Nerve agent sarin: Invisible and indiscriminate

The poisonous nerve gas sarin is so potent that a tiny amount ― less than a grain of salt if it were a solid ― is enough to kill you. Sarin, which is stored as a liquid, not only can be inhaled, but also absorbed through the skin and eyes.

It is also as colorless and odorless as water when it evaporates in air, making it impossible to detect.

Sarin hinders breathing

Sarin mainly affects the body's autonomous nervous system, the involuntary muscles that control functions such as digestion, perspiration and salivation, as well as blinking, breathing or moving your arm.

"You'll find it extremely difficult to breathe because your brain will be telling your lungs to work in a normal way, but the message is getting interfered with and eventually you stop breathing," says organic chemist Rob Stockman of Nottingham University.

The signal to breathe, for example, is sent by nerve cells to muscle cells by chemical messengers called neurotransmitters. After the message has been sent, the neurotransmitter acetylcholine has done its job and is broken down by an enzyme. The byproducts are choline, which is recycled, and acetic acid, which reacts with water to become vinegar before being excreted.

The 'Pac-Man' molecule

Nerve agents such as sarin disrupt this process by blocking the work of the regulatory enzyme acetylcholinesterase, which is found at all nerve junctions. Stockman calls this enzyme the "Pac-Man" molecule.

"This Pac-Man molecule comes along and snips things in half [into choline and acetic acid]," says Stockman. "If you stop that [enzyme from functioning], you'll always have a very large level of neurotransmitters in the junction, so you'll have a constant 'on' switch for that muscle. The muscle will contract constantly. If it's your arm, it means you can't move it, no matter what your brain is sending in electrical signals down the wires. At the junction with the muscle, that signal's not getting through."

Stockman says the nervous system is like a stereo system that sends out electrical impulses in the form of music. Similarly, human nerve cells release neurotransmitters that tell a muscle cell what to do. If the chemical message tells the eye to release tears and sarin is inhibiting the Pac-Man molecule, the eye will keep producing a non-stop flow of tears due to a build-up of acetylcholine.

"Obviously, if you're telling the muscle to move, you want to stop it at some point," Stockman says.

Sarin is fatal when this constant "on" switch affects the diaphragm controlling the lungs. The nerve cells keep firing acetylcholine, which tells the muscle cells to breath in one direction only. The act of always breathing in, but not out, for example, can lead to death by asphyxiation.

If your exposure is relatively mild, it's possible to survive a chemical attack. One antidote is Pralidoxime, which works by detaching sarin from the Pac-Man enzyme. This reactivates the enzyme, allowing it to resume its normal function of breaking down acetylcholine.

Baking soda and water helps degrade sarin

What do you do if you don't have no access to an antidote?

One first aid remedy is to breathe through a towel soaked in a glass of water mixed with one tablespoon of baking soda. The alkaline solution degrades sarin.

"The more alkaline the water the better," Stockman says.

"The main problem with chemical weapons is that they're very difficult to control ― they are subject to wind, and the delivery is much more difficult than high explosives," says Martyn Poliakoff, research chemist at University of Nottingham. "You don't know where exactly it's going to go. [Chemical weapons] often persist in the environment, so they can kill other people. It is this anonymous way of killing random people that everyone finds so repugnant."

Environmental factors influence sarin's effects

If the temperature outside is below 20 degree Celsius (68 degrees Fahrenheit), the amount of time that sarin lingers in the air ranges from two to 20 days. At 25 degrees Celsius (77 degrees Fahrenheit), that lifetime is halved, to between one and 10 days. Under cold weather conditions, at 0 degrees Celsius (32 degrees Fahrenheit) for example, nerve gases can remain in the atmosphere for as long as a year.

"If it is cooler," says Stockman, "you get less thermal cycling of gas in the air ― the cooler air sits closer to the ground. But the other thing is the environmental lifetime of sarin. The hotter the temperatures, the quicker it reacts to moisture in the atmosphere and the quicker it will degrade and it will naturally react with water in the atmosphere."

Other climatic conditions help degrade sarin, such as humidity and the amount of sunlight.

Difficult to deploy

It's possible that sarin and other nerve gases have only seldom been deployed as a weapon of war because the outfall is so unpredictable.

Sarin was discovered by the Nazis in the late 1930s. But they never deployed nerve gases on the battlefields, or in the extermination camps, where Zyklon-B, a cyanide gas, was used.

Chemical weapons have made headlines during the Syrian civil war in the 2010s, where the Assad regime used them in several attacks to kill its own citizens. In Japan, a terrorist cult unleashed homemade sarin in a Tokyo subway attack that killed a dozen people in 1995. In the 1980s, Saddam Hussein deployed nerve agents in the Iran-Iraq war, killing thousands of civilians.