Keypoints: This post will review the basic science of smell and summarize research supporting that certain aromas may trigger specific neurotransmitters that effect positive moods and reduce the sensation of pain.

Aromatherapy: Introduction

The reaction to smell is dichotomous, as personal as it is universal. Though, I imagine that most people walking into a French bakery might begin to smell the aroma of freshly baked bread and instantly know why they are there or even think of happy thoughts.  Food that is putrid can repel someone away from a possibly dangerous situation.  Our instinctual curiosity drives us to check the odors of milk before we drink it, day old food before we eat it, and half-worn socks to see if they can be used another time when the clothes need to be laundered and you are tired and don’t want to (and other crazy things that you have done).

Aroma can also evoke specific memories. I remember when I traveled to Africa for the first time. I wanted to bring an oil to use that smelled fresh. I was recommended Egyptian musk.  Africa was an incredible time and each day I used the oil. After that experience, every time I either wear Egyptian musk or smell it on someone else, those memories of Africa pour out.

An odor can trigger memories and feelings. But how does our body detect an odor and how does it interact with our brain to induce memories. Are there any positive therapeutic effects from the use of essential oils ?  Is there any scientific validity to the claims of aromatherapy – or is it all pseudoscience?

Essential oils have been used for thousands of years, either as topical applications or for their aromas. An essential oil, as defined by the International Standards Organization (ISO) is a product obtained from a plant source, either by a process of distillation or by mechanical measures.

The Neurobiology of Smell

Our brain processes smells in a way not so different from other organisms, demonstrative of the fact that olfactory sense is a tool for survival.  Smell can draw us toward a safe source or repel us away from a dangerous one.  Our prefrontal cortex stores memories of smells, which can affect our emotional response and direct a behavior.   

Even a primitive form of smell can be seen in bacteria and other single-celled organisms and how they register various chemical gradients in their habitats, known as chemotaxis.   And even bacteria show some degree of “learning” or memory from the experience.   The overarching principal is that an organism decides from how it interacts with environment through its various senses whether something is safe or harmful.

What happens when someone smells something?  The nose comes across odorant molecules (not usually one type of molecule – e,g, smell of chocolate is a blend of 600 chemicals).  Our brains signal to us what it is smelling usually cued in by the eyes, as it recalls the memories of the expected odor when triggered by sight and other cues.  That is to say, that smells are usually referenced from prior experiences.  The response to odorant molecules are affected by timing, whether they are mixed with other odors and wind currents.  Certainly, if someone has rhinitis (nasal membrane inflammation) from allergies or a viral infection, the scent may be blunted.

A sniff from the nose directs the air and various molecules carried in, or odorants, to come into exposure with olfactory sensing neurons (OSN).  The circulation of air can be retronasal (from the mouth) or orthonasal (directly from air to nose).  Each neuron expresses a specific gene (1000 types in rodents, 350 in humans, 100 in fish) and are scattered in the olfactory epithelial tissue.

Aromatherapy: The Research Base

Let’s start with the fact that research on odors is fraught with study design problems and challenges in drawing conclusions. It sounds appealing and intuitive to think that one’s mood can suddenly improve or that one can benefit simply by smelling a unique essential oil for an occasion. However, what differentiates a substance with anecdotal or individual benefits to one that has measured and observed benefits is the gold standard study design, the double-blind, randomized, placebo-controlled trial. As of 2017, there were a total of 549 original and review articles on aromatherapy published in 287 peer-reviewed journals with a steady increase in submissions yearly. A search on pubmed as of October 16, 2019 revealed 1,366 published studies with the key word “aromatherapy”.  Most of the research on aromatherapy hasn’t met and may never be able to meet the rigors of scientific studies – and maybe we actually need less aromatherapy research.

There are several inherent problems as well as entry points for bias with a study design in aromatherapy research. First, essential oils are not easy to conceal – the odor declares itself (unlike a white, odorless pill). Many people are aware of common aromas; an odorless oil may not be the best placebo either. Therefore, it would be difficult to separate out how much the patient’s knowledge of the essential oil is affecting the result. Second is the research dilemma of determining the extent the odor of an essential oil triggers a response, such as calmer state or livelier mood, and what extent is it from the person’s reaction and memory recall to that odor. The active chemical ingredient is producing an effect by being sensed – physiologically – but the brain’s reaction is inherently tied in with it. Third, along this same direction, is many of the tools to assess activity are qualitative measures (trying to be quantitative) of improved state, such as Likert scales, which makes it challenging to draw conclusions on reproduceable results; objective measures of effect may also be more challenging to rely on. This is in contradistinction to a glucose lowering medication, in which a response is assessed by measurable levels (glucose or HbA1c) compared with an inert substance.

Another question is does it even matter whether the effect originates in our brain, since the brain has an active and powerful control of our physical responses anyways and most certainly exerts an effect on other organs (take the example of Takotsubo Cardiomyopathy).  Thus, a positive response to a pleasant odor in one’s repertoire may lessen cortisol, epinephrine and norepinephrine release to produce a more relaxed state.  Sensations such as pain require both the sensory trigger and the brain’s neurotransmitter response to modulate the severity — so a direction toward greater relaxation could conceivably reduce the pain experience.  Though, this mind over matter stunt is purely natural and authentic not greater (though great enough!).

