Isoprene is by far the most abundant biological volatile organic compound (VOC) in the atmosphere; it accounts for about half of all biological VOCs and is heavily implicated in our understanding of atmospheric science.[i] This makes sense because it happens to be the main building block of terpenes, a broad class of molecules that make up most of the non-macro natural products that we know of, including the infamous cholesterol.[ii] In fact, cholesterol could accurately be classified as a triterpenoid because it is made up of six isoprene units, which when fused together end-to-end are known as squalene. Monoterpenes are made from two isoprene units (or technically one geranyl-pyrophosphate, hence the prefix), diterpenes are made from four (or two GPP), and triterpenes are made from six (three GPP). We add the suffix -oid to the word terpene almost arbitrarily in practice, but for consistency it will be used herein to describe terpenes that are oxidized (i.e., contain one or more oxygen atoms), and also non-specifically to refer to any volatile molecule that derives from isoprene.

            In any case, terpenes and terpenoids are ubiquitous in nature. Our focus now is on plant-derived terpenes, which constitute a large fraction of the pleasant plant smells that we are familiar with. An uninitiated yet marginally intelligent person might recognize terpenes primarily as the chemical things that make cannabis plants smell so strongly. A little bit deeper and they might also know that terpenes like humulene are what give real beer its hoppy flavor (indeed, hops are also in the Cannabaceae family). Deeper still and they may recognize specific molecules like linalool as the relaxing scent of lavender, limonene as the sweet scent of citrus, or menthol as the cool scent of mint, just to name a few.

The point is, we know (or at least can easily test) that terpenes and terpenoids are capable of evoking positive feelings and sensations, but can we say with certainty that they are medicinal? Can they be used therapeutically in a healthcare setting? What about outside of a healthcare setting? Do we get the same benefits from sniffing plants that we might get from swallowing a plant extract? Though perhaps we cannot answer each of these queries with a hard yes, there is clearly a place for terpenoid therapy as a part of a holistic wellness regimen. Let us examine some literature…

            At this point in our education, we may call it common knowledge that rosemary extract has a positive effect on memory, even in Alzheimer’s patients.[iii] Although we often state this, many available studies that have demonstrated the positive effects only report results over a short period of time following acute administration of a plant extract. Though plant extracts are clearly powerful and beneficial, we do often tend to suffer from confirmation bias. One example of this is a study of 144 participants orchestrated and analyzed by Moss, et al., which found that people performed significantly better on a working memory assessment in a cubicle that smelled of rosemary oil than they did in a cubicle without any specific aroma.[iv] The researchers compared rosemary aroma, lavender aroma, and no aroma groups; they found that both rosemary and lavender aromas improved the moods of the study participants, but only rosemary improved their working memories. The lavender aroma decreased the working memory performance of participants compared to the no-aroma control, which is consistent with lavender relaxing them. Interestingly, alertness for both rosemary and lavender groups also decreased, even though total memory quality increased for the rosemary group. This suggests that olfactory stimulation may consume a significant part of total brain processing capacity, and all the results together remind us that essential oils should be used mindfully to maximize the benefit(s) that we expect from them. A drawback to this study (though by no means limited to this study) is that, although volatile oils were used, whole plants were not. Perhaps potted desk plants would change the outcome. We also do not know the specific intensity of the olfactory stimulation. We may hypothesize that, if the partial pressure of essential oil were unnaturally high, using real plants could decrease the total olfactory load, thus also decreasing potential negative effects reported in the experiment. However, a lower total olfactory load might also render the positive effects more subtle. Indeed, another study involving spearmint and peppermint extracts determined that both cognitive performance and mental stamina were increased upon oral consumption of encapsulated mint oil in a dose-dependent manner.[v] Although only two doses were queried, the beneficial results did not appear to taper off within their experimental range of concentrations. Interestingly, though, two of the three performance metrics appeared to decrease 6 hours after an initial dose of the lower concentration oil relative to the placebo group (but not the higher concentration). This could be due to a variety of factors, including the actions of mint oil on GABAA receptors, or perhaps a contrast between stimulated and baseline states of neurons. It could even be due to a compensatory effect in expression of relevant receptors. We cannot discount the fact that there were only 24 participants in this study either, which means that the results may not be generalizable to the broader population. While the active components of mint oil are different from those in rosemary oil, most notably due to the presence of menthol and menthone, many of the minor terpenoids are similar. Indeed; mint, rosemary, and sage (which is also commonly studied for its neuroprotective effects) are all members of the Lamiaceae (mint) family.

