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Carbon monoxide poisoning: a hidden danger

Carbon monoxide is an odourless, colourless and tasteless gas. People exposed to high levels are likely to require emergency management. However, lower levels of exposure may go unnoticed. People who are repeatedly exposed to carbon monoxide may present to primary care with a range of non-specific symptoms such as headache, lethargy, memory difficulties, sleep disturbances or mood changes. Consider carbon monoxide poisoning as a potential differential diagnosis by asking patients about possible sources of exposure in their home or workplace.

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Published: 4 October 2019

Key practice points:

  • Exposure to carbon monoxide can occur via many common household or occupational activities, e.g. heating, some cooking appliances and exhaust fumes
  • Repeated exposure to low levels of carbon monoxide may be overlooked as people do not realise they are exposed and symptoms are non-specific, such as headache, lethargy, dizziness or loss of balance, mood changes, difficulty concentrating or sleep disturbances.
  • To assess the likelihood of carbon monoxide poisoning: establish a potential source of exposure, look for a pattern of symptoms, e.g. worsening while at home in the evening and improving when out of the house, and enquire whether other people in the same location have symptoms
  • People with repeated low-level exposure typically experience a resolution of symptoms once the source of carbon monoxide is eliminated, unless there has been underlying neurological damage.
  • Testing of carboxyhaemoglobin levels is unlikely to be useful for patients with low level carbon monoxide exposure as levels begin to decrease once a person leaves the source; results can also be confounded by smoking
  • Pulse oximetry is not useful for diagnosing carbon monoxide poisoning as it is unable to distinguish between carboxyhaemoglobin and haemoglobin bound to oxygen so will give falsely high readings
  • All cases of suspected carbon monoxide poisoning must be notified to the Medical Officer of Health, which can be done using the Hazardous Substances Disease and Injury Reporting Tool available via Medtech, MyPractice and Profile Patient Management Systems. Contact your local Public Health Unit directly to report the exposure if your practice does not have access to the tool.

Carbon monoxide is produced by combustion of fossil fuels such as liquid petroleum gas (LPG), diesel and petrol, as well as other combustible materials such as wood and paper. Exposure can occur from a range of activities involving heating, cooking and transport, particularly if appliances are not correctly ventilated, are faulty or poorly maintained (Table 1). The use of appliances in an inappropriate manner, such as moving a barbecue inside or using a gas oven for heating, is also a cause of exposure. Carbon monoxide cannot be detected by smell, taste or irritation of the airways, therefore people can be unaware of exposure, particularly if the symptoms are not severe enough to be recognised as an acute poisoning event.1

Table 1: Potential sources of exposure to carbon monoxide6, 7

Potential source Precautions to prevent exposure
  • Un-flued fuel heaters, e.g. LPG, wood or charcoal
  • Portable, un-flued gas cookers, e.g. for camping
  • LPG-powered fridges without venting, e.g. in caravans
  • Diesel generators or diesel or petrol-powered machinery used indoors, in an attached garage or with exhaust entering an open window
  • Running a vehicle in an enclosed garage, particularly if attached to a house
  • Using petrol or diesel chainsaws or cutting equipment or welding in confined spaces
  • LPG powered forklifts or platforms, particularly if used in poorly ventilated spaces such as cold storage rooms
  • Working on fossil fuel powered vehicles or machinery, e.g. a car mechanic
  • Inhalation of smoke from a fire, e.g. bonfires or burn-offs

These sources normally produce carbon monoxide and require adequate ventilation to prevent exposure

  • Gas or wood burning for heating or cooking:
    • Open fireplaces or wood burners
    • Flued LPG gas fires
    • Flued gas cookers or gas ranges with rangehoods
    • Pellet fires
    • Outdoor barbecues
    • Boilers or central heating systems
    • Kilns
  • Vehicles with faulty exhaust systems or with a blockage in the exhaust, such as snow or mud from driving off-road

These sources are typically associated with little risk but can result in exposure if equipment is faulty, poorly maintained or used improperly, e.g. a BBQ or patio heater used indoors, a gas oven used with the door open for heating

  • Cigarette smoking
  • Waterpipe (hookah) smoking, which results in more carbon monoxide exposure than cigarette smoking8

These activities involve inhaling carbon monoxide and quitting or smoking less is the only way to reduce exposure

High levels of carbon monoxide exposure requires emergency management

Exposure to moderate to high levels of carbon monoxide can rapidly cause severe symptoms and progress to fatality, due to hypoxia and other factors associated with carbon monoxide poisoning (see: “Effects of carbon monoxide poisoning”). In New Zealand, carbon monoxide exposure resulted in 379 deaths from 2006 to 2014, with 96% of these intentional exposures.2 The severity of carbon monoxide poisoning depends on the level and duration of exposure, although some people are more susceptible than others.

