Contact Us

Our experienced team can help.

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 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.

Definitions of Types of Co Exposure
Acute CO Poisoning - Exposure to CO ccurs 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

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

Symptoms of Occult CO Poisoning

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

Occult - "hidden from view, secret, concealed, not pulged". 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.

Subjective Symptoms

Symptom Frequency %
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.


* 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)

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.
Differences From Acute Poisoning

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

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

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

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 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.

Longterm 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

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, building 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