1. Search Strategy and Study Selection
A comprehensive literature search was conducted using PubMed, ScienceDirect, and Google Scholar databases. Keywords such as “aromatic scents,” “essential oils,” “fragrance compounds,” “neurological effects,” “respiratory response,” and “cardiovascular impact” were used. The inclusion criteria were as follows: 1) studies published in peer-reviewed journals up to 2023, 2) studies involving either human or animal subjects, and 3) studies focusing on inhaled or topical fragrance exposure. The exclusion criteria included articles that discussed the ingestion of fragrance compounds, review papers without original data, and studies not available in English.
2. Executive Summary
These findings indicate that aromatic scents contribute to mood regulation, cognitive enhancement, and physiological balance through specific neural and biochemical pathways. Despite the promising outcomes reported across several studies, the review also identified notable inconsistencies in the experimental design, limited longitudinal data, and a lack of standardized dosages.
This review highlights a significant gap in the existing literature. Most studies have focused primarily on psychological aspects or individual physiological effects, often overlooking the integrated impact of complex, blended fragrances. Furthermore, current research tends to focus on individual scent components rather than on interactions among various aromatic ingredients. This underscores the need for future studies to explore the effects of compound fragrances and the combined effects of their ingredients on human health.
Although some laboratory, clinical, and community-based studies have been conducted, they remain limited in scope. A future study, which will be published in a subsequent paper, will aim to address these gaps by incorporating comprehensive research methodologies to explore the holistic effects of aromatic scents and their potential therapeutic applications.
In conclusion, while aromatic scents hold significant potential in therapeutic contexts, a more nuanced and evidence-based understanding is required to maximize their benefits and mitigate their possible risks.
Accordingly, this review addresses the following research question: To what extent do aromatic scents affect the neurological, respiratory, and cardiovascular systems and how do these effects vary between therapeutic and potentially harmful outcomes?
3. Introduction
Aromatic scents and candles have gained immense popularity in recent years, not only because of their pleasant fragrance, but also because of their perceived therapeutic benefits [1]. Humans can smell and identify 10,000 different odors, each determined by their unique chemical structures. Additionally, each odorant interacts with a group of receptors and each receptor type responds to a variety of odorant molecules. Olfactory receptors are sensitive to the molecular and physical characteristics of odorant molecules [2] [3].
Essential oils are extracted from plants or fruits and used for aromatherapy and as fundamental fragrance components [4].
Many cosmetics and household products contain natural and synthetic fragrances, such as body lotions, shampoos, laundry detergents, air fresheners, and disinfectants [5].
Since fragrance is typically a complex mixture of several chemicals, it is difficult to confirm its safety [1], and its potential side effects on the body, particularly on vital systems such as the brain, lungs, and heart, have raised concerns among researchers and healthcare professionals.
The industry’s testing procedures for FPs focus on assessing their effects on the skin, disregarding their impacts on respiratory, neurological, or systemic functions [6]. In addition, owing to the complex components of FPs, which are typically composed of a diluent (usually ethanol) and a fragrance formulation consisting of hundreds or more Fragrance Raw Materials, the safety of these mixtures is generally assessed based on the information available for the individual ingredients. This is because each element in fragrance compositions can be arranged in various ways, and because of the commonality of these ingredients in different compositions [7].
Fragrance molecular weight is less than 300 Da, so humans can detect it using the olfactory system [3]. Typically, a quality perfume’s fragrance lingers for 6 to 8 h, whereas the scent of laundry products can endure for weeks [6].
It is important to note that the safety data sheet and label of fragmented consumer goods do not have to disclose all content to the public [8]. This means that less than 10% of all ingredients found in product analyses are declared, if any, at all [9]. The lack of full disclosure is a significant concern for consumers.
More than 156 volatile chemical compounds were found to be emitted from 37 fragmented consumer items. The most prevalent terpenes are (e.g., limonene, alpha-pinene, and beta-pinene). However, less than 3% (of over 550 volatile substances emitted into the atmosphere, including dangerous air pollutants such as formaldehyde) were disclosed on product labels and safety data sheets. Furthermore, even so-called green, organic, and natural fragrance products release hazardous air pollutants [10].
Fragrance can affect the brain and nervous system. Some of these effects are instantaneous and temporary, encompassing both the physiological and emotional aspects. These components often overlap, complicating the evaluation of whether an effect is solely physiological or is influenced by subjective factors [6].
4. The Mechanism of Fragrance Absorption
Various routes, including skin absorption, inhalation, ingestion, and olfaction, enable the fragrance to enter the body, as illustrated in Figure 1. These substances can affect any organ or system upon entry into the body [6].
