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Leonard Pearlstine, 2006. Aromatherapy Science. AromaScents Journal 35(Fall):17-24.

Aromatherapy Science

Aromatherapy is a shadowy world of romantic illusion. Its magic easily dispelled by the harsh light of science. Its quaint notions may have a value for patients’ equivalent to a child’s belief in Father Christmas. It would be uncharitable to break the spell too soon. (King 1994 p. 413)

In randomized, double-blinded, controlled clinical trials, Hay et al. ( 1998) found that daily aromatherapy scalp massage with thyme, rosemary, lavender and cedarwood showed significant improvement in patients with hair loss due to alopecia areata compared to a control group using only carrier oils (jojoba and grapeseed). In contrast to conventional treatments, no significant adverse effects were reported from the aromatherapy massages which lasted 7 months with a 3 and 7 month follow-up. Yet, the U.S. National Institutes of Health informational web site for alopecia areata, in listing the pros and cons of treatments, only had this to say about aromatherapy:

Alternative therapies--When drug treatments fail to bring sufficient hair regrowth, some people turn to alternative therapies. Alternatives purported to help alopecia areata include acupuncture, aroma therapy, evening primrose oil, zinc and vitamin supplements, and Chinese herbs. Because many alternative therapies are not backed by clinical trials, they may or may not be effective for regrowing hair. In fact, some may actually make hair loss worse… (National Institute of Arthritis and Musculoskeletal and Skin Diseases 2003)

Therapeutic treatment with essential oils is often regarded with skepticism or simple dismissal by the medical profession, even, as seen above, when carefully controlled clinical trials offer evidence of effectiveness and safety. The above quote also points to one of aromatherapies problems. It is frequently lumped into a broad category of “alternative therapies” which are most often regarded by scientists as being guided by faith and romanticism rather than science (e.g., McCracken 1999).

Plant materials have always been an important component of medicine and pharmaceutical firms continue in the search for plant medicinal properties. Much of the legitimate skepticism of aromatherapy comes not from questioning the efficacy of plant-derived treatments, but from questions of the effectiveness of essential oils applied outside the body to aiding internal disease and discomfort and the anecdotal nature of much of the evidence of its therapeutic successes (e.g., McCracken 1999). When we are sure that something works, we notice when we are getting better while using the something, but we have a remarkable ability to not remember when it fails. Perhaps more importantly, it is easy to overlook all the other circumstances surrounding the event (my daughter-in-law got pregnant after recovering from a cold, but I feel fairly certain that the two events are not related. I am certain, however, that a large volume of anecdotal evidence could be collected of women catching colds and becoming pregnant shortly thereafter). Aromatherapy has a rich history of centuries of anecdotal evidence, however, that has given many scientists and physicians reason to want a closer look.

To illustrate the state-of-the-science, this paper reviews some of the scientific research into aromatherapy, principally for mental discomfort, and evidence for its effectiveness. To increase readably, just the common names are used in this paper when referring to essential oils. The taxonomic names of referred to essential oils are listed in the appendix.

Aromatherapists have occasionally balked at the idea that you can confirm or reject aromatherapy using scientific approaches because science is reductionist and aromatherapy depends on a holistic approach (Schnaubelt 1998). I completely reject this argument and believe it harms the profession. First, science does not have to be reductionist. The science employed depends on the questions. “Does the constituent limonene have significant motor relaxant effects when applied to the skin?” and “Do patients receiving holistic aromatherapy care respond better to treatment?” and “Is the activity of the natural plant extract greater than the combined activity of individual components in their naturally occurring ratios?” are all legitimate scientific questions that can be approached with controlled experiments.

Secondly, reductionism and holism are not mutually exclusive. Reductionist experiments can inform holistic methods. Knowing, for example, that synergistic interactions between monoterpenoid constituents of sages aids memory, but the terpenoid thujone is toxic in large doses (e.g., Tildesley et al. 2005) may lead the aromatherapist to experiment with Salvia lavandulaefolia rather than Salvia officinalis because of the known composition of those two species, but that knowledge does nothing to stop the aromatherapist from continuing to engage the patient as an individual with preferences and environmental influences that should be brought into the complete therapy.

