An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove. An oil is "essential" in the sense that it contains the "essence of" the plant's fragrance--the characteristic fragrance of the plant from which it is derived. Essential oils do not form a distinctive category for any medical, pharmacological, or culinary purpose. They are not essential for health.
Essential oils are generally extracted by distillation, often by using steam. Other processes include expression or solvent extraction. They are used in perfumes, cosmetics, soaps and other products, for flavoring food and drink, and for adding scents to incense and household cleaning products.
Essential oils have been used medicinally in history. Medical applications proposed by those who sell medicinal oils range from skin treatments to remedies for cancer and often are based solely on historical accounts of use of essential oils for these purposes.
The potential danger of an essential oil is generally relative to its level or grade of purity. Many essential oils are designed exclusively for their aroma-therapeutic quality; these essential oils generally should not be applied directly to the skin in their undiluted or "neat" form. Some can cause severe irritation, provoke an allergic reaction and, over time, prove hepatotoxic. Non-therapeutic grade essential oils are never recommended for topical or internal use.
Essential oils should not be used with animals, as they possess extreme hepatotoxicity and dermal toxicity for animals. Some essential oils, including many of the citrus peel oils, are photosensitizers, increasing the skin's vulnerability to sunlight.
Industrial users of essential oils should consult the material safety data sheets (MSDS) to determine the hazards and handling requirements of particular oils. Even certain therapeutic grade oils can pose potential threats to individuals with epilepsy or pregnant women.
Essential oils can be aggressive toward rubbers and plastics, so care must be taken in choosing the correct handling equipment. Glass syringes are often used, but have coarse volumetric graduations. Chemistry syringes are ideal, as they resist essential oils, are long enough to enter deep vessels, and have fine graduations, facilitating quality control. Unlike traditional pipettes, which have difficulty handling viscous fluids, the chemistry syringe has a seal and piston arrangement which slides inside the pipette, wiping the essential oil off the pipette wall. This improves accuracy, and the inside of the pipette is easy to clean and reuse immediately. Chemistry pipetting syringes are equal in accuracy to the best laboratory equipment and are available in sizes from 1 mL through 25 mL.
The use of essential oils in pregnancy is not recommended due to inadequate published evidence to demonstrate evidence of safety. Pregnant women often report an abnormal sensitivity to smells and taste, essential oils can cause irritation and nausea.
Estrogenic and antiandrogenic activity have been reported by in vitro study of tea tree oil and lavender essential oils. Case reports suggest the oils may be implicated in some cases of gynecomastia, an abnormal breast tissue growth, in prepubescent boys. However, these claims have been challenged [unreliable medical source?] and the European Commission’s Scientific Committee on Consumer Safety has dismissed the claims saying "Since the hormonal active ingredients of Tea Tree Oil were shown not to penetrate the skin, the hypothesized correlation of the finding of 3 cases of gynecomastia to the topical use of Tea Tree Oil is considered implausible." 
There is some concern about pesticide residues in essential oils, particularly those used therapeutically. For this reason, many practitioners of aromatherapy buy organically produced oils. Not only are pesticides present in trace quantities, but also the oils themselves are used in tiny quantities and usually in high dilutions. Where there is a concern about pesticide residues in food essential oils, such as mint or orange oils, the proper criterion is not whether the material is alleged to be organically produced, but whether it meets the government standards based on actual analysis of its pesticide content.
Essential oils are used extensively as GRAS flavoring agents in foods, beverages and confectioneries according to strict Good Manufacturing Practice (GMP) and flavorist standards. Therapeutic grade essential oils are generally safe for human consumption in small amounts. Pharmacopoeia standards for medicinal oils should be heeded. Some oils can be toxic to some domestic animals, cats in particular. The internal use of essential oils can pose hazards to pregnant women, as some can be abortifacients in dose 0.5–10 ml, and thus should not be used during pregnancy.
The flash point of each essential oil is different. Many of the common essential oils, such as tea tree, lavender, and citrus oils, are classed as a Class 3 Flammable Liquid, as they have a flash point of 50–60 °C.
The following table lists the LD50 or median lethal dose for common oils; this is the dose required to kill half the members of a tested population.LD50 is intended as a guideline only, and reported values can vary widely due to differences in tested species and testing conditions.
This information is for educational purposes only, it is not intended to treat, cure, prevent or, diagnose any disease or condition. Nor is it intended to prescribe in any way. This information is for educational purposes only and may not be complete, nor may its data be accurate.
As with all essential oils, never use them undiluted. Do not take internally unless working with a qualified and expert practitioner. Keep away from children. If applying an essential oil to your skin always perform a small patch test to an insensitive part of the body (after you have properly diluted the oil in an appropriate carrier.