The term “in vitro” refers to experimental data collected by testing components of a living organism that have been isolated from their normal biological environment. An easier way to understand in vitro analysis is to envision it as an experiment occurring in a petri dish or test tube. For this very reason, in vitro data has limitations and does not perfectly reflect the same biological responses that occur in the whole organism. Instead, in vitro studies set preliminary groundwork for future, in vivo studies. It is important to keep the results in context and not draw final conclusions when interpreting in vitro studies because of their many limitations.

For example, two in vitro studies measured the cytotoxicity of lavender (Lavandula angustifolia) and clove (Syzygium aromaticum) essential oils on human skin cells. The whole oils as well as their major constituents were tested. Because isolated cells are much more fragile than cells in the normal physiologic conditions of the human body, the essential oil had to be heavily diluted before application. At a dilution of 0.25 percent, whole lavender oil was found to be toxic to the cells, though at lower concentrations, the oil was safe. Similarly, at very low concentrations, clove oil was found to be safe; however, when the concentration reached 0.03 percent or more, the oil induced cytotoxic effects. In comparison with lavender, clove essential oil has approximately eight times more powerful cytotoxic effects, which can likely be attributed to its high phenol content. This study gives us helpful information about the overall potency of different essential oils, which accordingly may provide clues for determining relative dose and dilution. However, because the experiment was not conducted on living organisms, it is not conclusive as to human use or toxicity. For example, isolated skin cells are very different from human skin. The skin is composed of multiple layers that include a protective covering of dead cells on top, collagen and elastin to bind everything together, glands, hair, blood vessels, nerves, etc. The specialized structure of skin allows it to handle a much higher dose of essential oil at lower dilutions than isolated cells. Determining that dose then requires in vivo toxicity studies.

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