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International Journal of Neuroscience
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Aromatherapy Facts and Fictions: A Scientific Analysis of Olfactory Effects on
Mood, Physiology and Behavior
Rachel S. Herz a
a Department of Psychiatry and Human Behavior, Warren Alpert Medical School, Brown University,
Providence, Rhode Island, USA
Online Publication Date: 01 February 2009
To cite this Article Herz, Rachel S.(2009)'Aromatherapy Facts and Fictions: A Scientific Analysis of Olfactory Effects on Mood,
Physiology and Behavior',International Journal of Neuroscience,119:2,263 — 290
To link to this Article: DOI: 10.1080/00207450802333953
URL: http://dx.doi.org/10.1080/00207450802333953
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International Journal of Neuroscience
, 119:263–290, 2009
Copyright C
2009 Informa Healthcare USA, Inc.
ISSN: 0020-7454 / 1543-5245 online
DOI: 10.1080/00207450802333953
AROMATHERAPY FACTS AND FICTIONS: A
SCIENTIFIC ANALYSIS OF OLFACTORY EFFECTS
ON MOOD, PHYSIOLOGY AND BEHAVIOR
RACHEL S. HERZ
Department of Psychiatry and Human Behavior
Warren Alpert Medical School
Brown University
Providence, Rhode Island, USA
A systematic review of scientific experimentation addressing olfactory effects on
mood, physiology and behavior was undertaken. From this review, 18 studies
meeting stringent empirical criteria were then analyzed in detail and it was
found that credible evidence that odors can affect mood, physiology and behavior
exists. To explain these effects, pharmacological and psychological mechanisms
were explored and a psychological interpretation of the data was found to be
more comprehensive. Methodological problems regarding dependent measures
and stimuli, which led to inconsistencies in the data were discussed, as were the
mediating variables of culture, experience, sex differences, and personality.
Keywords Behavior, mood, odor, pharmacology, physiology, psychology
INTRODUCTION
The aim of this article is to review and analyze the current literature on
aromachology, the scientifically documented study of olfactory effects in
humans, and thus to determine whether and how the inhalation of aromatic
chemicals can influence mood, physiology and behavior. The methods,
Received 10 August 2007.
The research review conducted for this paper was supported by Firmenich, SA.
Address correspondence to Rachel S. Herz, Ph.D., 89 Waterman Street, Brown University,
Providence, RI 02912, USA. E-mail: Rachel Herz@Brown.edu
263
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264 R. S. HERZ
mechanisms and theories that produce and explain these effects, as well as
cultural, individual and olfactory factors that may influence the outcomes of
various manipulations will be discussed.
To begin with, the term “aromatherapy” will be defined and contrasted
to “aromachology.” Aromatherapy is the use of essential oils extracted from
plants for the treatment of physical and psychological health. The concept
of aromatherapy is ancient. Aromatic plant-based compounds were used by
the Chinese in the form of incense, by the Egyptians in embalming the
dead, and by the Romans in their baths. The term “aromatherapy” (originally
aromatherapie) was coined by the French chemist Rene-Maurice Gattefosse,
in the late 1920s, who began the exploration of essential oils for their
healing powers after an explosion in his laboratory left his hand badly
burned and the accidental soaking of his injury in pure lavender oil produced
a rapid and miraculous healing. Contemporary aromatherapy proposes that
various plant-based aromas have the ability to influence mood, behavior, and
“wellness.”
It is true that many plants have therapeutic properties. For example, the
use of acetyl salicylic acid, the active ingredient in aspirin, came from the
discovery that chewing willow bark alleviated pain, inflammation, and fever.
However, aromatherapy is based not on the ingestion of various plant-based
products, but rather on inhaling their aromas, which are claimed to have a
range of therapeutic and psychological properties. Here are some examples
of the purported effects for several commonly “prescribed” essential oils:
Sandalwood aroma is sedating and relaxing and is beneficial for treating
anxiety, depression, and insomnia. Rosemary clears the mind and stimulates
memory. Lavender is uplifting, soothing and helpful for reducing stress, anxiety,
depression, and insomnia. Sweet marjoram is calming and sedating and helpful
in relieving a variety of negative emotional states, including anxiety, irritability
and loneliness. Clary sage is uplifting and relaxing as well as helpful in relieving
depression, anxiety, fatigue and calming irritable children. Indeed authors
of aromatherapy textbooks make a number of extraordinary and sometimes
contradictory assertions in the absence of systemically collected data for a
range of cures. For example, juniper oil is claimed to have 17 different properties
ranging from aphrodisiac to sedative (Price, 1991).
Aromatherapy today is usually practiced in conjunction with massage,
and one of the major claims of aromatherapy is a reduction in anxiety. There
is considerable evidence from randomized trials that massage alone reduces
anxiety (Field et al., 1992). If massage is effective, then aromatherapy plus
massage, unless essential oils are harmful (which seems unlikely), is also
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AROMATHERAPY FACTS AND FICTIONS 265
effective. So “aromatherapy works, even if it doesn’t”, as several reviews of
the literature have noted, at least for the reduction of anxiety (Cooke & Ernst,
2000; Vickers, 2000).
In contrast to aromatherapy which is not scientifically supported, aro-
machology, a term that was coined by the Sense of Smell Institute in 1982,
refers to the scientific analysis of olfactory effects on mood, physiology and
behavior. Aromachology research must meet the following empirical criteria:
(1) theory guided goals and clear hypothesis testing, (2) fragrances are tested
using appropriate experimental methodology, (3) sufficient and representative
subject populations and appropriate contrasting control groups are used, (4) data
are analyzed using suitable statistical methods, and (5) the results have been
vetted by scientific peers and accepted for publication in reputable journals.
Note that there are numerous “research” reports that claim to demonstrate the
effects of various aromatic compounds on mood, mental state, and behavior,
however, most of these studies are vexed with problems that prevent them from
being scientifically meaningful.
