estimates Based on Statistical Data: 2

  • 5,700 - 10,000 people seen in emergency rooms for suspected CO poisoning, 1992-94.
  • 200 CO-related fatalities during the same period.
  • 7850 / 200 = 39.25

Thus, for every CO death, this suggests there are 39.25 people who present to the ER for CO poisoning. How many more people with CO poisoning don't go to the ER, and thus are not found in the record?

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Symptoms of Occult CO Poisoning

  • Headache
  • Fatigue
  • Dizziness
  • Paresthesias
  • Chest pains
  • Palpitations
  • Visual Disturbances

Occult - "hidden from view, secret, concealed". Most chronic CO poisoning is of this type, at least at first.

Paresthesias - "abnormal or morbid sensation, as with burning, prickling, etc., but without objective symptoms.

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Subjective Symptoms

SymptomFrequency %
Fatigue 92
Headache 87
Dizziness 69
Sleep Disturbances 66
Cardiac Symptoms 62
Apathy 54
Nausea, vomiting 42
Memory Disturbances 40
Reduced Libido 22
Loss of Appetite 17

From: Jain, K.K. (1990) Carbon Monoxide Poisoning, Warren H. Green, Inc., St. Louis, MO

Chronic CO poisoning often masquerades as lethargy, listlessness, lack of motivation, sleepiness, etc. and is often characterized as chronic fatigue syndrome, clinical depression, or an endocrine disorder. The changes are frequently subtle and only recognized as being related to CO exposure after a period of time. Recognition of CO involvement often only occurs by accident or by happen-stance and documentation of abnormally elevated CO in the air and blood is frequently not possible.

Source: From Study A, Penney, D.G. Carbon Monoxide Toxicity, D.G. Penney, ed., CRC Press, 2000. Back to Top


  • Often goes long undetected
  • Masquerades as flu, fatigue, etc.
  • Often many people "sick" simultaneously
  • May go away upon leaving poisoning site (to work, on vacation, etc.)
  • Nearly always misdiagnosed by physicians
  • May involve pets "sick", dead at same time
  • Rarely involves sinus congestion, cough (when present, it may be due to other compounds {eg., NOx, SO2} in exhaust gases)

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Clues to Discovery

  • Lethargy, headache, etc. of long duration
  • Long-standing "illness" intractable to medical solutions
  • "Illness" that suddenly improves when leaving site
  • Multiple cases at one location
  • Morbidity/mortality of pets
  • CO alarm sounding, once or repeatedly
  • Presence of malfunctioning furnace, water heater, etc.

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Differences From Acute Poisoning

  • may not elicit the typical symptoms of (acute) CO poisoning:
    • headache
    • nausea
    • weakness
    • dizziness
    • mucous membranes almost never cherry pink
  • COHb is usually not excessively elevated
  • CT and MRI generally not useful

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Common Misdiagnoses

  • Chronic fatigue syndrome
  • Viral or bacterial pulmonary or GI infection
  • "Run-down" condition
  • Endocrine problem
  • Immune deficiency
  • Psychiatric/psychosomatic problem
  • Allergies
  • Bad/tainted food

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Problems in Dealing With Chronic CO Poisoning

  • Fact of exposure usually recognized only later
  • Good COHb level measurements usually not obtained
  • Air CO level measurements often not obtained
  • Residual effects commonly occur, but often subtle; thus usually unrecognized by physicians.
  • Less medical/scientific literature available than for acute CO poisoning
  • Seldom produces damage recognizable by high-tech scanning techniques (MRI, CT, SPECT)
  • Changes seen by neuropsychological testing usually most useful
  • Considerable variability of effects from one inpidual to the next

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Why is CO Poisoning Not Better Recognized by the Medical Profession?

