By Martin Watt
Original version published in Aromatic Thymes. 1995. Vol. 3. No. 2. 11-13.
Original © 1995 Martin Watt. Revised 2021.

Note: An extra article has been added at the foot of this one.

Introduction:
I am concerned that people do not misconstrue my articles as indicating that I
don't think aromatherapy works. This is far from the truth as I wholeheartedly
agree the therapy can have wonderful healing benefits. However, I am certain
some of the traditionally held views on how it works are very misleading.

Please also take note of the original date of this article. In the intervening
years I had only heard of a handful of research projects where adequate
methods have been used to prevent the inhalation of the oil vapours. In 2021 I
have still seen nothing to convince me that aromatherapy has got anything to
do with the skin absorption of essential oils. Where a few so called 'essential
oils' such as birch and wintergreen are known to be absorbed, they also
present toxicological and immunological problems.

Beware of many scientific research papers where they cite some of the
references mentioned here as evidence of skin absorption, in particular #10
below. That is a sure sign of one of the numerous papers written by trawling
up references from the internet without bothering to check their relevance.
Frequently this is a sign of University students preparing a thesis without
really knowing the subject or caring as long as it gets them a degree or fame.

Aromatherapy can be a potent tool for:
• Unlocking the brains inhibition of normal bodily processes caused by various
emotional factors.

• It is excellent for giving relief from many musculo-skeletal ailments.

• Essential oils can achieve spectacular results when treating some types of
skin damage and infections.

However, much confusion and misinformation exists about two relatively
separate forms of treatment:

1. Massage.

2. The use of aromatic oils with or without massage.

Therapeutic activity
Some of the essential oils used in aromatherapy do have well-documented
therapeutic actions. However, many of the oils for which aromatherapists claim
physiological medical activity, in fact possess no recorded historical medicinal
actions. Oils such as: Moroccan chamomile, citronella, clary sage, geranium,
rosewood, vetiver and ylang were originally produced solely for the perfumery
and fragrance trades

On the other hand, certain oils such as aniseed, cubeb, dill, fennel,
peppermint, rose, sandalwood, etc. have been used over several hundred
years for a variety of ailments. However, such oils were mainly used internally
as medicinal agents. The majority of reports within aromatherapy about the
therapeutic activity of these oils are based on information gleaned from the
oral consumption of herbal remedies which differ a lot from essential oils.
Most aromatherapists claim explicit physical effects after massaging with oils,
for example: "Fennel is diuretic," "Geranium regulates the hormonal system,"
"Grapefruit is good for cellulite." However, none of these effects have been
proven when these oils are applied to the skin during massage.

One example of the misleading hype is found with fennel oil. It is well known
for producing an increase in urine output when it is taken as a medicine.
However, when the volume of fennel oil used in the average massage is
applied, it is doubtful that enough can be absorbed through the skin to elicit
any such diuretic action. If very large amounts are used on the skin, or it is
occluded - such as with compresses - or the essential oil is used in hot humid
environments, then I am prepared to accept some oil may get into the
subcutaneous tissues. Diuresis has though been proven to occur following
ordinary massage without the use of any essential oils. (1)

I believe the actions of essential oils used in aromatherapy are:
1. The psychotherapeutic effects of the oils on the olfactory system and the
brain.

2. The absorption into the circulation of some of the oils constituent chemicals
via the nasal membranes and lungs.

3. For muscular problems; if much higher percentages of essential oil than are
normally used in massage are 'rubbed in' or applied on compresses.

4. Damaged skin can often benefit from using 'healing' essential and fixed oils.
This form of treatment may not be strictly 'aromatherapy', but it is closely
allied. This is because the essential oils can have a direct pharmacological
action on damaged tissues, as well as indirect beneficial effects on the mind if
the aroma is perceived as pleasant.

The effects of essential oils on the brain via the olfactory system:
This is the basis on which the perfumery trade functions, and is the way I
believe most aromatherapy works. The fragrance trades have sponsored
substantial research on the psychological effects of aromatic substances. It has
been clearly demonstrated in animals and humans that brain wave patterns are
affected to quite a remarkable degree when aromatic vapours are inhaled. It
has even been shown that brain wave patterns are altered, when human
subjects inhaled aromatic vapours at such a low level that they said: "they
could not smell the substance that was being administered." This experiment in
particular clearly demonstrates that the human sense of smell is much more
acute than it is normally credited with.

