Introduction

The majority of the global population dwells in cities1, where rates of mental illness and stress disorders are higher than in rural areas2,3. Consequently, the need to integrate health and well-being into urban planning and governance is growing4,5 and is highlighted in global policies and guidelines suggesting how to ‘ensure healthy lives and promote well-being for all at all ages’ for a more sustainable future6. Exposure to urban green spaces or nature is one potentially important way forward towards better health7. There is increasing evidence that exposure to nature can have positive effect on physical and mental health, promoting increased well-being8,9, stress reduction10,11 and positive long-term benefits for chronic diseases and cognitive functions12. However, the full extent of how different qualities of nature affect us, through self-evaluation or psychophysiological measures, is not fully understood. By investigating psychological and physiological responses elicited by natural stimuli, it is possible to better understand the details of what enhances potential health benefit in natural settings. This is crucial if we aim to design and manage healthy environments close to where we live. Health benefits include physiological changes in individuals, such as effects on blood pressure, the immune system, or the autonomic nervous system, as well as psychological benefits, such as supporting mental health and well-being by reducing stress levels or eliciting positive feelings7,13. Inputs from all our senses, such as hearing, sight, touch, taste and smell, play an important role in how humans experience the world and connect with nature. Sight is considered the dominant sense in humans, and processing visual information seems to dominate over the processing of information from other sensory modalities14. Accordingly, most research on urban green spaces and human health has primarily focused on the visual qualities of the surrounding environment15, while less is known about the importance of sounds, smells, and tactile elements in the beneficial effects of experiencing nature16,17,18. However, the senses do not function independently but operate together, providing integrated multisensory information17. All senses influence mental health and well-being in various ways (e.g. reducing distress and monotony, fostering a sense of belonging and safety), emphasizing the need to consider multisensory experiences in urban planning to support mental health and well-being in cities19.

Additionally, human senses are much more receptive to smell than previously thought20. Smells provide important information about potential hazards, social interactions, and strongly influence how people act, think and behave21. Smells also play a vital role in perceiving the surrounding environment (e.g. the smell of smoke), shaping how people interact with it22. The experience of smells is essential in people’s everyday lives, and smells are often considered key elements for fully experiencing an environment, especially when combined with stimuli from other sensory modalities23. One way to describe the relationship between human experiences of smells and the surrounding environment is through the concept of “smellscape”. Smellscape is defined as “the totality of the olfactory landscape as perceived and understood by an individual”24. Xiao et al.25 reveal that smells are “spatial-emotional intermediary, bridging interpersonal experiences of smells and the social-spatial structure of place”. Thus, smellscape are an intangible element of a place that contribute to creating place-identity for communities, enhancing space qualities, and valuing its cultural heritage26,27. Smellscapes play a vital role in experiencing a place and may induce a sense of belonging28. Different cultures have different traditions for creating and value smellscapes, as seen in the “One Hundred Sites of Good Fragrance” across Japan or the creation of odour-themed features in the streets of Grasse, France24.

The sense of smell, or olfaction, stands out from other sensory systems. It is more potent than our other senses in evoking long-term memories, modulating the autonomic nervous system, and triggering both negative and positive emotions29. Much of these effects can largely be explained by the unique aspects of neuroanatomy, where the olfactory system has unfiltered access to the brain via its non-obligatory thalamic relay, as well as the fact that the amygdala (a brain structure implicated in processing saliency and emotion), hippocampus (memory encoding), hypothalamus (body homeostasis regulation) are all only two synapses away from the olfactory receptors30. The perception of smells is highly subjective, influenced by many individual aspects (e.g. age, gender, culture, past experiences, familiarity), as well as environmental features (e.g. weather, temperature), spatial patterns and human activities31,32. Looking on objective physiological measurement, smells seem to have even greater effects in facilitating physiological health benefits than other senses. In a multisensory study in a virtual reality comparing three different environments (a forest, a park and an urban area), Hedblom et al.33 found that olfactory stimuli were more effective in reducing stress than visual or auditory stimuli. The stress-reducing effects were higher in the two natural environments compared to the urban area. The olfactory stimuli were congruent to the specific environment, such as smell of grass in the park, fir and mushrooms in the forest and tar and diesel in the urban area, thus smells from nature seem to be particularly important.

To date, smells in cities have mainly been highlighted for its negative effects, focusing on the elimination of bad odours and control of odour pollution rather than for their potential positive benefits34. Existing policies and legislation related to smells in cities focus on controlling odour emission of substances that result to be harmful to humans, or on management of air quality levels24,35. There´s no current distinction on how to managed smells perceived as pleasant or unpleasant35. Urban planning focusing on the elimination of smell pollution may result in a deodorization of places with a possible loss of sense of place and a collection of sensorial experiences28. Thus, by acknowledging in current policies the importance of positive aspects of smells, it is possible to design more meaningful environments28. Understanding the potential health benefit of experiencing smells in cities will permit us to use smells as a resource to create healthier and sustainable environments.

