One of the factors that determines the purity of an oil is its chemical constituents. These constituents can be affected by a vast number of variables, including: the part(s) of the plant from which the oil was produced, soil condition, fertilizer (organic or chemical), geographical region, climate, altitude, harvesting methods, and distillation processes. For example, common thyme (Thymus vulgaris) produces several different chemotypes (biochemically unique variants within on species) depending on the conditions of its growth, climate, and altitude. One chemotype of thyme will yield an essential oil with high levels of thymol, depending on the time of year it is distilled. The later it is distilled in the growing season (ie., mid-summer or fall), the more thymol the oil will contain. Proper cultivation assures that more specific chemotypes like Thymus vulgaris will maintain a good strain of thyme on the same mountainside. An example of this was shown in studies at the University of Ege botany department in Izmir, Turkey where it was found that among Oreganum compactum plants within a 100 square foot radius, one plant would be very high in carvacol and another would be high in another compound. Wildcrafting plants cannot guarantee the same chemotype even on the same hillside. The key to producing a therapeutic-grade essential oil is to preserve as many of the delicate aromatic compounds within the essential oil as possible. Fragile aromatic chemicals are easily destroyed by high temperature and pressure, as well as contact with chemically reactive metals such as copper or aluminum. This is why all therapeutic-grade essential oils should be distilled in stainless steel cooking chambers at low pressure and low temperature. The plant material should also be free of herbicides and other agrichemicals. These can react with the essential oil during distillation to produce toxic compounds. Because many pesticides are oil-soluble, they can also mix into the essential oil. As we begin to understand the power of essential oils in the realm of personal, holistic healthcare, we will appreciate the necessity for obtaining the purest essential oils possible. No matter how costly pure essential oils may be, there can be NO substitutes. Although chemists have successfully recreated the main constituents and fragrances of some essential oils in the laboratory, these synthetic oils lack therapeutic benefits and may even carry risks. Why? Because essential oils contain hundreds of different chemical compounds, which, in combination, lend important therapeutic properties to the oil. Also, many essential oils contain molecules and isomers that are impossible to manufacture in the laboratory. Anyone venturing into the world of therapy using essential oils must use the purest quality oils available. Inferior quality or adulterated oils most likely will not produce therapeutic results and could possibly be toxic. In Europe, a set of standards has been established that outlines the chemical profile and principal constituents that a quality essential oil should have. Known as AFNOR (Association French Normalization Organization Regulation) and ISO (International Standards Organization) standards, these guidelines help buyers differentiate between a therapeutic-grade essential oil and a lower grade oil with a similar chemical makeup and fragrance. When seeking quality essential oils, one important question should be, "Does the company utilize gas chromatography (GC) in their quality control process?" AND "Does their GC equipment utilize a column of at least 50 to 60 meters in length in order to properly identify the hundreds of natural constituents found in an essential oil?" Almost all labs in the United States use a 30-meter column that is not long enough to achieve proper separation of all the essential oil constituents. A gas chromatogram shows a pattern of the separated constituents of an oil and helps a trained technician determine its purity and therapeutic quality. How to Be Sure Your Essential Oils are 'TRUE' Therapeutic Grade  Are the fragrances subtle, rich, organic, and delicate? Do they "feel" natural? Do the fragrances of your oils vary from batch to batch as an indication that they are pure and painstakingly distilled in small batches rather than industrially processed on a large scale? Does your supplier send each batch of essential oils it receives through up to five different analyses before it is released? Are these tests performed by independent labs? Does your supplier grow and distill its own organically grown herbs? Are the distillation facilities part of the farm where the herbs are grown so they are freshly distilled, maintaining their potency? Does you supplier use a low pressure and low temperature to distill essential oils and preserve all of their fragile chemical constituents? Are the distillation cookers fabricated from costly stainless steel alloys to reduce the likelihood of the oils chemically reacting with metal? Does your supplier have representatives traveling worldwide to personally inspect the fields and distilleries where the herbs are grown and distilled? Do they scrutinize the facilities to check that no synthetic chemicals are being used in any of these processes? If your answer to all of these question is YES, you can feel confident that your oils are therapeutic grade, the purest available. Reference: Essential Oils Integrative Medical Guide by D. Gary Young, N.D.