Aromatherapy for Pain, Happy Thoughts and Cognitive function

There are multiple studies in this area. One randomized controlled study by Soden et al. looked at four weeks of aromatherapy plus massage compared to massage alone in a hospice setting. Forty-two patients were randomly assigned to these two groups: one with lavender essential oil and massage and another with inert carrier oil and massage and other groups with inert oil or no intervention. The results did not show any benefit in pain, anxiety and depression scores (multiple used) in either aromatherapy or massage, though sleep scores were significant improved in the massage groups. This was a negative study (no benefit) for lavender oil.

A recent study by Kouzuki et al. published in October 2, 2019 in Psychogeriatrics evaluated the effect of aroma oil (blend of lavender and sweet orange) on cognitive function. This was a small study with only 35 patients (10 with Alzheimer’s disease and 25 with mild cognitive impairment) enrolled and analyzed. Furthermore, the groups were divided into 0.1%, 0.5% and 1% aroma bath salts. The study was conducted over 24 weeks and various dementia scales were checked 4 weeks before and 4 weeks after the intervention. There were no significant differences – another negative study.

There was a positive (slight) study with the use of essential oils to improve sleep for patients in cardiac rehabilitation in a recent article out of Journal of Alternative Complementary Medicine. Forty-two participants were studied, using cotton balls infused with lavender, bergamot and ylang ylang compared to placebo. There was a crossover after five days. There was no difference in duration of sleep or sleep efficiency, though sleep quality was reported as improved by those given the essential oils.

On the basic science side, Komiya et al. in 2006 measured the serotonin and dopamine activities in mice after exposing them to lemon oil, rose and lavender. Lemon oil was found to have the strongest anti-stress effect. With various medications, they deduced that the anti-stress effect of lemon oil was related to the suppression of dopamine activity related to enhanced serotonergic neurons. In a nutshell, the lemon oil caused the mice to have less stress through a pathway that has been shown in humans to relate to stress-reduction.

Karan,NB. studied the influence of lavender oil inhalation on vital signs and anxiety in a recent article published in September 2019. One hundred twenty-six patients undergoing wisdom tooth removal were assigned to lavender oil or control. Various tests for anxiety were assessed and vital signs were noted before, during, and after the operation. There were changes in blood pressure observed in the lavender oil group post-operatively (p<0.05). Pre-operative anxiety levels were comparable in both groups. There was patient satisfaction with the lavender oils but there was no mention in improved anxiety scores.

Aromatherapy for Perennial allergic rhinitis (PAR)

A 2016 study from Evidence Based Complementary Alternative Medicine by Seo Choi and Kyungsook Park investigated whether the use of inhaled aromatherapy containing sandalwood, geranium, and almond oil would help allergic rhinitis symptoms. This was a randomized, double-blinded study. In the experimental group after an intervention of 7 days, the Total Nasal Symptom Score (TNSS) suggested significant improvement, especially in obstruction (“stuffiness”) symptoms, where a significant p-value was reached (p=0.031). After review of this article, I wouldn’t say that this was a profound practice-changing result, though definitely fulfills an alternative option.

Aromatherapy for labor pain

An August 2019 article by Chen SF et al. in Women Birth entitled Labour pain control by aromatherapy: A meta-analysis of randomized controlled trials reviewed a total of 17 trials on this very question. Specifically, aromatherapy reduced pain during labor into the active phases, with a trend of reducing the time in the second stage (or active pushing). As a metanalysis, there was some variability in the blinding, but most were single-blinded or unclear. These studies were performed worldwide, including countries such as Thailand, Iran, Egypt, Pakistan, India, Korea and Italy. Some of the oils used were jasmine, lavender, salvia, rose, sweet almond and mixed essential oils. I am not sure in the United States whether most women in labor would favor this over an epidural or other pain regimen. But why not add some essential oil to the experience?

Aromatherapy, Phytoncides, and Natural Killer Cell Function

Phytoncides are released by conifers and have added benefit to the practice of “Forest Bathing” or Shinrin-yoku. Q Li and other researchers demonstrated an increased NK cell, an important cell in our innate immunity with anti-cancer activity, lasting for several days, when 12 male subjects experienced a day in a forest park. They walked for approximately 4 hours and had their blood and urine assessed for levels of NK and its products to show activity (perforin, granulysin and granzymes) T cells, and adrenaline concentrations. The researchers confirmed that phytoncides were present in the air of the forest. There were increased levels of NK cells and decreased CD4(+) T cells, which lasted even 7 days after the trip, when reassessed. Q Li and others did an interesting article to see if this benefit could be “bottled up”. They assessed natural killer (NK) cell activity after twelve healthy male (not sure if same) subjects stayed in an urban hotel for 3 nights and were subjected to vaporized aromatic volatile substances (phytonsides) compared to controls. Afterwards, their blood samples were checked for NK activity. In his book, Into the Forest, Q Li wrote of an increase in NK activity from 17% to 26.5% and NK cell number from 440 to 661, both of these approximately a 50% increase after subjects went on a three day, two night hike in the forest.

But the question that remains is whether the process of escaping the stressors of the city or the benefits of a relaxing scent was responsible for these changes or whether it was a signature effect of these phytoncides. This is an inherent challenge in trying to discern from where the response came – internal or external. We can all agree that a calming walk hike in the park with healthy, conditioning exercise, comes with many health benefits. In fact, similar findings of increased NK activity have been seen with exercise as well.