  • It almost invariably presents with too many disparate, seemingly unrelated and often non-specific symptoms. This tends to confuse physicians who act mainly on pattern recognition of one or a few symptoms to come up with a probable diagnosis, or at least a "short list". The result of being presented with 5, 10, 15 or more symptoms is likely to yield a diagnosis of hypochondriasis (faking), psychiatric condition, or both.
  • Presentation in urgent care settings is such that it usually appears not to require emergency measures - absence of unconsciousness, no obvious provoking agent, low or normal COHb values, skin/mucous membranes not pink, etc.
  • It has been difficult to study in animal models because rats, mice, etc. are far more resistant to CO than humans, and also are unable to report the many psychological, cognitive and emotional changes that result. Thus we have little understanding of the underlying cellular mechanisms at play.
  • Lack of training in the area, thus a low index of suspicion for the condition and the resultant shockingly high rate of misdiagnosis.

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Long Term Effects (Based on CO Support Data)

  • Tiredness, weakness
  • Pains, cramps
  • Headaches
  • Nausea, sickness
  • Loss of Concentration
  • Dizziness
  • Digestive Problems
  • Cardiac Problems
  • Flu Symptoms
  • Difficulty Breathing
  • Pins & Needles, Stiffness
  • Vision Problems
  • Memory Loss
  • Personality, Emotional Problems
  • Sleep Disturbance
  • Mouth/Throat Problems
  • Unable to Walk / Work
  • Clumsiness
  • Hallucinations, Zombie-like State
  • Depression
  • Panic Attacks
  • Loss of Hearing
  • Trembling

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Furnace Concerns – U.K. vs. U.S.

Chronic carbon monoxide problems are potentially worse in the U.K. than in the USA, because of the many very old buildings and the past and present construction approach which consists of building solid walls, floors, and ceilings. This usually precludes the use of ducted forced air heating/cooling. Instead, buildings are fitted with "gas fires", ie. gas heaters that are usually located in old fireplaces, exhausting into the fireplace chimney.

Problems with Gas Fires/Fireplaces

  • Most use air from within living space for combustion
  • Inadequate installation / maintenance
  • Possible exposure of inhabitants to heat, flame, and fumes
  • Possible leakage of unburned heating gas into living space

Other Specific Problems With Gas Fires

  • Chimney outlet too low
  • Cold chimney, leading to water condensation, then rusting of metal parts
  • Exhaust fan creating negative pressure in living/combustion space
  • Unusual geography near chimney
  • Wind conditions around chimney
  • Doors/windows open, additions to structure

Exhaust Gas Removal

  • Leakage of fumes from flue - masonry/metal/plastic (lined/unlined)
  • Partial/complete blockage of flue - cement, condensates, birds nests, etc.
  • Age of fire/furnace, flue, and chimney

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Hypothetical Case Report

Mrs. Betty Jones is a 35-year-old homemaker. She and her husband George, 37 years old, live in a city in the mid-west. She has an Associates degree in accounting, while her husband has a Masters degree in Business Administration. Neither of them are smokers.

In early 1995, they purchased a home in a suburban community through a real estate brokerage company. The home was built in 1958. It was inspected and major appliances in the home were guaranteed for 5 years. The home has three bedrooms, a living room, a family room, and a glassed-in back porch. It is heated by a forced-air, natural gas furnace in the basement. Hot water is provided by a gas-fired water heater, also in the basement.

Beginning in the autumn of 1995, Betty Jones began having headaches and feeling very tired. Her two children, John (12 years of age) and Cathy (9 years of age), and her husband George occasionally awoke in the morning with headaches, dizziness, and nausea. They believed that they all had a touch of "flu" or had eaten tainted food.

Mrs. Jones continued to feel "out of it" for the remainder of 1995 and into the spring of 1996. Her physician, Dr. Blackstone, gave her a "physical", obtaining chest X-rays, blood for complete CBC, and samples for a Pap smear test. He found nothing wrong, saying that "flu" has been going around. A furnace company that regularly serviced the heating system found "everything in good working order."

During the summer of 1996, Betty Jones and the whole family felt much better, although she and the children continued to have frequent headaches and to feel slightly fatigued. They felt better when they went away for vacation for two weeks.

