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Early carbon monoxide intoxication: happy to be poisoned?
  1. S F J Clarke1,
  2. A Crosby2,
  3. D Kumar3
  1. 1Health Protection Agency, Division of Chemical Hazards and Poisons (London), London, UK
  2. 2London Ambulance Service NHS Trust, London, UK
  3. 3North West London Health Protection Unit, London, UK
  1. Correspondence to:
 Dr S J Clarke
 Consultant in Emergency Response, Health Protection Agency, Division of Chemical Hazards and Poisons (London), Avonley Road, London, SE14 5ER; sfjclarke{at}


Carbon monoxide poisoning is the commonest cause of death by poisoning in the UK and chronic exposure is thought to be a frequently missed diagnosis. Early recognition of carbon monoxide poisoning is vital to institute prompt treatment and to prevent exposure to others. An incident of mass exposure to carbon monoxide is presented where euphoria, lasting several hours, was the only symptom reported in approximately one quarter of the casualties. This has not been reported previously and we believe that mild carbon monoxide intoxication should be included in the list of differential diagnoses of inappropriate euphoria.

  • Acute poisoning
  • carbon monoxide
  • euphoria

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Carbon monoxide poisoning is the commonest cause of death by poisoning in the UK,1 and chronic exposure is thought to be a frequently missed diagnosis. Although much has been written about the signs and symptoms of both acute and chronic intoxication, the features of early, mild, acute exposure have received much less attention. Early recognition of carbon monoxide poisoning is vital to institute prompt treatment2 and to prevent exposure to others.3

We describe an episode of mass exposure to carbon monoxide that resulted in previously unreported changes in mood, which may provide a useful warning symptom.


The emergency services were called to a secondary school (for children aged 11–18 years) with special needs. A high proportion of the children had severe learning difficulties and physical disabilities. At lunchtime, the catering staff at the school had noticed a strange smell and feelings of dizziness, mild headache, and euphoria; this gradually became worse over approximately 30 minutes, at which point the area was evacuated and the kitchen boiler was switched off. After this, no one developed further symptoms, although the emergency services were called about 1 hour later because of persistent symptoms in some of those exposed.

Monitoring was carried out by the local authority within the first hour of the alarm being raised, and this showed the presence of carbon monoxide in the dining room area. At the time, 43 pupils and 35 staff member had been in the dining room; all were considered to have been exposed, although none had become drowsy, lost consciousness, or experienced fits. The affected individuals were gathered together in the school gymnasium, which was well ventilated and away from the source of the exposure, and triage was carried out there. It was deemed impractical to try to take blood samples from so many severely disabled children who seemed to be otherwise well. The staff agreed to have venous blood taken for carboxyhaemoglobin levels after the National Poisons Information Service was consulted to confirm that venous levels were accurate enough for the purposes of screening.4 The appropriate bottles were collected from the biochemistry laboratory at the local hospital, and the attending paramedics took the samples 3–4 hours after first exposure. All of the results were <5% (1–2% can be considered normal for urban residents, while smokers may have levels of 6–8%). The half life of carbon monoxide in a subject breathing air is 5 hours5 (none of the patients was given supplemental oxygen), so these results were in the upper range of normal, which was thought to indicate mild exposure only.

Of the 35 staff members tested, 8 (23%) reported feeling light headed and euphoric. The teachers and the carers were questioned by two of the paramedics about any changes to the behaviour and demeanour of their children. Only five children raised concerns, all about conjunctival injection, and were assessed by the medical team from the Chemical Hazards and Poisons Division of the Health Protection Agency who had attended the scene. The children were reassured and sent home, apart from one child, who was sent to the local emergency department because he suffered from Eissenmenger’s syndrome and was complaining of persistent mild headache. He was subsequently discharged after a period of observation.

Because of the high dependency needs of the children, carers routinely escort them home on school buses. The local consultant in communicable disease control and the on call public health team contacted the parents/carers of all the children later in the day to enquire about any unusual symptoms in the children and to address concerns. All parents/carers of the 41 children were contacted; of these, 10 (24%) reported that the children appeared happier or more elated than usual for the rest of the evening.


It is well recognised that carbon monoxide poisoning is commonly missed as a diagnosis.6 A literature search using the Medline and Embase databases was undertaken, as was a hand search of the papers collected in the carbon monoxide file at the National Poisons Information Service, London. This revealed a large number of articles concerning the effects of acute and chronic exposure to carbon monoxide. The majority consisted of serious cardiorespiratory and neurological effects due to significant or prolonged poisoning, and, although a small number of papers described unspecified neuropsychiatric sequelae,2,7,8,9,10,11 no mention of euphoria could be discovered.

It is interesting to note that it was the exceptional circumstances of this episode of carbon monoxide exposure that prompted the follow up health questionnaire to be performed, and that euphoria was repeatedly reported in both the children and the staff. As mood elevation has not been reported as a symptom of carbon monoxide poisoning, not all the parents were specifically asked about euphoria; the symptom was reported by the parents when they were questioned about any unusual behaviour in the children.

Euphoria tends to be associated with ingestion of certain drugs of misuse and with forms of acute psychosis, and we suggest that the list of potential differential diagnoses should include early carbon monoxide poisoning. It is an easily overlooked diagnosis because many of the clinical features are non-specific, but it is an easy one to exclude as blood levels are readily measurable in most hospitals.

In this case series of 78 people with early acute intoxication with carbon monoxide, euphoria was noted to be a relatively common occurrence, a symptom that to our knowledge had not been previously reported.



  • Competing interests: none declared