Perfume manufacturers have based their business around the effects that
certain perfumes can have on the emotional state of both the wearer and
people they come into contact with. Therefore, businesses that worldwide are
worth billions of dollars, are largely based on the psychological and emotional
effects of fragrance. It is therefore somewhat peculiar that this most important
aspect of the use of fragrant plant oils, is not the linchpin of aromatherapy.
Rather, most courses insist on dogmatically sticking to the hypothesis that the
oils achieve a pharmacological effect by being absorbed through the skin and
into the circulation - a fundamentally flawed concept

Skin absorption of essential oils:
I remain extremely sceptical that this is a route by which significant volumes of
most essential oils can enter the body. After years of looking at so called
'scientific' research, I have failed to find one trial where the methodology used
has been adequate (2). Generally researchers have taken no precautions to
prevent the inhalation of the volatile molecules. This is the critically important
area that I have found time and time again being overlooked by researchers.
They always fail to understand the fundamental nature of most essential oils,
which is that they are extremely volatile substances. As such, they quickly
find their way into the respiratory tract epithelium and thence to the
bloodstream.

Currently a lot of theoretical skin biology is being taught in aromatherapy
courses. Most tuition is based on theoretical models of how essential oils may
be metabolised once they have gained access to the layers of skin where
enzymatic reactions are known to occur. As a small number of drugs are now
administered in the form of skin patches, this is promoted as being "conclusive
evidence" that essential oils are freely absorbed in a similar manner. Yet, even
hormone patches require the solution of the hormones in alcohol or other
solvents in order to permit their absorption.

Scientific references supplied by various authors about 'evidence of
skin absorption' frequently refer to experiments of little relevance to
aromatherapy such as:

1. Individual fragrance chemicals (usually synthetic) are used - not the WHOLE
oil with its hundreds of different chemicals.

2. The substance being tested has often been applied under occlusion
(covered) (4), which does force the substance into the skin. However, this
ignores the fact that when essential oils are used in massage, body heat will
quickly evaporate the vast majority of the highly volatile chemicals away from
the skin, thus permitting quick inhalation.

The use of a vegetable carrier oil probably makes little difference to the
amount of essential oil absorbed by the skin. This is because the volatile
chemicals in essential oils evaporate within seconds of application to a warm
area. Also, the rate of evaporation from the skin is likely to be substantially
enhanced by the heat generated by the massage. I have to remind you that
even when using carrier oils you can quickly smell the essential oils used. The
mere fact that you can smell them means the vapours are gaining
immediate access to the respiratory tract.

3. Of Major importance, is the most fundamental error of all research that I
have come across which is that inadequate precautions have been taken
to prevent inhalation of the essential oil vapours. I have read all of the
paper published by Rommelt et al in 1974 (5). However in the oft quoted 1974
paper, aromatherapy writers and some scientists, simply overlook the fact that
150 ml. of a Pine bath oil was added to the bath of the subject, and no
mention was made of how he breathed. It does not surprise me that he
excreted a-b-pinene and camphene for several days. How on earth can anyone
compare the effects of 150 mls. with the few drops of essential oils used in the
average massage.

This team published a subsequent paper (10) on absorption of essential oil
compounds from a bath, but this time inhibiting breathing of the vapours. They
subsequently detected fragrance chemicals in the blood. However the use of
essential oils in a bath is nothing like the same as their use in aromatherapy
massage. In the presence of heat and more importantly humidity, the skin will
absorb compounds. Again in this experiment far larger volumes of oil seem to
have been used than are used in massage.

The same researchers indicated there might be some absorption of essential
oils from ointments. Indeed, there may be a little absorption by this method,
but I do not know if the inhalation factor was excluded in any trials. Ointments
have an extremely ancient history of being used as local applications for
musculo-skeletal problems, but there is little sound data suggesting that
the volume of essential oils so absorbed, can have anything other than
a localised effect.

I am not aware of any evidence suggesting that enough essential oil is left in
the bloodstream to have any effects on other organs. Until experiments are
conducted with the people being massaged having an air supply under
pressure and from a remote source, then all these tests are unreliable.
Interestingly no one in complementary medicine seems interested in
sponsoring such a simple trial, I wonder why?

4. In fact, there is far more evidence to support the opposing view, which is
that most essential oils are not freely absorbed. Human skin seems to more
readily permit the absorption of a number of water soluble plant chemicals
such as the nicotine anti-smoking patches - nicotine being a water soluble
alkaloid unrelated to essential oils. Many National pharmacopoeias contain
formulations for lotions, creams and ointments for painful conditions such as
sciatica, neuralgia & arthritis based on water soluble plant alkaloids. There is
however little evidence to support the theory that human skin will readily
permit the passage of the lipid (fat) soluble portions of plants - barring a few
exceptions. In traditional medicine we find few examples of plant oils being
used for anything other than localised treatments. Fixed and volatile plant oils
have always been used principally for cosmetic and skin care purposes.

