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.

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?

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] .

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

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] .

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.

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.