Well established ideas from historical anecdotes can cause aromatherapists to make misinformed decisions. The myth that 1,8-cineole is a major irritant in tea tree oil (Melaleuca alternifolia) has been passed from author to author, however, repeated research among investigators has dispelled that association (see Carson et al. 2006). Aromatherapists benefit from the knowledge gained in these reductionist studies that essential oils containing 1,8-cineole are safe to the skin and the main cause of skin irritation appears to be oxidation products in aged or improperly stored oils.

Finally, aromatherapist have an ethical responsibly to favor evidence-based, controlled studies over anecdote and, when necessary, their own beliefs, when treating others. Aromatherapy is gaining acceptance in some medical practices, particularly nursing. However, science is, by definition, rationally skeptical. To continue growing as an accepted practice complementing conventional medicine, aromatherapy methods must be vetted against carefully controlled clinical and laboratory trials to demonstrate their efficacy.

Mechanisms of action

It is generally understood that essential oils may improve skin and scalp by external application and they may improve mood simply by offering a pleasant aroma as with a perfume or room spray. Internal therapies may be elicited from direct ingestion of essential oils (practiced in European aromatherapy, but rarely in North America). Inhaled aromatic molecules are another pathway to reach internal tissues. The inhaled molecules react with nerves in the olfactory bulb and relay nerve messages to the limbic system or are absorbed into the blood stream by thin membranes of the nose, bronchioles and lungs. It is also now well accepted that essential oil components can be absorbed through the skin to reach internal organs. In fact, the use of skin patches has become a common mechanism for dispensing pharmaceuticals. These various mechanisms have lead one author to remark that she prefers the phrase “essential oil therapy” to “aromatherapy” because the oils are not always inhaled and don’t necessarily smell good (Halcon 2002).

More recent literature has supported and expanded our knowledge of the details of these mechanisms. Richard Axel and Linda Buck won the 2004 Nobel Prize in Physiology or Medicine for their research (Axel & Buck 1991) clarifying in molecular detail the gene coding of odorant receptors. An unexpected result was that of all the genes that code for olfactory receptor molecules, each individual olfactory receptor cells expresses only one gene. Different odors are detected by different combinations of receptors. It is the combinatorial power of multiple receptors, each distinguishing a limited piece of the odorant code, that results in our ability to distinguish and form memories of more than 10,000 different odors (Nobelprize.org 2004).

Absorption through the skin was observed by Jager ( 1992). When a 2% solution of lavender was applied to the abdomen, 10% of the lavender was absorbed into the general blood circulation with plasma levels peaking after 20 minutes. Levels of linalool and linalyl acetate, active constituents of lavender oil, dropped to zero after 90 minutes. During this period after application, the lavender oil constituents were circulated to tissue via capillaries. Potentially higher levels of absorption are likely across the highly vascular cribiform plate in the nose with a direct pathway to the brain (Jager 1992).

Many anesthetics, who’s uptake and distribution mechanism is known (Eger 1998), are aliphatic hydrocarbon chains, as are many essential oils. Geiger speculates that the action of anesthetics may conceivably be applied to explain some of the actions of essential oils at the cellular level. Intranuclear protein synthesis from DNA may be involved in the action of constituents of scent at the cellular level (Frondoza et al. 2004).

A growing number of in vitro and in vivo studies document the specific actions of essential oils, particularly anti-inflammatory, antibacterial, and anti-fungal properties. Baylac and Racine ( 2003) suspected that the mechanism for anti-inflammatory properties of some essential oils is inhibition of enzymatic reactions in the epidermis and other tissues. They evaluated 32 essential oils, 10 absolutes and 26 chemical constituents in vitro for their ability to inhibit 5-lipooxygenase, an important enzyme in a complex case of inflammatory events. Many of the oils used in aromatherapy for inflammation (e.g., myrrh, Copaiba balsam, Himalayan cedar, sandalwood, juniper berry and German Chamomile) had strong to good activity. The authors were surprised to find other essential oils, primarily Citrus species, also had strong activity in vitro, but were not reportedly used in aromatherapy for inflammation. Roman chamomile, which is used for inflammation in aromatherapy had poor activity, which suggest that other modes of action are responsible for its anti-inflammatory activity. The authors were also able to compare the activity of individual constituents of the essential oils (Baylac & Racine 2003). Anti-inflammatory effects in vivo due to both lipoxgenase and cyclooxygenase inhibition have been reported earlier for clove essential oils (Saeed & Gilani 1994).

