Use only pure, properly distilled essential oils. The oils should be tested, with a chromatogram, to insure that all the healing constituents are intact, and have not been damaged during distillation. Because the Americans don't understand essential oils, but they do understand marketing. Intuitively we are drawn to essential oils, because on a deep subconscious level we know they can help us, and on a conscious level they make us emotionally feel good. We usually fall for the marketing, rather than research for ourselves.
Lavender is a good example, because it is a universal oil and the most widely sold.
Just to give you an understanding, there are four hybrids of lavenders , but they're called lavandins . They're not therapeutic for aroma therapy use, but that's what's sold in the United States predominantly, because people here don't know the difference. If that is not bad enough, the improper distillation of these inferior essential oils fracture the healing constituents. Distillation needs to be done with low or no pressure, low temperature, and for longer periods of time. Back to our example of lavender, it has to be in the chamber for a minimum of one and a half hours, better two hours.