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Limonene is a chemical compound that can manifest in two ways, as l-limonene or d-limonene. The two are mirror images of one another chemically, and have the same properties, but different scents. The "d" version typically smells like a citrus fruit, and is used in food grade products, cleaning products, and beauty products, while "l" tends to have a more sour turpentine-like scent mixed with pine, and is used primarily as an alternative solvent for cleaning products. Both versions can be produced as technical grade, and are produced in the same manner, but chemically separated by scientists. The molecule itself is the compound C10H16.
The name comes from "lemon" and "lime," since it is most commonly found in the rinds of citrus fruit. The most common source it is extracted from is orange rind. The rinds are juiced first to create a food grade limonene product, then they are sent into a steam extractor. The extractor presses out more oils. The steam is compressed down, and the oil floats to the top where it can be collected. This oil is technical grade.
Technical grade limonene has been used in cleaning products since the 1990s. The orange essence makes for a pleasant smelling cleaner. Many consumers welcome a natural product as opposed to man-made alternatives. Those interested in conservation find the use of the chemical appealing, since the rinds of oranges used as food products were formerly discarded, but have now found new use. Limonene has been found to be non-caustic on metal surfaces, but can be degrading to plastic. This fact does make it an effective adhesive remover. It is a colorless liquid that is not water soluble.
The boiling range of straight limonene is 310-352 degrees Fahrenheit (154-177 Celsius). The flash point is 119 degrees Fahrenheit (48 Celsius), which classifies it as a flammable product. This means it is also classified as a hazardous material for disposal purposes.
Limonene, in its pure form, is classified as a light skin irritant, but is much safer for the skin than alternative cleaning solvents. There have not been significant studies done on either the safety or danger of the chemical to humans on a casual basis. Studies on long-term exposure have shown it to be a mild respiratory irritant.
Some studies are being done to see if the food grade "d" version of the substance can be used to treat cancer. In some studies, mammary tumors in rats seemed to shrink after a regiment of ingestion. The effectiveness has yet to be proven on human patients, but tests are being conducted to see if the animal results can be duplicated.