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j. Cosmet. Sci., 51,141-151 (March/April 2000)
Fragrance perception: From the nose to the brain
PAMELA DALTON, Monell Chemical Senses Center, 3500 Market
Street, Philadelphia, PA 19104.
Accepted for publication February 15, 2000. Presented to the New York
Chapter of the Society of Cosmetic Chemists, November 4, 1998.
Synopsis
Fragrance is a major component in many personal products, influencing their acceptability and, in some
cases, their perceived efficacy. Despite claims that certain fragrances can relax or energize, there is surpris-
ingly little scientific evidence in support of direct, physiological effects of fragrances. Rather, recent research
suggests that psychological factors, such as personal experience, expectations, and the surrounding context
may be among the most important factors that determine how a fragrance is perceived.
INTRODUCTION
Among humans, the ability to perceive volatile chemicals through our sense of smell is
often considered to be far less important than perception via other sensory modalities
such as sight or sound. In stark contrast to this view, considerable scientific, anthropo-
logic, and economic evidence exists to suggest that stimulation of olfaction through the
perception of environmental volatiles, fragrance materials, or scented products is of
paramount importance to humans. Odor perception provides information that guides
our responses to the environments in which we live (1) and the individuals we encounter.
At a commercial level, the significance of our response to olfactory stimulation is
illustrated by the fact that the fragrance industry is a multibillion dollar industry that
supplies products to scent shampoo, deodorants, tissues, soaps of all types, hand creams,
leather products, toys, air fresheners, cleaning products, and many other commodities.
In many commercial contexts, the addition of fragrance can serve a primary or secondary
purpose. For example, in some applications, such as air fresheners or perfumes, the
delivery of a pleasing scent (and the masking of an unpleasant one) is often the primary
function of fragrance. In other products, such as shampoos, lotions or soap, both the
immediate and lingering scent provides a secondary dimension that, in addition to its
hedonic impact, can also serve to reinforce perception of the product's purpose and
efficacy.
From an anthropological perspective, the meaning and significance of odors in everyday
human experience has historically been quite varied. Prior to the discovery of germ
theory, for example, unpleasant odors were deemed to be carriers of disease and good
141
142 JOURNAL OF COSMETIC SCIENCE
odors were viewed as potentially curative, or at least protective (2,3). For example, the
habit of perfuming gloves and handkerchiefs seems to have originated as a way of
protecting the wearer from the foul and "disease producing" smells permeating Europe
during the Middle Ages and Renaissance. In an attempt to protect his troops from
airborne disease, Napoleon commissioned a "bad smell map" of Egypt to guide their
movements in the early nineteenth century (4). We should, perhaps, not find it sur-
prising that many of these attitudes and beliefs still exist, being embodied in reactions
to odors that range from concerns about becoming sick from exposure to environmental
odors (5,6) to attempts to be healed through exposure to natural aromas (7,8).
Some of the most extreme claims for the positive benefits of fragrance derive from the
field of aromatherapy. The resurgence of interest in the use of essential oils and volatiles
highlights the persistence of the belief that certain fragrances have beneficial effects on
health, mood, and mental well-being (7-10). Although anecdotal experience informs us
that exposure to fragrances that we like or fragrances that are associated with happy
memories or favored individuals can elevate our mood, aromatherapy's claims go beyond
such second-order associative or placebo effects of fragrance. Aromatherapists contend
that positive benefits are derived from the actual properties and characteristics of these
derivatives, which are active, independently therapeutic agents (11). As might be ex-
pected, this assertion has provoked considerable controversy between aromatherapists
and the scientific and medical establishment. Although inhalation or dermal absorption
of lipophilic compounds can introduce these chemicals into the bloodstream, many
scientists believe that the amounts that will be absorbed in these applications are not
physiologically significant doses and, thus, that any systemic effects (positive or nega-
tive) from inhaled or absorbed oils are negligible. Unfortunately, there is little in the
way of scientific evidence to support or refute this idea. However, it is equally plausible
that the effects of scent can exert powerful effects on mood and well-being through
psychological mechanisms alone, and in this domain, there is considerable scientific
research to inform us.
