Pesticides as Poisons: Analysis of the Gebrev Case

There has been a recent investigation here on Bellingcat, of a poisoning incident in Bulgaria in April 2015, where an individual named Emilian Gebrev was posioned. An angle of this investigation has been to see if it is at all connected to the attempted murder of Sergei Skripal. One particular aspect of recent publicity has been to examine whether or not a “Novichok” series nerve agent was responsible for the 2015 poisoning incident.

The respected Finnish research institute Verifin was asked to analyse serum and urine samples taken from Mr. Gebrev. There does not appear to be a physical sample of the actual poison used in the samples sent to Finland. Therefore, the analysis performed was not examining the actual poison, but its biological descendants (metabolites) that remained in the human body. Although the report is not available online at the time of writing, I have read a copy of the report.

We may not ever actually learn whether or not “Novichok” agent such as A-234 was used in this particular incident. However, it is definitely worth examining the various chemicals mentioned in the Verifin report, as these chemicals actually are poisonous and have potential for deliberate use to make people ill or even kill them. Pages 18 and 19 of the Verifin report provide a list of organophosphate pesticides that the Finnish institute believes might have been responsible for this particular poisoning. The report lists the chemicals detailed below , although they have many synonyms as trade names. For reference, I include their CAS numbers. Chemical Abstract Service (CAS) numbers are basically an indexing scheme for specific chemicals, to provide an exact reference as nomenclature does vary widely. 

General notes: All of these chemicals are in the general family of organic phosphorus compounds which attack the human nervous system in ways similar to military nerve agents. The properties and availability of these chemicals vary greatly from one another. Some of them have numerous synonyms and trade names. Some are more suited for poisonings than others. All of these chemicals are poisons to humans, although their effectiveness by various routes of exposure vary greatly.

Chlorethoxyphos (CAS 54593-83-8) is a pesticide used to control agricultural pests in soil, including corn rootworm and white grubs. It is used in the USA and is found in dilution (in 2.5% and 5% strengths) in the DuPont pesticide “Fortress”. It’s use in the USA is restricted by Federal regulations and some states. It is not allowed in the EU.   This chemical is deadly if ingested or inhaled. LD50 for ingestion is 1.8 mg per kg of body weight in rats. As chlorethoxyphos is a solid, there would need to be some sort of mechanism for inhalation to happen. The substance has toxicity by dermal absorption through the skin. However, toxicity by this route is inefficient, and relatively large amount (at least a gram of pure) would be needed to be absorbed through the skin for possible lethality.

Chlorpyrifos (CAS 2921-88-2) is an insecticide which is widely used. It was first developed and marketed by Dow Chemical (US) and is considered a highly effective insecticide. It has come under criticism for various long term health effects, which are from different mechanisms than its acute toxicity as a nerve agent. The status of a ban on it is controversial in the USA, and is in litigation. It was formerly used in widespread ways in household and commercial buildings, but it is now largely used in agriculture. It is banned in many places, but allowed in others. It is approved in the EU. It is a solid, but it is not very soluble in water. It is highly soluble in solvents like toluene or methanol. Although it does have toxicity in mammals, the amounts needed for account poisoning by oral exposure are quite high (66 mg per kg of body weight in rats.) If an aerosol of particles could be achieved, it would be quite dangerous by inhalation, but this would need some mechanism to disperse it. It’s dermal toxicity is inefficient and low relative to other poisons in this class. It would need an impractically high amount of material to be absorbed through the skin in order to have a reasonable chance of acutely poisoning someone.

Coumaphos (CAS 56-72-4) is used as a dip or a wash to combat fleas, ticks, and related parasites. It is allowed in the US for restricted use, and is not permitted in the EU. It is soluble in solvents but not in water. Its oral toxicity (80 mg/kg of body weight in rabbits) makes a dangerous oral dose for an adult human rather large and improbable. Inhaled toxicity requires a large amount over a long period of time, and is thus impractical.   Likewise, the dermal toxicity is as high as several grams per kg of body weight so someone would have to literally roll around in powdered coumaphos to achieve a viable poisoning scenario.

Diazinon (CAS 333-41-5) was originally developed in the 1950s in Switzerland by Ciba-Geigy (a predecessor to Novartis) as a replacement for DDT. It is a broad-spectrum general purpose insecticide useful against a wide variety of pests. It is considered an environmental menace, and is no longer allowed in the EU. It is allowed in some agricultural uses in the US, but not for household use. It is soluble in water and its properties as a water pollutant are well known. It is far more toxic to birds than mammals, by several orders of magnitude. It’s toxicity in mammals is rather too poor for its use as an efficient acute poison. An adult person might have to eat 75 or 80 grams in order to face a lethal dose. It’s toxicity by dermal absorption is quite low (several grams per kg of body weight). Also, it is a skin irritant, so it would likely be washed off long before a dangerous dose could be absorbed.

Disulfoton (CAS 298-04-4) was manufactured by Bayer and was introduced in 1956. It was designed for agricultural use against sucking insects on various crops, such as cotton, tobacco, and potatoes. It is no longer allowed to be used in the EU. It is restricted in the USA and many of its agricultural uses there were discontinued. It has been noted for its hazards to household pets. It is no longer widely available in commercial quantities, although various specialty vendors will sell laboratory samples for analytical use. It is an oily liquid at room temperature. It has low solubility in water, but is mixable with various solvents. In relatively pure form, it significantly toxic through ingestion, inhalation, or contact. Well under a gram could make a person seriously ill and dermal absorption is a viable threat with this material.

