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He had begun to suspect organophosphates - a group of chemicals including nerve agents such as Novichok - by Monday evening, after remembering what he had learned while "studying for lots of exams".

Mr Darch told the inquiry that poisoning from organophosphates, a toxic chemical compound, can come from several sources including pesticides, fertiliser, and flammable retardants.

For example, organophosphates and carbamates, two similar classes of insecticides that have been used for decades, interfere with the nervous system.

Acephate and malathion belong to a class of chemicals called organophosphates, which U.S. farmers have used for decades because they efficiently kill aphids, fire ants and other pests.

They identified chemicals that selectively damaged oligodendrocytes belong to two classes: organophosphate flame retardants and quaternary ammonium compounds.