Historically, fragrances have been considered as skin allergens and irritants. Until the late 1970s, it was believed that the skin acted as a strong barrier against fragrance absorption. However, it is now known to be an ineffective barrier against a wide range of chemicals. This serves as the body’s entrance point for materials. Once they enter the system, systemic effects can occur [6].
Fragrance interactions with the skin are influenced by numerous factors. Individual ingredients can produce vastly different effects compared to intricate mixtures. In addition, certain components utilized in perfumes can modify the skin surface tension, allowing for deeper skin penetration. Research has shown that
Figure 1. Illustrations of the fragrance absorption mechanism.
some terpenes can significantly boost dermal uptake of medications. It is believed that they disrupt the stratum corneum, thereby facilitating greater skin penetration. Terpenes are prevalent in scented products and are likely to enhance the absorption of other ingredients within these products and their fragrance components [11].
Various fragrance substances have been recognized for their potential to induce respiratory sensitization. Fragrance compounds function as haptens on the skin, linking them to body proteins to form allergens [12]. Skin contact contributes to respiratory sensitization. The same mechanism is believed to play a role in the onset of respiratory sensitization to certain chemicals [13].
Nerve impulses from receptors in the nose follow a direct route to the brain, where smells are recognized. The olfactory pathways establish the most straightforward link to the brain compared to any other pathway and allow toxic substances to access the brain. Trigeminal nerve receptors are expressed in the olfactory epithelium. Trigeminal receptors are located in the eyes, nose, mouth, face, scalp, and airways, whereas olfactory receptors are exclusively located in the nose. The trigeminal nerve is stimulated by approximately 70% of odorants, which cause cold, hot, tingling, or irritation sensations. Trigeminal nerve stimulation materials can irritate the nose and airways, and cause eye tearing [6].
108 out of 1027 (10.5%) households randomly surveyed in eastern North Carolina reported that one or more household members had negative reactions to perfumes [14].
5. The Effects of Fragrance Inhalation
5.1. Neurological Effects
According to one study, exposing a group of male Swiss-Webster mice to emissions of five FPs for 1 h caused neurotoxicity and acute respiratory difficulties, such as sensory and pulmonary irritations. Neurotoxicity worsened after the mice were repeatedly exposed to FPs. Gas chromatography-mass spectrometry (GC/MS) analysis of FPs showed the presence of some chemicals that have notarized their irritant and neurotoxic properties. Based on the experimental data and chemistry, it is predicted that human exposure to these FPs could have side effects, including eye, nose, and throat irritation; respiratory difficulty; and Central Nervous System (CNS) reactions such as dizziness, incoordination, confusion, and fatigue [15]. This was further substantiated by the identification of specific neurotoxic compounds within the fragrance products, which likely contributed to the adverse neurological outcomes observed in this study. However, the lack of standardized fragrance concentrations and reliance on animal models limit the applicability of these findings to humans.
Some fragrances and chemicals, such as musk ambrette, have been found to have neurotoxic properties and can cause photosensitivity and contact sensitization in rats [16]-[18].
Inhaling fragrances has been shown in one study to affect the adrenal system via the sympathetic nervous system (SNS), which may be utilized as a mild regulator of SNS dysfunction. In that study, 43 healthy females were exposed to inhalation of six types of EOs, which are used widely in fragrance and cosmetics, including rose oil, pepper oil, and patchouli oil for 3 - 7 min. The result showed that sympathetic activity was significantly stimulated by fragrance inhalation of pepper oil and others, while rose and patchouli oil inhibited this activity. They also found that fragrance inhalation of rose oil lowered adrenaline levels, whereas inhalation of pepper oil increased adrenaline concentration [19]. Pleasant fragrances may induce relaxation and decrease sympathetic activity, but unpleasant fragrances may increase it [19].
The differential impact of essential oils on sympathetic activity was demonstrated by Haze et al., where rose oil reduced adrenaline levels, whereas pepper oil increased them. Although this suggests the potential for regulating stress responses, the study’s short exposure time and small sample size limit the generalizability of the results.
Some perfumes show increased neuronal proliferation and differentiation rates at very low concentrations. Because perfumes bind olfactory neurons and produce a pleasing fragrance, they selectively influence the development of olfactory neurons. One feature of autism spectrum disorder is an underdeveloped olfactory bulb. The brains of many autistic children grow more quickly than typical, both during perinatal development and in utero. Subsequent brain growth is more normal, or perhaps comparatively slower, in childhood. Early in prenatal life, exposure to some perfumes may have neurostimulatory effects on the newborn brain. As a result, it is likely that the neurostimulator effects that start during pregnancy continue to impact after birth. Therefore, they believed that these perfumes may contribute to the development of autism [20] [21].
According to some studies, fragrance compounds have the same frame-of-mind effects as alcohol and tobacco by acting on brain receptors, which raises the intriguing possibility of fragrance addiction [6].