LITERATURE REVIEW: METHODS AND RESULTS
To determine the current state of aromachology research, the literature on
the psychological and physiological effects of “essential oil,” “fragrance,”
“aroma,” “scent,” and “odor or odorant” was searched using Pubmed
(www.pubmed.gov) databases, a service of the National Library of Medicine
and the National Institutes of Health. The results of this search yielded several
hundred articles. The abstracts of these articles were read and sorted according
to whether they were: (1) discountable aromatherapy studies or (2) possibly
viable aromachology research. Discountable aromatherapy studies were those
not meeting three or more of the five criteria described above. Other reasons
for exclusion included: studies of nonspecific odor effects or nonspecific odors,
studies where the effects of odors were not pertinent to mood or mental state,
studies involving animal subjects, studies that examined the effects of odors
on psychiatric or medical conditions or involved clinical trials, and studies
that were more than 10-years old. The articles that passed this first level of
examination were then reviewed in detail and from this analysis 18 studies were
chosen as representative of valid aromachology experimentation, and/or which
addressed important conceptual or methodological issues in aromachology
research.
Table 1 provides a synopsis of the aromachology studies that were reviewed
in detail for this article.
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Tab l e 1 . Aromachology studies reviewed
Authors Date Title and journal Odor(s) tested and dependent measures Effects/findings
Diego et al. 1998 International Journal
of Neuroscience
Essential oil of Lavender and Rosemary.
Mood arithmetic computation, EEG.
Pre vs post “aromatherapy” comparison of mood,
math computation and EEG. Contrary to
predictions, both odors increased speed in math
computations, increased relaxation, and
decreased anxiety. However, EEG changes
were interpreted as consistent with the idea that
lavender is sedating and rosemary is alerting.
No control group. Sex differences were not
assessed.
Lehrner et al. 2000 Physiology &
Behavior
Essential oil of Orange (citrus sinensis).
Subjective emotional states.
Women undergoing a stressful procedure while
exposed to ambient orange oil experienced
decreased state anxiety, improved mood, and
increased calmness. Presence of orange oil had
no effect on the well-being of male participants
in this situation.
Heuberger et al. 2001 Chemical Senses (+)limonene, (−) limonene, (+) carvone,
(−) carvone.
Physiological measures, subjective states,
odor hedonic ratings.
Inhalation of (+) limonene led to increased
systolic BP, subjective alertness and
restlessness.
(−) limonene increased systolic BP. (+)
carvone increased systolic and diastolic BP.
(−) carvone increased diastolic BP, pulse and
restlessness. Effects were attributable to odor
hedonic perception. Sex differences were not
assessed.
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Ilmberger et al. 2001 Chemical Senses Peppermint, jasmine, ylang-ylang,
1,8-cineole [100 µl], 1,8-cineole
[10 µl], menthol.
Reaction time and motor time.
Subjective odor ratings.
Alertness as a function of odor presence
was measured by a reaction time task.
Subjects exposed to any odor, except
cineole 100 µl, did not differ in reaction
time from control groups inhaling water
(no- odor). Subjects exposed to cineole
100 µl had faster reaction times than
control subjects. Subjective
ratings (pleas, intensity, stimulation, relax-
ation) were obtained for all stimuli and
any/all odor effects were seen to be psycho-
logical. Sex differences were not assessed.
Raudenbush et al. 2001 Journal of Sport & Exercise
Psychology
Synthetic peppermint oil.
Physical performance.
Both male and female athletic college
students exposed to peppermint oil in a
patch on their upper lip experienced
enhanced physical performance,
specifically generating more push-ups
and running faster (400m dash),
compared to when they were exposed to
an un-scented patch. No sex differences.
Motomura et al. 2001 Perceptual and Motor Skills Lavender essential oil
HR, BP Subjective mood and arousal.
Lavender decreased negative mood and
arousal during a 20 min stressful
situation compared to the stressor
without lavender. No effects on
physiological measures. Sex differences
were not assessed.
(Continued on next page)
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Tab l e 1 . Aromachology studies reviewed
Authors Date Title and journal Odor(s) tested and dependent measures Effects/findings
Raudenbush et al. 2002 International Sports
Journal
Synthetic peppermint oil, jasmine oil,
dimethyl sulfide (3ppm in water).
Physiological measures, subjective
athletic performance.
During a 15 treadmill exercise, exposure to any odor
had no effect on physiological parameters (BP,
pulse, oxygen saturation). However, peppermint
odor significantly reduced perceived workload and
effort and increased self-evaluated physical
performance and energy among athletic college
students. No sex differences.
Bensafi et al. 2002 Chemical Senses Isovaleric acid, thiophenol, pyridine,
menthol, isoamyl acetate, 1–8
cineole.
Physiological measures,
ANS responses (e.g., heart-rate, skin conductance)
were correlated with subjective odor ratings. Odor
pleasantness predicted HR changes. The more
pleasant the odor was rated, the lower the subject’s
HR. Odor arousal correlated with skin
Subjective odor ratings. conductance. Interaction between trigeminal and ol-
factory odor factors suggested. Sex differences were
not assessed.
Moss et al. 2003 International Journal
of Neuroscience
Essential oil of Lavender and
Rosemary.
Objective cognition measures,
Subjective mood.
Lavender decreased working memory and impaired
reaction time on memory and attention tasks
compared to control (no odor). Rosemary
enhanced performance for memory and secondary
memory factors, but also impaired reaction time
compared to control. After testing, the rosemary
group was more alert than the lavender or control
group and both aroma groups were more content
than the control group. Subjects did not evaluate
odor hedonics. Sex differences were not assessed.
268
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Villemeure et al. 2003 Pai n Self-selected pleasant and unpleasant
synthetic odors.
Subjective measures of mood and
pain.
Subjects were exposed to painful heat
under various odor conditions.
Self-selected pleasant odors improved
mood and decreased anxiety and pain
unpleasantness, while an unpleasant odor
worsened mood, anxiety and pain
unpleasantness. Odor condition did not
affect the perception of pain intensity. No
sex differences.
Campenni et al. 2004 Psychological Reports Synthetic lavender, neroli, placebo
(no-odor).
Subjective mood, Physiological
measures
The suggestion that an odor would be
“stimulating” increased HR and skin
conductance. The suggestion that an
odor would be “relaxing” decreased HR
and skin conductance. Effects were seen
regardless of the odor, including the
no-odor condition. No self-report mood
effects were seen as a function of
odor/suggestion condition. However, over
time in the experiment mood improved for
all subjects. Female subjects only.
Heuberger et al. 2004 Neuropsych-
opharmacology
(−) Linalool via transdermal
absorption. Inhalation of (−)
linalool was prevented by a face
mask.
