Malathion

September 26, 2022
I’m a 70-year-old pesticide.
What molecule am I?
Image of Malathion 3D Image of Malathion

Malathion1 is an organophosphate insecticide that was developed in the early 1950s by Jack T. Cassaday and co-workers at American Cyanamid (Stamford, CT). It and other dithiophosphate–maleate esters were claimed as compositions of matter in US Patent 2,578, 652 (1951). It acts as an acetylcholinesterase inhibitor to poison insects.

A liquid with a skunk- or garlic-like odor, malathion is still on the market today for commercial and domestic use despite findings that it and similar pesticides are probable carcinogens. It is considerably safer to use than the similar organophosphate parathion2, which has been banned or restricted worldwide.

In 2017, the US Environmental Protection Agency found that malathion would adversely affect almost all endangered species and their habitats. In 2021, based on EPA’s findings, the US Fish and Wildlife Service determined that the insecticide is likely to harm 78 species and 23 critical habitats. But during this period, EPA, Fish and Wildlife, and pesticide manufacturers collaborated to develop mitigation measures for malathion.

This past March, Fish and Wildlife concluded that, if the mitigation measures are put in place, threatened and endangered species would not be harmed by malathion applications. EPA expects to approve new malathion labels by late 2023. Environmental advocacy groups, however, strongly objected to Fish and Wildlife’s decision and would like stronger controls to be enacted. One advocate said that the agency ignored “the best available science” and chose “to rely on unenforceable promises of good behavior by the pesticide makers rather than real on-the-ground conservation measures.”

1. SciFinder: butanedioic acid, 2-[(dimethoxyphosphinothioyl)thio]-, 1,4-diethyl ester.
2. CAS Reg. No. 56-38-2.

Malathion acid hazard information

Hazard class* GHS code and hazard statement
Acute toxicity, oral, category 4 H302—Harmful if swallowed Chemical Safety Warning
Skin sensitization, category 1 H317—May cause an allergic skin reaction Chemical Safety Warning
Carcinogenicity, category 1B H350—May cause cancer Chemical Safety Warning
Short-term (acute) aquatic hazard, category 1 H400—Very toxic to aquatic life Chemical Safety Warning
Long-term (chronic) aquatic hazard, category 1 H410—Very toxic to aquatic life with long-lasting effects Chemical Safety Warning

*Globally Harmonized System of Classification and Labeling of Chemicals. Explanation of pictograms.

Malathion  fast facts

CAS Reg. No. 121-75-5
Empirical formula C10H19O6PS2
Molar mass 330.36 g/mol 
Appearance Yellow to brown liquida
Melting point 3 °C 
Boiling Point 156–157 °C (0.7 Torr)
Water solubility 143 mg/L

a. Colorless when pure.

MOTW updates

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1. CAS Reg. No. 5119-48-2.
2. CAS Reg. No. 27570-38-3.
3. CAS Reg. No. 1972-08-3.

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