Research on the effects of exposure to smells have largely focused on stimuli that mainly include natural essential oils, aromatic, volatile liquids extracted from different parts of plants (e.g. flowers, stems, leaves or roots) or wood material36,37 through various techniques and named according to the plant from which they are derived38. Inhalation of these essential oils has been shown to result in changes in brain activity39 and autonomic nervous system responses40, an increase in mental and physical relaxation36,41, positive effects on the immune system42, beneficial influence on several conditions such as anxiety, insomnia, stress and pain, with a general increase in well-being43,44. Although essential oils are products extracted from part of plants, they lack the complexity of smells of nature that we as humans experience from real settings. Furthermore, smells are perceived within a context, and smells may be tied to an individual´s sense of place, often calling to mind associations with, or memories of, specific landscapes45. The chemical profile of an essential oil varies depending on the technique used for extraction, and several elements can influence the quality, quantity and composition of the extracted product46, making it almost impossible to reproduce the existing smells found in natural settings. Moreover, extracting essential oil requires large quantities of natural materials, equipment, energy and time, and using smells emitted directly from natural sources represents a more sustainable approach to creating healthier environments. Nonetheless, our main encounter with smell from nature is through exposure to the real natural sources (e.g. various plants, trees, water and soil) in daily life and not through perfumes or essential oils. Thus, this review focuses on smells that are non-artificial and undiluted. Although we acknowledge that animals and humans are considered to be part of nature, this paper does not focus on animal or human smells. Instead, the focus is on those smells of nature that can be implemented, modified, and managed in urban environments. Thus, in this paper, the term smells from nature covers all scents emitted by components and phenomena within vegetation (e.g. different species of plants, trees or flowers), water bodies (e.g. rivers, lakes, marshes, rain) and originated from the ground (e.g. soil, moss, decaying leaves…).

Nature is rich in various smells, which represent a significant component of the natural environment and delineate its unique smellscape. Smells from nature are greatly influenced by environmental factors such as weather conditions, temperature, wind, the different seasons and time of day. Elevated temperatures, for instance, cause more molecules to evaporate from their sources, increasing the production of smell24. Plants and flowers may emit smells spontaneously or after being triggered by an external stimulus, such as rain, wind, high temperature or physical touch47. The changing of seasons and day-night cycles also influence the amount, concentration and variety of smells from nature emitted by plants, with some species releasing more smells during different times of the day, morning, evening or night31, or in specific seasons47. Smells from nature are not distinctive of natural environments but can also be found in urban environments, within different spaces such as gardens, parks, flower beds, trees or plants spread throughout the cities, as well as in plants, wood or flowers in indoor environments. This suggests that smells from nature are accessible and readily available in everyday life experiences. Smells from nature are commonly perceived positively25,48 and are frequently associated with positive feelings16. However, not all smells from nature are considered to be pleasant, for example, the smells of decaying leaves or stagnant water, although the perception of smell pleasantness is highly influenced by individual preferences49. Investigating the effects of both pleasant and unpleasant smells will help to understand the possibilities in implementing smells from nature in cities. Urban planning should consider how to implement positive smells, but at the same time how to possibly control and manage negative smells by eliminating, blocking or masking those smells from nature that result to be harmful or unpleasant for human. Additionally, implementing smells that result to be positive will aid in identifying and selecting plant species whose integration into urban planning may better support mental health and well-being.

One common method for evaluating the perception of smells is self-reported assessments conducted during a smellwalk, a sensory walk where participants focus on identifying, describing and evaluating the perceived smells in their surroundings24. In-situ studies evaluating the experience of smells in cities, such as smellwalks, are more ecologically valid than indoor studies, meaning they are more generalizable to real-world situations, making them more relevant for understanding the practical implication of smells in everyday life. Smellwalks focus on the collection of various existing smells but, beyond rating the simple degree of pleasantness of individual smells, they do not further investigate the emotions connected with or elicited by these smells50. As Parker et al.50 noted in their review, current studies on smellwalks are exclusively centred around the urban context, with some including urban parks or open spaces to create olfactory profiles of various places in cities. There are some studies evaluating people’s experience of natural smellscapes, although these studies also highlight the need to fill this gap further16,18,24.

Regularly experiencing smells from nature is important for people’s daily lives and the occurrence of these smells is linked to the amount of greenery in the area34. It has also been argued that ’as humanity becomes ever more urban and experiences ever less nature7, we are cut off from an evolutionary library of olfactory experiences’22, highlighting the need to integrate smells from nature in the urban context. Recent studies suggest that, given the history of olfactory responses to natural environments during evolution, smells from nature may be more important for human well-being than previously thought22,33. Bratman et al.22 propose a conceptual framework that integrates olfaction into the understanding of the effects of the natural environment on human well-being. In their work, they highlight the various individual and environmental factors that mediate olfactory perception and how exposure to nature through olfactory pathways may affect a range of well-being aspects, such as emotion regulation, quality of life, and dietary choices22. Exposure to smell from nature may, therefore, represent a unique and important pathway between nature and well-being22.