Physiological measures, subjective
measures of mood/mind.
Trandsdermally absorbed (−) linalool
induced mild de-activating effects on BP
and skin temperature, but did not have
any effects on subjective measures of
well-being or mood. Sex differences
were not assessed.
(Continued on next page)
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Tab l e 1 . Aromachology studies reviewed (Continued)
Authors Date Title and journal Odor(s) tested and dependent measures Effects/findings
Burnett et al. 2004 Psychological Reports Essential oils of rosemary and lavender
Physiological measures, subjective mood
No effects on physiological measures, however some
subjective mood changes were observed; e.g.,
lavender led to higher vigor/activity than rosemary.
Rosemary increased anxiety. Sex differences were not
assessed.
Lehrner et al. 2005 Physiology & Behavior Essential oil of Orange (citrus sinensis)
and lavender.
Subjective emotional states.
Both men and women undergoing a stressful procedure
experienced decreased state anxiety, more positive
mood and greater calmness when exposed to either
orange or lavender oil in ambient air compared to
no-odor or music.
Ho & Spence 2005 Neuroscience Letters Synthetic peppermint oil.
Cognitive tasks.
Peppermint aroma improved performance on a difficult
but not on an easy cognitive task. Female subjects
only.
Kuroda et al. 2005 European Journal of
Applied Physiology
Jasmine tea lavender essential oil (R)-(−)
linalool (S)- (+) linalool
Jasmine tea, lavender and (R)-(−) linalool decreased
HR, increased positive mood and decreased negative
mood. (S)- (+) linalool increased negative mood and
Physiological measures, Subjective mood. decreased positive mood. No sex differences.
Goel et al. 2005 Chronobiology
International
Lavender oil sleep physiology measured
by polysomnograpy, subjective
sleepiness and mood ratings.
Lavender increased slow-wave-sleep and subjects
reported higher vigor upon awakening compared to
control. No effects on rated sleepiness. Sex
differences on some sleep measures.
Goel & Lao 2006 Biological Psychology Peppermint oil sleep physiology measured
by polysomnograpy, subjective
sleepiness, mood and odor ratings
No main effects on physiological sleep, but individual
perception of peppermint (e.g., stimulating, intensity)
correlated with polysomnography measures.
Peppermint reduced fatigue and depression for all
subjects. Sex differences on some sleep measures.
Studies are listed chronologically. ANS =autonomic nervous system; HR =heart rate; BP =blood pressure.
270
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AROMATHERAPY FACTS AND FICTIONS 271
All experiments used subjective and/or behavioral and/or physiological
measures to obtain quantitative data on the effect of odors on mood, mental
states, physiology and behavior. In addition, for discussion and comparison,
one study was included where odor was delivered via transdermal absorption
rather than inhalation (Heuberger, Redhammer, & Buchbauer, 2004).
A careful review of these studies revealed reliable empirical evidence that
various inhaled aromas can significantly affect mood, cognition, physiology
and behavior/performance. However, inconsistencies were frequently observed
both within studies and across laboratories for various dependent measures and
specific odors. The remainder of this paper attempts to address the questions
of: (1) how and why odors produce the observed effects, (2) how methodology
influences outcomes, and (3) what the relevant mediating factors are.
THEORIES AND MECHANISMS
The most important question for the field of aromachology is how and why
odors produce the effects that have been observed on various aspects of mood,
behavior and physiology. Two primary mechanisms for the psychodynamic
and physiological effects of odors have been proposed: (1) a pharmacological
hypothesis, and (2) a psychological hypothesis.
Pharmacological Hypothesis
The pharmacological hypothesis proposes that the effects of various aromas on
mood, physiology and behavior are due to the odor’s direct and intrinsic ability
to interact and affect the autonomic nervous system/central nervous system
and/or endocrine systems. In support of the proposition that odors could behave
pharmacologically, lavender has been shown to act postsynactpically where
it is suggested to modulate the activity of cyclic andenosine monophosphate
(cAMP) (Lis-Balchin & Hart, 1999). A reduction in cAMP activity is associated
with sedation. Linalool, a principal component of lavender, has also been found
to inhibit glutatmate binding, which may have sedative effects (Elisabetsky,
Marschener, & Souza, 1995). It is therefore possible that lavender produces
sedative effects via these neuropharmacological mechanisms.
In order for a volatile compound to act pharmacologically it must enter the
bloodstream by way of the nasal or lung mucosa, or diffuse directly into the
olfactory nerves and the limbic system of the brain. Although the level of active
compounds that can be absorbed by these routes is low compared to more typical
modes of administration, such as ingestion or injection, aromatic compounds
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272 R. S. HERZ
have been detected in the bloodstream of rodents exposed to the vapors
of essential oils (Jirovitz, Buchbauer, Jager, Raverdino, & Nikiforov, 1990;
Jirovitz, Buchbauer, Jager, Woiiieh, & Nikiforov, 1992; Kovar, Gropper, Friess,
& Ammon, 1987). It has also been shown in rats that after surgically reducing
olfactory function, inhalation of cedrol, a major component of cedarwood
oil, marked sedative effects were still seen, suggesting that the mechanism
of action is via a pathway other than the olfactory system (Kagawa, Jokura,
Ochiai, Tokimisu, & Tsubone, 2003).
The pharmacological hypothesis has some compelling components,
however, there are a number of problems as applied to human olfactory
responses. For one, there is no data in humans suggesting that inhaled volatiles
become present in bloodstream or any other physiological system. This may
be because the concentration of odors that one is normally exposed to is much
lower than would be required for their presence to be detected in bloodstream.
Nevertheless, this means that in the normal context of “aromatherapy,” or odor
experimental manipulations, odor levels do not reach the appropriate doses to
be pharmacologically active/detectable. Extrapolating data from rodents and
expecting similar outcomes in humans is confounded by the relative body-size
to chemical concentration ratio between rats and humans, the rat’s much
greater olfactory sensitivity and number of functioning olfactory receptors
as well as experimental conditions that are neither practical nor ethical for
humans. Moreover, the mode of chemical administration in animal studies
where pharmacological effects have been demonstrated has involved direct
physiological infusion, not inhalation (e.g., Elisabetsky et al., 1995; Lis-Balchin
& Hart, 1999).
