Only since the mid-20th century has the skin been recognized to be a permeable organ. Where it was once considered impermeable, the skin is now known to be a poor barrier to lipophilic substances, and essential oils in a base oil - when applied to the skin - do get absorbed into the bloodstream. Although the skin has developed a barrier to water which comprises 90% of all cells, it is slightly permeable to water-soluble substances, water, and lipids.

Most essential oils used in aromatherapy are lipophilic substances and as such are readily absorbed through the skin. Once the essential oil molecules have passed through the epidermis and entered the complex of lymph and blood vessels, nerves, sweat and oil glands, follicles, collagen, fibroblasts, mast cells, elastin, etc., they are carried in the circulation to every cell in the body. Factors to consider in this regard are:

1. The skin's very large area makes it possible for large amounts of oils to be applied to the skin and thereby taken into the body.
2. Easier penetration may occur in parts of the body where the skin is thinner such as behind the ears, eyelids, and inside the wrist. The soles, palms, forehead, scalp, and armpits are more permeable than the legs, buttocks, trunk, and abdomen.
3. Hydrophilic molecules can get through the skin via the sweat glands, and lipophic molecules can use the sebaceous glands and through the thin skin of the forehead and scalp. The number of follicles and sweat glands is one more factor that plays a role in the speed and capacity of access to the body. When sweating, the body's intake of oils is slower since it is exuding.
4. Essential oils may be stored in the body in separate quarters so that there are reservoirs of essential oils in the outer layers of the epidermis and subcutaneous fat which may persist for some time.
5. Enzymes present in the skin can activate and inactivate different drugs and foreign chemicals, as well as activating the body's own natural chemicals, including hormones and steroids.
6. Skin that is damaged or inflamed is highly permeable, and intake is very quick through cuts, abrasions, ulcers, psoriasis, burns, etc. Skin that is aged or dehydrated through extended exposure to the sun is not easily permeable to essential oils.