Chlorothalonil

Uses

In the US, chlorothalonil is used predominantly on peanuts (about 34% of usage), potatoes (about 12%), and tomatoes (about 7%), although the EPA recognizes its use on many other crops. It is also used on golf courses and lawns (about 10%) and as a preservative additive in some paints (about 13%), resins, emulsions, and coatings.

Chlorothalonil is commercially available in many different formulations and delivery methods. It is applied as a dust, dry or water-soluble grains, a wettable powder, a liquid spray, a fog, and a dip. It may be applied by hand, by ground sprayer, or by aircraft.

Mechanism of action

Chlorothalonil reacts with glutathione giving an glutathione adduct with elimination of HCl. Its mechanism of action is similar to that of trichloromethyl sulfenyl fungicides{{Citation

Toxicity

Acute

According to the EPS, chlorothalonil is a toxicity category I eye irritant, producing severe eye irritation. It is in toxicity category II, "moderately toxic", if inhaled (inhaled 0.094 mg/L in rats.) For skin contact and ingestion, chlorothalonil is rated toxicity category IV, "practically nontoxic", meaning the oral and dermal is greater than 10,000 mg/kg.

Carcinogenic

International Agency for Research on Cancer IARC has assessed chlorothalonil as a Group 2B "possible human carcinogen", based on observations of cancers of the renal and gastrointestinal systems in laboratory animals fed diets containing chlorothalonil.

Environmental

Chlorothalonil was found to be an important factor in the decline of the honey bee population, by making the bees more vulnerable to the fungal pathogen Nosema ceranae.Jeffery S. Pettis, Elinor M. Lichtenberg, Michael Andree, Jennie Stitzinger, Robyn Rose, Dennis vanEngelsdorp "Crop Pollination Exposes Honey Bees to Pesticides Which Alters Their Susceptibility to the Gut Pathogen Nosema ceranae" PLOS One, 24 July 2013, Online: 9 April 2014. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0070182

Chlorothalonil is highly toxic to fish and aquatic invertebrates, but not toxic to birds.

At a concentration of 164 μg/L, chlorothalonil was found to kill a species of frog within a day.

Contaminants

Common chlorothalonil synthesis procedures frequently result in contamination of it with small amounts of hexachlorobenzene (HCB), which is toxic. US regulations limit HCB in commercial production to 0.05% of chlorothalonil. According to the EPA report, "post-application exposure to HCB from chlorothalonil is not expected to be a concern based on the low level of HCB in chlorothalonil. 2,3,7,8-Tetrachlorodibenzodioxin being one of the most potent carcinogens known is also a known contaminant".

Environmental contamination

Chlorothalonil has been detected in ambient air Prince Edward Island, as well as in groundwater in Long Island, New York and Florida. In the first three cases, the contamination is presumed to have come from potato farms. It has also been detected in several fish kills in Prince Edward Island.

The main breakdown product of chlorothalonil is SDS-3701 (structure shown below). SDS-3701 has been shown to be 30 times more acutely toxic than chlorothalonil and more persistent in the environment.{{Citation |archive-url = https://web.archive.org/web/20101110064601/http://www.pesticide.org/get-the-facts/pesticide-factsheets/factsheets/chlorothalonil |archive-date = 10 November 2010}} Laboratory experiments have shown it can thin the eggshells of birds, but no evidence supports this happening in the environment.

In 2019, a review of the evidence found that "a high risk to amphibians and fish was identified for all representative uses", and that chlorothalonil breakdown products may cause DNA damage. Agrochemicals are claimed to be the strongest factor in bumblebee population decline.

File:2,4,5-trichloro-6-hydroxyisophthalonitrile.svg|SDS-3701 File:R417888.svg|R417888 File:R471811.svg|R471811

Gag order

Syngenta sued to stop communication by Swiss health authorities with the Swiss public regarding the "relevance" of specific metabolites of chlorothalonil that Swiss authorities detected in high concentrations in the groundwater from which hundreds of thousands of Swiss people obtain drinking water. The court banned Swiss health authorities from communicating with the public about the dangers posed by some of the metabolites.

Bans

In March 2019, as a result of the previously mentioned research, the European Union banned the use of chlorothalonil dated to take effect May 20, 2020. Switzerland followed in December 2019.

Production

Chlorothalonil can be produced by the direct chlorination of isophthalonitrile or by dehydration of tetrachloroisophthaloyl amide with phosphoryl chloride. It is a white solid. It breaks down under basic conditions, but is stable in neutral and acidic media. Technical grade chlorothalonil contains traces of TCDD and hexachlorobenzene, both are persistent organic pollutants banned under the Stockholm Convention.

References

References

1. "Chlorothalonil".
2. [http://www.ncfap.org/ncfap/nationalsummary1997.pdf Pesticide use in U.S. crop production: 1997] {{webarchive. link. (10 December 2006 National Center for Food and Agricultural Policy, 1997.)
3. [https://archive.epa.gov/pesticides/reregistration/web/pdf/0097red.pdf Reregistration Eligibility Decision for chlorothalonil], US EPA, 1999.
4. Siegel, Malcolm R.. (1970). "Reactions of certain trichloromethyl sulfenyl fungicides with low-molecular-weight thiols. ''In vivo'' studies with cells of ''Saccharomyces pastorianus''". J. Agric. Food Chem..
5. [http://www.inchem.org/documents/ehc/ehc/ehc183.htm Environmental Health Criteria 183], [[World Health Organization]], 1996.
6. Taegan McMahon, Neal Halstead, Steve Johnson, Thomas R. Raffel, John M. Romansic, Patrick W. Crumrine, Raoul K. Boughton, Lynn B. Martin, Jason R. Rohr "The Fungicide Chlorothalonil is Nonlinearly Associated with Corticosterone Levels, Immunity, and Mortality in Amphibians" Environ Health Perspectives, 2011, Online: 4 April. {{doi. 10.1289/ehp.1002956
7. (2006). "Ambient air concentrations of pesticides used in potato cultivation in Prince Edward Island, Canada". Pest Manag. Sci..
8. "Island Fish Kills from 1962 to 2016".
9. (January 2018). "Peer review of the pesticide risk assessment of the active substance chlorothalonil". EFSA Journal.
10. Carrington, Damian. (2019-03-29). "EU bans UK's most-used pesticide over health and environment fears".
11. "European Commission votes to ban fungicide Chlorothalonil".
12. Le Monde, 27 Sept. 2022 [https://www.lemonde.fr/en/environment/article/2022/09/27/by-products-of-pesticide-banned-in-2019-found-in-swiss-drinking-water_5998346_114.html "By-Products of Pesticide Banned in 2019 Found in Swiss Drinking Water, Syngenta, The Producer of The Fungicide Chlorothalonil, Has Sued Swiss Health Authorities for Making Public The Danger Posed by Certain Derivatives of The Pesticide,"]
13. "Swiss ban widely-used pesticide over health and environment fears". [[swissinfo]].
14. Franz Müller. "Fungicides, Agricultural, 2. Individual Fungicides".
15. [[National Farmers' Union of England and Wales]]. "Chlorothalonil use-by date approaching".