Another issue is that the emotional and behavioral effects of aromatic
compounds that are reported in the studies reviewed here indicate that responses
to odors are immediate. If the mechanism of action is via bloodstream then the
effects would require at least 20 min to be seen, as this is the time necessary
for chemically active substances to be circulated through blood and then cross
the blood-brain barrier.
One study that allows evaluation of this point in humans was conducted
by Heuberger and colleagues at the University of Vienna who investigated the
effects of (−) linalool a primary chemical in lavender scent, via transdermal
absorption (Heuberger et al., 2004). Although the presence of volatiles in
internal systems was not measured in this study, when (−) linalool in peanut oil,
as compared to peanut oil alone, was applied to the ventral skin of participants,
mild central nervous system effects were seen. Inhalation of the odorant was
prevented by having participants wear a surgical facemask throughout the
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AROMATHERAPY FACTS AND FICTIONS 273
procedures. Subjective mood and mental measurements were taken when (−)
linalool or pure peanut oil was first applied to the participant’s abdominal skin
surface and then at the end of a 20-min trial. Physiological ratings were acquired
throughout the 20-min trial. No subjective emotional, mental, or mood changes
were observed at any time. However, after application of (−) linalool systolic
blood pressure was significantly reduced, which may be due to its deactivating
effect on the autonomic nervous system (ANS). However, this effect was not
seen until the end of the trial. That is, 20 min postapplication, consistent with
the time required for physiological action.
Transdermal absorption of linalool is comparable to the experience of
aromatherapeutic massage. Thus, through skin massage and hence absorption,
as opposed to inhalation, an aromatic oil may have the possibility of producing
deactivating central nervous system effects. It was noted earlier that massage
alone has been validated to be physically and emotionally relaxing (Cooke
& Ernst 2000). Therefore, massage coupled with transdermal absorption
of certain compounds may have some real physiological and psychological
benefits.
Proponents of the pharmacological hypothesis also suggest that odorants
could exert their effects through direct, immediate, nonconscious interactions
with neural substrates. The most critical issue for the pharmacological
hypothesis in this respect is receptor-ligand binding. That is, the specific
chemical structure of the aromatic compound is critical to the effects
produced, and a structure-function relationship must exist between odorant
and response. In order to test the structure-function relationship aromatic
compounds with different structural properties need to be directly compared, as
do molecules that smell identical but which have different chemical/structural
composition.
The critical test of whether molecules that smell the same but are
chemically different produce the same effects (or not) has not been directly
tested. However, as will be further discussed, the fact that many different
variants of the same “odors” have been used across laboratories with similar
results suggests that it is the psychological perception of the odorant not
the chemical structure of the molecule that is important. The structure-
function issue has, however, been examined by testing chemically identical
molecules with minor differences in molecular orientation. Chirality refers
to the fact that subtly different aspects of a molecule’s orientation (e.g.,
mirror images) exist. If odor molecules act pharmacologically then odor
chirality will influence the outcomes because the different orientations of
chiral molecules means that they will bind differently to the system’s receptors.
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274 R. S. HERZ
Olfactory receptors are especially geared towards shape-fit interactions, which
is why chiral forms of odor molecules smell different from one another.
For example, (+) carvone smells like caraway and (−) carvone smells like
spearmint.
Heuberger and colleagues (Heuberger, Hongratanaworakit, Bohm, Weber,
& Buchbauer, 2001) examined the structure-function issue by testing how
odor chirality for limonene and carvone influenced autonomic and self-
evaluation measures of arousal and mood in healthy young adults. Autonomic
measures were: skin temperature, pulse rate, breathing rate, and blood pressure.
Subjective mental and emotional states were: mood (cheerful-bad tempered),
calmness (calm-restless), and alertness (alert-tired). The participants also rated
the two chiral forms (enantiomers) for pleasantness (pleasant-unpleasant),
intensity (weak-strong), and stimulation (stimulating-tiring) on visual analog
scales.
The physiological results showed that (+) limonene led to increased
systolic blood pressure, subjective alertness, and restlessness, while (−)
limonene caused increases in pulse rate, diastolic blood pressure, and
restlessness. Note that the hedonic qualities of (+) and (−) limonene are rather
different; (+) limonene is characterized as a fresh citrus orange note, and
(−) limonene as a harsh turpentine lemon note. Inhalation of (+) carvone
(caraway scent) led to increases in systolic and diastolic blood pressure but
had no effects on any of the emotional/mental measures. Inhalation of (−)
carvone (spearmint scent) led to increases in pulse rate, diastolic blood pressure,
and subjective restlessness. In terms of mental or emotional effects, none of
the enantiomers led to distinctive effects on mood or mental states, though
individuals responded differently to the various odorants on the hedonic
rating scales. Most importantly, correlation analyses showed that both the
psychological and physiological effects produced by the odorants could be
explained simply by the subjective hedonic evaluations that each odorant
received. For example, in the (+) limonene condition, subjective ratings of
the fragrance’s pleasantness were positively correlated with how alert it made
the participants feel, and how much blood oxygen saturation increased during
the trial. Intensity ratings to (+) limonene were positively correlated with
skin conductance and systolic blood pressure changes (Heuberger et al., 2001).
Note too that because enantiomers smell different, perceptual-psychological
and structural-chemical factors are confounded, and thus a pure test of
structures-function relationships is not achieved.
To this end, Kuroda and colleagues (Kuroda et al., 2005) examined
the enantiomeric components of lavender essential oil, (−) linalool and (+)
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AROMATHERAPY FACTS AND FICTIONS 275
linalool, and also found different results; (+) linalool is perceived as a sweet
floral and (−) linalool is a woody lavender note. Jasmine tea, lavender essential
oil, and (−) linalool (a component of both jasmine tea and lavender oil)
decreased heart-rate and improved subjective mood, whereas (+) linalool was
found to increase heart-rate and negative mood. Notably, the authors stated that
the concentrations of (+) linalool and (−) linalool tested were below detection
threshold, and therefore subjects could not smell the difference between them,
though subjects could smell the odors of jasmine tea and lavender oil.