Mood

Aromatherapy is perhaps most well known for its potential to alter mood. Some studies have gathered evidence demonstrating essential oils possess pharmacological effects on brain function.

Gurgel do Vale et al. ( 2002) found that the constituents citral, myrcene and limonene decreased activity in mice and presented sedative as well as motor relaxant effects. Vale et al was studying a Brazilian herb, cidrcira, however, many essential oils contain one or more of these constituents including clary sage, lavender, geranium, fennel, lemongrass, and palmarosa. Muscle relaxation was observed at the higher doses of citral and myrcene and even at the lowest doses of limonene. Citral and myrcene increased barbiturate sleeping time compared to the control. Citral did not increase onset of sleep, however, it increased duration of sleep. Limonene has similar effect at higher doses. An advantage to studies using mice is that they are free of placebo effects that might affect a trial on a human subject (Mantle 2002). Relaxation of smooth muscle tissue was also observed in laboratory mice by Aqel ( 1992) with rosemary essential oils and Lis-Balchin et al. ( 1998) testing multiple species of geranium essential oils.

The addition of lavender oil to a 10 minute hot foot bath caused delayed, but significant changes in autonomic activity associated with relaxation beyond that observed with a hot foot bath alone (Saeki 2000). This randomized crossover controlled study contrasted with previous studies that have found psychological benefits, but could not find physiological effects with the addition of essential oils. It was in agreement with those same previous studies in finding no significant change in measures such as blood pressure and heart rate, however physiological effects were demonstrated with the more sensitive tests of autonomic function evaluated using spectral analysis of heart rate variability. It is likely, as the author notes, that very little of the essential oil penetrated the skin during the short bath and most of the effect was from inhalation of vapors.

Lewith ( 2005) found inhalation of diffused lavender oil to be effective in improving sleep quality as measured by standard physiological questionnaires in a randomized, single-blinded, crossover design controlled for belief in aromatherapy. A common criticism of “blinded” studies in aromatherapy is that the distinctive odor of essential oils defeats attempts at blinding and the study is thus subject to the placebo effect. That was certainly possible in this study, however before treatment, subjects were less confident that lavender oil would help than they were about the sweet almond oil control. One subject found the smell of lavender unpleasant and yet recorded significantly enhanced sleep quality.

Hudson ( 1996) found a sedative effects of lavender placed on elderly patients pillows. Sleep quality improved in 84% and increased daytime activity levels and alertness in 70% of patients. Another study of sleep quality in young, healthy individuals, presented intermittent stimulus with lavender oil or a distilled water control (Goel et al. 2005). Both polysomnographic and questionnaire data were collected. In addition to a measured increase in deep sleep and higher vigor reported by subjects of both genders in the morning after lavender exposure, this study demonstrated a gender difference between subjects. Women experienced increased light sleep, decreased rapid-eye movement sleep, and decreased amount of time to reach wake after first falling asleep. Men experienced the opposite effects.

The psychological benefits of aromatherapy massage when contrasted with massage alone are often subtle. Kuritama et al. ( 2005) used an extensive set of blood chemistry tests and psychological questionnaire measures to evaluate differences between aromatherapy massage with lavender, cypress and sweet marjoram in sweet almond oil versus a control massage of only sweet almond oil every 2 weeks over a 4 month period. Both groups showed a significant reduction in anxiety and self-ranked depression with no difference between the aromatherapy and control groups. The aromatherapy group, however, exhibited a significant post-treatment increase in peripheral blood lymphocytes, CD8+ and CD16- lymphociyes. The control group did not show this difference which could be beneficial in immunological disease states that require augmentation of CD8+ lymphocytes (Kuriyama et al. 2005).

