People don’t die of carbon monoxide poisoning any more – do they?

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Carbon monoxide (CO) is always produced when any fuel containing carbon burns. It is present in smoke in addition to its more thoroughly oxidized cousin, carbon dioxide (CO2). Tobacco smoke or any other smouldering smoke contains lots of this deadly gas. Carbon monoxide is more dangerous today than before for two reasons. Today’s buildings are constructed to be more air-tight than before, and so the old habit of closing the damper of the chimney flue when there is still some dying embers in the fireplace can be a killer in a modern house. In addition, many people are blissfully unaware of the dangers of carbon monoxide, and the happy owner of a new romantic fireplace or somebody sleeping in a trailer may have absolutely no appreciation of the dangers associated with careless use of fire. For example, in Finland about a hundred people are killed each year by carbon monoxide.


Outdoor carbon monoxide originates mostly from traffic and small-scale heating with solid fuels. The role of traffic is declining, because cars equipped with oxidizing catalytic converters are becoming more common and the proportion of diesel vehicles is increasing. For instance in Helsinki, the carbon monoxide concentrations have decreased to one third of their levels 10–15 years ago. Unfortunately the role of space heating has not decreased as a carbon monoxide source, even in highly developed industrial countries. In the developing countries, the situation is terrible.

Indoor carbon monoxide comes from incomplete burning processes. Any indoor combustion produces carbon monoxide, if smoke is not led out of the house via chimneys (gas stoves or furnaces, gas lamps and heaters, kerosene lamps and stoves, smoking, candles etc). Even chimneys are not a guarantee of safety, if the updraft is not efficient or it is hindered e.g. by closing the damper too early.

Parents with ice-hockey players in the family should be concerned about ice resurfacing machines with internal combustion engines in ice arenas or motor engines running in other enclosed spaces such as karting halls. Carbon monoxide can cause harmful exposure both indoors and outdoors, but immediate danger at exposure levels of 100 to 1000-fold higher than outdoor exposures can only occur with major indoor sources such as gas equipment, stoves or combustion engines.

If there are no specific indoor sources, then the exposure levels are usually related to outdoor concentrations. On the streets, exposure may be higher but because exposure among traffic is usually brief, the contribution to the total exposure is usually limited. In towns where local heating is decreasing (introduction of regional heating systems or electrical heating) also carbon monoxide exposures have decreased. Another beneficial factor has been the ban of smoking in public places – this has clearly decreased carbon monoxide exposures.

In a comparison study of seven European cities, variation between cities was about threefold (average 4–15 mg/m3, peaks up to 100 mg/m3). The highest concentrations were found in Athens both in the average and peak samples. In Mexico City, the concentrations may be even higher (100–200 mg/m3).

Health effects

Carbon monoxide binds to the haemoglobin in blood and prevents it from carrying oxygen. High concentrations cause unconsciousness or death within minutes due to lack of oxygen – and there are few warning signs. Low concentrations, which in themselves do not cause any detectable decrease in arterial oxygen concentration, may still cause various compensatory reflex mechanisms in the lungs and blood vessels.

In a few time series studies, a statistical association has been found between sudden cardiac deaths and the variation in outdoor carbon monoxide concentration. However, one has to wonder about how reliable these reports are, even though the correlation would be logical since carbon monoxide does obstruct the oxygen supply.

In indoor air, chronic exposure levels may be many times higher than in the outdoor air; all it needs are local sources such as poorly operating or inappropriate heating devices. The carbon monoxide levels may rise surreptiously to hazardous levels, because this gas is both invisible and odourless. Even in our so-called advanced European societies, carbon monoxide is still the chemical that causes most deaths from poisoning (if one excludes alcohol and drug-related deaths).

Some people may experience warning symptoms such as headache or nausea before they fall unconscious. It is not possible to acquire any adaptation to the toxic effects, and it is not possible to protect oneself by wearing gas masks or any filtering devices.

Carbon monoxide readily passes through the placenta. Since carbon monoxide has an even higher affinity for foetal haemoglobin than its adult counterpart, the same exposure may cause a more serious intoxication in the foetus than in the mother. This must be taken into account when treating a case of poisoning in a pregnant woman.

Carbon monoxide poisoning can be easily diagnosed by analysing blood carboxyhaemoglobin. However, these analyses are done very occasionally. An English clinical study in the 1990s on the carbon monoxide poisonings and deaths suggested that most of the poisonings and even many of the deaths go undiagnosed. Therefore the causes of even recurring poisonings may not be corrected. In Finland, several carbon monoxide group poisonings which caused tiredness and other symptoms were detected in young skaters using ice arenas and in enthusiastic go-kart drivers in karting halls.


Carbon monoxide is one of the least appreciated environmental exposures even though it is known to be dangerous, and this underestimation means that many human lives are needlessly lost every year. The limit values of urban air in the European Union are based on short term experimental studies e.g. in angina pectoris patients experiencing chest pain during exercise. If more recent results from population studies are confirmed, the limit values probably will have to be re-evaluated downwards.

Limit values help only for outdoor exposure, most carbon monoxide deaths are caused by short-term indoor exposures. These can be prevented by enacting changes in building codes, but this takes time to have any impact. A more realistic approach is to demand that carbon monoxide detectors with alarms should be installed in every home with a fireplace or other possible source of carbon monoxide. Warm carbon monoxide is less dense than air and therefore a detector fixed to the ceiling responds very well.

Stoves and heating devices may not function properly or they may be misused causing a real risk of poisoning. Furthermore, regular obligatory checks may only concern stoves and fireplaces with a chimney. The only way to avoid accidents is that all devices producing burning gases indoors should be banned, and gas stoves and similar should be equipped with adequate ventilation, and carbon monoxide detector should be used in all rooms with a fireplace. Finally, the general public needs to be educated on the correct use of these potentially dangerous devices.

The problem of ice resurfacers with internal combustion engines came as a kind of surprise, since these devices produce amounts of carbon monoxide clearly higher than urban levels. The ventilation of ice arenas should be on even during training practices and not only during matches when there are spectators present. Catalytic converters and propane fuel decrease the carbon monoxide concentrations somewhat, but the logical final solution is to change to electrical motors in these machines.

Carbon monoxide remains as one of our most risky chemicals, and poisoning deaths are more common than is generally thought. Better education, removal of dangerous sources and detectors with alarms are needed to prevent intoxications.

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