The Kuroda et al. (2005) results are the most compelling for a structure-
function relationship and hence the pharmacological hypothesis because
subjects could not smell the different enantiomers of linalool yet physiological
and psychological differences were observed. However, in all other studies
reviewed the scent of the compounds was perceived and variations of natural
and synthetic compounds were used. In addition to confounding the question
of structure-function relationships, the issue of chemical inconsistencies
across laboratories may also explain why inconsistencies for particular odors
were observed across laboratories. For example, although lavender has been
generally shown to have relaxing and positive effects on mood, physiological
and psychological indices have not been consistent. Goel, Kim, and Lau (2005)
found that lavender produced some sleep enhancing physiological effects but
had no concomitant mood effects. In contrast, Motomura, Sakurai, and Yotsuya
(2001) found that lavender positively influenced mood but had no physiological
influences.
The use of certain experimental techniques such as electroencephelograms
(EEG) has been interpreted by some (e.g., Diego et al., 1998) as indicating
direct neurological responding and hence pharmacological effects. However,
this measure is also influenced by psychological factors. Contingent negative
variation (CNV) is a central metric used in EEG research. Changes in the
participants’ CNV magnitude after presentation of an odor as compared to a
blank indicate whether the odor has a stimulatory or sedative effect on the
brain. Decreases in CNV amplitude indicate a sedative effect and increases in
CNV amplitude indicate a stimulatory effect. Torii and colleagues (Torii et al.,
1988) examined odor-induced alternation in CNV as a measure of aromatic
effects on mood and well-being. Twenty different essential oils were tested
by this procedure and most of them produced changes in CNV magnitude
that corresponded to their attributed aromatherapeutic effects. For example,
jasmine which is said to have stimulatory properties significantly increased
CNV magnitude, while lavender which is believed to be sedating led to a
decrease in CNV magnitude.
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276 R. S. HERZ
The value of CNV as an indicator of the stimulatory or sedative properties
of essential oils has, however, been seriously questioned by Lorig and Roberts
(1990) who replicated the study of Torii and colleagues but introduced an
additional odor condition. As in the experiments of Torii et al. (1988), changes
in CNV after inhalation of jasmine and lavender were measured. Additionally,
responses to a mixture of the two fragrances were recorded. Subjects were led
to believe that they would be exposed to high or low concentrations of the
odors. In the high concentration condition, pure fragrances were administered,
while in the low concentration condition, the mixture was used. For the high
concentration conditions changes in CNV replicated the findings of Torii et al.
(1988), however, in the low concentration condition changes in CNV reflected
the subjects expectation of odor effects rather than the chemicals that were
present; indicating that beliefs and expectation about an odor can crucially
influence changes in CNV independent of the odor that is present.
Psychological Hypothesis
The psychological hypothesis proposes that odors exert their effects through
emotional learning, conscious perception, and belief/expectation. The central
claim of the psychological hypothesis is that responses to odors are learned
through association with emotional experiences, and that odors consequently
take on the properties of the associated emotions and exert the concordant
emotional, cognitive, behavioral, and physiological effects themselves (Herz,
2001, 2004a). Evidence to support the associative learning hypothesis for
olfactory emotional and behavioral effects comes from a multitude of studies
and has been reviewed in detail elsewhere (see Herz, Beland, & Hellerstein,
2004a; Mennella & Beauchamp, 2005). It has also been shown that the
emotions elicited by pleasant and unpleasant odors affect physiological
correlates of emotion as expected (Alaoui-Ismaili, Robin, Rada, Dittmar, &
Vernet-Maury, 1997; Ehrlichman, Kuhl, Zhu, & Warrenburg, 1997; Miltner,
Matjak, Braun, Diekmann, & Brody, 1994). For example, an odor that triggered
anxiety elicited electrodermal changes that were consistent with fear but
only among participants who had a fearful association to the specific smell
(Robin, Alaoui-Ismaili, Dittmar, & Vernet-Mauri, 1998). Physiological effects
produced by odors are therefore simply the physiological sequelae of the
psychological-emotional responses elicited by the odor, and are expected due
to mind-body interactions.
That odors are highly associable and emotionally evocative is consistent
with the neuroanatomy of the limbic system. Olfactory efferents have a
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AROMATHERAPY FACTS AND FICTIONS 277
uniquely direct connection with the neural substrates of emotional and memory
processing (Cahill, Babinsky, Markowitsch, & McGaugh, 1995). Only two
synapses separate the olfactory nerve from the amygdala, a structure critical
for the expression and experience of emotion and human emotional memory;
and only three synapses separate the olfactory nerve from the hippocampus,
involved in the selection and transmission of information in working memory,
short-term and long-term memory transfer, and in various declarative memory
functions (Eichenbaum, 2001). Classical conditioning of specific cues to
emotion is also mediated by the amygdala (Otto, Cousens, & Herzog, 2000).
Moreover, olfactory processing is primarily localized to the orbitofrontal cortex
and the orbitofrontal cortex and amygdala have been shown to play a major
role in stimulus reinforcement association learning.
A large body of literature indicates that mood can influence behavior. In
general, positive mood is linked to an increase in productivity and the tendency
to help others (Clark, 1991; Isen, 1984; Wright & Staw, 1999); while negative
mood reduces prosocial behavior (Underwood, Froming, & Moore, 1997).
Notably, prosocial behavior and productivity are also enhanced in the presence
of positive ambient odors. For example, people exposed to pleasant ambient
odors in a shopping mall (baking cookies, roasting coffee) were more inclined to
help a stranger than people not exposed to an odor manipulation (Baron, 1997).
Baron (1990) also found that participants who worked in the presence of a
pleasant ambient odor reported higher self-efficacy, set higher goals and were
more likely to employ efficient work strategies than participants who worked
in a no-odor condition. Conversely, Rotton (1983) found that the presence
of a malodor reduced participants’ subjective judgments and lowered their
tolerance for frustration. Participants in these studies also reported concordant
mood changes.
The observed behavioral responses in the presence of pleasant and
unpleasant odors are due to the effect that various ambient odors have on
an individual’s mood. In the research reviewed here, participants who did not
show the expected hedonic response did not experience the mood or behavioral
change (e.g., Burnett, Solterbeck, & Strapp, 2004). Personal liking or disliking
of an odor is directly related to the mood change that occurs. For example,
Villemeure and colleagues (Villemure, Slotnick, & Bushnell, 2003) found that
only odors that participants self-selected as pleasant were able to improve mood,
decrease anxiety and pain unpleasantness, whereas a disliked odor worsened
mood and the emotional effects of pain.