Despite the growing acknowledgement of the importance of experiencing smells from nature, little is known about how different smells from nature influence psychological and physiological responses across various environments. Bringing together expertise from multiple fields, Bratman et al.22 propose the following consensus statement: ´A better understanding of the relationship of human beings with natural olfactory environments can promote appreciation and revitalization of the natural world—and can thereby contribute to human well-being´. Thus, this study intends to better understand the relationship between humans and smells from nature. This paper aim to review the existing scientific literature on the effects of exposure to smells from nature in both indoor and outdoor environments, and explore the different ways in which smells from nature can affect psychological and physiological responses. The overall objective is to highlight both the methodological and application challenges, as well as the potential health benefits associated with smells from nature, to strengthen their implementation in cities to create healthier ecosystems.

Methods

Search strategy

The search was conducted in a structured way, using a combination of search terms. This structure consisted of three lists of terms, each one within different domains of interest: one list with terms related to smell and olfaction (e.g. “Smell”, “Scent”, “Odour”, “Odour” …), a second one with terms related to the natural and urban environments (e.g. “Nature”, “Outdoor”, “Landscape” …) and a third one with terms describing human experiences (e.g. “Perception”, “Wellbeing”, “Stress” …). Each search string was composed of one term from the first list, one term from the second list and all terms from the third list combined using Boolean operators. The full list of search terms and combinations is presented in the Supplementary Information file.

The electronic databases searched included Web of Science (All Databases selected), Scopus and APA PsycINFO. The search was performed within “Topic” (Web of Science), “Article title, Abstract, Keywords” (Scopus) and “Document title & abstract” (APA PsycINFO). Only accessible publications written in English were selected, without any limitations on the publication year. The literature search was conducted between December 2023 and January 2024. Considering the multidisciplinary nature of the topic, no specific research areas were initially selected. However, if the number of documents retrieved for each search string was substantial, then the search was refined by subject area with some exclusion criteria applied to simplify the process. The total number of strings searched was 92 (see Supplementary Information for the search strategy).

In order to achieve a more successful search strategy, the original database search was then complemented with a snowballing methodology51, using the references cited in some articles that were deemed of interest for the research. In addition, a screening and selection of documents citing some of the most relevant publications were made to further refine the results.

Selection criteria

Studies fulfilling the following five criteria were included in the review: (1) The study focuses on smells from nature emitted by the real natural source; (2) The study is conducted in an indoor or outdoor environment, and the source of the smell is derived from the real natural component or part of it (e.g. plants, flowers, wood) or the study took place in an outdoor environment with the presence of smells from the natural surroundings; (3) The participants are humans; (4) There is a focus on psychological and/or physiological responses to the smells from nature in the analysis; (5) In multisensory contexts, there is an analysis of the specific influence of olfactory stimulation.

Following these criteria, studies were excluded if they: (1) Focused on artificial smells; (2) Used essential oils or odorants not derived from a real natural source (or part of it); (3) Involved animals as subjects; (4) Lacked an investigation about the human experience of smells, for example, studies that focus on creating a smell evaluation of the place, mapping an olfactory profile of a specific environment, or improving or evaluating marketing and tourism; (5) Evaluated the general effects of a multisensory setting without specific details on olfactory influence and experience.

Study selection

After an initial screening of titles and abstracts, publications deemed irrelevant to our research topic were excluded. The full text of the relevant studies was then reviewed by the first author and included if it full filled the inclusion criteria (Fig. 1). The final number of studies included in the review was discussed and confirmed by all authors.

Fig. 1: Literature search, selection process and inclusion criteria.
figure 1

(*) The numbers refer to the total results when searching the full combination of search terms and excluding non-relevant research areas (see Supplementary Information).

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Results

The database searches resulted in more than 4800 papers. After the abstract and title screening, 30 studies met the inclusion criteria (Fig. 1).

Publication year

Of the selected studies, six were published before 2014, with the earliest dating back to 2008; eight were published between 2015 and 2020, and the remaining 16 studies were published after 2021. This data trend indicates that research on smells from nature is relatively new and that interest in the topic has grown in recent years.

Sample and study design

Details about the sample of the reviewed studies (i.e. sample size, mean age, gender, and population) are provided in Table 1. Sample size across studies is diverse, spanning from 752 to 470 participants53, which could be explained by differences in sample population (e.g. specific population clusters such as patients with dementia52) and the difference in time required depending on the methodology used (e.g. a single questionnaire compared to multiple physiological measurements).

Table 1 Overview of the major findings of the included reviewed articles linked to outdoor and indoor environments
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The reviewed papers in an indoor setting are experimental studies. Of these, 12 out 18 include a control condition in their study design, thus increasing the validity and reliability of the findings (see Table 1). In the reviewed study in outdoor environments, there is greater diversity in the setting, structure, objectives and methodology. Thus, the typology of study is diverse and includes case studies and experimental research. In an outdoor setting, only 3 out of the 12 studies include a control condition54,55,56. Examples of control conditions used, in both settings, include the administration of air as an olfactory stimulus, a visually similar environment without the presence of smells, or a group of participants not taking part in the olfactory experience.