The previous two studies focused on younger, healthy individuals, but when we talk about neurodegeneration, we are usually concerned with older adults and a longer time scale. Another study by Dehghan, et al. focused on memory issues in hemodialysis patients and compared the effectiveness of aromatherapy on memory. They experimented with rosemary, lavender, and orange oils over the course of one month during dialysis and followed up one month after therapy.[vi] The results of the study are mixed, but rosemary was clearly the most effective treatment overall, as it appeared to improve prospective memory and retrospective memory scores both immediately following the regimen and one month following. However, the improvements are only statistically significant (p-value < 0.05) for retrospective memory. All three aromatherapy oils had a noticeable effect on prospective memory compared to the control group; although there was little to no improvement in memory, there was also little to no decline in memory, as observed in the control group. All three oils also had a positive effect on retrospective memory compared to the control as well, however as noted previously, rosemary had the only statistically significant effect, as judged by the p-value. Still, the order of effectiveness appears to be rosemary > lavender > orange > control. The study also measured medication adherence, but it appeared to not be correlated much. In fact, if anything, it appears that the decrease in adherence could be slightly more prevalent one month following in the aromatherapy groups than in the control group, which is consistent with apparent drawbacks determined from the other studies examined herein, but not statistically significant.

Following the literature search, we note that very few available studies (nearly none) report on the effects of live plant-derived terpenoids, as opposed to an essential oil or oral extract, on high cognitive function, although we do know that plants can help relieve stress. Still, we note that live plants may exhibit significantly altered terpenoid profiles compared to their extracts, and may also confer different benefits. Obviously, we need to know more, but it will be difficult to isolate the proper experimental variables.

Though this literature search did not turn up many rigorous studies of desk/house plants, one particularly notable in silico study that did turn up by Roviello, et al. sought out to discover whether any houseplants could provide a true “indoor forest bathing” experience. The researchers determined by molecular dynamics calculations that there are at least two volatile compounds emitted from the peace lily, a common houseplant, that likely outperform known inhibitors of the SARS-CoV-2 main protease.[vii] Their study is entirely computational, but based on previous findings that eugenol, the characteristic clove flavor also found in nutmeg, cinnamon, and basil (among others), and umbelliferone, a compound found in many umbellifers (Apiacieae family) like carrot and also in citrus (Rutaceae family), are both inhibitors of the COVID19 protease. The researchers gathered computational docking data from a crystallized protein structure(s), then experimentally docked at least 15 volatile molecules from common houseplants near the known binding site of eugenol. Their findings indicate several compounds that may also inhibit the protease, some of which were experimentally validated in other studies, including α-pinene, farnesene, and linalool oxide. The most stable protein-ligand interactions indicated in the study are with β-costol and sesquirosefuran, which are distinct volatile sesquiterpenoids emitted appreciably by the peace lily. The authors conclude that “indoor forest bathing” certainly may be beneficial, and would probably encourage those in charge of maintaining any public places to make room for peace lilies in their floor plans.

We have now examined a healthy variety of terpenoid studies and can formulate some valuable conclusions regarding the effectiveness of terpenoids in horticultural therapy. Returning to the questions posed in the introduction: Yes, terpenoids can be used medicinally. They are powerful plant-derived compounds that, just like pharmaceuticals, can be very effective in certain situations, but also potentially detrimental in other situations. For example, lavender terpenoids may elicit a feeling of well-being in patients, but they may also decrease cognitive performance on mentally demanding tasks. We know now that rosemary has, indeed, been used in healthcare settings to mitigate memory loss in older patients, and we have seen data that suggest it and mint oils can improve cognitive performance outside of a healthcare setting as well. Finally, we introduced compelling evidence that keeping houseplants, specifically peace lilies, may mitigate a COVID19 infection, in addition to smelling nice. Herein, we have only scratched the surface of the wonderful world of terpenes and terpenoids. With this knowledge in mind, and a love for the natural world, the reader is encouraged to do their own field research to learn more… And by that, I mean go get out in the gardens and sniff some plants!

References:

[i] Guenther, et. al, The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1): an extended and updated framework for modeling biogenic emissions, Geosci. Model Dev., 5, 1471-92. (2012)

[ii] Wade, Map: Organic Chemistry (Wade) Complete Semesters I and II, <https://chem.libretexts.org/>. (2025)

[iii] Lim, et al., Natural Products and Their Neuroprotective Effects in Degenerative Brain Diseases: A Comprehensive Review, Int. J. Mol. Sci., 25. (2024)

[iv] Moss, et al., Aromas of rosemary and lavender essential oils differentially affect cognition and mood in healthy adults, International Journal of Neuroscience, 113(1), 15–38. (2009)

[v] Kennedy, et al., Volatile Terpenes and Brain Function: Investigation of the Cognitive and Mood Effects of Mentha × Piperita L. Essential Oil with In Vitro Properties Relevant to Central Nervous System Function, Nutrients, 10(8). (2018)

[vi] Dehghan, et al., The effects of lavender, rosemary and orange essential oils on memory problems and medication adherence among patients undergoing hemodialysis: A parallel randomized controlled trial, Explore, 18, 559-556. (2022)

[vii] Roviello, et al., Evaluating In Silico the Potential Health and Environmental Benefits of Houseplant Volatile Organic Compounds for an Emerging ‘Indoor Forest Bathing’ Approach, Int. J. Environ. Res. Public Health, 19, 273. (2022)