The sense of smell is a fascinating and complex subject. Like EOs, it has recently been “rediscovered” with entire institutes and scholarly careers devoted to understanding its intricacies. To properly administer TIEO, a basic knowledge of this sense, and in particular issues relating to odor processing, is imperative.
Smells are uniquely processed in the brain, and elicit both memory and emotion. It is the only sense that’s stored in this fashion, and this is thought to be due to the fact that our ancient ancestors relied heavily on olfaction for survival. Interestingly, olfaction is also our most rapidly processed sense. While visual and auditory input is first routed through the thalamus before arriving in the brain’s interpretive center, aroma messages are fast tracked along specialized pathways, directly to the olfactory cortex, for instant processing (10). The olfactory cortex is embedded inside the limbic system and amygdala, the areas responsible for generating emotional context as well as storing emotions. This is the reason that smells, emotions and memories are so profoundly intertwined.
The strongest associations with smells are made before the age of 10, and can persist throughout a lifetime (11). For example, researchers found that older Americans tended to like the smell of wintergreen, because it reminded them of a candy popular when they were young. Conversely,
British citizens of the same age were reminded of a wintergreen flavored cold medicine they took as children, and subsequently tended to dislike the smell. These scent biases can take an unexpected turn, as one researcher found; a woman reported enjoying the scent of skunk, as it reminded her of childhood summer walks with her mother. Conversely, the smell of roses was repugnant to another research participant, whose mother died unexpectedly when she was a child, subsequently causing her to associate their scent with the sadness and grief she felt at her mother’s funeral (12).
The phenomenon of scent bias is important to understand for the clinical setting. In Post Anesthesia Care Units (PACUs), nurses often hold isopropyl alcohol pads under a patient’s nose to stop nausea. This technique works quickly, and can have an efficacy rate of 50% (13). Unfortunately, patients often associate the smell of isopropyl alcohol with unpleasant experiences, such as needle sticks, because these pads are commonly used to disinfect the skin prior to injections. It is not uncommon for patients to refuse the alcohol pad intervention due to their previous associations with the smell.
Scents can also influence how someone perceives an event. Studies in the retail industry show that people remember 35% of what they smell, as opposed to only 5% of what they see (14). Researchers demonstrated that customers rate their experience as more tolerable, and evaluate service significantly higher, when a pleasant aroma is diffused while they wait in a long line (15).
Investigational reviews done with TIEO for treatment of post-operative nausea and vomiting (PONV) have shown similar findings. Patients consistently rate their satisfaction with PONV management higher when given pleasantly scented TIEO for treatment, even if their PONV is not completely relieved (16).
Another important consideration is the phenomenon of conditioned aversion. This is when the mind develops a strong resistance towards something, such as a taste or smell, after associating it with feeling ill. For example, if someone eats oysters, then later develops gastrointestinal distress, they will associate the two events, either consciously, or subconsciously, and can develop a strong aversion to oysters in the future. Researchers believe that humans are biologically predisposed
to rapidly make these associations as a survival mechanism (17). Because taste and smell are intrinsically linked, this effect also occurs with smells. Nurses who used peppermint essential oil to treat pediatric patients’ nausea, found that when their patients subsequently smelled peppermint in any form, it actually made them nauseous. The aroma of peppermint became paired in their mind with nausea, creating a conditioned aversion (18). This is why it is best practice to blend a number of essential oils together when formulating an EO combination for treating nausea, as the more complex aromas show the least conditioned aversion effect (19).
Patients with environmental sensitivities present a challenge when utilizing TIEO. This is a largely self- reported condition, with few measurable physiological markers, where sensitivity and intolerances
to odors are the presenting condition. The most commonly reported sensitivity triggers are cleaning products (88%), followed by tobacco smoke (82%) and perfumes (81%). Studies find the degree of irritancy to smells was rated much higher when research subjects were given negative messages about the odors, showing that perception can be influenced by expectations (20). Although sensitivities clearly have a strong psychological component, patients can display physical symptoms such as headaches, coughing, rhinitis and hoarseness when exposed to an environmental trigger.
Migraine sufferers have also been found to have higher olfactory sensitivity than non-sufferers. In fact, odors can trigger the headaches nearly 50% of the time, with the majority of the triggers being perfumes. (21). Females of reproductive age are 3 times more likely to have migraines than males, suggesting a possible hormonal link (22). Some studies have shown that TIEO can relieve the pain of migraines and the nausea that often accompanies it, primarily when peppermint and lavender are used. While studying the skin application of peppermint EO for migraine treatment, researchers concluded its pain relieving effects may be due, in part, to the inhaled molecules acting in the brain (23). Additionally, participants inhaling the vapor of lavender essential oil, reported significant relief from their migraine pain, compared to the placebo group (24). The mechanism of action is likely due to the anxiolytic and antispasmodic properties of lavender. While a history of environmental sensitivities and migraines are not a contraindication to TIEO, cautious introduction and positive encouragement are suggested when initiating essential oil therapy these patients.