Essential Oils and Their Lack of Skin Absorption
The article below was sent to the I.J.A. in April 1995 but was never published. It is a little dated but most points are still as valid today.
Cutaneous Absorption (or the lack of) of Essential Oils
by Sylla Sheppard Hanger and Martin Watt
Do essential oils penetrate the skin into the bloodstream?
This is a very confusing issue in the aromatherapy literature as most sources say the skin readily absorbs essential oils into the bloodstream and this is the route by which significant volumes of oils can enter the body. They say this is not only because the skin is the largest organ, but also because some essential oils are extremely lipophilic in nature, and the molecular structure of essential oils is very small.
Research has proven the penetration ability of several drugs now administered in the form of patches, and it is assumed that essential oils are similarly absorbed. However, by investigating a wide range of dermatological literature, one can find much more evidence to support the opposite view. Whole essential oils are not freely absorbed and very few essential oil constituents are absorbed through the skin into the bloodstream. Most chemical constituents would be absorbed into the circulation in reasonably significant amounts via nasal membranes and lungs if the concentration in the atmosphere is at an appreciable level.
Human skin has developed as a highly effective barrier. Little evidence shows, with a few exceptions, that the skin will readily admit lipid soluble portions of plants. Almost without exception (an exception may be chamazulene), the small number of natural chemicals present in essential oils which are slightly absorbed by the skin, are also well documented as causing adverse dermal reactions. This would seem to indicate that a wide range of essential oils are alien to the immune system when taken into the body via the skin. Nevertheless, the important question remains whether sufficient quantities enter the body through the skin to have any beneficial pharmacological effects. And, yes, in spite of this, aromatherapy still works! It is just the method it is assumed it works by is not correct. Certainly it works symptomatically by external application on superficial skin layers (i.e., as antiseptic, anti-inflammatory, etc.) and more importantly in some cases, via the mind.
Based on theoretical models of how the skin is constructed and of how it should work, it is assumed that essential oils can pass through this tough barrier. Yet, skin absorption of whole essential oils has not been definitely shown. It is assumed, at least in theory that substances with low molecular weight will penetrate the skin. Essential oils and their naturally occurring constituents are mostly below 1000 m (m = molecular weight). It is also said that “due to their lipophilic nature”, some essential oils and vegetable oils have an affinity to the skin, thereby allowing their passage into the bloodstream. It is also said that “due to this lipophilic nature, essential oils are soluble in fat and some fat-dissolving oils (high in ketones) are said to he able to dissolve the myelin sheath around nerves promoting “neuro”-toxicity”.
First of all, if essential oils managed to get into living cells in sufficient quantity to dissolve the fat then it would certainly also kill the cell(s). The test of putting a drop of thuja on chicken fat and seeing it dissolve (as advocated by some) is not considered “proof” and cannot at all be compared to use in aromatherapy (at least any more than watching a drop of lavender eat a hole in a styrofoam cup.
Citing patch medications and garlic applied to the feet (then detecting on breath) is also incorrect as evidence of penetration through the skin. Application of occluded concentrated chemicals cannot be compared to essential oil application in aromatherapy. And garlic has a chemical composition totally different to any other essential oil. Because of its fantastic volatility, inhalation is impossible to avoid (most people have to leave the room when a bottle is opened). Yes the oil gets in the body, but by being inhaled and then excreted on the breath for hours afterwards. In spite of all this, there remains no substantial evidence of whole essential oils having been detected in the bloodstream in clinically active amounts following skin application.
Secondly, we use carrier oils for skin application; the molecules of vegetable oils are much larger than essential oils, and in spite of their lipophilic nature, are also unable to pass through the skin (1). The theoretical model advocated is that skin is relatively permeable to fat soluble molecules and impermeable to water and salts; and because cell membranes have a lipid bilayer, it seems possible that fat soluble molecules can pass through. However, most vegetable oils (including essential oils contained within them) have been found to reside in the outer dead layer of skin without penetrating into the bloodstream. In addition, viscosity or degree of saturation of the vegetable oil plays a part in ability of penetration. Almond and olive oil were shown to penetrate the outer skin layer slowly (mono-unsaturates) in abdominal guinea pig skin, while linseed, rich in polyunsaturates, penetrated rapidly. Because of this fact, the external emollient qualities of vegetable oils cannot be denied, therefore they are highly suitable as carriers for essential oils.