Aim and scope of the studies

Most of these studies evaluate psychological and physiological responses to exposure to the smell of a specific plant, flower or natural element57,58,59,60,61, or from the comparison of different smells of various species62,63,64,65,66. Some studies further analyse differences in responses to varying concentrations or intensities of the same smell67,68. Other studies investigate the relationship between the natural smellscapes and participants’ subjective perceptions69,70,71,72,73, with a specific focus on the connection between smells from nature and affective states (any experience of feelings or emotions)54, perceived restorative effects73 or on how smells from nature contribute to human self-reported well-being74. Other studies examine the therapeutic effects of smells from nature within rehabilitation programmes or horticultural therapies, using these smells from nature as stimulation during nature-based interventions for healthy adults75 or in specific population clusters, such as maladjusted soldiers56,76, patients with mental stress-disorders77 or patients with dementia52. Finally, some studies focus on the effects of olfactory stimuli in a multisensory environment, aiming to analyse both the combined and separate effects of stimulating different senses52,53,55,78,79,80,81.

Countries

Of the 18 studies conducted in indoor environments using natural components, 17 were conducted in East Asia (China, Japan, South Korea, and Taiwan), except for one study conducted in Israel69. In contrast, the 12 studies conducted in outdoor environments were more spread out among different countries, with a more balanced division between Asia (China, Korea, and Taiwan) and Europe (United Kingdom, Sweden, Poland, and Austria), unlike the indoor experiments.

Environments, setting, stimuli, smell source and time of exposure

Of the 30 studies, 18 were conducted in an indoor environment or in a laboratory. One indoor study68 also included an outdoor condition, but the chosen scented area was a limited, enclosed space with features resembling a laboratory more than a real-world environment. Twelve studies were conducted in an outdoor environment, of which four took place in an urban context. The chosen environments for in-situ studies outdoors included woodlands, multisensory or rehabilitation gardens, and urban areas. Four of the outdoor environments were in an urban context, all conducted in China (see Table 1), specifically urban forest parks73, urban parks71,72 and an urban street with traffic noise55. Thus, very few studies exist on smell in-situ from nature within urban contexts.

In indoor environments, the sources of smells from nature are always clearly defined by selecting specific species of plants or flowers (e.g. “plants of lavender”, “fresh rose flowers”, “fresh herbs of geranium”), resulting in stimuli-specific responses. The natural smell sources used indoors include real plants (e.g. Lavender, pelargonium, primula), fresh flowers (e.g. Lily, rose, osmanthus), flower petals (e.g. Japanese plum blossom, jasmine, rose), tree needles (Pine tree), wood chips (Hinoki cypress), leaves (needle fir), fresh herbs (e.g. geranium, citrus, herbal tea) and even sediment from an urban riverbed (see Table 1). Of the four outdoor studies in an urban context, two studies investigate the effect of specific species, specifically lilac55, magnolia, tree peony, syringa, osmanthus and wintersweet71. The other two studies72,73 evaluate the effects from exposure to the whole smellscape, including an urban park with scents of e.g. flowers, grass, pine, bamboo, water, soil as well as three urban forests with e.g. pines, cypresses, ginkgoes, mountain apricots, maples and greasy pines. In other outdoor in-situ studies conducted in a natural context, the plants, trees or herbs that release the smells within the chosen environments are also specified. These include gardens (e.g. geranium, rosemary, pine, lemon balm, lavender)52,54,65,77, as well as forests (e.g. coniferous forest, oak woodland, subtropical evergreen broadleaf trees)53,74,76. Some studies refer more generally to smells from nature, describing them as “forest smell” or “smells of plants”56.

The time of exposure to various smell sources varies between 18 s64 and 4 h76,77, with most studies ranging from 5 to 30 min (see Table 1).

Timeframe and season

Only a few studies investigate changes in the perception of smells from nature across different times and seasons. Xiong et al.60 examined the effects of smells from nature at different times of the day (morning, afternoon, and evening). They found that exposure to certain smells from nature in the afternoon reduces negative emotions more significantly, while evening exposure to the smell enhances physiological relaxation. Weber and Heuberger54 reported that the beneficial effects of smells from nature are more pronounced at night when vision stimulation is limited. Two in-situ studies investigated how the perception of smells from nature varies across seasons. Pálsdóttir et al.77 conducted a five-year longitudinal study across different seasons, while Bentley et al.74 investigated the changes in the smellscape of the same two forests in winter, spring, summer and autumn. Their results show that the intensities and types of smells from nature vary with the seasons, with less smells observed during winter. They also observed seasonal influences on participants’ sensory experiences and perceived well-being, with smells of autumn contributing the most across different domains of well-being (physical, emotional, cognitive, spiritual and global), followed by those of summer, spring, and winter. Song et al.71 selected plants that bloom in all four seasons to cover the whole changes in the smellscape. Weber and Heuberger54 had different experimental conditions in different months (from May to July) with smells elicited from different seasonal plants. Most of the remaining in-situ studies have been conducted only during one season, specifically spring55,73,76, winter56, autumn53, or summer72.

Indoor environment studies

Most studies include both physiological and psychological measures in their experimental design, but three studies only examine physiological responses59,62,80, and one study includes only psychological measurements70.

The subjective evaluation of the perception of smells from nature in an indoor environment includes a positive effect on mood and emotions, an increased sense of satisfaction, greater human comfort and positive feelings, together with a decrease in negative feelings58,60,61,67. Smells from nature can therefore help alleviate psychological stress, induce relaxing feelings and improve mental states68. Moreover, they have been shown to enhance cognitive performances, with significant improvements in spatial working memory81.