Several studies in my laboratory have directly shown how an odor that
has been associated to a specific emotional state can later influence behavior
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278 R. S. HERZ
in an emotionally consistent manner. Both children and adults who were
exposed to a novel odor while engaged in a frustrating experience later
showed less motivation to complete an unrelated task when re-exposed to
that odor (Epple & Herz, 1999; Herz, Schankler, & Beland, 2004b). These data
indicate that emotional experiences associated to odors can in turn influence
behavior in a mood congruent way. In sum, through associative learning
odors can elicit emotional, behavioral and physiological responses that are
equivalent to the responses that an emotionally involving event itself would
produce.
Other psychological factors, such as belief and expectation, have significant
influences on aromatherapeutic outcomes as well. To illustrate the power
of belief and expectation in the creation of aromatic effects, Campenni and
colleagues (Campenni, Crawley, & Meier, 2004) conducted a very clever
study. Lavender, neroli and placebo (no-odor) and the “suggested” effects of
these odors (relaxing, stimulating, none) were examined in 90 female college
students. Mood was assessed by the physiological measurements of heart rate
(HR) and skin conductance and by a self-report mood questionnaire. Results
revealed that regardless of what odor was present, or even whether an odor was
there, the suggestion that an ambient odor was “relaxing” decreased HR and
skin conductance, and the suggestion that an ambient odor was “stimulating”
increased HR and skin conductance. Notably, no self-report changes in mood
were seen as a function of odor presence. However over the one-hour course
of the experiment, all subjects rated their mood as having improved. It should
be underscored that the placebo no-odor condition was able to produce the
predicted physiological changes associated with relaxation or stimulation
simply by “suggestion,” and that lavender which is a culturally denoted
“relaxing” odor was able to elicit stimulatory effects when so designated.
Knasko (1995) also found that the suggestion of a pleasant odor in a room
(when none was there) had positive effects on mood. These findings illustrate
that the chemical nature of the odorant itself plays a secondary role in the
emotional and subjective changes that occur in the presence of an odor, and
that it is the meaning of the aroma that induces the consequent psychological
and/or physiological responses.
Associative learning, perceptual experience, and expectation can ac-
count for the emotional, behavioral, and physiological effects pro-
duced by odor inhalation that are reported in Table 1. Thus the
psychological hypothesis is currently the best explanatory model for
how odors produce emotional, cognitive, behavioral, and physiological
responses.
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AROMATHERAPY FACTS AND FICTIONS 279
ANALYSIS OF AROMACHOLOGY METHODOLOGY
This section reviews the main methodological issues that need to be considered
for the future of aromachology research and discusses problems with the data
reviewed here. Although all the studies selected for review met a basic level of
scientific legitimacy, there are still methodological problems endemic to this
area of research, particularly if one wants to compare results across different
laboratories. Statistical issues are also a major factor that must be considered
when assessing the validity of aromachology data; the central problem in this
area being statistical power (1-ß); the probability of correctly rejecting Ho,
the null hypothesis. Cooke and Ernst (2000) analyzed the extant aromatherapy
literature and focused on the problem of statistical power in detail, as such the
present paper does not do so and the reader is referred to Cooke and Ernst
(2000) for further review.
Dependent Measures
The most common dependent measures obtained in the studies reviewed were
those involving subjective self-report of mood and mental state. Physiological
recordings of autonomic nervous system (e.g., heart rate) or central nervous
system (e.g., skin conductance) reactivity were often recorded in addition to, or
independently of, subjective emotional states. Less frequently, actual physical,
or cognitive performance was behaviorally measured. Rarely was EEG used
and neuroimaging has not yet been employed.
The most meaningful data are those where both physiological and
psychological measures are taken and correlated with one another. However
this was rarely observed in the studies reviewed. For example, Raudenbush,
Corley, and Eppich (2001) found that athletic young adults ran faster and
did more push-ups in the presence of peppermint aroma, and in a similar
study found that peppermint aroma enhanced self-evaluations of vigor and
perceived performance on a tread-mill running task (Raudenbush, Meyer, &
Eppich, 2002). Regrettably subjective measurements of energy and strength
were not obtained in Raudenbush et al. (2001) and in Raudenbush et al. (2002)
subjective changes in response to odor exposure occurred but no physiological
changes were observed. Motomura et al. (2001) found emotional changes in
the presence of lavender odor but no physiological effects, whereas Campenni
et al. (2004) observed physiological changes in the presence of lavender and
neroli odors but no concomitant changes in reported mood. Diego et al. (1998)
found effects on EEG and mood in the presence of lavender and rosemary
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280 R. S. HERZ
but they were inconsistent. Both lavender and rosemary increased subjective
relaxation among subjects, but EEG evaluation were interpreted as rosemary
being stimulating. Kuroda et al. (2005) and Goel et al. (2005) are the only
studies reviewed here where both objective and subjective measures of aromatic
chemicals were found to correlate as expected.
Physiological recordings are useful for demonstrating changes in physio-
logical reactivity in the presence of odors, but they are only meaningful once
they have been correlated with subjective evaluations. There are many different
emotional situations that can lead to very similar nervous system changes
and only by asking someone what they are feeling do you know whether
the person showing heightened heart-rate is ecstatic, anxious, or angry.The
lack of correlation between physiological responses and subjective responses
makes the meaningfulness of the physiological recordings alone questionable.
Subjective measures alone have more validity because behavioral changes in
response to aromas have been shown to be dependent upon subjectively altered
mood (Herz et al., 2004b), and unless one has reasons to suspect that participants
are lying, subjective reports are a valid measure of internal state (Vincent &
Furedy, 1992).
Odorant Characteristics
Aromatherapy makes much of the superior properties of natural versus synthetic
chemicals, yet no studies compared natural and synthetic versions of odors to
address this claim. Moreover, the studies reviewed here varied widely with
respect to whether synthetic or natural chemicals were used and where the
odorants were derived from. The lack of consistency with respect to chemical
stimuli makes designating effects to specific odorants less achievable.
