The pyrethrins are a pair of natural organic compounds normally derived from Chrysanthemum cinerariifolium that have potent insecticidal activity. Pyrethrins are neurotoxins that attack the nervous systems of all insects. When present in amounts not fatal to insects, they still appear to have an insect repellent effect. Pyrethrins are gradually replacing organophosphates and organochlorides as the pesticide of first choice. They are non-persistent, being biodegradable, and break down on exposure to light or oxygen. The chemical structure of pyrethrins inspired the production of a variety of synthetic insecticides called pyrethroids such as bifenthrin, permethrin, and cypermethrin.
History and biosynthesis
Well after their use as insecticides, their chemical structures were determined by Hermann Staudinger and Lavoslav Ružička in 1924.Pyrethrin I and pyrethrin II are structurally related esters with a cyclopropane core. Pyrethrin I is a derivative of (+)-trans-chrysanthemic acid. Pyrethrin II is closely related but one methyl group is oxidized to a carboxymethyl group, the resulting core being called pyrethric acid. Knowledge of their structures opened the way for the production of synthetic analogues, which are called pyrethroids. In terms of their biosynthesis, pyrethrins are classified as terpenoids, being derived from dimethylallyl pyrophosphate, which combine by the action of the enzyme chrysanthemyl diphosphate synthase.
Most of the world's supply of pyrethrin and Chrysanthemum cinerariaefolium comes from Kenya, which produces the most potent flowers. Other countries include Croatia (in Dalmatia) and Japan. The flower was first introduced into Kenya and the highlands of Eastern Africa during the late 1920s. Now, Kenya produces as much as 70% of the world's supply of pyrethrum. A substantial amount of the flowers are cultivated by small-scale farmers who depend on it as a source of income. It is a major source of export income for Kenya and source of over 3,500 additional jobs. About 23,000 tons were harvested in 1975. The active ingredients are extracted with organic solvents to give a concentrate containing pyrethrin I and II, cinerin I and II, and jasmolin I and II. Supercritical carbon dioxide has also been used as an extractant.
As an insecticide
Pyrethrins are used in many varieties of insecticide, fogging products and in some pet products and have been used as an insecticide for over 100 years. In the 1800s, it was known as "Persian Insect Powder," "Persian Pellitory", and "Zacherlin". They affect the flow of sodium out of the nerve cells in insects, resulting in repeated and extended firings of the nerves, causing the insects to die. Piperonyl butoxide, a synergist, is often used in combination with pyrethrin, making the mixture more effective by not allowing the insect's system to detoxify the pyrethrin. Although pyrethrin is a potent insecticide, it also is an insect repellent at lower concentrations. Observations in food establishments demonstrate that flies are not immediately killed but are found more often on windowsills or near doorways. This suggests that, due to the low dosage applied, insects are driven to leave the area before dying. Pyrethrin and the synergists are biodegradable and rapidly disintegrate in sunlight and air, thus assuring that there will be no excessive build-up of insecticides dispensed in the area being treated.
The United States Department of Agriculture, as of 1972, has stated that synergized pyrethrum is "probably the safest of all insecticides for use in a food plant"  and that " a pyrethrum formulation is approved for use around foodstuffs." All pyrethrins are easily hydrolyzed and degraded by stomach acids in mammals, so toxicity following ingestion by pets is very low. However, pyrethrins are dangerous for cats and fish. Toxicity is usually associated with applying much more of the product than directed. Care should be taken to observe direction labels when using this substance around humans and animals. Overdose and toxicity can result in a variety of symptoms, especially in pets, including drooling, lethargy, muscle tremors, vomiting, seizures and death. Toxicity symptoms in humans include asthmatic breathing, sneezing, nasal stuffiness, headache, nausea, incoordination, tremors, convulsions, facial flushing and swelling, and burning and itching sensation. The latest information regarding toxicity of piperonyl butoxide has determined that it can pose a distinct health risk when it becomes airborne and pregnant women are exposed during the third trimester. This leads to delayed mental development in young children. A 2011 study found a significant association between piperonyl butoxide (PBO), a common additive in pyrethroid formulations, measured in personal air collected during the third trimester of pregnancy, and delayed mental development at 36 months. Children who were more highly exposed to PBO in personal air samples (≥4.34 ng/m3) scored 3.9 points lower on the Mental Developmental Index than those with lower exposures. The lead researcher stated, regarding PBO, "This drop in IQ points is similar to that observed in lead exposure. While perhaps not impacting an individual's overall function, it is educationally meaningful and could shift the distribution of children in the society who would be in need of early intervention services." 
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