Cutaneous Absorption (or the lack of) of Essential Oils

 
by Sylla Sheppard Hanger and Martin Watt

This article was sent to the International Journal of Aromatherapy in April 1995
but was never published. It is a little dated but most points are still as valid
today. At a later date, other articles by other authors, suddenly started
questioning the concept of skin absorption!!

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 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 or sauna)
Therapy room
Therapists hands
Bath water
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.

SUMMARY
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.

References:
(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.

Bibliography:
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; OUT OF PRINT and personal communications.

 
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