OLFACTORY RESEARCH AND FRAGRANCE PERCEPTION: WHAT'S
THE CONNECTION?
Correlated with the increased interest in aromatherapy and other therapeutic uses of
fragrance is a proliferation in the number of scientific studies of odor perception in
humans and the impact of odors on mood, memory, and perceived well-being. Between
1990 and the present, MedLine (the National Library of Medicihe's [NLM] premier
bibliographic database covering the fields of medicine, nursing, dentistry, veterinary
medicine, the health care system, and the preclinical sciences) lists more than 650
published, scientific articles on the subject of human oilaction, suggesting that there
have been considerable scientific advances in our understanding of both the mechanism
and function of the human olfactory system and the effect of odors on mood, mental
state, or performance. Among these references, however, there are no studies that provide
unequivocal evidence that exposure to specific fragrance materials at concentrations that
are commonly encountered in perfumes or scented products produces measurable direct
effects (positive or negative) on human health or well-being. Instead, much of the
evidence suggests that the first, and perhaps the foremost, impact of fragrance is through
the conscious perception of odor. Because our ability to perceive odors has an indisput-
FRAGRANCE PERCEPTION 143
able influence on our willingness to use or shun certain products or to linger in or avoid
certain environments, effective commercial use of fragrances can benefit from an under-
standing of the myriad factors that contribute to or determine individual sensitivity and
hedonic responses to odor. One aim of basic research in olfaction is to identify and
describe the sources of variation in odor perception, which can lead to a more complete
understanding of the many factors that determine our perception of fragrance. The
following sections provide a review of the current psychological perspectives on the way
odors are processed and the factors that contribute to the variation in odor experience,
at both the neural and psychological levels.
TOP-DOWN AND BOTTOM-UP PROCESSING OF ODORS
Because the perception of volatile chemicals is such an evolutionarily ancient and ru-
dimentary sensory system, it is tempting to think that what we perceive and how we
interpret the resulting olfactory sensation occur without much involvement from higher-
level cognitive or emotional processes. Similarly, any observed variation in ability to
perceive an odor or in its impact has typically been ascribed to biological factors such as
genes, age, or disease. However, as anyone who has failed to recognize the identity of an
odor until locating its source can attest, olfactory perception, like perception in other
sensory systems, is determined not just by the sensory properties of a fragrance, but by
information that the stimulus activates in memory, our current expectations, or even our
affective or emotional state. Thus, it is appropriate to examine the variation induced
both by biological and psychological mechanisms.
INDIVIDUAL VARIATION IN SENSITIVITY TO ODORS
GENETICS AND AGING
It is widely recognized in the fragrance industry and, to a lesser extent, among the
general public, that individuals can vary dramatically in their sensitivity to odors. This
variation is partly due to a number of predisposing factors. For example, the sensory
world of the older adult is often distinguished by a marked decrement in the detect-
ability or intensity of many odors (12,13), although there is evidence to suggest that the
decrease in sensitivity may not be uniform across all odorants (14) (see Figure 1). In
addition, genetic differences among individuals appear to account for a significant
amount of olfactory variability. The most dramatic manifestation of genetic influences
on olfaction is the inability of some people to smell a specific odorant or class of odorants,
known as specific anosmia (15,16). The most well-known of the selective anosmias in-
volves the perception of the steroid, 5ot-androst-16-en-3-one (androstenone). It is esti-
mated that approximately 40-50% of individuals worldwide cannot smell androstenone
at any concentration; among those who can smell it, however, a majority report it as
having a urinous, sweaty odor (15).
Although it is believed that specific anosmia is a common occurrence among individuals
with normal olfactory function, there is only limited information on the range and
diversity of odorants that cannot be smelled by one or more individuals with otherwise
normal olfactory function (16,17). Most people's specific anosmias probably go uniden-
144 JOURNAL OF COSMETIC SCIENCE
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Figure 1. Variation in olfactory function across the lifespan, as measured by odor identification ability for
six odorants from the National Geographic Smell Survey. After C.J. Wysocki and A. N. Gilbert (14).
