Malathion is an organophosphate parasympathomimetic which binds irreversibly to cholinesterase. Malathion is an insecticide of relatively low human toxicity.
In the former USSR it was known as carbophos, in New Zealand and Australia as maldison and in South Africa as mercaptothion.
However, malathion breaks down into malaoxon, which is 60 times more toxic than malathion. For this reason, if malathion is used or somehow enters an indoor environment, as it breaks down into malaoxon, it can seriously and chronically poison the occupants living or working in this environment. Malathion present in untreated water is converted to malaoxon during the chlorination phase of water treatment, so malathion should not be used in waters that may be used as a source for drinking water, or any upstream waters.
In 1976, numerous malaria workers in Pakistan were poisoned by isomalathion, a common impurity in malathion, which is capable of inhibiting carboxyesterase enzymes in those exposed to it; the original toxicity evaluation for malathion had not anticipated isomalathion coexposure.