Most essential oils are made by collecting plant material (leaves, flowers, roots, bark, seeds, resin, etc.) and passing steam through the material which is then cooled. The cooled water then contains the volatile chemicals from the plant material which floats on top. Some essential oils are collected by putting the plant material directly in the water and then heating it, some are collected by dry heat, and still others (such as the citrus oils) are collected by cold pressing the material (rind). Plant material can also be subjected to chemical extraction process such as with hexane and ethanol but they would be considered an absolute and not an essential oil. Once collected, the essential oils are then drawn off and bottled. The chemical properties will vary slightly depending on the exact species of plant grown, the country of origin, and other variables such as weather and altitude.
Adulteration can occur at any stage during this process. Farmers can adulterate the product by including plant material that is not typically used to produce the essential oil such as the stems and leaves when the root is what is typically used. Oils are also adulterated by the addition of one or more essential oils that have similar chemical properties, color, or odor. A good example of this would be using de-mentholated cornmint oil in place of peppermint oil. This is commonly done when prices within the market fluctuate or certain oils become more scarce. Lemongrass essential oil is often exchanged for another essential oil called Litsea cubeba and vice versa depending on market pricing and which oil is cheaper at any one time. Oils can also be adulterated by adding synthetic chemicals or diluting with a carrier oil. All of these methods of adulteration would change the chemical constituents of the resulting oil and can be verified by gas chromatography and mass spectrometry analysis. Many independent labs exist that will verify whether or not what is put on the label matches what is in the bottle.
The quality of essential oils is important to anyone who is using them therapeutically. That is why it is important that the consumer (or the retailer) verifies that the essential oil supplier they use are not adulterating their oils. Essential oils should be clearly labeled with the latin genus and species name, the country of origin, the method of extraction and whether or not any carrier oils are used in the final product (which is often the case for blends that are to be used topically such as in massage). Even with this level of labeling it is not a bad idea to either request to see a GC/MS analysis of the oil or to send the oil yourself to an independent laboratory. Most general consumers will not do this but a retailer or health professional should consider it if they are recommending or selling the oils directly to consumers.
Proper labeling should also include any warnings or contraindications for use. Many essential oils, even if they are pure and unadulterated, can cause skin sensitization, photosensitivity, contact dermatitis, acute oral toxicity, cellular toxicity or other adverse reactions. The risks associated would depend greatly on the method of exposure (inhalation, oral or topical use), the susceptibility of the individual (age, stature, health status), and the amount of exposure and frequency of use (either accidentally or intentionally). Again, I want to emphasize that these potential risks exist for many essential oils regardless of the supplier. Adulteration always carries risks and the information presented on the label does not match what is in the bottle, but even 100% pure steam distilled oils carry with them some inherent risk due to the chemical composition of the plant material itself.
Be safe with essential oils. Know and trust your supplier or go the extra step and make sure using independent verification. And remember that even if your essential oil is pure and unadulterated it should be used with respect for what it is: concentrated chemical constituents from medicinal plant material.
To Your Health!