Of utmost importance, is not if essential oils are absorbed into the superficial
dead layers of the skin, as clearly this does occur. But, does sufficient find its
way into the body via the skin to have any clinical effects? My investigations of
dermatological literature have led me to the following conclusion: When a few
natural chemicals in essential oils are absorbed by the skin, with a few
exceptions, it is found that those same essential oils are well documented as
causing adverse dermal and systemic reactions. This seems to me to indicate
that many essential oils are alien to the immune system when they are taken
into the body via the skin.

I offer the following evidence on skin absorption or the lack of it:
The monographs published by the IFRA provide the following unless indicated.
There is insufficient space to give full references, but they are available in their
monographs. I must add here, that even where absorption of volatile chemicals
has been indicated, without exclusion of the inhalation factor the results must
still remain questionable.

Note on the chemicals below: Frequently these are lab grade synthetic
chemicals.
Absorption tests are via the skin of animals, but of note is that human skin is
far less permeable than animal skin.

CHEMICALS - ABSORBED:

Benzyl acetate, benzoic acid, camphor, d-carvone, cinnamic acid, coumarin,
para-cymene, d-limonene, methyl salicylate, a-phellandrene, terpineol, a -b
-pinene & camphene.
With d-limonene only 3% was absorbed in vitro across isolated human skin,
while in rats the figure was 6%. (6). Note: One probably gets higher levels of
d-limonene in the blood from eating orange flavoured drinks, candies, cakes,
liqueurs, etc.

CHEMICALS - NOT ABSORBED:
linalool within 2 hours of application. (7) ***
d-pulegone in pennyroyal.
carvacrol in some thymes and mints.
eugenol, isoeugenol & methyl benzoate in clove, tuberose and ylang.
fenchone in anise, fennel & some lavenders.
geraniol in geranium & palmarosa.

WHOLE ESSENTIAL OILS - ABSORBED:
Cumin, Tansy.

WHOLE ESSENTIAL OILS - NOT ABSORBED:
Lavender (***see reference above on linalool), Tolu balsam oil, Copaiba
balsam oil, Parsley seed, Patchouli, Pimenta berry and leaf.

The absorption of aromatic molecules via the nasal passages and
lungs:
This method by which aromatic molecules in essential oils gain access to the
body has been demonstrated: Rosemary oil vapours were introduced into the
atmosphere of caged mice. It was shown that their blood contained a
substantial proportion of one of the chemicals present in the inhaled essential
oil. This proved the volatile chemicals in essential oils can gain access to the
bloodstream in significant amounts if the concentration in the atmosphere is at
an appreciable level. (9).

UPDATE: Since the time of writing, trials on humans have confirmed that
indeed, significant volume of essential oils do gain access to the blood via the
respiratory tract.

As the brain is a 'blood hungry' organ then clearly the first port of call for
aromatic molecules absorbed via the olfactory epithelium is likely to be the
brain. It is of course well known that certain drugs are known to act extremely
quickly when they are sniffed up the nose.

CONCLUSION:
I believe it is likely that we get a complexity of effects when essential oils are
inhaled:
1) A potential pharmacological effect via the blood supply to the brain.

2) An indirect effect via the olfactory nerve pathways to the brain.

3) The beneficial effects from the massage and the touch receptors.

4) The powerful placebo effect, caused by client therapist interactions.

5) Possibly, a regulation of body energy flows.

With that kind of bombardment, it's not surprising that aromatherapy can
achieve such excellent results. The therapy is clearly potent at reducing the
brains capacity to inhibit the body from carrying out its routine regulating and
healing activities.

(1). E. Ernst M.D. et al. 1987, Physiotherapy vol. 73, no. 1.

(2). J. Buchbauer et al. Jan-Feb. 1992. J. of Am. Soc. of Cos. Chemists, 43; 49-54.

(3). Bronaugh et al. 1990. Fd. & Chem. Box. 28, (5), 369-373.

(4). Hitchhike et al. 1992. FD.& Chem. Box. 28, (6), 443-447.

(5). H. Roomette et al. 1974. Munch. Med. Waster. 116, 537.

(6). S. Hitchhike, St.Mary's Hospital, London. New Scientist, Jan 1994, p.24-27.

(7). Meyer & Meyer 1959, Arzneimittel-forsh 9,516.

(8). E.J.Lee et al. Arch. Dis. in Childhood 1993,68: 27-28.

(9). K. Kovar et al. 1987. Planta Medica 53, 315-318.

(10). H. Roemmelt, H. Drexel and K. Dirnagl Die Heilkunst, Vol 91, no. 5, 1978.