What’s your poison?

It’s been a method of murder for thousands of years. But what is a poison, exactly, and how do we detect them?

Forensic toxicology is the study of the effects of foreign substances (alcohol, illicit drugs, medicines, poisons, noxious materials and various other substances such as solvents) on the body in a forensic context.
What is a poison one may ask?
Well, as the science saying goes, all substances are poisonous — the only thing that distinguishes them is the dose.

This is still a good working definition for poisons and it is worth bearing in mind the next time you drink water, because the consumption of a large volume of water could be, and has been, fatal.

In the early days of ecstasy use in the UK a number of fatalities were actually attributed to excessive water consumption.

Different substances have vastly different effects on the body. Medicines are usually effective in milligram quantities (a milligram is a thousandth of a gram) whereas some substances, such as bacterial toxins, may be fatal in far smaller amounts.

On the other hand it takes more than several litres (or kilograms) of water to be harmful, so it can be considered relatively innocuous — fortunately for us.

There is a full spectrum of effects, good, bad or negligible between these extremes. Some of these effects may be important in a criminal or forensic context and it is the job of the expert witness in forensic toxicology to try to describe these and place them in context for the benefit of the courts.

The first part of this job is to attempt to identify the substances that may have had an effect on behaviour, or injured someone or even produced death.

With a limitless number of substances involved, identifying what is involved could prove a time-consuming and expensive but fortunately there are often good pointers to what may be involved and a series of tests that can be performed relatively cheaply and quickly. These are often used in conjunction with circumstances and symptoms, if any, to guide the analyst.

Having identified the potential substances they must be confirmed using a scientifically independent method and measured in an appropriate sample, such as blood or urine.

Further tests may be carried out to confirm and measure the substances in additional samples such as body tissues — for example the liver or muscle.

Sometimes in the case of fatalities hair segments can be used to test for previous exposure over time since many substances leave traces in the hair shaft as it grows out of the scalp.

Some tests are only applicable to certain substances so a variety of tests may be used depending on the circumstances and requirements of the case.

The analytical tests include visible and ultra-violet spectroscopy, immunoassays, and mass- spectrometry, usually combined with liquid-chromatography or gas-chromatography to assist in the separation of the substances from the matrix.

Traditional colour and spot tests, organoleptic testing (sniffing the odour), pH (acidity/alkalinity) and microscopy may also be useful especially for adulterated foodstuffs.

The second part of the job of the court-going forensic toxicologist is to interpret these findings in the context of the circumstances of the particular case.

This can be fairly simple, such as in a fatality caused by an overdose of a medicine, or may be much more complex when multiple substances and tolerance to some or all of these is involved.

The significance of these and any unusual findings must be considered carefully by the expert witness.

Notable poisoning cases in the last century include the notorious Graham Young who killed his workmates in 1971 by lacing their tea with the salts of the toxic, heavy metal thallium.

He had previously carried out experiments on his own family members!

Ricin was apparently used to kill Georgi Markov, a Bulgarian, who had fled from the Communist regime in his native land to live as a writer in London.

The ricin was said to have been injected into his leg in a pellet with a specially adapted umbrella while he was waiting at a bus stop near Waterloo Bridge in 1978.

Ricin, produced from the castor oil bean is a particularly potent poison requiring only a couple of milligrams to produce a fatal outcome when injected.

More recently, in 2006, polonium 210 was used to kill the Russian ex-spy Alexander Litvinenko who had sought political asylum in London.

Although polonium is similar to certain toxic heavy metals it is particularly lethal because it emits harmful alpha radiation that damages the body’s tissues.

Thus tiny quantities can produce irreparable radiation damage over a period of time.

It is said to be several hundred times more lethal than cyanide.

However because polonium 210 is only produced as a by-product in nuclear reactors, its availability is somewhat limited, and it requires special and very careful handling because of the radiation danger.


Dr Ardrey is Senior Research Fellow in the School of Applied Sciences, University of Huddersfield.