Forensic toxicology and carbon monoxide poisoning

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Carbon monoxide is often called a ‘silent killer’, a gas without smell or taste than can accumulate in the blood and cause death.  It is an often encountered poison in forensic toxicology.


This week a Coroner for Shropshire, UK, recorded a verdict of accidental death in the case of a girl who had taken a barbeque into her tent to keep warm on a family camping trip. A fatal level of carbon monoxide (CO) built up in the tent, poisoning the girl. This type of scenario is not that uncommon and it is one I have encountered several times, along with several other methods of poisoning by CO.



Carbon monoxide is a silent, odourless deadly gas. It is formed by incomplete combustion of fuels. When a fuel is burnt efficiently in the presence of oxygen it will form carbon dioxide and water. If insufficient oxygen is present carbon monoxide is also formed. Barbeques, gas boilers, open fires and car engines all produce carbon monoxide gas in small amounts due to inefficient combustion. It is also present in cigarette smoke.


Why is carbon monoxide toxic?


Carbon monoxide binds to the haemoglobin in the blood, forming a molecule called carboxyhaemoglobin (COHb). The binding of CO to haemoglobin prevents oxygen being bound and carried around the body as needed. CO has a higher binding affinity for haemoglobin than oxygen which means it is much harder to remove once bound and will not resdily be replaced by oxygen. Also at higher concentrations of CO it has an effect of increasing the binding of oxygen to haemoglobin, meaning less oxygen is released from haemoglobin and available for use.

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The symptoms of CO poisoning (shown above) are headaches, dizziness, vomiting and shortness of breath which increase with increasing exposure at a relatively low level. These symptoms are quite generic though and are often confused with flu or food poisoning, for example.


Type of cases involving carbon monoxide


The story above of a barbeque in a tent is not one that I believe I have encountered personally.  In New Zealand I was asked to investigate several fatalities where trampers (NZ hikers) took a barbeque into a hut to get warm. There is a great network of tramping huts in NZ, often these are in remote areas without heating which makes it understandable that you might want to use your disposable barbeque to keep warm once you have cooked your dinner. With little or no ventilation the CO will build up in the hut and in the blood of anyone in there.


Suicide by barbeque. This seems to be a method of suicide more common in the East- japan and Hong Kong for example. A charcoal barbeque is taken into a room, usually the bathroom, which is then sealed (often with towels around the door frame) and then the CO level builds up.


The most commonly encountered case involving CO is the car exhaust suicide. A hose is attached to the exhaust pipe and fed into the car, with the engine running the car soon fills up with fumes.  This case type can often bring complications though – how do you ensure the person was alive when the hose was attached?  The majority involve a suicide note of some sort that can be verified but there are occasions where a full toxicology screen would be undertaken to check for an overdose or other poison.


Victims of house or office fires may be overcome by CO or other manners of death.  CO will be measured post mortem to help establish the cause of death here.  Victims of flash fires have lower levels of CO saturation whilst slower burning fires produce high saturation levels.


In New Zealand all pilots who die in an accident are tested for CO to help the accident investigator.  If an engine was faulty or badly vented it could cause build up of CO in the cockpit leading to dizziness or even death.


Less common but more concerning to many is the build up of CO in the home.  Gas cookers and boilers that are inadequately ventilated or not working efficiently can produce CO.  This is why it is recommended that you have a CO alarm in the house and have any gas appliances serviced regularly.


Blood saturation levels and detection


In the general population you would not expect to find a CO saturation level above 5 %, rising to 10 % or so in smokers.  A fatal saturation of CO is usually above 35 %.  One thing a pathologist and toxicologist will look out for is the colour – carbon monoxide often turns the blood a characteristic cherry red.

With some exhaust suicides the saturation level can be lower if the car is fitted with a catalytic converter.  These converters mean carbon dioxide is present rather than carbon monoxide.  The suicide can still be successful as carbon dioxide depletes the oxygen level to the extent that asphyxiation can occur.

In the laboratory the analysis to determine CO saturation is undertaken in a various ways. The simplest is a carboximeter – an automated machine for analysis of blood.  We found these machines often got blocked by mortem samples that are decomposed or putrefied (given the case circumstances the body may have been exposed to lots of heat before the samples are collected).

There are various other methods available.  The method we used as a back up to the automated machine was  UV analysis.   The sample gets divided into two portions. One part has CO gas bubbled through to become fully saturated with COHb (100 % saturated).  The other portion is analysed without pre treatment with CO.  The COHb has a specific UV wavelength, so is able to be measured accurately.  A comparison of the two portions allows the percentage saturation to be determined.


In summary


Carbon monoxide is a deadly gas which humans cannot see taste touch or smell.  Deaths from CO are largely avoidable.

Do not take unflued fires or barbeques into tents or enclosed spaces.

Have gas equipment serviced regularly.

Get an audible alarm for the house – the spot ones will not wake you up!


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About Dr Tom Bassindale

Dr Tom Bassindale is a forensic scientist, and the founder of We Are Forensic. He is currently the subject lead for chemistry and forensic science at Sheffield Hallam University. He's managed hundreds of forensic toxicology cases, and is an experienced court witness. He has specialist expertise in forensic toxicology and drug testing in sport. And yes... he watches CSI.