How could Novichok have poisoned people four months after the Skripal attack?
The arcane issues of how long chemical warfare agents could last in storage or in the environment after use are usually considered arcane and best left to the handful of specialists in the field. However, events in the last few years, including Sarin attacks in Syria and the use of “Novichok” nerve agents in the United Kingdom have brought the issues into everyday discussions. Commentary on how chemical warfare agents behave in the environment has largely been uninformed, ranging from understandable ignorance of an esoteric subject all the way through to convoluted conspiracy theories. For example, some have questioned how someone could stand near a shell crater in Khan Sheikhoun, Syria, some time after the event. Others query how a “Novichok” nerve agent could still be a potent threat in Wiltshire after a period of up to four months. There are reasonable answers to these questions. Two different fundamental issues are involved here, which can be simply called “shelf life” and “persistency”.
This is an easy concept to understand. Chemicals sealed up in a container can last a very long time. If you deprive a chemical of exposure to air or water, and the chemical itself is not corrosive to the container that it is in, chemicals can last a very long time. Nerve agents in a storage container lack any reasonable chemical process by which they will degrade, provided that the agents are made properly. It seems unreasonable to expect that a major country would spend vast resources to develop a chemical warfare agent that only lasted a short time in storage. This would create enormous logistical difficulties, including disposal of stockpiles that have passed their expiry date. However, purity is important, and with many of the nerve agents, some of the byproducts of their production would have to be purified out of the agent. In some cases, such as with Sarin, a sophisticated refining process and additives are needed to ensure that the agent does not degrade from the acids left over from production or to ensure that the residual acid does not corrode the container or weapon in which it is stored. When Saddam-era Iraq made nerve agents, it did not master these techniques and was left with agents that only lasted a few months at best before degrading to a very low purity level.
Well-made nerve agents in containers or weapon systems can easily last many decades. For example, the US Army manufactured well over a million Sarin-filled weapons between 1953 and 1957. Shells destroyed in the late 1990s and 2000s mostly contained Sarin that was as deadly as the day it was made decades prior. The same was found with the hundreds of thousands of shells filled with VX that had been made in the 1960s. The idea that Novichok in a jar or bottle can last for decades is consistent with experience, particularly given the reported high purity of the material.
Chemical warfare agents are divided very arbitrarily into persistent and non-persistent. “Persistent” when used in the context of chemical warfare agents and chemical weapons simply means that a particular substance lasts a long time in the environment when it is released. How long is a “long time” is a matter of subjective opinion and different references use varying definitions. Generally speaking, non-persistent means hazards that go away in minutes or at best a few hours. The US military’s definition of a persistent agent is one that “remains able to cause casualties for more than 24 hours to several days or weeks” and this definition is as good as any. It is possible that some chemicals could fall into either category based on specific scenarios, such as high or low temperature.
As a broad rule of thumb, non-persistent chemicals were designed for causing immediate casualties on the battlefield, primarily through inhalation. The nerve agents Tabun and Sarin are classic textbook examples of non-persistent agents. They act quickly and easily provide a hazard that can be inhaled. Persistent agents tended to have been developed for other uses. While they might cause some immediate casualties, their usual intended roles were as area denial weapons. Persistent agents could be used to contaminate roads, runways, port infrastructure, and other things to degrade an enemy’s movements and operations. They could be used to contaminate equipment like tanks and artillery so that soldiers would need to wear protective equipment and take time and resources to decontaminate the dirty items.
There is no point asking “how long does Agent ABC last” as a question, because the answer depends on many different factors. They can be broadly summarized as: quantity, configuration, properties of the chemical, the surfaces the agents are on, and environmental conditions. Quantity is obviously important. A large amount will last longer than a small amount. If you open a full bottle of something volatile like acetone or alcohol on a hot day and you open a bottle with only a small amount, it is quite intuitive to most of us that the bottle with a small amount will be empty first. Configuration is very important too. By this I mean the shape and distribution. A large pool is likely to last longer than a mist of small droplets. It should be noted that military chemical munitions, such as bombs and artillery shells, were usually designed to spread drops of agent over an area, for the purposes of contamination. Such weapons, if they worked as designed, were not intended to make a large pool of agent. The reason why I write this is that the vast majority of military studies on how long chemical warfare agents last in the environment study this kind of dispersion rather than how long a puddle lasts. So, if one were to look up these studies (and some are available on sites like dtic.mil) you’d find persistency figures. However, bear in mind that the persistency of a puddle will likely be longer than a field of dispersed droplets the size of raindrops or smaller.