The physiological responses following exposure to smells from nature emitted from real natural sources in an indoor environment, measured using an electroencephalogram (EEG), show significant changes in brain activity. These include an increase in the ratio of α waves and β waves, and an increase in emotional relaxation, improved attention and better physiological states66,67,78,80,81. Significant effects are also observed in the autonomic nervous system, with an increase in both the sympathetic and parasympathetic systems58,61,62. These changes can produce positive effects, such as a decrease in heart rate, systolic pressure, and skin conduction, as well as inducing physiological relaxation and improving vascular functions59,68. Significant changes are also found in the oxyhemoglobin saturation level in the right prefrontal cortex, as measured by near-infrared time-resolved spectroscopy, contributing to physiological relaxation63,78. In addition, muscle comfort, as assessed by the surface electromyography index, increases following exposure to aromatic plants, indicating muscle and body relaxation68. Finally, the inhalation of smells from nature affects the functions of the immune and endocrine systems by reducing salivary cortisol and proinflammatory cytokines levels, which, in turn, decrease stress levels and reduce the body’s inflammatory response. These two systems appear to be more influenced by olfactory stimulation than by visual stimulation, which instead elicits a greater response in the nervous system. This difference suggests that olfaction may have a more direct and significant effect on the immune and endocrine systems than vision81.

Perception of smell in indoor environments

Changes have been observed in participants´ evaluation, arousal, attention levels, muscle comfort, oxyhemoglobin saturation level and the amplitude of α waves, caused by variations in the intensity and concentration of each smell, as well as by comparisons of smells from different varieties of plants or flowers62,63,66,68,75,78. General satisfaction was higher with a slightly-scented plant compared to the greater intensity of a strongly-scented plant78. Furthermore, various species of flowers (e.g. jasmine, rose or lily) can evoke feelings of happiness, relaxation, serenity, energy or refreshment64,66,75, as well as produce contrasting effects on participants’ arousal62.

However, some studies do not show any significant effects from exposure to smells from nature. No significant results were found in participants´ evaluations of positive feelings or their satisfaction levels69,79. Moreover, in some studies, there was no significant effect on the physiological responses, with no activation of the autonomic nervous system, heart rate, blood pressure or skin conductance indices57,61,67,69. As explained in these studies, the lack of significant effects could be due to the short duration of exposure to the smell in the study design and its concentration.

Outdoor environment studies

The four studies conducted in an urban context were all subjective, including questionnaires without physiological measurements. In the study by Li et al.73, vision, sound and smell were compared in three urban forest parks. While smell did not play a major role in psychological restoration as vision did, it was still beneficial to the human body. The second study by Wang et al.72 emphasised a ‘…remarkably strong association…between the degrees of olfactory, visual, and overall experiential satisfaction’, thus highlighting the importance of multisensory experiences. The third study55 revealed more positive evaluations of street sounds with higher concentrations of lilacs (either Syringa amurensis or Syringa oblata). This study confirmed previous research on sound–odour interactions conducted in laboratory settings82. The fourth study71 was conducted in a park and revealed that the evaluation of five different plants, magnolia, tree peony, syringa, osmanthus and wintersweet, showed that all had a distinct smell linked to its plant type.

Experiences of smells from nature were predominantly positively evaluated in outdoor settings, resulting in improved long-lasting emotional and affective states54, eliciting highly-positive personal associations65, evoking positive feelings and mental relaxation77, affecting perceived restorativeness53,73, and influencing physical, emotional and global well-being, leading to a general improvement in general health74.

Only three of the 12 outdoor studies measure physiological responses while experiencing smells from nature in-situ. Koura and Ikeda52 assessed the responses of the autonomic nervous system in people with dementia while walking in a horticultural therapeutic garden, measuring heart rate and heart rate variability. Their findings show an increase in the parasympathetic nervous system activity. Kim et al.56,76 evaluated physiological responses, respectively, the salivary cortisol level and brain activity, at the end of exposure to smells from the forest during a therapy programme, showing a significant decrease in stress levels and tension.

Perception of smell in outdoor environments

Different species of plants and flowers (e.g. osmanthus, magnolia, and tree peony) emit distinctive smells, which participants evaluate with varying characteristics and ratings71. These differences can be influenced by individuals’ personal experiences (sensitivity of the receiver) and the physical features of the environment, such as seasons, weather conditions, or ecological settings74, as well as planting density, space size and openness71. Some smells are vaguely described, such as ‘scent of plants’56, or ‘oak woodland and a mixed deciduous and coniferous plantation woodland’74, while others are more precise, providing lists of species54. Remarkably, all the selected studies conducted in outdoor environments found some effect from exposure to smells from nature (this was not always the case in indoor environments). However, few of these studies compare the results with a control condition (see Table 1).

Discussion

This review found a total of 30 studies investigating smells from nature. Of these, only 12 studies were conducted in-situ natural settings, and 4 were conducted in urban areas, specifically an urban forest73, urban parks71,72 and a street with traffic55. Thus, studies on natural complex smellscapes in outdoor environments seem very rare, and there seems to be a lack of research on smells from nature.

