June 14, 2021
I offer a cautionary tale about pesticide use.
What molecule am I?
Image of Fipronil 3D Image of Fipronil

Fipronil is a broad-spectrum insecticide. It disrupts insects’ central nervous systems by blocking chloride channels that are gated by γ-aminobutyric acid (GABA) or glutamate. These receptors are weaker or nonexistent in mammals.

Fipronil has a chiral center at the sulfur atom in the sulfoxide group; the two enantiomers have been isolated and verified. The article of commerce is a racemic mixture of the two.

Fipronil formulations sold worldwide include flea-control products for pets, cockroach traps, and insecticides for agrciultural crops, lawns, golf courses, and the like. But despite its wide range of uses and relative harmlessness toward mammals, fipronil is a significant environmental hazard. It is highly toxic to fish and other waterborne wildlife, birds, and beneficial insects. It has been cited as one of the primary causes of honeybee die-off.

In 2017, fipronil was the culprit in a food-contamination scandal in the Netherlands. Molecule of the Week’s Dutch reader who suggested this molecule provided this narrative:

  • Egg-laying chickens in the Netherlands are plagued by red mites (bloodmites, Dermanyssus gallinae), which cause anemia and reduce egg output.
  • To combat these mites, farmers turned to a tiny company in the Netherlands called Chickfriend. The company offered an effective way to exterminate the mites by fumigation.
  • Chickfriend helped hundreds of chicken farmers get rid of the mites for months.
  • But—it turned out that Chickfriend’s fumigant contained fipronil, which was banned in the Netherlands. Contamination of Dutch eggs, which are sold worldwide, plunged the industry into a tailspin, with estimated damages running into the hundreds of millions of euros.

A recent article in the Dutch newspaper De Stentor chronicles the fipronil scandal, including the subsequent legal fallout and a better way to combat red mites. (Note: The article is in Dutch, but it is easily translated by major Web browsers.)

Fipronil hazard information

Hazard class*Hazard statement
Acute toxicity, oral, category 3H301—Toxic if swallowedChemical Safety Warning
Acute toxicity, dermal, category 3H311—Toxic in contact with skinChemical Safety Warning
Serious eye damage/eye irritation, category 2AH319—Causes serious eye irritationChemical Safety Warning
Acute toxicity, inhalation, category 3H331—Toxic if inhaledChemical Safety Warning
Specific target organ toxicity, repeated exposure, oral, category 1H372—Causes damage to organs through prolonged or repeated exposure if swallowedChemical Safety Warning
Short-term (acute) aquatic hazard, category 1H400—Very toxic to aquatic lifeChemical Safety Warning
Long-term (chronic) aquatic hazard, category 1H410—Very toxic to aquatic life with long-lasting effectsChemical Safety Warning

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

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fast facts

CAS Reg. No.120068-37-3
1H-Pyrazole-3-carbonitrile, 5-amino-1-[2,6-dichloro-4-(trifluoromethyl)
Empirical formulaC12H4Cl2F6N4OS
Molar mass437.15 g/mol
AppearanceWhite to yellow powder
Melting point200.5 ºC
Water solubility2 mg/L

MOTW update:
April 24, 2023

Fipronil1 is a broad-spectrum insecticide with several known environmental hazards, including contamination of chicken eggs in the Netherlands in 2017. Earlier this month, Weidong Qu and colleagues at Fudan University (Shanghai), the China National Center for Food Safety Risk Assessment (Beijing), Qingdao University (China), and Oxford University (UK) reported that fipronil and its metabolites in drinking water can produce thyroid-disrupting effects in humans.

1. CAS Reg. No. 120068-37-3.

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