The physical and chemical properties of a chemical weapon are extremely important. A liquid in the environment could do one of several things. It could evaporate and blow away in the wind. It could wash away. It could react with things in the environment. Every chemical compound has properties that make these outcomes more or less likely.
First, lets look at evaporation. The related characteristics vapour pressure and volatility are very important. Broadly speaking, these are the tendency of a liquid (or solid) to transform into a vapour. It should be noted that the word “volatile” has been frequently misused by laypersons to mean “dangerous”, but that’s not its meaning in chemistry and physics. An example is useful here. Water has a certain vapour pressure, and it evaporates in a way we all understand. Alcohol has a higher vapour pressure than water. Pour some alcohol on the floor and some water on the floor, and under the same overall conditions, the alcohol will evaporate faster than the water. Most of the nerve agents, and ALL of the ones considered persistent agents have vapour pressures significantly lower than water. Vapour pressure is usually measured in millimetres of mercury (mmHg), the same as a barometer reading. While no accurate reference value for the various Novichoks has been published, US military references give the persistent nerve agent VX a vapour pressure value of 0.0007 mmHg at 20º C. For comparison purposes, the vapour pressure of water at the same temperature is about 17.5 mmHg, i.e. 25,000 times higher.
Another important factor is reactivity. If a chemical warfare agent can react with things in the environment, like air or water, then it will have lower persistency. Obvious, something that reacts quickly with air is not likely to be terribly useful as a chemical warfare agent, especially not a persistent once. With nerve agents, there is a chemical process called hydrolysis, which is reactivity with water. Nerve agents react with water over time. Some react more quickly than others. Sometimes this is just down to chemistry – in other words, the various bonds within the molecule may be more or less reactive to water. In other situations, it’s more to do with the mechanics of the situation. A glob of a thick, gel-like liquid that does not mix well with water is going to react with moisture a bit differently than something like Sarin which is thin and runny.
The surface on which a liquid chemical agent has landed is an important factor in the equation. Chemical agents can soak into things like soil, paint, building materials, and vegetation. Depending on the material, the absorbed agents will “desorb” at varying rates. For example nerve agents, with all other factors being equal, they will last longer in sandy soil than on grass. Porous things like leather (on footwear) and paper can absorb chemical agents as well. The US military spends much money on “CARC” – chemical agent resistant coating – a special paint for combat vehicles that is resistant to chemical agent absorption.
Finally, environmental conditions matter. Temperature, wind, humidity, and precipitation are important. Higher temperatures, higher wind, and the presence of water in the air and in the form of rain all serve to reduce the persistency of nerve agents in the field.
It can be instructive to look at an example from hard data. The West simply does not have a large amount of hard data on the persistency on the so-called “Novichok” chemicals. There is actually a fair amount of data available on the nerve agent VX, which may have served as a benchmark against which the design specifications of the Novichok family were judged. As the Soviet Union could make VX, there is little logic in setting out to make an agent that is supposed to be more persistent than VX and ending up with one that is less. Anecdotal evidence from multiple sources, which I cannot confirm, indicate that the Novichok series are MORE persistent than VX.
As a thought exercise, I will use an example from US Army field manual (see table 2-5) to estimate the persistence of VX dispersed from an artillery shell. This would be a burst of droplets, the size of a raindrop or smaller. At 20º C this VX could be expected to last up to 26 days on a tank painted with that “resistant” CARC paint. Using the same charts and tables, the same VX would last 46 days on grassy terrain. In sandy soil, this VX would last up to 118 days. In warm weather, these times are shorter. At 30º C, it is 10 days, 17 days, and 45 days, respectively. At lower temperature (10º C) it would last much longer. The corresponding figures are 74 days on the chemically “resistant” tank, 129 days on grassy terrain, and 11 months in sandy soil. Below zero, it will last even longer. These are very long periods of time. Although the comparable figures for Novichok agents are known only to their inventors, I suspect that they are likely even longer.
The idea that a Novichok agent, somewhere in England could last a few months in the wild is certainly plausible. If it was in a container, it almost certainly could last far longer.