What does penetrate the skin?
Because sensitisation reactions have been experienced, it seems some components of essential oils must pass into the skin. However, it is very difficult to find substantial scientific evidence that the absorption is of pharmacological significance (i.e., the amount absorbed causing systemic reactions such as diuretic). The whole oil does not appear to be absorbed into circulation through the skin at all. The main reference sources cited as evidence that skin absorption occurs have several areas of uncertainty that cannot be denied.
Certain constituents (i.e., a-pinene, camphene) are both absorbed in possibly significant amounts, as shown in tests with pine oil in a hot bath (combined with heat and humidity) and using a nose clip to avoid nasal inhalation. Both chemicals were found to be excreted in the urine for up to twenty-four hours later.(2). However, this experiment in a hot tub cannot be equated with an aromatherapy massage.
Benzoic acid (benzoin) is well known to be readily absorbed and has been used to test variability of absorption over the body. Inner forearms and back were found to be the best sites. Benzyl acetate (jasmine) has been recovered from urine 24 hours after neat application and both jasmine absolute and synthetic jasmine have been shown to produce irritation reactions. Benzyl benzoate (ylang ylang) and benzyl alcohol (up to 30% of Peru balsam) were absorbed within 24 hours. Cinnamic alcohol (cinnamon leaf & benzoin), cinnamic acid (cinnamon bark), both severe sensitising agents, and safrole (sassafras), a carcinogen, are also absorbed. Cinnamic aldehyde (cassia, cinnamon bark) was absorbed well following neat application and is not advised due to strong sensitising potential. Methyl salicylate (sweet birch, wintergreen) is also freely absorbed, especially in the presence of water. The ease of absorption of the above constituents may be why they are irritants or sensitisers. However only a few molecules are required to trigger a sensitisation reaction.
Many of the skin permeability tests used occluded (covered) isolated concentrated synthetic aroma chemicals as the prototype for essential oils,(3) which cannot compare to the whole essential oil. And no studies found on skin absorption used a mask to prevent absorption into the bloodstream from breathing the volatile components in through the lungs; with the exception of the pine oil bath test cited above (which inhibited nasal breathing and fails to state how the person breathed as there was no mention of oxygen apparatus in the methodology).
Some of the dermatological tests may be unreliable when compared to humans as they were unfortunately conducted on animals. Animal skin absorbs chemicals more readily than than human skin.(3). Tests in vitro on isolated chemicals mentioned previously are unreliable when compared to the whole oil, and when compared to in vivo situation; however, care should be taken when using oils, which contain these chemicals in significant quantities.
In vitro studies on human (breast tissue, foreskin) and animal skin showed benzyl acetate was rapidly absorbed, creating a reservoir in upper dead cell layer. (4). This indicates that a major proportion of essential oils applied to skin may leave by evaporation, rather than passage through. Ethyl alcohol, a major part of perfumes, enhances percutaneous absorption and a recent study showed 75% of fragrance was absorbed through the skin when occluded (covered) regardless of the fragrance, as opposed to uncovered.
Testing with the whole oil (lavender) diluted (2%) and applied to human skin, still only showed a few individual chemicals (linalool and linalyl acetate) in the bloodstream. The test methodology did not indicate any other amounts detected or if other constituents were even tested for. (5). Again, no breathing apparatus was used to prevent lung absorption, thereby, not satisfying the question of skin absorption of whole essential oils. Note: Since writing a similar test was done using a single chemical and breathing apparatus. The volume of chemical in the bloodstream was of no pharmacological significance when related to the volume an aromatherapist would use in a massage.
In addition, because the skin is an important site of metabolism of drugs and solvents, essential oils may be changed before the permeable constituents can enter. Essential oils may undergo molecular transformation by skin enzymes (P450’s). Benzyl acetate (jasmine) is broken down very effectively by esterase enzymes in the skin.
Increasing the permeability of the skin is possible and can be achieved by several means. It must be remembered that increased permeability can also mean increased irritation and sensitisation with any essential oil. Hypersensitive or atopic persons (i.e., those suffering from hayfever, allergic rhinitis, eczema, asthma, wool or animal intolerance, or a family history of any of these) require greater caution. Caution is needed with irritating or sensitising oils when any of the following factors are present.
The following have been shown to increase permeability of the skin.