Most people today live in urban areas, where nature experiences are constantly being reduced due to urbanisation83. Additionally, people living in more green areas report better mental health than those living in urban areas84. Here, we present findings indicating that smells from nature, in general, had positive effects on both psychological and physiological responses in humans, with potential benefits for health and well-being, regardless of whether the studies were conducted indoors or outdoors. Smells from nature can affect people´s experience of the environment and influence multi-sensory perception.

Cities ‘do…not know how to incorporate nature and nature contribution to people into city planning’85. Currently, planning and design of urban green environments are dominated by visual aspects86,87. However, incorporating stimulation from all senses (vision, hearing, smell, touch and taste) in urban green planning could create a richer sensory experience of an environment increasing its overall satisfaction72. Thus, multisensory urban design represents a way to promote mental health and well-being19 and create more restorative environments53,73 by maximizing the positive effects of nature on human health88. One application could be seen in Singapore´s initiative to create therapeutic gardens that stimulate all the senses, shifting the focus of garden design towards promoting mental health (https://www.nationalgeographic.com/travel/article/singapore-wellness-mental-health). As explained by Roe and McCay19, one approach to multisensory design for mental health is to maximize salutogenic sensory design while minimizing exposure to negative sensations across all senses, both individually and in combination, integrating sensory stimulation in a cohesive yet engaging way.

Looking at the influence of smells from nature on other senses in experiencing an environment shows mixed results53,55,79,80,81, suggesting that the relationship between stimuli from different senses is a complex phenomenon. The reviewed in-situ studies suggest that, for example, smells can affect the perception of noisy urban environments, increasing both the auditory and olfactory qualities and improving overall street comfort55. However, a pleasant smell may not be perceived as positive if it clashes with other sensory stimuli in the environment, and the results may differ from that of the same smell within an environment with congruent stimuli. What appears clearer is that exposure to combined multisensory stimuli, including visual, sound, smell, touch and taste, results in more positive effects compared to single-sense stimuli alone, regardless of the specific sense involved79,80,81, and these results are similar to the findings from other studies89,90. Therefore, attention must be given to the quality of these stimuli and how they interact with each other. Furthermore, according to Wang et al.72, smells from nature ‘…are able to form an organic whole, rather than existing in fragments, interspersed with the industrial odours of the city…’, suggesting that a complete smellscape integrates better with other sensory elements.

Emerging research explores how to incorporate smells from nature into the design of both indoor and outdoor environments91,92 and highlights their importance to connect with nature within urban contexts93. Wang et al.72 suggest a need to manage and construct smellscapes in urban parks, not only to control negative smells in cities but also to promote the creation of, for example, different gardens with positive smells that can enhance immersive, multisensory experiences that support relaxation and emotional well-being. Krzeptowska-Moszkowicz et al.65 found that associating smells with positive memories can improve mood and perceived well-being for individuals in cities. Thus, experiencing smells in sensory gardens through various species of fragrant herbs can improve the quality of life for residents in big cities. However, numerous factors that can affect smell diffusion must be considered when allocating fragrant plants. The reviewed studies emphasize that, to enhance positive smell from nature in urban areas, city management and design should consider which species to plant, the density of trees to optimize smell diffusion, and the spatial size and openness of the area71. Previous research explains that designing with smell requires evaluating local plant species, their growing conditions, seasonality, environmental factors, functions, and bloom periods91. Human perception aspects should also be considered, such as the distance and height from the source, the physical context, and the engagement required to fully experience the smell91. However, attention must also be given to the possible harmful effects of certain fragrant plants, as they can increase the risk of allergic reactions or other individual issues94. Thus, investigating the harmful effects of various species is relevant to select those species that better mitigate these risks. Understanding the relationships between all these elements that shape the complexity of smell experiences is essential if we want to create sustainable environments that incorporate smells from nature.

