Cutaneous Absorption of Essential Oils
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. For many years it has been said it is most “unlikely that any essential oils fail to reach the bloodstream.” 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 have an affinity to the skin, thereby allowing their passage into the bloodstream and that due to this nature, essential oils are soluble in fat and some oils (ketone-rich) can dissolve the myelin sheath around nerves promoting “neurotoxicity. 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 is not proof and cannot at all be compared to use in aromatherapy (at least not any more than watching a drop of lavender eat a hole in a Styrofoam cup). Secondly, citing patch medications and garlic applied to the feet is also incorrect as evidence. Application of concentrated medication chemicals cannot be compared at all to essential oil application (in quantity or constituents), 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) therefore it is excreted on the breath (from inhalation) for hours afterwards. In spite of all this, there remains little substantial evidence of whole essential oils having been detected in the bloodstream after skin application.

Because sensitization reactions have been experienced, it seems some components 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 a test with a massive dose of pine oil (over 100 mIs.) in water (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.

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 sensitizing 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 sensitizing 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 sensitizes.

Many of the skin permeability tests used occluded isolated concentrated synthetic aroma chemicals as the prototype for essential oils,7 which can not compare to essential oils. And, most importantly, 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 methodology). Some of the dermatological tests may be unreliable when compared to humans as they were unfortunately, conducted on animal skin, which is considered by leading dermatologists to be far more absorbent than human skin. Tests in vitro on isolated chemicals mentioned previously are unreliable when compared to the whole oil, and when compared to in viva situation; however, care should be taken when using oils which contain these chemicals in significant quantities.

In vitro studies on human breast tissue and foreskin (Hotchkiss, 1 992)~ and animal skin showed benzyl acetate was rapidly absorbed, creating a reservoir in upper dead cell layer. 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, has been shown to enhance percutaneous absorption. A recent study showed 75% of fragrance applied was absorbed through the skin when occluded

Testing with the whole oil (lavender) diluted (2%) and applied to human skin, still only showed a few individual chemicals (Iinalool and linalyl acetate) in the bloodstream, although the test methodology did not indicate other amounts detected or if other constituents were even tested for.8 Again, no breathing apparatus was used to prevent lung absorption, thereby, not satisfying the question of skin absorption of whole essential oils.

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 even enter. Essential oils can undergo molecular transformation by skin enzymes (P450's). Benzyl acetate is broken down very effectively by esterase enzymes in the skin.

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, since essential oils are highly volatile, therefore appearing in blood or urine. However, of the few constituents that may penetrate, besides being mostly irritant and sensitizing, the amounts received in aromatherapy massage or needed to cause systemic pharmacological actions remains to be determined, It may be, therefore, the actions claimed for many essential oils may be due to inhalation (or internal ingestion), may apply to the herbal remedy 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.

In conclusion, just because whole essential oils may not be absorbed into the bloodstream creating a systemic reaction, 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 are quite efficient at treating many minor complaints. Certainly the beneficial mental effects induced when using a pleasing fragrance cannot be denied.

The safest and most pleasant method of delivery is the external use of essential oils (highly diluted) usually in the form of massage (self or professional).

Dilution: Essential oils are usually diluted for safe applidation by the percentage method, between 1 % and 5 %. In other words, fifteen drops of essential oil in 30 ml. (one ounce) of vegetal carrier or base oil produces a 2.5% blend, which is suitable for face and body. Percentage can be increased to 5% if covering a smaller area (e.g., facial oil, reflex points) and decreased for a larger application (full-body massage).

Frequency: Many factors have to be considered, and the question of frequency of application depends on individual essential oil toxicity ratings, the extent of skin covered, and the efficiency of the particular essential oils chosen. Once a week is generally sufficient; twice a week should be regarded as maximum for regular treatments. For small skin areas (face) and safe (non-toxic, non-irritant) essential oils, daily application (including micro-diffusion for environmental purification and psychological purposes) is quite safe. In very serious pathologies (acute infections, etc.) under strict medical supervision, which is beyond the scope of this work, professionals have used from three milliliters to 40-48 ml per 24 hours (which represents 90-1450 drops).

Increasing 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 sensitization. This can occur with any oil under the following conditions, and most likely with damaged skin, 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). The following have been shown to increase permeability of the skin, thus caution with irritating or sensitizing oils:

• temperature - skin (hence caution after exercise, sauna, sunbathing) room
  therapists hands bath water
  
• presence of water - after soaking for a while the normally water-resistant skin becomes hydrated, thus more permeable addition of detergent, soaps and solvents
  
• damaged or abraded skin absorbs easier (i.e., psoriasis, eczema, wounds) thus irritation, sensitization reactions are more likely
  
• occlusion - covering the area reduces evaporation

The following chart is a guide on appropriate measurements for external methods. Keep in mind that this is an approximate rough guide. Of course, babies (up to 1 8 months) and elderly persons are very sensitive; avoid excessive use on either (1% or less dilution).