FRAGRANCE PERCEPTION 145
titled, because the majority of real-world odor experiences rarely involve smelling a
single odorant. Nonetheless, an individual's specific anosmia has the potential to influ-
ence the perception of fragranced products because the inability to smell one or more
fragrances can greatly alter the overall intensity and quality of a complex fragrance (18).
EXPOSURE HISTORY AND THE PERCEPTION OF FRAGRANCE
Manufacturers of fragranced products such as lotions, perfumes, and air fresheners fre-
quently hear complaints that the perception of the fragrance in their product fades in a
remarkably short time. Although the consumer may suspect that the product fragrance
is less long-lasting than the manufacturer claims, the simple truth is that continued or
repeated exposure to any odor stimulus will result in a decrease in the perceived intensity
and detectability of that odor--a normal sensory process known as "olfactory adaptation"
(and sometimes referred to as "odor fatigue").
Odor adaptation is a common occurrence in natural environments when individuals who
work or live in odorous contexts cease to smell odors that are readily perceived by new
visitors. This decrease in odor perception does not appear to be due simply to a decrease
in the amount of attention paid to a familiar fragrance (although that may certainly play
a role), but appears to reflect specific, often persistent, changes in sensitivity and re-
sponse to the odorant that may be occurring both at the level of the olfactory receptors
as well as in higher cortical structures in the brain. Studies of individuals who are
occupationally exposed to a chemical have shown that workers exposed to acetone or
styrene have significantly higher detection thresholds for those chemicals, respectively,
than do non-exposed controls but show no difference in their sensitivity to another
chemical to which neither group is exposed (19,20).
Consistent with anecdotal observations, long-term adaptation has also been documented
following exposure to "air-freshener" type odors in the home. Participants whose bed-
rooms were continuously odorized with either isobornyl acetate or citralva for a two-
week period showed a marked decrease in sensitivity to this odor, but an increased
sensitivity to a control odor (see Figure 2). These results suggest that individuals who
regularly use the same scented products can experience long-lasting changes in the
perception of those fragrances that can alter both their olfactory experience and use of the
product. Of course, individuals such as perfumers or fragrance evaluators whose profes-
sions require them to maintain sensitivity to fragrances even as they sniff and sample
them regularly often limit their exposure duration in order to minimize the effect of
adaptation on their sensory systems.
Paradoxically, brief, repetitive exposures can also increase sensitivity to an odorant, as is
often experienced by individuals like perfumers or flavorists who find themselves able to
detect ever-lower concentrations of volatiles with continued experience. Although the
phenomenon of sensitization has been less well-studied than its counterpart, adaptation,
there is experimental evidence of increased sensitivity following certain types of odor
exposure. For example, some individuals who were anosmic to the volatile steroid
androstenone could be induced to smell androstenone following repeated exposure to its
odor (21,22). And among individuals who were already able to smell an odorant,
repeated testing at threshold frequently led to a lowering of the olfactory threshold for
the target odorant (23,24).
146 JOURNAL OF COSMETIC SCIENCE
_
_
10-
12-
18-
20
ß Adapting Odorant
[] Control Odorant
bl b2 al a2 rl r2
Test Session
Figure 2. The effect of long-term exposure to an odorant on sensitivity as measured by olfactory thresholds.
Ten subjects were exposed for two weeks in their home to a single odorant: half were exposed to isobornyl
acetate (IBA) and half to citralva (C); at weekly intervals they were given threshold tests for their adapting
odorant (IBA or C) and a control odorant (the alternate odor). bl and b2 refer to baseline tests, prior to
odorant exposure; al and a2 refer to tests during adaptation phase, and rl and r2 refer to tests following
removal of the odorant from their home (recovery). Thresholds were significantly elevated to the exposure
odorant, but not to the control odorant. Reprinted, by permission, from reference (24).