A randomized controlled trial comparing aromatherapy massage using neroli oil and massage with plain vegetable oil on post-cardiac surgery patients found equal significant benefit was derived from both the aromatherapy massage and vegetable oil massage groups compared to control groups on day 1 (Stevensen 1994). A follow-up on day 5 post surgery indicated a trend towards greater and more lasting psychological benefit from the massage with neroli oil compared to plain vegetable oil. Kyle ( 2006) evaluated the effectiveness of massage with 1% sandalwood oil when compared to massage with sweet almond oil alone or diffused sandalwood oil in reducing anxiety in palliative care. Two psychological measures gave consistent results in finding that both the aromatherapy massage and diffusion of sandalwood oil showed steady and sustained declines in anxiety over a 4 week period in contrast to the control massage which had little or no decline.

Umezu ( 1999) has shown that lavender and rose essential oils decreased conflict behaviors in mice, suggesting an anti-anxiety effect. The potential to reduce agitated behavior with aromatherapy has received a fair amount of attention. A placebo-controlled, observer-blind rating of agitation in 15 individuals with severe dementia resulted in 60% improvement and 1 patient worsening when treated with 2% lavender massage (Holmes et al. 2002). Melissa was used in another study (Ballard et al. 2002) of 72 patients and resulted in an overall improvement in agitation of 35% of the essential oil patients compared to 11% in the placebo group. In a randomized controlled trial of 21 patients assigned to aromatherapy and massage, conversation and aromatherapy, or massage only, the aromatherapy and massage group showed the greatest decrease in excessive motor behavior (Smallwood et al. 2001). However, when subjects in another study with dementia and behavioral challenges where exposed to aromas of lavender, sweet orange, tea tree, and no aroma, there was no significant difference in resistive behavior resulting from aromatherapy (Gray & Clair 2002). Snow et al. ( 2004) tested whether effects of aromatherapy were the result of skin absorption of the oils or smelling the aromas. This study found no support for use of purely olfactory form of aromatherapy to decrease agitation in severely demented patients and cites previous literature finding that persons with dementia have impaired olfactory abilities.

Uncontrolled, unblinded aromatherapy trials have been reported from hospital birthing and neonatal centers. While the limited design of these studies does not allow firm conclusions to be reached, it does allow the hospital to inexpensively sample a large cohort of patients over an extended period of time. As a result, they are valuable as exploratory trials whose results can focus the questions and experiments of controlled clinical and laboratory studies. One of the most cited studies involved evaluation of 8,058 mothers over 8 years using aromatherapy during childbirth compared to group of 15,799 mothers not using aromatherapy in the same teaching hospital (Burns et al. 2000). Overall, 50% of the mothers found aromatherapy helpful and 14% found it unhelpful. For reduction in anxiety lavender (50%), frankincense (44%) and rose (71%) were rated as helpful. In reducing pain in labor, lavender (54%) and frankincense (64%) were rated as helpful. The study suggested that clary sage augments the strength of contractions in dysfunctional labor, reducing the requirement for oxytocin infusion, but controlled trials are needed. There was a very low number of associated adverse symptoms reported (1%) and aromatherapy was reported to be a very inexpensive care option.

A recent study of postpartum depression treated 36 healthy, first-time mothers to a 30-minute aromatherapy-massage (neroli and lavender at 0.5% concentration) on the second postpartum day. A control group had 20 mothers which stayed with their babies in their hospital room receiving standard care. Psychological questionnaires where completed before and after the massage. Scores significant decreased in the aromatherapy massage group for depression, anxiety and the “Conflict Index of Avoidance/Approach Feeling toward Baby”. The results suggest aromatherapy massage is effective in improving physical and mental status of new mothers and facilitating mother-infant interaction (Imura et al. 2006). This trial did not attempt to differentiate the effects of massage alone versus massage with essential oils.