Temperature -skin (hence caution during or after exercise, sauna)
Humidity, presence of water - after soaking for a while the normally water-resistant skin becomes hydrated, thus more permeable.
Addition of detergent, soaps and solvents have been shown to increase permeability.
Damaged or abraded skin absorbs easier thus irritation and sensitisation reactions are possible (i.e., eczema, psoriasis, wounds).
Occlusion - covering the area aids penetration by preventing evaporation.
It still remains to be shown that whole essential oils penetrate through the skin into the bloodstream, especially in any significant amounts, although single constituents may. Therefore, the statement that “whole essential oils pass into the bloodstream through the skin” is incorrect. They do enter the bloodstream very quickly through respiration, if the amount in the air is appreciable, therefore appearing in blood or urine. However, of the few constituents that may penetrate, besides being mostly irritant and sensitising, the amounts received in aromatherapy massage or needed to cause systemic pharmacological actions remains to be determined. It may be, therefore, that many of the actions claimed for essential oils may be due to inhalation, or to unreliable extrapolation of the internal ingestion (of oil or herbal extract) and may not apply to amounts absorbed through skin application. The question of VOLUME of oil or its constituent chemicals getting into the bloodstream via the skin is of critical importance because most people take in far more essential oils in food and drink.
Just because whole essential oils may not be absorbed via skin into the bloodstream creating a systemic reaction, does NOT mean beneficial skin affects and certainly the mental effects (relaxation) are very much possible with essential oil treatments. Skin treatments affect the external layers where many problems are found. The antimicrobial and anti-inflammatory properties, as well as others, have proven essential oils remain quite efficient at treating many minor complaints. And the relaxing mental effects produced from the joy of using a pleasing fragrance will never be denied.
(1). From “Application of Oil in Prevention of Fatty Acid Deficiency in Pre-term Infants”. Lee F. Gibson K. Zimmer K. Food & Chemical Toxicology, Vol. 28, Jan. 1985. pgs. 27-28, & Vol 6, 1 990. Showed topical application of safflower oil does not decrease fatty acid deficiency; found no evidence at all of trans-dermal absorption.
(2). ‘Percutaneous absorption of Essential oil and Compounds’. Rommelt, et al 1974, Kosmet Database.
(3). Bronaugh, et. al, Food & Chemical Toxicology, 28 (51, 369-373, 1990. ‘In vivo percutaneous absorption of fragrance ingredients in rhesus monkeys and humans’.
(4). Hotchkiss at. al. Food & Chemical Toxicology. 28 (6), 443-447, 1990. ‘Percutaneous absorption of benzyl acetate through rat skin in vitro. Validation of an in vitro model against in vivo data’; and Hotchkiss et. al. Food & Chemical Toxicology. 30 (2), 145-153, 1992, ‘Percutaneous absorption of benzyl acetate through rat skin in vitro. Effect of vehicle and occlusion’. In which only 4% was absorbed (Bronaugh. 1990)
(5). Buchbauer, J. G, Jirovetz. Firtzer, Journal of Society of Cosmet/c Chemists, 43;49-54, Jan-Feb 92; “Percutaneous Absorption of Lavender from a Massage Oil;” after a 10 minute massage on a male adult stomach area, showed traces of linalyl acetate and linalool (only) within five minutes in the blood, with maximum concentration in 20 minutes and elimination within 90 minutes.
Buchbauer, J.G., Jirovetz, Firtzer, Journal of Society of Cosmetic Chemists 43:49-54, Jan-Feb, 1992.
Hanger, Sylla Sheppard, Lisin, G., Watt, M., Moyler, D., (eds.), The Aromatherapy Practitioner Reference Manual, 1995.
Hotchkiss et al. Food & Chemical Toxicology 28 (6), 443-447, 1992 and 30 (2). 145-153, 1992.
Bronaugh,et al. 28 (5), 369-373, 1990. Kosmet Database reports from IFSCC (International Federation of the Societies of Cosmetic Chemists), National Societies of Cosmetic Chemists, technical publications from professional associations; Delaport House, Luton, Bedfordshire, UK.
Lee, Gibson, & Zimmer, Food and Chemical Toxicology, (28), 27-28. 1985 & (6), 1990.
Watt, M. Plant Aromatics Data & Reference Manual on Essential Oils and Aromatic Plant Extracts. 1994; and personal communications.
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