CULTURAL EXPERIENCE AND THE PERCEPTION OF FRAGRANCE
Anthropologists have observed that people of different cultures "inhabit different sensory
worlds" (25-28). Thus, it is not surprising that cultural-specific experience can evoke
different patterns of cognitive and emotional experience of an odor, and together, play
a role in one's odor perception. Indeed, the available studies suggest that odor familiarity
and dietary habits can influence both the identifiability and the perceived pleasantness
of an odor. For example, more than 90% of American subjects (regardless of their ethnic
background) correctly identified the odors of lemon and cherry (29), but only 23% of
Taiwanese subjects correctly identified lemon odor (30) and only 65% of Japanese
subjects correctly identified cherry odor (29). In addition, odors familiar to the Japanese
(e.g., India ink, dried fish, roasted tea) were rated as more pleasant by Japanese than by
German subjects (31). Conversely, odors familiar to the Germans (e.g., Catholic church
incense, anise, almond) were rated more pleasant by Germans than by Japanese subjects.
FRAGRANCE PERCEPTION 147
Results from the 1986 National Geographic Smell Survey showed that a significantly
larger number of Asians from Malaysia, Singapore, and Thailand said that they would eat
something that smelled like mercaptan (an unpleasant rotten-egg smelling odor) than
did the Caucasian respondents from those same countries (32), perhaps reflecting the
prevalence of fermented foods in Asian diets.
Odor familiarity may even influence the perceived intensity of an odor. For example,
Japanese subjects perceived a familiar Japanese food odor, dried fish, as much less intense
than did German subjects (31). Thus, just as culture-specific experience can account for
some of the variation in both the sensory and hedonic response to odors, it can also play
a major role in judgments of preference and appropriateness of fragrances for specific
contexts.
EXPECTATIONS, EMOTIONS, AND THE PERCEPTION OF ODORS
Although any odor sensation is based, in part, on properties inherent in the chemical
stimulus (e.g., concentration, quality), it can also be influenced by the mental set or
expectations that we bring to any situation where odors are present. Odor detectability,
perceived intensity, and even quality are meaningfully influenced by a variety of dif-
ferent types of "top-down" information that can color the sensory information available
from the odorant stimulus itself. For example, Knasko and colleagues (33) have shown
that describing an aerosol application of deionized water as either a pleasant or unpleas-
ant odorous substance produced reports of odor experience and mood effects that were
congruent with the given hedonic characterization. And, in several studies that exam-
ined more indirect influences of expectation on odor perception, adding color to an
odorous solution increased the likelihood that an individual reported detecting the odor
(34) and increased the perceived odor intensity (35).
In a clear demonstration of the malleability of odor experience, Dalton and colleagues
(36-38) have conducted a series of studies examining how information about the source
and consequences of exposure to a volatile chemical influenced people's sensory and
somatic response to an odor. In the basic paradigm, individuals were exposed in a
chamber to a steady-state concentration of an odorant and were asked to rate the
perceived intensity of odor and any sensory irritation (i.e., to eyes, nose, and throat) at
regular intervals during exposure. Although all subjects were exposed to the same
chemical, different groups were given different characterizing information about the
odorant to which they would be exposed. Subjects assigned to a positive-bias condition
were told that the odorant was a natural extract. In contrast, those assigned to a
negative-bias condition were told that the odorant was an industrial chemical, while
those assigned to a neutral-bias condition were given no characterizing information
about the odorant. Perceived odor and irritation ratings were measured during exposure,
and reports of health symptoms were obtained following exposure. In all cases, infor-
mation provided to the subjects greatly influenced their reported odor experience during
and following exposure (see Figure 3). Specifically, individuals exposed to the odorant
under a "positive" expectation exhibited the most adaptation to odor and the lowest
perceived irritation; following exposure they reported the fewest health symptoms. In
contrast, individuals given the negative expectation rated higher levels of odor intensity
and reported the most overall irritation; following exposure they reported significantly
more health symptoms than the other groups. Similar results were observed when the
148 JOURNAL OF COSMETIC SCIENCE
60
'•50
ß 40
• 3o
ß lO
ODOR
--O-- Neutral
•--EI-- Positive
--•- Negative
IRRITATION
I I I I
5 10 15 20
5 10 15 20
-- VS
--S
--M
--W
Exposure Duration
(minutes)
Figure 3. Average intensity ratings of the perceived odor and irritation of a 20-minute chamber exposure
to methyl salicylate (wintergreen) for participants given a positive, neutral, or negative bias about the nature
of the odorant. Reprinted, by permission, from reference (36).