5.2. Respiratory Effects
It is well known that several scent materials may sensitize the respiratory system [6]. Limonene is one of the most widely used terpenes in fragrances, air fresheners, and cleaners. Thus, it is considered a relatively safe additive. Limonene inhalation is the most common route of exposure. Exposure to 4500 μg/m3 limonene may lead to sensory irritation by producing chemicals that can induce skin and lung sensitivity when oxidized. Therefore, manufacturers frequently use antioxidants to prevent the formation of sensitizing chemical compounds [6] [22]. Air fresheners emit substances that enter the respiratory system by inhaling and reaching the alveoli. The eyes, nose, and skin were directly impacted when air fresheners were used [22].
A cross-sectional study of 112 fragrance industry workers in the UK was conducted to compare lung function between workers exposed to fragrances who work in production and non-exposed workers who work in administrative functions. The results showed no significant effects on lung function [23].
Peppermint essential oil has been found to have highly positive effects when inhaled by patients with disseminated and infiltrative pulmonary tuberculosis undergoing a combined multidrug therapy [24] [25]. Inhalation of the essential oil of peppermint (Mentha piperita L.) for 20 min for 2 months as a supplement to combined multidrug therapy for pulmonary tuberculosis has an in vitro antimycobacterial effect. Additionally, it has demonstrated prior positive lung X-ray changes and attenuation of intoxication syndrome, which suggests that patients with disseminated and infiltrative pulmonary tuberculosis may benefit from the use of peppermint essential oil in combination with antibiotic therapy [25]. However, these findings require validation through double-blind placebo-controlled trials to establish their efficacy and safety.
5.3. Cardiological Effects
Inhalation of EOs is considered effective in reducing stress and blood pressure in patients with essential hypertension [26]. Lavender contains substances that calm and relax. Mice that inhaled linalool, a substance present in lavender, showed 40% reduction in motility. Caffeine stimulation did not restore activity to normal levels. In another study, they found that after inhaling lavender for ten minutes in females, the blood flow increased, and the systolic blood pressure and galvanic skin conductance decreased, suggesting a decrease in sympathetic nerve activity followed by a drop in blood pressure [6] [27].
Inhalation of citrus aroma leads to a significant increase in breathing rate, heart rate, and diastolic blood pressure in healthy individuals because it is considered a stimulating flavor [28]. In contrast, another study found that exposing patients to different essential flavoring substances, including citrus, decreased SBP [29]. It also regulates BP and reduces SBP by altering neuronal circuits related to anxiety and pain in patients with acute myocardial infarction (AMI) [30].
5.4. Other Physiological Effects
Some essential oils such as lavender, bergamot, and lemon balm have shown analgesic effects in mice, rats, neonates, and premature rats [31]. Other studies have reported that inhalation of rose oil decreases anxiety-like behavior in rats [32].
A database of 153 fragrance chemicals in children’s products was recently released. Fragrance chemicals are classified based on their chemical structures, sources, chemical origins, odor profiles, physicochemical properties, and predicted data on absorption, distribution, metabolism, excretion, and toxicity (ADMET). Several fragrance chemicals have been identified in children’s products as pollutants of concern because they have been linked to potential carcinogenic, endocrine-disrupting, neurotoxic, phytotoxic, and skin-sensitizing effects [33].
Long-term exposure to air fresheners leads to the accumulation of their emitted compounds in fatty tissues, which can make weight loss difficult as a defense mechanism to protect the immune system because the toxic substances stored in these tissues when they decompose release these toxins [34]. In a study on the effects of the scent of lavender oil (SLVO), they found that it affects autonomic nerves and increases appetite and body weight by decreasing lipolysis and energy consumption [35]. The toxic effects of these VOCs are caused by their lipophilicity [36], which can result in their accumulation in the lipid bilayer of the cell membranes with prolonged exposure [37].
Reactive oxygen species (ROS) are cytotoxic agents that cause oxidative stress by damaging the cell membranes and DNA [38]. VOCs injure the lungs, brain, and other organs by producing reactive oxygen species (ROS) [37]. According to one study, oxidative stress plays a role in VOCs genotoxicity [37]. Oxidative stress accelerates cell aging by damaging fibroblasts as well as nervous and muscle cells. In addition, it leads to cell death by damaging the antioxidant system, promoting lipid peroxidation of the cellular membrane, and damaging the cellular organelles [39] [40]. Most studies on odors have examined single VOCs or one EO at high concentrations. Therefore, there is an urgent need to examine chemical mixtures at low concentrations [41].
5.5. Summary of Reviewed Studies
Table 1 summarizes all studies mentioned in this paper that examined the effect of fragrance inhalation on the body.
Table 1. Summary of studies on aromatic scents and their effects.