In late October 1996, Betty Jones again began to have frequent severe headaches and to become extremely fatigued. She was becoming so lethargic that she could not accomplish her normal housework. She was forgetting tasks that needed doing, and finding it increasingly difficult to maintain the family checkbook. She was also feeling depressed and defeated in her daily life.

On several visits to Dr. Blackstone, she was told that there was nothing wrong with her. He said her perceived state was psychosomatic, and that she should seek counseling or schedule regular visits with a psychiatrist.

By spring 1997, the Jones' children John and Cathy, previously excellent students, were on academic probation at school. John, a 7th grader, was in danger of failing and being held back a year. Cathy was now getting C's and D's in her classes in elementary school and her teachers were concerned. Mr. Jones, who all his life had been an ambitious and successful employee at a national insurance company, believed he now was in danger of being fired.

To gain extra space in their modest 1300 square foot home, the Jones family contracted to have a fourth bedroom added during the summer of 1997. Because the old furnace in the home was the original unit and would not be adequate to heat the new larger house, the contractor installed a new one. In doing so, he discovered that the heat exchanger in the old furnace was badly rusted through, that the near horizontal run of flue pipe to the chimney was also rusted through, and that the old brick chimney was oversize, unlined, and partially blocked near the top.

Upon learning of these problems, Mr. Jones asked that the old furnace be fired up and measurements of CO made by the gas company. He had recently seen a program on TV about the dangers of CO and wanted to be sure. With the family safely outside, CO levels in the house were observed to attain 176 ppm after one hour. The whole family then went to see Dr. Blackstone, who drew blood for the measurement of carboxyhemoglobin. COHb levels came back at between 0.5% and 1.4%. The physician, not familiar with the effects of the gas, told them that since the CO was now out of their bodies, they would be well again.

Mrs. Jones continued to suffer from severe headaches, fatigue, depression, and irritability. She also continued to have cognitive and memory problems, and began to develop muscle and joint pain, to hear a buzzing sound in her head (Tinnitus), and to have various visual problems. Mr. Jones continued to find it difficult to do his job. He could not make decisions (loss of executive functioning) and lost track of details in his work. The children continued to struggle academically and socially - cognitive testing at school suggested recent significant declines in I.Q. in both children.

As of early 1999, the Jones family is attempting to recover from the health problems caused by their old, leaking furnace. They have been seen by a number of health professionals with varying results: neurologists, toxicologists, and neuropsychologists. To the Jones', it appears that few people in the medical community have much understanding of the long-term health effects of chronic CO exposure. They have retained legal counsel and are discussing options that might lead to compensation from responsible parties. Fortuitously, they have kept the old furnace, flue and other parts as evidence.

What Important Points does this Case Illustrate?

  • Have a thorough inspection when you buy a house, especially an older house.
  • The multiple symptoms reported (headache, dizziness, nausea) should have increased suspicion of CO poisoning.
  • Similar symptoms in several people should also increase the suspicion of CO poisoning.
  • A CO detector should have been purchased and installed in the home.
  • The physician should have been strongly encouraged to promptly order COHb tests.
  • Furnace and "gas" inspectors should always test for CO.
  • Fatigue and lethargy combined with headaches are strong indicators of CO presence.
  • If you can't get satisfaction with one physician, see another - a G.P. or a specialist with experience in CO poisoning.
  • While the leaking furnace, flue ducts and faulty chimney were discovered by chance, Mr. Jones did the right thing to immediately have the house tested for CO.
  • Blood samples for COHb measurement were taken way too late, ie. they must be done within 2-4 hrs. after leaving the site of the poisoning).
  • The residual effects elicited by all members of the Jones family are consistent with chronic CO poisoning.
  • The health effects of the CO poisoning continue at least 1-1/2 years after the CO poisoning was discovered/ended.
  • Mr. Jones was wise to have kept the faulty furnace, flues, and other parts, should legal action be necessary.

Back to Top Used with permission from the author David G. Penney, PH.D

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