People who are particularly vulnerable to the effects of carbon monoxide poisoning include:3

  • Infants and children, due to a higher respiration rate for their body mass
  • Pregnant females
  • Older people with frailty
  • Those with co-morbidities such as cardiovascular disease, respiratory conditions or anaemia

Carbon monoxide is inhaled in cigarette smoke, and people who smoke appear to have an increased tolerance to carbon monoxide exposure, with fewer symptoms until higher concentrations are reached.4

For further information, see: “Emergency management of carbon monoxide poisoning

Lower levels of carbon monoxide exposure may go unnoticed

It is uncertain how many people are exposed to non-fatal levels of carbon monoxide, as not all exposures are recognised or reported, and some people do not seek treatment or receive a diagnosis.

Depending on the specific circumstances, low level repeated exposure to carbon monoxide may result in a combination of both acute and chronic symptoms (Table 2).4 There is little evidence available regarding the long-term health effects of this type of exposure. Patients may have a complete resolution of symptoms once the source is discovered and removed.4 However, in some instances repeated low level exposure has caused underlying neurological damage resulting in permanent symptoms.5 People who are unknowingly being exposed are also at risk of a potential fatal or severe poisoning event if the level of exposure increases.

Table 2: Symptoms of exposure to carbon monoxide. Adapted from Ashcroft et al.4

Acute exposure, i.e. short-term inhalation of moderate to high concentrations Chronic exposure, i.e. repeated exposure to lower concentrations
  • Headache
  • Tiredness
  • Nausea and vomiting
  • Abdominal or chest pain
  • Shortness of breath
  • Delayed neurological symptoms
  • Memory difficulties
  • Fatigue
  • Mood changes
  • Hearing loss*
  • Ataxia, coordination difficulties, tremors and slow movement

* Exposure to carbon monoxide can cause hearing loss and may also increase the risk of noise-induced hearing loss in industrial settings17

For further information, see: “Effects of carbon monoxide poisoning”

There can be a wide range of potential causes of non-specific symptoms associated with carbon monoxide exposure, such as headache, dizziness, loss of balance, mood changes, difficulty concentrating, memory loss or visual disturbances (Table 2). Differential diagnoses could include a variety of neurological, infectious or endocrine conditions. However, the possibility of carbon monoxide poisoning should always be considered.

Assessing the likelihood of carbon monoxide poisoning begins with establishing if there is a potential source (Table 1) and identifying whether the patient’s symptoms have a pattern of onset or worsening consistent with exposure to that source. In published cases, people with repeated exposure to carbon monoxide typically experience worsening of symptoms such as headache in the hours after the exposure starts, and symptoms reduce within hours after leaving the source.4 A carbon monoxide detector installed at a patient’s home* can provide evidence of a source of exposure. If potential exposure at work is suspected, workplaces can arrange for employees to wear portable carbon monoxide detectors or have other assessments carried out to establish whether safe working levels of carbon monoxide have been breached, with guidance from Worksafe New Zealand.7

A definitive diagnosis of carbon monoxide poisoning can usually only be made after a source is eliminated, resulting in resolution in the patient’s symptoms.

A simple four-letter acronym which can guide questioning is COMA:15

  • Co-habitants or companions: Do other people living at the same address or at the same workplace have similar symptoms? This could also include pets with signs of lethargy, difficulty walking or loss of balance.
  • Outside or away: Do symptoms improve when outdoors or away from the normal place of work or residence for a number of hours?
  • Maintenance: What sources of heating are used, are gas heating or cooking facilities or fireplaces well maintained and regularly serviced?
  • Alarm: Is there a carbon monoxide alarm in the house or workplace?

* Available at hardware stores for approximately $60-$70. Some smoke alarms also detect carbon monoxide.