characterizing information was conveyed by the behavior, symptoms, and verbal reports
of a "confederate" subject (an actor whose positive, negative, or neutral verbalizations
and symptom reports were scripted) (39).
These results suggest that odor experience may be particularly prone to the influence of
extrasensory factors, such as expectations and the emotional states and vigilance induced
by such expectations. Moreover, based on the studies reviewed here, responses to odors
appear to depend strongly on context and, accordingly, will vary among different in-
dividuals and across time within the same individuals as a function of how odor infor-
mation is interpreted through different cognitive and emotional filters.
INTERPRETING AND UTILIZING THE SOURCES OF VARIATION IN
OLFACTORY PERCEPTION
Given the wide variety of factors that appear to influence individual sensitivity and
hedonic responses to any odorant, one may be left to wonder whether it is possible to
utilize fragrance in any systematic fashion or whether the variation in response to a select
group of odorants will render the perception and response to these fragrances hopelessly
idiosyncratic. Clearly, as more research is conducted on fundamental mechanisms in
olfaction and sources of variation in odor perception, better information will be available
to guide the systematic use of fragrances. In the meantime, there is considerable evidence
from extant olfactory research to indicate that fragrance is a powerful tool to elicit
hedonic reactions (both positive and negative) and different mood states (40,41), and
that perhaps through mood induction, specific odors can become associated with a
FRAGRANCE PERCEPTION 149
particular environment or emotional state (42). The association of odors with emotional
states through conditioning may well be one of the most potent methods to endow
specific odors with the ability to promote relaxation and health benefits (43•47).
Exposure can also significantly modify the sensory and affective response to a fragrance,
although the specific direction of the effect will depend greatly on the duration and
frequency of the exposure and the other stimuli that are present at the time of exposure
(23,24,48).
Moreover, although there can be substantial variation across different cultures or ethnic
groups, within a particular culture or age-group there can be considerable agreement
over what odor sensation constitutes a "clean" smell or a "calming" smell, owing perhaps
to the homogeneity of lifestyles and experiences that promote associations with odors
(31,49). Thus, following a careful evaluation of the target audience, the appropriate
fragrances and their optimal intensities can be selected for many different commercial
applications.
CONCLUSION
Despite the popular perception that olfactory sensation and information is of limited
importance to humans, who rely heavily on sight and sound, there is considerable
evidence that multiple facets of human experience and behavior are impacted, both
positively and negatively, by our ability to perceive odors and fragrances. This ability can
warn us of possible airborne chemical hazards (50), orient us to food sources, and allow
us to recognize kin or other close relationships (51). Perhaps most significantly, olfactory
sensations can rapidly and dramatically alter mood (40,52) and anxiety (46). This
proclivity for impacting affectlye responses has served as a powerful argument for in-
corporating olfactory cues into many products or environments.
All of these applications can benefit greatly from an increased understanding of the
determinants of human odor perception and response. What should emerge clearly from
this review is that psychological mechanisms are as important as biological mechanisms
for understanding and predicting the human response to olfactory sensations. Moreover,
psychological determinants of odor perception appear to hold greater promise for modi-
fying olfactory experiences to address the variability found across different individuals
and cultures than do biological determinants. Fragrance research and development that
acknowledges and pursues both mechanisms will undoubtedly profit from this insight.
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