The perceived hedonic value of the odors tested was, however, found
to be important. Burnett et al. (2004) specifically controlled for subjective
differences in pleasantness evaluations to the aromatic conditions in their
study; lavender, rosemary, and water, without which they would not have
seen any effects. Notably, while the majority of participants in the lavender
and rosemary groups rated their scent as pleasant, 20% of subjects in the
water condition also rated water as somewhat or very pleasant. Bensafi, Rouby,
Farget, Bertrand, Vigouroux, and Holley (2002) showed that odor pleasantness
predicted HR changes. The more pleasant the odor was rated, the lower the
subject’s HR. Indeed, odors need to be liked in order to produce positive
effects on mental states. Villemeure et al. (2003) showed how the subjective
evaluation of an odor’s pleasantness was directly related to the emotional
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AROMATHERAPY FACTS AND FICTIONS 281
experience of pain. In their study, subjects experienced painful heat while
exposed to various odors. Odors that were self-selected by participants as
pleasant improved mood, decreased anxiety and pain unpleasantness, but an
odor that the participant disliked worsened mood and the emotional effects of
pain. Similarly, Heuberger et al. (2001) examined chemical chirality in relation
to subjective and physiological responses and found that odor hedonics could
explain most of the results. For example, the more pleasant (−) limonene was
judged to be the less calm participants felt, and the more stimulating it was
judged to be the more skin temperature decreased. Note, however, that the
effects are not predictable as one would assume that the more pleasant an odor
was the more calming it would be, and the more stimulating it was the more it
would increase ANS responses.
Other subjective aspects of odor perception were also shown to be relevant
to the effects that odors can produce. For example, Goel and Lau (2006) found
that half of their participants evaluated peppermint as a sedating aroma and
the effects of peppermint on sleep were different than among participants who
evaluated peppermint as stimulating. Notably, those who rated peppermint as
sedating took longer to achieve deep sleep, than those who rated it stimulating.
Moreover, participants who rated peppermint as very intense had more total
sleep time than those who rated it as only moderately intense. Importantly, the
authors note that peppermint odor had no effect on sleep physiology when these
subjective factors were not considered. Bensafi et al. (2002) found that the more
arousing an odor was rated the more skin conductance increased. Ilmberger et al.
(2001) who studied the effects of various aromas on reaction time, and included
water as a no-odor control, found that the subjective evaluations given to the
odors were largely responsible for their effects. A striking illustration of this
point is that the more stimulating water was judged to be, the faster participants’
reaction times were on a task that measured alertness. How participants interpret
the terms “intense,” “arousing,” and “stimulating” and whether there are any
real differences between them is, however, a further concern for this area of
research, particularly when comparing data across laboratories.
In the studies reviewed here, there was no analysis of the trigeminal versus
non-trigeminal quality (cranial nerve 5, that gives the tactile quality to an
odor, e.g., cooling, burning) of the odors tested and how this may influence
outcomes. It is highly likely, particularly in the case of stimulating effects
such as peppermint, that trigeminal activation is involved. Raudenbush et al.
(2002), tested jasmine, peppermint, and dimethyl sulfide for physiological and
subjective effects and found that only peppermint enhanced vigor and feelings
of enhanced performance. Peppermint is a pleasant odor that also produces a
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282 R. S. HERZ
strong trigeminal cooling sensation. Jasmine, a pleasant but nontrigeminal odor,
produced effects that trended towards the positive consequences of peppermint
but did not reach it. Dimethyl sulfide, an unpleasant and nontrigeminal odor,
had neutral effects on subjective and objective measures of mood and energy.
In another study, Ilmberger et al. (2001) found that 100 µL cineole, the
aromatic chemical responsible for cinnamon scent and a warming trigeminal
stimulant, increased reaction time on a test task relevant to alertness, whereas
the other odorants in the study did not. Together these findings suggest
that the complementary effect of trigeminal activation with certain odors
(e.g., peppermint cooling, cinnamon warming) may enhance the associative
responses elicited by them, which then augments that odorant’s emotional and
behavioral output. At present this hypothesis is purely speculative. In order
to establish the role of trigeminal activation in the enhancement of various
emotional-behavioral effects a variety of odors and test tasks now need to be
assessed.
INDIVIDUAL DIFFERENCE FACTORS
Culture
It has already been discussed that odor perception is mediated by learned
associations and culture provides a substantial framework upon which this
learning takes place. A striking example is the difference between people in
Britain and North America for their hedonic responses to the odorant methyl
salicylate (wintergreen mint). A comparison of two studies illustrates this point.
In the mid-1960s in Britain, Moncrieff (1966) asked adult respondents to
provide hedonic ratings to a battery of common odors. A similar study was
conducted in the United States in the late 1970s (Cain & Johnson, 1978).
Included in both studies was the odorant methyl salicylate (wintergreen).
Notably, in the British study, wintergreen was given one of the lowest
pleasantness ratings, whereas, in the US study it was given the highest
pleasantness rating. The reason for this difference can be explained by history.
In Britain, the smell of wintergreen is associated with medicine and particularly
for the participants in the 1966 study, with analgesics that were popular during
WWII, a time that these individuals would not remember fondly. Conversely,
in the US, the smell of wintergreen is exclusively a candy mint smell and one
that has very positive connotations.
To this end, the predictive effects of an aroma are limited to the culture(s) in
which the acquired associations fit the aromatherapeutic response. Lehrner and
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AROMATHERAPY FACTS AND FICTIONS 283
colleagues (Lehrner, Eckersberger, Walla, Potsch, & Deecke, 2000; Lehrner,
Marwinski, Lehr, Johren, & Deecke, 2005), who conducted their studies in
Vienna, found that orange and lavender essential oil produced significant
increases in positive mood among people undergoing a stressful situation.
Similarly, Kuroda et al. (2005) found that aroma of jasmine tea had relaxing
effects on Japanese participants. As a function of the cultural associations
that have been learned to orange and jasmine tea among European and
Asian populations, respectively, one cannot assume that these effects would
necessarily be seen cross-culturally. It has already been shown that there
are widely differing hedonic responses to “everyday” odors across cultures
(Ayabe-Kanamura et al., 1998). Further research investigating and defining
cultural differences in response to specific aromas is now needed.
Experience
Idiosyncratic experiences with odors are a primary cause for unexpected
aromatic reactions. Any given individual within a specific culture may not
have the predicted response to an odor due to their own personal associations.