Study/Authors |
Focus |
Subject |
Findings |
Type of Substance |
Dosage |
Methodof Use |
Target System(s) |
Anderson & Anderson, 1998 |
Fragrance inhalation |
Mice |
Neurotoxicity, respiratory issues, CNS symptoms |
Fragrance Products(FPs) |
Not specified |
Inhalation |
Brain, Lungs |
Cronin, 1984; Parker et al., 1986; Spencer
et al., 1984 |
Neurotoxicity of musk ambrette |
Rats |
Photosensitivity, neurotoxicity |
Muskambrette |
Not specified |
Skin contact |
Brain |
Continued
Haze et al., 2002 |
Fragrance and SNS |
43 females |
Modulation of adrenaline and sympathetic activity |
Essential Oils (Rose, Pepper, Patchouli) |
Inhalationfor 3 - 7 min |
Inhalation |
Heart |
Bagasra et al., 2013; Sealey
et al., 2015 |
Perfumes and neurodevelopment |
Hypothetical/Autism link |
Neurostimulator effects may contribute to autism |
Perfumes |
Prenatal exposure (hypothetical) |
Inhalation during prenatal stage |
Brain |
Bridges, 2002a |
Fragrance effects |
Humans |
Possible addiction, respiratory sensitization |
Various Fragrance Compounds |
Not specified |
Inhalation/Skin contact |
Brain, Lungs |
Kim et al., 2015 |
Air fresheners and respiratory effects |
General |
Sensory irritation, VOC emissions |
Air fresheners (limonene) |
4500 μg/m3of limonene |
Inhalation |
Lungs |
Dix, 2013 |
Occupational exposure in fragrance industry |
112 workers |
No significant lung function effects |
Occupational fragrance exposure |
Workplace exposure |
Inhalation |
Lungs |
Shkurupiĭ et al., 2002, 2006 |
Peppermint oil and TB |
TB patients |
Improved symptoms and lung function |
Peppermint Essential Oil |
20 min/dayfor 2 months |
Inhalation |
Lungs |
Hwang, 2006; Saeki, 2001 |
Lavender and blood pressure |
Humans |
Reduced BP, sympathetic activity |
Lavender Oil |
10 min |
Inhalation |
Heart |
Goepfert et al., 2017; Cha et al., 2010; Rambod et al., 2020 |
Citrus aroma and cardiovascular effects |
Humans |
Varied effects on BP and heart rate |
Citrus aroma, essential oils |
Variable exposure |
Inhalation |
Heart |
Sattayakhom
et al., 2023; Ueno et al., 2019 |
Analgesic and anti-anxiety effects |
Rodents |
Pain relief, anxiety reduction |
Lavender, bergamot, lemon balm |
Low concentrations |
Inhalation |
Brain |
Ravichandran et al., 2022 |
Fragrance chemicals in children’s products |
Children’s products |
Presence of harmful chemicals |
Various fragrance chemicals |
Varied concentrations |
Use in children’s products |
General (all systems) |
Chiang et al., 2016;Shen et al., n.d. |
Fragrance, metabolism and weight gain |
Rodents |
Weight gain, fat accumulation |
Lavender oil |
Not specified |
Inhalation |
Brain, Metabolism |
Wang et al., 2013; Kabuto
et al., 2003 |
VOCs and oxidative stress |
Rodents |
Genotoxicity, cell damage |
VOCs |
Low-dose exposure |
Inhalation |
Brain, Lungs |
Suk et al., 2002 |
Chemical mixtures in odors |
Review |
Need for research on chemical mixtures at low concentrations |
Various VOCs |
Low concentrations |
Inhalation |
Brain, Lungs |
6. Conclusion
In conclusion, this review underscores the significant yet complex role of aromatic scents in vital human systems. Although many findings suggest beneficial effects, variability in methodology and reporting standards limits the ability to draw definitive conclusions. The review recommends further research that focuses on standardized approaches, controlled clinical environments, and exploration of the biochemical pathways involved. Future studies should also aim to determine the specific aroma compounds and their optimal applications in therapeutic contexts.
7. Research Gaps
Several research gaps have been identified that warrant attention based on a critical review of existing studies:
1) Fragrance exposure parameters—such as concentration, duration, and method of delivery—are rarely standardized, making cross-study comparisons difficult.
2) Many investigations focus solely on individual fragrance compounds rather than examining real-world mixtures, which more accurately represent consumer exposure.
3) There is a lack of long-term, longitudinal studies evaluating the chronic effects of fragrance exposure on the nervous, respiratory, and cardiovascular systems.
4) There is minimal exploration of the synergistic or antagonistic interactions between fragrance components and their effects on various physiological pathways.
Addressing these gaps is crucial to establish a robust and reliable scientific framework for the application of aromatic scents in therapeutic and consumer settings.