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How do chemical weapons affect the human body?



All organophosphate chemicals act the same way, but weapons-grade chemicals are several thousand times more potent (Source: Sergei78/iStockPhoto)

How do 'nerve gases' like sarin affect the human body? And how can you tell whether someone has been poisoned by sarin or another chemical?

There is growing concern that chemical weapons have been deployed against civilians in Syria.

From the available video, it appears they have been exposed to a type of chemical known as an organophosphate, says Dr David Caldicott an emergency physician and senior lecturer at the Australian National University.

"It'll probably become very obvious very quickly whether an organophosphate has been used, more difficult than that will be determining what sort of organophosphate that was, and even more difficult than that who was responsible for its release," says Caldicott.

Organophosphates are a class of chemicals that include many of the insecticides we use every day, as well as the much more exotic weapons-grade chemicals such as sarin, soman, tabum, and VX.

The weapons-grade chemicals, which were developed by Germany in the lead-up to WWII, are a "several thousand times" more potent than everyday organophosphate insecticides.

"[Sarin] is literally one of the few agents in the world where even a drop put onto your skin could kill you," says Caldicott.

"They are possibly some of the most dangerous things that humans have ever made, after the atom bomb."

All these chemicals work in much the same way by inhibiting an enzyme called acetylcholinesterase at the nerve junction (synapse).

Under normal circumstances, acetylcholinesterase regulates the amount of the neurotransmitter acetylcholine crossing nerve synapses — so it effectively works as an off-switch.

Acetylcholine acts mainly on the body's autonomic nervous system — the body's involuntary nervous system — which controls things such as heart rate, respiratory rate, salivation, digestion, pupil dilation, and urination.

"You can imagine that if you block one of the major 'off-switches' of the body, and are left with all the lights turned 'on' all of the time, the body might run into trouble. With an extremely rapid build up of acetylcholine in the synapse, things like secretions, respiratory problems, and muscular dysfuntion can go on unattenuated," explains Caldicott.

"And that's really how people suffer and die."

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How can you tell if someone has been exposed?

Caldicott says it is possible to detect whether or not someone has been exposed to organophosphate chemicals by taking urine and blood samples.

"Early on following the exposure to a military organophosphate you may well see the breakdown products of metabolism in the urine, but after it's been secreted in the wee it's very difficult to detect.

"If someone has got very low levels of functioning acetylcholinesterase in their blood, then they've probably been exposed to an organophosphate, because the poison has bound to it and inactivated it.

"Depending on the toxicity of the agent used, how much was involved, how long patients were exposed and how they were exposed, enzyme levels can start to return to normal levels from several days to several weeks post-exposure.

"What is more difficult and more problematic, the later we are in the process of analysis, is working out what sort of organophosphate has been used.

"That is the real test for the inspectors, particularly a week down the line."

Caldicott says it is unclear whether or not military-grade chemicals have been used.

"You could mimic this effect by using a high concentration and large volumes of a simple insecticide," he says.

Another factor sparking some scepticism, he says, is the lack of protective clothing worn by medical staff treating casualties.

Organophosphates can spread by simple contact or inhalation.

In its gas form it would persist for the same time as it takes water to evaporate.

"There's no real evidence of patients being decontaminated in the way that we would consider appropriate decontamination, yet none of the treating or transporting personnel are wearing any protective gear at all.

If treating personnel were not wearing protective gear and were exposed to weapons grade organophosphates "we would expect them to be injured in some way," says Caldicott.

"We now know the phenomenon of 'off-gassing', which is the delivery of dangerous products to the environment by contaminated patients, is not that important to us in the emergency department for simple insecticide poisoning.

"But, because of the near thousand-fold potency of these weapons, it is very important for first responders dealing with exposure to sarin or another militarised organophosphate."

However, he says, further video evidence suggestive of a mass casualty chemical event can be seen in what has been released to date.

"There are patients or bodies seen in the corridors who have been intubated [a plastic tube placed in their trachea to help them breathe] but are not attached to oxygen or any form of ventilation. We might expect that when the resources of a hospital are overwhelmed in a toxicological mass casualty event — it would be very unusual in normal practice.

"What we're seeing on the video footage is not the sort of medicine that we are used to in normal circumstances; it's more like veterinary medicine. It's mass casualty medicine."

Dr David Caldicott is an emergency physician at the Calvary Health Care in Canberra and a senior lecturer at the Australian National University. He spoke to Genelle Weule.


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Published 27 August 2013