Mirex

Mirex has been listed as persistent, bioaccumulative, and toxic pollutants target by EPA. It is a manufactured insecticide which is white crystalline odorless solid. It was used to control fire ants and as a flame retardant in plastic, rubber, paint, paper and electronics. It is lipophilic compound and is stored in adipose tissue to a greater extent. Mirex is transported across placenta and is excreted with milk. Photomirex and kepone are major degradation products. Mirex is a fully chlorinated organic compound, based on two linked five member carbon rings. It is highly stable, doesn't burn easily or react readily with acids, bases, chlorine or ozone. Combustion products of mirex include carbon dioxide, carbon monoxide, hydrogen chloride, chlorine, carbon tetrachloride, phosgene, and hexachlorobenzene.

Application of Mirex
Mirex was first synthesized in 1946 by Prins but was not used in pesticide formulation until 1955. Mirex is made by the dimerization of hexachlorocyclopentadiene in the presence of aluminium chloride. It is a stomach insecticide. The insecticidal use was focused on Southeastern United States in order to control the imported fire ant Solenopsis saevissima richteri and Solenopsis invicta a closely related species of former. To combat the problem, approximately 250000 kg of mirex was applied to fields during 1962-75 (US NRC, 1978). Most of the mirex was in the form of 4X mirex bait, which consists of 0.3% mirex in 14.7% soybean oil mixed with 85% corncob grits. Application of the 4X bait was designed to give a coverage of 4.2 g mirex/ha and was delivered by aircraft, helicopter or tractor. 1x and 2x bait were also used. Use of mirex as pesticide began in 1962 in the United States and all uses of mirex as pesticide were banned in 1978.

The Stockholm Convention banned production and use of several persistent organic pollutant and Mirex was one of the dirty dozen, naming the worst offenders.

Mirex is still used in the USA mainly as a flame-retardant in plastics, rubber, paint, paper and electronics. Common trade names include Ferriamicide and Dechlorane. It has also been used to combat leaf cutters in South America, harvester termites in South Africa, mealy bug of pineapple in Hawaii. Mirex has never been registered for use as an insecticide in Canada.

Biodegradation of Mirex
Mirex is very resistant to microbiological degradation and is only slowly dechlorinated to a monohydro derivative by anaerobic microbial action in sewage sludge and by enteric bacteria in monkeys. There have been no reports of evidence of metabolic degradation by soil microorganisms.

Bioaccumulation and Biomagnification of Mirex
Mirex is highly cumulative and amount depends upon the concentration and duration of exposure. There is evidence of accumulation of mirex in aquatic and terrestrial food chains to harmful levels. After 6 applications of mirex bait at 1.4 kg/ha, high mirex levels were found in some species; turtle fat contained 24.8 mg mirex/kg, kingfishers, 1.9 mg/kg, coyote fat, 6 mg/kg, oppossum fat, 9.5 mg/kg, and racoon fat, 73.9 mg/kg. In a model ecosystem with a terrestrial-aquatic interface, sorgum seedlings were treated with mirex at 1.1 kg/ha. Caterpillars fed on sorgum seedlings and their faeces contaminated the water which contained algae, snails, Daphnia, mosquito larvae, and fish. After 33 days, the ecological magnification value was 219 for fish and 1165 for snails.

Mirex Toxicity
Mirex is moderately toxic in single-dose animal studies (oral LD50 values range from 365 - 3000 mg/kg body weight). It can enter the body via inhalation, ingestion, and via the skin. The most sensitive effects of repeated exposure in experimental animals are principally associated with the liver, and these have been observed with doses as low as 1.0 mg/kg diet (0.05 mg/kg body weight per day), the lowest dose tested. At higher dose levels, it is fetotoxic (25 mg/kg in diet) and teratogenic (6.0 mg/kg per day). Mirex was not generally active in short-term tests for genetic activity. There is sufficient evidence of its carcinogenicity in mice and rats.

Environmental and Health Consequences
There are no good environmental effects for the use of Mirex, other than ridding the infected area of ants. Mirex has mainly negative environmental effects. Effects on organisms combined with its persistence suggest that mirex presents a long-term hazard for the environment. Mirex induces pervasive chronic physiological and biochemical disorders in various vertebrates. Aquatic crustaceans show extreme sensitivity to the compound, and game birds and fish feeding close to manufacturing plants accumulate enough mirex to constitute a health hazard. No acceptable daily intake (ADI) for mirex has been advised by FAO/WHO. IARC (1979) evaluated the carcinogenic hazard resulting from exposure to mirex and concluded that "there is sufficient evidence for its carcinogenicity to mice and rats. In the absence of adequate data in humans, based on above result it can be said that it has carcinogenic risk to humans”. Mirex is one of the most stable of the organochlorine insecticides.  Although general environmental levels are low, it is widespread in the biotic and abiotic environment. Mirex is both accumulated and biomagnified. Mirex is strongly adsorbed on sediments and has a low water solubility. The delayed onset of toxic effects and mortality is typical of mirex poisoning. The long-term toxicity of mirex is uniformly high. Mirex is toxic for a range of aquatic organisms, with crustacea being particularly sensitive. Mirex induces pervasive long-term physiological and biological disorders in vertebrates.     Although no field data are available, the adverse effects of long-term exposure to low levels of mirex combined with its persistence suggest that the use of mirex presents a long-term environmental risk. No data on human health effects are available in connection with occupational exposure to mirex. Based on the findings in mice and rats, this chemical should be considered, for practical purposes, as being potentially carcinogenic for human beings.Effects on the organisms studied, as well as its persistence, suggest that mirex presents a long-term hazard for the environment.