Understanding essential oil chemistry can give us insight to why we choose essential oils. While it's great to feel positive effects, the results are not quite measurable. Through scientific methods, one can identify the molecules found in essential oils. Scientist research to identify active ingredients, components which are shown to produce a desired effect. Examples include menthol found in peppermint 1,8 cineole a.k.a eucalyptol found in eucalyptus which produces a cooling sensation. As we know peppermint essential oil is made up of more components than just menthol and eucalyptus oil is made up of more components than just eucalyptol. Essential oil enthusiasts love using essential oils in it's entirety rather than it's parts. They synergy and quenching.
Synergism: cooperation or interaction between two or more different substances, such that the total effect is greater than the sum of the individual effects An example of synergism is an in vitro study of star anise essential oil and 6 different essential oil components: β-eudesmol, farnesol, β-caryophyllene and β-caryophyllene oxide. Out of 6 different essential oil components, only trans-anethole, β-caryophyllene and farnesol only was shown to supports healthy immune function*. Interstingly, star anise essential oil was shown to be more effective* than all six essential oil components by themselves.
Quenching: where antogonism from one substance is inhibited by the presence of another substance Quenching is usually a term described extinguishing a fire. Likewise the the theory is that if there is a negative effect by an individual component, a different component would negate it. This theory would support the belief that essential oils are better as a whole than as a part. There are some studies that show that compounds like limonene or eugenol may "quench" the skin sensitization effect of cinnamaldehyde. Dr. Robert Tisserand gives an example in his book Essential Oil Safety:
An antagonistic effect in essential oils is exemplified by the reduced toxicity of carvacrol in the presence of thymol (Karpouhtsis et al 1998). This apparently manifests in thyme oil high in thymol and/ or carvacrol which contains a combined total of 30.9– 79.9% thymol plus carvacrol. The rat oral LD50 values of these constituents are 980 mg/ kg and 810 mg/ kg, respectively. If we assume an average LD50 (median lethal dose) for each of 895 mg/ kg, then the LD50 of a thymol/ carvacrol CT thyme oil would range from a possible 1,118– 2,887 mg/ kg. However, the rat oral LD50 of this type of thyme oil is 4,700 mg/ kg, making it about half as toxic as would be predicted from the thymol and carvacrol content.
Because many essential oils are made up of hundreds of compounds, it is difficult to pinpoint which compounds has interactions that exhibit signs of synergism, quenching or even other interactions such potentiation and antangonism or if those interactions exist. Hopefully, future studies can shed some light to the exact mechanisms on how these components interact with each other, but at least we know what the general thought and reasoning behind why essential oil enthusiasts love to utilize essential oils in its purest form.