For example, if one’s first association with the scent of rose were at a funeral,
one’s hedonic and emotional associative responses to that aroma would be
negative and the expected positive effects on mood and physiology would not
be observed (Herz, 2005). The issue of individualistic hedonic responses to
odors was addressed by Villemeure et al. (2003), and as discussed previously
only aromas that were self-selected as pleasant had positive effects. The findings
of Goel and Lau (2006) also underscore this issue, as subjective differences in
the perception of peppermint aroma led to physiological outcomes for sleep
only when individual differences were taken into consideration. Thus, if an
individual does not like the scent of lavender she will not find it relaxing,
regardless of how well and widely lavender aroma has been marketed as
“relaxing.”
The problem of individual differences is not restricted to psychological
effects. Idiosyncratic responses are also a serious mitigating factor for the
effectiveness of drugs. Up to 30% of the population, for example, does not
respond to the current pharmacopoeia of SSRI’s (selective serotonin reuptake
inhibitors) used to treat depression. To this end, it is possible that individuals
differ genetically in olfactory receptor expression and that this may also
have some influence on odor hedonic perception, particularly as it relates to
odor intensity. That is, individual differences in odor receptor expression may
predispose some individuals to be more or less sensitive to specific chemicals
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284 R. S. HERZ
and accordingly may affect susceptibility to learning particular emotional
associations to certain odors. When more is learned about how much and
what genetic variation in the expression of olfactory receptors exists within the
population or within specific ethnic or racial groups (Gilad & Lancet, 2003),
better determination of the role of genetics can be made.
Sex Differences
Women are more sensitive than men to odors at certain times during the
menstrual cycle (Doty, Snyder, Huggins, & Lowry, 1981), and this varying
sensitivity may modulate the effectiveness of aromas on physical and emotional
states. Women have also been shown to be more emotionally reactive to odors
(Chen & Dalton, 2005), and more susceptible to emotional conditioning with
odors (Bell, Miller, & Schwartz, 1992; Dalton, 1999; Kirk-Smith, Van Toller,
& Dodd, 1983). To this end, Lehrner et al. (2000) found that women, but not
men, experienced less anxiety and a calmer and more positive mood when they
were exposed to orange essential oil while waiting for an anxiety provoking
dental appointment.
In most of the studies reviewed in this report sex differences were not
considered, so an analysis of this issue is limited. However, in an extended
replication of Lehrner et al. (2000), Lehrner et al. (2005) tested both orange
and lavender essential oil in contrast to a music or no-odor condition and
found that both odors were able to improve mood and reduce anxiety equally
among men and women. It is unclear why this inconsistency in sex differences
occurred or what it means; the authors themselves do not offer any explanation.
Goel et al. (2005) and Goel and Lau (2006) also found differences in the
way that men and women’s sleep was affected by the scents of lavender and
peppermint, respectively. However, here too, the effects could not be predicted
across experiments. It is most likely that when sex differences are observed
they are due to an interaction between psychological and physiological factors.
At this time the basis for these differences has not been well elucidated.
The degree to which one is susceptible to the emotional connotation of an
odor and forming associations to it will modulate the effectiveness of an
aroma to influence mood, physiology and behavior. Further aromachology
studies now need to explicitly address whether different responses are elicited
by men and women, and if they occur, to what aromas and under what
conditions.
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AROMATHERAPY FACTS AND FICTIONS 285
Personality
There is evidence to suggest that personality can modulate the degree to which
odors elicit emotional states. Devriese and colleagues (Devriese et al., 2000)
found that neurotic individuals (individuals with high negative affectivity) were
more likely to generalize acquired somatic symptoms in response to odors. For
example, neurotic participants were more likely to hyperventilate in response to
an odor that had not been previously paired with hyperventilation. There is also
some evidence that emotionally labile (unstable and emotionally changeable)
individuals have greater absolute sensitivity to some smells (Pause, Ferstl, &
Fehm-Wolfsdorf, 1998). Most recently, Chen and Dalton (2005) showed that
women high in trait anxiety perceived hedonically polarized odors (odors rated
as distinctly pleasant or unpleasant) as more intense than a neutral odor, and that
men who were high in neuroticism or anxiety detected hedonically polarized
odors faster than a neutral odor. In sum, neurotic, labile and anxious individuals
may respond more intensely and selectively to emotionally meaningful odors
than individuals without these personality traits. Note, however, that the
intensified responses of these individuals tends to be toward negative reactivity
rather than augmented positivity responses. The issue of neurotic, labile and
anxious personality traits also has implications for the disorder of multiple
chemical sensitivities. More research is needed to determine whether it is a
significant mediating factor in applied aromachology.
SUMMARY
Aromatherapy is a folkloric tradition espousing the beneficial properties of
various plant-based aromas on mood, behavior, and “wellness.” Aromachology
is a term that was coined by the Sense of Smell Institute and designates
only olfactory effects that have been scientifically demonstrated to affect
mood, physiology and behavior. For the purposes of the present review
valid aromachology research was defined by the following five criteria:
(1) theory guided goals and clear hypothesis testing, (2) fragrances are tested
using appropriate experimental methodology, (3) sufficient and representative
participant populations and appropriate contrasting control groups are used,
(4) data are analyzed using suitable statistical methods and indicate adequate
statistical power, and (5) the results have been vetted by scientific peers
and accepted for publication in reputable journals. Eighteen studies that met
the criteria for aromachology were reviewed in detail for this report. The
results indicated that various odors can significantly affect mood, cognition,
physiology and behavior, though inconsistencies between dependent measure
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286 R. S. HERZ
and laboratories were observed. An analysis of the two theoretical mechanisms
that have been proposed to explain these effects (pharmacology or psychology),
concluded that a psychological explanation could best account for the data
obtained in the studies reviewed here. The perceived quality of the odor was
the most relevant factor for determining how an individual would respond
to it both emotionally and physiologically. The methods that were used in
the various aromachology studies were analyzed to explore the problems
and inconsistencies that were observed. A central finding was a lack of
consistency in the specific chemicals used to evaluate the effects of a particular
odor (e.g., natural, synthetic, isomeric molecules). As such, validation and
generalization of the effects of odors is currently limited. Several factors were
also found to mitigate and modulate the responses elicited to an odor including
culture, experience, gender, and personality. At present more consistent
methodological practice and further research is needed to fully elucidate how,
under what conditions, and to whom, specific aromatic chemicals can alter
mood, physiology and behavior.
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