Citral is the molecule that gives  lemons, citronella, lemongrass, and many others their familiar scent.

What Are Essential Oils?

Plants take the light of the sun, the minerals of the earth, and the carbon dioxide exhaled by humans and animals and, through photosynthesis, transform them into the building blocks of medicine. Among the most important therapeutic compounds manufactured by plants are essential oils. These volatile oils contain a variety of active constituents and are also responsible for each plant's unique fragrance.

Fragrance Molecules

The basic elements of carbon, hydrogen, and oxygen combine to form the different organic molecular compounds that produce aromas. So far, more than 30,000 of these molecular compounds have been identified and named. Most individual essential oils consist of many different aromatic molecular compounds. In fact, the essential oil from just one plant may contain as many as one hundred different fragrance molecules. In nature there are thousands of plants, all with unique fragrances that are comprised of different combinations of these molecules.

Plants that smell similar to one another usually contain some of the same molecular compounds. Lemon verbena, lemon balm (melissa), lemon thyme, lemon eucalyptus, citronella, lemongrass, and lemon itself, for instance, all smell like lemon because they contain a lemon-scented molecule called citral. But it is the other aromatic molecules they contain that give each plant its unique fragrance.

Aromatic compounds are grouped under larger classes of compounds such as terpenes, phenols, aldehydes, alcohols, ketones, acids, esters, coumarins, and occasionally, oxides. Citral is an aldehyde; eugenol is a phenol. Each molecular compound has characteristic scents and actions on the body. Some may be cooling and relaxing, while others are warming and stimulating. Some are better for treating indigestion, while others are antiseptic.

Every effect of an essential oil has a chemical explanation. These effects include their biological activity in the body (beneficial, irritating, or toxic), their solubility (in oil or alcohol, for instance), how rapidly they evaporate in air or are absorbed through the skin, and how well different oils combine as scents. Aldehydes such as those found in cinnamon and lemongrass, for example, have a slightly fruity odor and may often cause skin irritation and allergic reaction. Ketones found in fennel, caraway, and rosemary are not metabolized easily and may pass unchanged into the urine. The phenols found in clove and thyme are very likely to be irritating.

The proportion of aromatic compounds in a particular type of plant is not necessarily constant. This proportion can change from year to year depending on the plants' growing conditions, including geographic location, elevation, climate, soil quality, and the methods used to harvest it and extract the essential oil. Consistent variations found in the same species are called chemotypes, or chemical types (CT). Aromatherapists often take advantage of these natural alterations, selecting a certain chemotype over the standard for its special attributes.

The Physiology of Scent

Essential oil molecules enter the body through the nose and the skin. Since these molecules are extremely small and float easily through the air, you can simply inhale them into your lungs, which then disperse them into your bloodstream. The blood quickly carries them throughout your body. Essential oil molecules are also small enough to be absorbed through the pores of the skin.

Once absorbed, some molecules enter the bloodstream, while others remain in the area of application or evaporate into the air. How much goes where depends on the size of the essential oil molecules, the method of application (massage increases absorption), and the carrier containing the essential oil, be it alcohol, vegetable oil, vinegar, or water. This makes essential oils perfect for healing a specific skin problem as well as the entire body.

The sense of smell has its own important mechanisms. High in the nose is the olfactory epithelium, two smell receptors about the size of dimes. The receptors pick up volatile and lipid-soluble molecules using tiny filaments called cilia, which may actually be able to identify odor molecules by their "shape." It is believed that these odor receptors are coded by a huge family of genes to sense particular components of smell that produce a characteristic "fingerprint" pattern of activity in the brain.

From the olfactory mucus membrane, signals travel to olfactory bulbs that extend forward like tiny spoons from the brain. An electrical impulse then goes directly to the limbic system, which is part of what is called the primitive or "old" brain. Smell, it seems, was our first sense, and our old brain actually evolved from the olfactory stalks. Because recognition of smell moves directly into the old brain, it completely bypasses areas that control reasoning and the central nervous system.

Thus, it directly influences survival mechanisms such as "fight or flight" reactions and the autonomic functions of the body, including heartbeat, body temperature, appetite, digestion, sexual arousal, and memory -- the functions we can't control by will or reason. It also affects instincts such as emotions, attraction/repulsion, lust, and creativity. The senses of hearing and vision, by contrast, first stimulate the thalamus, which registers only warmth and pain. Furthermore, the old brain is directly connected to the hypothalamus and pituitary glands, and therefore to our immune system and hormones, which is why smell affects them so powerfully.

Damage to the limbic system of the old brain has been found to adversely affect memory and cause eating disorders and sexual dysfunction. Thus, medical researchers hope to someday treat such memory disorders as Alzheimer disease with fragrance. Other treatments being researched include those for fatigue, migraine headaches, food cravings, depression, schizophrenia, and anxiety.

On our final page, you will learn about how essential oils and aromatherapy work together.

To learn more about Aromatherapy and other alternative medicines, see:
  • Aromatherapy: Here you will learn about aromatherapy, how it works, what part essential oils play, and how to use aromatherapy.
  • How Essential Oils Work: In this article, you will learn how essential oils are produced, the difference between essential oils, and how to buy and store essential oils. How to Treat Common Conditions With Aromatherapy: Aromatherapy can be used to treat a number of conditions, from asthma to depression to skin problems. Here you will learn how to treat some common medical problems with aromatherapy.
  • Home Remedies: We have gathered over a hundred safe, time-tested home remedies for treating a wide variety of medical complaints yourself.
  • Herbal Remedies: Herbal remedies and aromatherapy can be very similar, and they stem from similar historic roots. On this page, you will find all of our herb profiles and instructions for treating medical problems with herbal remedies.

This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.