An Introduction to Poisons

Before you can even begin to look at how certain poisons affect the body and what the tell tale signs of them are the obvious is question is what are poisons and how do they work. The first thing to do here is to define what we mean by a poison. the dictionary definition of poison is:

"A substance that when introduced into or absorbed by a living organism causes illness or death."

That sums up what poisons are used for but there is a major flaw in this definition of poison. Anything in absorbed or introduced in a great enough quantity will cause illness or death to a living organism. I'm not just talking about drugs but everything from the air we breath (there have been documented cases of CO2 poisoning for example) to food that we eat (obesity and diabetes are both food based illnesses which may result in death). Thus I think a better definition would be the definition put forward by Andrew and Julie Jackson in their textbook, imaginatively titled forensic science:

"A poison is any substance  that produces an injurious or lethal effect when administered to, or otherwise taken up by an organism in too high a quantity".

This definition nicely acknowledges the fact that basically anything can be a poison if you ram enough of it into the victim, however this still runs into the problem of everything being a poison, which isn't helpful in a legal environment. Thus we need to put a number to how little of a substance must be deadly to qualify as a poison. The number used by the United States and the number which will be used in the rest of this blog is 50mgkg^-1of body weight or less. This means just 3/4 of a teaspoon would be enough to kill an average man. Now that's out the way we can get to the really interesting stuff. The types of poison I will be looking at I'm going to organise into groups based on how they attack and damage the body. This grouping system will make it easier to identify common traits between poisons and by extension common effects of these poisons on the body.

1. Enzyme Inhibitors: Enzyme inhibitors kill by slowing down enzymes through various means. When enzymes are slowed down the body loses the ability to properly carry out certain reactions. Within minutes this can lead to a wide range of equally unfortunate problems from liver failure to muscular paralysis depending on the enzyme blocked. Enzyme inhibitors cause rapid death and a lot of them are fairly easy to obtain which is why they are some of the most common poisons used in murders. Examples of enzyme inhibiting poisons are cyanide and deadly nightshade.

2. Nerve Agents: Nerve agents are among the most dangerous and gruesome poisons out there. The fact they are classified as a weapon of mass destruction by the U.S. is a testament to their lethality. Unsurprisingly nerve agents attack the nervous system of the human body, normally by blocking the receptors of the stream of sodium ions which is how the body communicates through nerves, however there are a variety of imaginative ways these substances interfere with the human body. The status of these poisons as weapons of mass destruction means a forensic pathologist will very rarely encounter them as obtaining them is practically impossible. Examples of nerve agents include the notorious sarin and tabun.

3. Corrosive Poisons: Corrosive substances are ones which damage other substances they come into contact with, usually causing chemical burns if the substance is organic. Corrosives are particularly dangerous as their effects are instantaneous, whereas other poisons kill over a longer time period giving more chance for a medical 'intervention'. Acid -base reactions are how corrosives break down organic tissue they come into contact with (amide and ester hydrolysis). Corrosives are usually acids or bases such as sulphuric acid or bleach.

4. Narcotics: Technically a just a substance with sleep inducing properties, narcotics account for a surprisingly large number of drug related deaths, primarily just due to how many narcotics there are. Narcotics often become deadly when they also fall under one of the other categories of poison. Narcotics are also especially linked to 'accidental' deaths or deaths linked to misuse of drugs, since carbon monoxide, heroine, chloroform and morphine are all narcotics.

These four categories, especially when used in conjunction cover every poison a forensic pathologist is likely to encounter. Knowing how these poisons work is a vital step in analysing them. For example a victim with no external injuries who is found to have died by asphyxiation, in their sleep with a rosy pink tint to the corpse immediately leads to a diagnosis of carbon monoxide. The fact they were asphyxiated points to a nerve poison, the fact they dies in their sleep points to it also being a narcotic and the rosy tint left on the corpse is a specific symptom of Carbon Monoxide. I aim to use these categories to help with my more in depth analyses of specific poisons. I will also dedicate at least one article to each of the categories, exploring in depth the chemistry behind each one. Stay tuned, I'll report in soon!