Alertness & Memory

Peppermint oil is believed to be effective for treating mental fatigue and the constituents of peppermint oil (1,8-cineol, menthone, isomenthone, menthol, (R)-(+)-pulegone, menthyl acetate and caryophyllene) have been found to significantly increased ambulatory activity in mice (Umezu et al. 2001). Spanish sage has been found by several authors to significantly aid memory (Perry et al. 2002,Tildesley et al. 2003,Savelev et al. 2003,Perry et al. 2003,Tildesley et al. 2005). Drugs for treatment of Alzheimer’s Disease inhibit acctylcholinesterase (AChE) resulting in increased levels of the neurotransmitter acctylcholine. Spanish sage has been shown to inhibit AChE in vitro and in vivo (Perry et al. 2002). Savelev et al. ( 2003) monitored synergist and antagonist interactions between components in Spanish sage and found evidence that synergy with individual terpenes measured as the same concentrations as existed in the oil was not as great as the whole oil. They found that high 1,8-cineole and low camphor contents in the oil may increase its anti-AChE activity. With oral administration of Spanish sage, Tildesley observed significant improved immediate word recall in two placebo-controlled, double blind, crossover trials (Tildesley et al. 2003) and consistent improvements in speed of memory and secondary memory in healthy young adults (Tildesley et al. 2005). He also found increases in self-rated alertness, calmness, and contentedness.

Perry et al. ( 2003) found significant reductions in neuropsychiatric symptoms and an improvement in attention in Alzheimer’s patients after 6 weeks of treatment with Spanish sage. The common spice, kitchen sage, has also been shown to provide some protection against declines in cognitive performance in Alzheimer’s patients (Akhondadch et al. 2003).

Conclusions

The psychological benefits of aromatherapy that were the focus of this paper are only one aspect of clinical trials testing therapeutic applications of essential oils. As suggested by the introductory example of scalp treatments, compelling, evidence-based science exist to suggest essential oil therapy efficacies and cautions for many diseases and discomforts. A few, very non-inclusive, examples include studies on pain (e.g., Buckle 1999), skin ulcers (e.g., Warnke et al. 2005), eczema (e.g., Anderson et al. 2000), epilepsy (e.g., Sayyah et al. 2002), herpes virus (e.g., Schuhmacher et al. 2003), weight loss and blood pressure (e.g., Shen et al. 2005), and numerous recent studies of immunological and antiseptic properties (e.g.,Standen & Myers 2004,Caldefie-Chézet et al. 2006).

Most of the clinical trials on humans, however, still suffer from lack of adequate control, small numbers of participants, and lack of repetition by independent researchers. Some standard and important elements of good study design include objective measures of outcome variables, partitioned effects of confounding variables and follow-up studies (Martin 1996). Investigators also need to be able to follow patients for longer periods of time if effects such as delayed positive or negative responses, sensitization, or acclimation are to be documented. Temporal responses may be critically important knowledge for aromatherapists if observed over a large cohort.

Well crafted scientific study and restudy provides care givers with the evidence necessary to ensure that they are giving patients safe and effective treatments. When all the available evidence is gathered, however, aromatherapy remains a holistic practice, treating not just the ailment, but the whole person. Cawthorn and Carter (2000) set the standard in their institutions. They demand safety in aromatherapy by using evidence-based practice and working within a protocol that defines acceptable practice. But essential oil treatments are just one aspect of their program of HEARTS: holding, empathy, aromatherapy, relaxation, therapeutic relationship, and stroking (modified massage) in cancer and palliative care.

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APPENDIX: Taxonomic names of essential oils mentioned in this paper.

Common NameGenus species
Balsam, CopaibaCopaifera officinalis
Cedar, HimalayanCedrus deodara
Cedarwood AtlasCedrus atlantica
Chamomile, GermanMatricaria recutila
Chamomile, RomanAnthemis nobilis
CidrciraLippia alba
Cinnamon LeafCinnamomum zeylanicum
Clary SageSalvia sclarea
CloveSyzygium aromaticum
CypressCupressus sempervirens
FennelFoeniculum vulgare
FrankincenseBoswellia carteri
GeraniumPelargonium spp.
Juniper berryJuniperus communis
Lavender, TrueLavendula agustifolia
LemongrassCymbopogon citratus
Marjoram, SweetOriganum marjorana
MelissaMelissa officinalis
MyrrhCommiphoria myrrha
NeroliCitrus aurantium, vurgaris
Orange, SweetCitrus sinesis
PalmarosaCymbopogon martini
PeppermintMentha piperita
Rose, CabbageRosa centifolia
RosemaryRosemarinus officinalis
Sage, KitchenSalvia officinalis
Sage, SpanishSalvia lavandulaefolia
SandalwoodSantalum spicatum and S. album
Tea TreeMelaleuca alternifolia
ThymeThymus vulgaris

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