Research on smells from nature is emerging, covering aspects from multiple disciplines. Thus, the methodology used is still under development and varies significantly depending on the quality of the study, its main purpose, and its objectives. The ephemeral aspects of smells, the complexity of their delivery, high subjectivity, and strong influence from environmental conditions and uncontrollable factors are some complications that arise when working with smells. Smellwalks, multisensory walks, and outdoor experiences represent important methods for collecting, mapping, and describing perceived smells and their connections to specific environments53,54,55,71,72,73,74. By using these methods, it is possible to identify the variety of smells present in an environment and how their perception may change over time depending on environmental factors (e.g. wind, temperature, rain). Smellwalks also help create an olfactory profile of cities, which can serve as a tool for designing and managing urban green spaces. Rehabilitation and therapeutic programs provide insight into the healing properties of smells from nature emitted by various plants and highlight the value of the relationship between patients and smells from nature52,56,65,76,77. Increasing knowledge about the therapeutic properties of smells by exploring how smells from nature may affect the rehabilitation process, can help classify the effects of fragrant plants and support their implementation in therapeutic gardens. Additionally, experimental studies in controlled settings provide insights into the effects of inhaling smells from specific and defined species to understand the differences among them. Adding in these study design the evaluation of physiological changes provide an objective and complementary approach to exploring the underlying mechanisms and potential health benefits of inhaling smells from nature. Since the perception of smell is highly subjective, the inclusion of physiological measurements can be even more crucial to identify physiological substrates involved when experiencing smells from nature that are common among different subjects. It would also help evaluating the subconscious changes that cannot be investigated through subjective assessments, as these do not always reach human awareness. For example, in an indoor laboratory experiment, Hedblom et al.33 found that people subjectively rated visual features as more important than smell features. However, the objective physiological measurements showed the opposite, with smell being the most strongly correlated with stress reduction. Several of the reviewed indoor studies have shown that exposure to smells from nature in a laboratory setting increases physiological relaxation and reduces stress levels59,66,67,68,80,81. However, investigating whether these physiological responses occur when experiencing smells from nature in-situ may be more ecologically valid than indoor studies. Ecologically valid studies to in-situ smells exposure, may also be more relevant for understanding the practical implications for people´s everyday life experience. Additionally, such studies can help identify environmental qualities that influence the human body at a physiological level, aiming to enhance health and well-being. Only three of the reviewed studies were found in this context52,56,76, none of which were conducted in urban settings. These in-situ studies explored the effects of smells during therapeutic activities, showing that exposure to the smells of a forest56,76 or favourite herbs (i.e. apple mint, lemon balm, rosemary and lavender52), respectively, reduces physiological stress and increases parasympathetic nervous activity. However, while Koura and Ikeda52 measured changes in heart rate in participants walking in the garden, Kim et al.56,76 evaluated physiological responses before and after the therapy in a pre-post design, rather than while experiencing forest smells. This aligns with the dominance of self-reported measurement over physiological responses in current studies on the benefits of nature exposure. Conducting in-situ studies in urban settings presents numerous challenges, including unpredictable occurrence, the use of portable devices, and the difficulty of isolating the effects of smells from other sensory stimuli. However, two recent studies demonstrate the feasibility of conducting in-situ research in urban nature combining physiological and subjective measurements, although their focus is not on smells. Korpilo et al.95 investigated multisensory restorative experiences in real-life environments by combining soundscape perception with physiological measurements of stress recovery across different urban settings. Similarly, Olszewska-Guizzo et al.96 examined the effects of passive exposure to urban green spaces with varying visual qualities using portable EEG devices.

The duration of exposure to the smells represents an additional element to consider when studying smells. Olfactory adaptation is the phenomenon in which olfactory receptors stop responding to an odorant, and the olfactory stimuli are no longer detected97. The time for adaptation to occur usually ranges from 1 to 20 min, although it depends on several factors, including the qualities of the smell (e.g. intensity, concentration) and on the individual perceiving it (e.g. perceived danger of the odorant, current mood)97,98. Thus, phenomena such as olfactory adaptation and cognitive habituation (i.e. the psychological process by which an individual no longer detects a smell after long-term exposure97 need to be considered when implementing smells from nature into urban design, as well as in the setup of studies investigating smells. Some considerations might include factors such as smell intensity, concentration, duration of exposure, or intermittently dispense the smell.

While olfactory stimuli used in laboratory studies mainly consist of smells from nature emitted by specific plants, outdoor studies focus mainly on the effects of the overall smellscape of the environment rather than analysing single stimuli-response effects of different smells. Although one might be walking in a forest with various tree species, Kim et al.56 describe the experience of smell only as “smells of forest” or “scent of plants”, without mentioning the specific species. Some studies, instead, define the species present in a forest environment more specifically (e.g. evergreen broadleaf tree, tropical species, pine, cypresses (See Table 1) but without going into further details about the precise effects due to different species within the environment. However, Bentley et al.74 found that the smells of an oak woodland were described differently and more frequently than those of a mixed deciduous and coniferous woodland, suggesting that smellscapes of different forest types are experienced differently. The results from various studies62,64,66,70,71,74 suggest that not all smells from nature are experienced in the same way, with some smells being preferred over others and potentially eliciting different emotions and feelings. Ba and Kang55, for example, revealed that the fragrance on streets planted with double rows of lilac was stronger than on the streets with single rows, and that the fragrance of Syringa amurensis was more intense than that of Syringa oblata at the same planting density. Considering the above, there is a need to specify plant species in in situ outdoor environments. In the more formal environments such as urban parks or gardens (compared to the less formal environments as e.g. forests), existing reviewed literature often provide more detailed descriptions of the species present, and it usually includes trees and shrubs (e.g. lilac, magnolia, osmanthus) or aromatic herbs (e.g. mentha, salvia, lemon balm). Weber and Heuberger54, for instance, used the smells of seasonal blooming plants of selected species in a fragrant garden as stimuli, showing that complex and natural smells increase calmness, alertness and mood in humans. Song et al.71 analysed the subjective perception of five different species of fragrant plants in an urban park, with an additional analysis of their main fragrance compound, type and diffusion process, and showed how these perceptions changed by species within the same park. For example, osmanthus exhibited a strong, sweet smell and received the highest positive perception rate, while tree peony was perceived as least pleasant with a woody and light medicinal fragrance. Krzeptowska-Moszkowicz65 defined the aromatic herbs in six different gardens and observed that lavender and mint were the preferred plants, and their smells elicited highly positive associations, while salvia brought to mind special memories for only a few respondents. However, few studies have investigated multiple defined species in cities, and these studies are limited to only a few species. Some smells from nature may target specific human states or cognitive functions (e.g. memory, emotions, stress…) more than others, but the characteristics and effectiveness of these mechanisms are not well understood. Additionally, little is known about how the complexity of overall olfactory perception of smells from numerous species is formed, and whether these combinations can provide additional benefits for human well-being. This is also something that Ba and Kang55 highlighted, acknowledging that they only examined one species in an urban setting and suggesting that other species might have similar effects, highlighting the need for further research.

