Because of the tiny molecular structure of the components of an essential oil, they are extremely concentrated. One drop contains approximately 40 million-trillion molecules. Numerically that is a 4 with 19 zeros after it: 40,000,000,000,000,000,000. We have 100 trillion cells in our bodies, and that's a lot. But one drop of essential oil contains enough molecules to cover every cell in our bodies with 40,000 molecules. Considering that it only takes one molecule of the right kind to open a receptor site and communicate with the DNA to alter cellular function, you can see why even one drop or inhaling a small amount of oil vapor can have profound effects on the body, brain, and emotions. Sometimes too many oil molecules overload the receptor sites, and they freeze up without responding at all, when a smaller amount would have been just right. This is why we say that when using oils, "sometimes less is better." Sometimes more is better, too. Knowing the difference is the art of aromatherapy.

Essential oils are mixtures of dozens, even hundreds, of constituents, all of which are composed of carbon and hydrogen and sometimes oxygen. All essential oils are principally composed of a class of organic compounds built of "isoprene units." An isoprene unit is a set of five connected carbon atoms with eight hydrogens attached whose formula is C₅H₈. The molecular weight or an isoprene unit is only 68 amu, which is very small, indeed.

Molecules built of isoprene units are all classified as "terpenes." Terpenes are what make essential oils unique in the world of natural substances.

Phenols and Phenylpropanoids are compounds of carbon molecules with a hexagonal ring incorporating one isoprene unit with a hydroxl radical (OH) attached. There are dozens of varieties of phenols and phenylpropanoids. They are found in Anise (90%), Clove (80%), Basil (70%), Oregano (70%), Thyme (48%), Calamus or Cane (30%), Cinnamon (25%), and Cassia (8%).

While phenols create conditions where unfriendly viruses and bacteria cannot live, the most important function performed by phenols and phenylpropanoids is that they clean receptor sites. Without clean receptor sites, cells cannot communicate, and the body malfunctions, resulting in sickness.