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Essential oils are often referred to as "volatile," a chemical property that explains some of the intense healing power of the oils.

It is because of their volatile nature that many oils are able to help in deep healing of the cells and that some oils appear to cross the blood-brain barrier and affect the emotions and the brain.
What Does Volatile Mean?

Essential Oils Evaporate Easily
The dictionary definition of "volatile" is "evaporating rapidly; passing off readily in the form of vapor." (Dictionary.com, accessed April 29, 2008) In chemistry, volatile chemicals are those that evaporate into gas at room temperature or that "flash off" quickly into the air.These chemicals often have strong fragrances because their strong presence in the air can be easily smelled.Any real, unmodified essential oil contains hundreds of different chemical molecules of varying sizes and shapes. Some of these chemicals – like esters and monoterpenes - are small, volatile chemicals, and some are heavier compounds such as sesquiterpenes or diterpenes.

Essential Oils and the Cell
Although most of the components of an essential oil have the power to heal, it is often the lightest chemicals that are able to penetrate first to the cells.Some, like the monoterpenols, carry oxygen, while others such as phenols help improve the functioning of the cells by removing wastes.


Essential Oils and the Brain
One of the most important implications of the light, volatile molecules in an essential oil is how it impacts the health of the brain. Most medications are unable to directly affect the brain: the blood-brain barrier filters most chemicals in the blood, blocking them from entering the brain.The light, small molecules in an essential oil are different: able to cross over the blood-brain barrier, they are used therapeutically to help students, those with memory loss or other brain disorders, Alzheimer's, depression, ADD/ADHD, heavy metal toxicity, and more.

Volatile Essential Oils Usually Smell Different After Exposure to Air
When using pure essential oils with no additives or artificial stabilizers, it is important to avoid leaving oil bottles open, or the chemical composition of the oil can change. Chemical compounds in the oil will evaporate straight out of an open bottle, starting with the most volatile, until only the heavier molecules are left.Because it is the synergy of the ingredients in an oil that enable it to heal with no side effects, keeping bottle caps tightly done up prevents wasting an essential oil or creating a different chemistry than the one that nature intended.

Reference:
Essential Science Publishing, Essential Oils Desk Reference, 2nd Edition; 2001