Due to COVID-19, many people suffered a reduced or temporary loss of the sense of smell, with a significant reduction in their quality of life, revealing how important smell is in everyday experiences99. Currently, the treatment with the greatest degree of supporting evidence for loss of smell is olfactory training100, which involves a twice-daily exposure to different sets of smells over 12 weeks, mainly conducted indoors with artificial smells. However, having access to nature close to where one lives might play an important role not only in promoting well-being and reducing stress but also in facilitating naturalistic olfactory training by using smells from nature. Moreover, due to the need to reduce transportation during the pandemic, close access to nature has gained increased importance for the health of the population101. Importantly, physical access to urban green is not equal among the population. Thus, smells from nature can represent a way to easily access nature within an urban context, even for people with reduced mobility, decreasing the need for travel to reach natural environments, hence promoting more sustainable cities. It is, therefore, of utmost importance to identify and define smells from individual plant species or perhaps dominant vegetation by family or genus if we are to create outdoor environments with the idea of improving the quality of an environment, user experience and promote health and well-being by increasing contact with nature through smells in cities.

Future research

The current number of studies on smells from nature is generally limited, leaving many knowledge gaps. The reviewed literature on smells from nature comes from several disciplines, including Landscape Architecture, Horticulture, Plant Studies, Health, Environmental Studies, Design, Forestry, Management, and Environmental Psychology. A multidisciplinary approach is needed to understand the details and health benefits of the smell experience, and the way smells from nature can potentially be integrated in urban nature.

One limitation of existing literature is the lack of studies specifying the effects of smells from nature released by various species. The reviewed studies show that not all smells from nature are perceived in the same way, but is not clear which smells are most beneficial for human well-being. There is a need to investigate how human responses vary to different smells and how the natural smellscape changes in different types of green environments (e.g. lawns, gardens, forests that are prone to timber production, or primeval forests). Additionally, future studies should investigate temporal changes of natural smellscapes across different seasons and times of day. These results could provide more knowledge on the relationship between smells and different habitats. Previous studies reveal that the visual perception of high biodiversity of plants is perceived as more attractive that areas with lower species richness102, and that a combination of singing by several bird species increase the positive perception of urban green areas more than singing by a single species103. It would be interesting to study if increased biodiversity of natural smells has similar effects as visual and soundscape features. Thus, future studies should investigate whether areas with a greater variety of smells from nature are perceived as being more positive than areas with fewer smells from nature. A practical implication would be to implement these findings considering local species, density, and spatial scale in the planning of public areas. By doing so, we can enhance the quality of life in cities and create healthier smellscapes in different types of natural and urban environments.

A limitation of current research is the limited number of in-situ studies that utilise smells emitted from real natural sources (e.g. plants, soil, water), incorporating physiological measurements using portable devices. More such studies are needed to enhance the generalizability and ecological validity of findings, and to better specify the complexity of smells in different types of urban green spaces. The reviewed laboratory studies measuring physiological responses reveal that smells from nature have positive effects, increasing relaxation and reducing stress levels59,66,67,68,80,81. Thus, there is a need to investigate whether the same effects occur in-situ in urban green spaces.

Another limitation of the existing literature is that, at the moment, studies on smells from nature seem to be centred towards Asia, especially the ones conducted in-situ within urban settings. Wang et al.72 highlight that smellscapes should be integrated with the local culture context, which might have an effect on, for example, the perception of scented plants. Hence, future research should examine the influence of more cultures on experiencing smells from nature in various urban areas all over the world. Thus, cultural aspects need to be taken into account when designing and managing urban green spaces with smells, in order to preserve the cultural and heritage values of various smellscapes.

Conclusion

The results of the reviewed articles suggest that exposure to smells from nature, emitted from real natural sources, positively influences human psychological and physiological responses. These include reducing stress levels, inducing relaxation, and enhancing positive emotions and affective states. However, research on smells from nature is still limited, particularly in defining the smell from specific species found in cities, and in conducting in-situ studies analysing human physiological responses in both urban and natural environments. Working with smells from nature in-situ presents several challenges, both methodological and in their implementation, such as the strong influence of environmental factors, the high subjectivity of smell perception, and the need for portable devices. Still, the literature highlights that a comfortable smell experience in cities can enhance overall satisfaction. The reviewed studies indicate that smells from nature have potential benefits for human health and well-being. Therefore, integrating smells from nature into urban environments could enhance nature exposure, improve environmental quality and potentially contribute to the creation of healthier ecosystems in a sustainable way.