Ethion (CAS 563-12-2) was first developed in the USA in the 1950s. It is an insecticide with broad applicability against various pests, including red spider mites, aphids, and some types of moths. It had a record of use in fruit trees. It is not allowed in the EU, but is allowable with restrictions in the USA. It is easily available in the developing world and can be easily purchased in India and China. It is a solid at room temperatures, with low solubility in water. While it is dangerous to bees, it has only moderate toxicity in mammals. For use as a viable deliberate poison, a fine aerosol of particulates by inhalation would work, but would take some engineering to achieve. By ingestion and dermal absorption, the doses for lethality would be rather large compared to the other pesticides on this list.

Parathion (CAS 56-38-2) is a very dangerous pesticide. It was originally developed in the 1940s by the German scientist Gerhard Schrader. He was the inventor of Tabun and Sarin, the first military nerve agents. The patent for Parathion was seized by the allies from I. G. Farben after the war. It is highly restricted and when it is used, it is used in weak dilution. It is extremely hazardous to humans, particularly through ingestion or inhalation, although inhalation would take some sort of mist or spray. The toxicity of Parathion through skin absorption is high enough that in the 1970s it was deliberately used by Rhodesian forces to poison insurgents by contaminating the crotch and armpits of items of clothing. Parathion is a prime candidate for deliberate poisoning. It is soluble in toluene and other organic solvents.

Phorate (CAS 298-08-2) is a pesticide banned in the EU, but it has relevance in control of leaf borers and nematodes. It has been used in corn and cotton farming in the past. It is highly restricted in the USA. It is a liquid at room temperatures and is somewhat soluble in water. It is extremely toxic by all routes of exposure. It has a strong odour and is irritating to the skin. (Poisons which are strong skin irritants but which also absorb slowly tend to not be effective as weapons as the target is likely to seek to wash off the material.) If it were not for the strong odour and immediate skin irritation effect, phorate would actually have many of the properties of a persistent chemical warfare agent. Phorate is certainly a strong candidate for poisonings. It appears to be commercially available in China.

Sulfotep (CAS 3689-24-5) is a pesticide developed after the Second World War by Bayer, based on the original nerve agent research done by I. G. Farben. It is a pale yellow liquid with a garlic-like odour. It is useful against aphids, thrips, spider mites, and white flies. It’s normal use formerly was in smoke generators which contained 14-15% Sulfotep. Burning these smoke generators created a toxic white smoke which would fumigate greenhouses without leaving residual contamination on plants. Formerly, it was produced by the German conglomerate BASF. It’s toxitciy to humans is high by inhalation or ingestion, but only moderate through skin absorption. It has been implicated in some accidental poisonings of greenhouse workers. It is banned in the EU. It’s availability now is poor, although in original packaging the shelf life of old products could be very long.

Terbufos (CAS 13071-79-9) is a liquid in its pure form, although it is marketed in solid granules for use in soil. Formerly produced by BASF, it’s use is banned in the EU. It is useful for control of pests in soil and has been used in corn, sugarbeet, potato, and peanut farming, among other uses. It is produced by American Cyanamid and is used in the developing world, where accidental poisonings are not unheard of. It is produced in China and is available for purchase online in powder and liquid form. It is toxic by every route of exposure and would certainly be a feasible poison. However, as a skin irritant, it’s practical utility as a poison by dermal exposure might be low.

Tetraethylpyrophosphate/TEPP (CAS 107-49-3): This chemical is banned in the EU and is limited to restricted use in the USA. Gerhard Schrader worked on it in the 1930s while conducting the research that led to the development of the military nerve agents. It is highly effective against aphids, mites, spiders, and similar pests. It is a liquid at room temperatures.   It is dangerous by inhalation, ingestion and dermal absorption. It is not as widely available as some of the other chemicals on this lists.

Amiton: (CAS 78-53-5) Amiton is actually a very deadly chemical. It is, chemically, a close cousin of the military nerve agent VX, so much so that it was even given a chemical warfare designation of “VG” by NATO. It was developed by the British company ICI, but was also known in the USSR as Tetram. It is the only pesticide on this list to rate being scheduled in the Chemical Weapons Convention. There are no manufacturers of it, having been completely withdrawn from the market for safety reasons in the 1950s. It is extremely dangerous. It has some issues with flammability which limited its utility for certain types of chemical weapons systems, and was less toxic than VX, hence it was not seriously pursued as a chemical warfare agent. Although extremely useful as a poison, it is far less available than the other chemicals on the list.

Demeton (CAS 126-75-0): Like Amiton, Demeton was investigated for use as a insecticide and is highly effective in that role. However, it’s toxicity in humans is too high and it has been discontinued. It is chemically similar to some of the V-series of military nerve agents. Several closely related compounds have been implicated in accidental poisonings in agricultural settings. Of the substances on the Verifin list, Demeton and Amiton should be considered the least probable due to its poor availability. However, a facility capable of synthesizing VX is likely to be able to synthesize Amiton should it so desire.

Any of these chemicals could have been used to poison food or drink. This was an active angle at the time. It should be noted that the Verifin report found hippuric acid in the urine sample, which could have been an indicator of toluene or a similar solvent. Such solvents can be used with many of the chemicals on this list.

Information sources used in this paper include but are not limited to the US Environmental Protection Agency, Pubchem at the National Library of Medicine (USA), the Extension Toxicology Network at Cornell University (USA) and the excellent Pesticides Properties Database at the University of Hertfordshire (UK).

Bellingcat’s research for this publication was supported by PAX for Peace.