Patterns of symptoms may be useful for diagnosis

There is no single symptom which is diagnostic for carbon monoxide exposure.10 Headache is commonly reported, however, there are no specific characteristics of headache caused by carbon monoxide exposure which can help diagnosis.16 Nausea or fatigue often accompanies headache with carbon monoxide exposure, but can also indicate migraine or many other causes.4

Patterns which could indicate exposure to carbon monoxide include symptoms which only occur:

  • After arriving home or during the evening (if the source is at home)
  • During the course of work (if the source is at work)
  • During winter, e.g. due to use of a heater which emits carbon monoxide
  • After moving to a different house or using a different form of heating

Patients exposed to low levels of carbon monoxide over a long period are unlikely to show any obvious signs on physical examination. Cherry coloured skin or lips was previously considered a tell-tale sign of carbon monoxide poisoning, however, it is not a reliable diagnostic sign as it is very rare and typically only occurs after exposure to very high levels or post-mortem.4

Testing for low-level exposure may be difficult

Testing carboxyhaemoglobin levels forms part of diagnosis during emergency management of carbon monoxide poisoning (see: “Emergency management of carbon monoxide poisoning”). However, this is likely to be less useful and more difficult to interpret in patients with repeated low-level exposures, as carboxyhaemoglobin levels will not be as elevated as in an acute poisoning event, and the delay between exposure and drawing blood for analysis means that test results will give falsely low readings. Detecting low-level exposure in people who smoke is further complicated by the fact that smoking results in the formation of carboxyhaemoglobin, with levels up to 9% classified as the upper limit of the reference range for smokers, compared to 1.5% for non-smokers.20

Pulse oximetry is not useful for detecting carbon monoxide exposure: Standard pulse oximeters use two wavelengths of light to determine oxygen saturation. These are unable to differentiate between haemoglobin bound to oxygen and haemoglobin bound to carbon monoxide (carboxyhaemoglobin). Therefore, they are not useful for the assessment of carbon monoxide poisoning as they will give a falsely elevated reading of oxygen saturation.21 Newer pulse oximeters have been developed using additional wavelengths of light which detect carboxyhaemoglobin, however, these are not widely available.

If low-level exposure to carbon monoxide is suspected and a potential source identified, develop a plan for the patient to avoid future exposure, such as switching to an alternative form of heating.

For patients with repeated low-level exposures, it would be reasonable to expect improvement in most symptoms within the first days of avoidance. However, some patients may have ongoing symptoms if exposure has resulted in underlying neurological damage, such as problems with vision, hearing loss or tremors.4, 12

General Practitioners are required to report suspected exposures to the Medical Officer of Health via the Hazardous Substances Disease and Injury Reporting Tool in the Practice Management System (PMS; see: “Notifying cases of suspected carbon monoxide poisoning”). Contact your local Public Health Unit directly to report the exposure if your practice does not have access to the tool. Patients with suspected exposure can be reported based on clinical suspicion and confirmation of exposure is not required when submitting a notification.22 Advise patients to discuss potential exposures in the workplace with their employer.

Notifying cases of suspected carbon monoxide poisoning

General Practitioners are required by law to report cases of suspected carbon monoxide poisoning, preferably using the Hazardous Substances Disease and Injury Reporting Tool; this is available via MedTech and the MyPractice and Profile PMS systems.23 In MedTech, go to:

  • "Module List"
  • "Hazardous Substances & Lead Notifications"
  • "Hazardous Subs & Lead Notifications"

The notification tool will show three tabs for clinicians to complete: “Exposure Event”, “Assessment” and “Notifier/Patient Details”. In the “Substance” category on the “Exposure Event” tab, carbon monoxide poisonings should be entered as “Other” with carbon monoxide poisoning specified in the text box below.

This tool will notify the local Public Health Unit (PHU) and Medical Officer of Health about a potential exposure; the PHU may take further action such as investigating further or forwarding notifications to Worksafe New Zealand.24

For further information on hazardous substances disease and injury notifications, see:

* Reporting cases of carbon monoxide poisoning to the Medical Officer of Health is required under Schedule 2 of the Health Act 1956, as it is an example of poisoning arising from chemical contamination of the environment.25 It is also required in the Hazardous Substances and New Organisms Act 1996 under Section 143.26

Figure 1: An example of reporting carbon monoxide poisoning in the Hazardous Substances Disease and Injury Reporting Tool

For further information about this module, see:


Thank you to Associate Professor Deborah Read, Environmental Health Indicators Programme, Centre for Public Health Research, Massey University for expert review of this article.

N.B. Expert reviewers do not write the articles and are not responsible for the final content.

Barnett Bond 14 Oct 2019 10:29

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