Individual and gender fingerprints in human body odour

Smell throughout the life course

The sense of smell is an important mediator of health and sociality at all stages of life, yet it has received limited attention in our lineage. Olfaction starts in utero and participates in the establishment of social bonds in children, and of romantic and sexual relationships after puberty. Smell further plays a key role in food assessment and danger avoidance; in modern societies, it also guides our consumer behavior. Sensory abilities typically decrease with age and can be impacted by diseases, with repercussions on health and well-being. Here, we critically review our current understanding of human olfactory communication to refute outdated notions that our sense of smell is of low importance. We provide a summary of the biology of olfaction, give a prospective overview of the importance of the sense of smell throughout the life course, and conclude with an outline of the limitations and future directions in this field.

Skin emitted volatiles analysis for noninvasive diagnosis: the current advances in sample preparation techniques for biomedical application

Human skin emits a series of volatile compounds from the skin due to various metabolic processes, microbial activity, and several external factors. Changes in the concentration of skin volatile metabolites indicate many diseases, including diabetes, cancer, and infectious diseases. Researchers focused on skin-emitted compounds to gain insight into the pathophysiology of various diseases. In the case of skin volatolomics research, it is noteworthy that sample preparation, sampling protocol, analytical techniques, and comprehensive validation are important for the successful integration of skin metabolic profiles into regular clinical settings. Solid-phase microextraction techniques and polymer-based active sorbent traps were developed to capture the skin-emitted volatile compounds. The primary advantage of these sample preparation techniques is the ability to efficiently and targetedly capture skin metabolites, thus improving the detection of the biomarkers associated with various diseases. In further research, polydimethyl-based patches were utilized for skin research due to their biocompatibility and thermal stability properties. The microextraction sampling tools coupled with high sensitive Gas Chromatography-Mass Spectrometer provided a potential platform for skin volatolomes, thus emerging as a state-of-the-art analytical technique. Later, technological advancements, including the design of wearable sensors, have enriched skin-based research as it can integrate the information from skin-emitted volatile profiles into a portable platform. However, individual-specific hydration, temperature, and skin conditions can influence variations in skin volatile concentration. Considering the subject-specific skin depth, sampling time standardization, and suitable techniques may improve the skin sampling techniques for the potential discovery of various skin-based marker compounds associated with diseases. Here, we have summarised the current research progress, limitations, and technological advances in skin-based sample preparation techniques for disease diagnosis, monitoring, and personalized healthcare applications.

Predicting Chronological Age via the Skin Volatile Profile

Skin volatile emissions offer a noninvasive insight into metabolic activity within the body as well as the skin microbiome and specific volatile compounds have been shown to correlate with age, albeit only in a few small studies. Building on this, here skin volatiles were collected and analyzed in a healthy participant study (n = 60) using a robust headspace-solid phase microextraction (HS-SPME) gas chromatography-mass spectrometry (GC-MS) workflow. Following processing, 18 identified compounds were deemed suitable for this study. These were classified according to gender influences and their correlations with age were investigated. Finally, 6 volatiles (of both endogenous and exogenous origin) were identified as significantly changing in abundance with participant age (p < 0.1). The potential origins of these dysregulations are discussed. Multiple linear regression (MLR) analysis was employed to model age based on these significant volatiles as independent variables, along with gender. Our analysis shows that skin volatiles show a strong predictive ability for age (explained variance of 68%), stronger than other biochemical measures collected in this study (skin surface pH, water content) which are understood to vary with chronological age. Overall, this work provides new insights into the impact of aging on the skin volatile profiles which comprises both endogenously and exogenously derived volatile compounds. It goes toward demonstrating the biological significance of skin volatiles and will help pave the way for more rigorous consideration of the healthy "baseline" skin volatile profile in volatilomics-based health diagnostics development going forward.

Influence of the human body odor compound HMHA on face perception

Body odors convey information about the individuals, but the mechanisms are not fully understood yet. As far as human reproduction is concerned, molecules that are produced in sexually dimorphic amounts could be possible chemosignals. 3-hydroxy-3-methylhexanoic acid (HMHA) is one of them-more typical of men. Here, we investigated the possibility that the perception of gender and attractiveness in human faces could be implicitly influenced by this compound. Clearly feminine, ambiguous and clearly masculine faces were primed with an odor of HMHA, a control odor or air. Based on 100-ms face presentation, 40 raters had to identify the face's gender as quickly as possible and provide attractiveness evaluations. 3-hydroxy-3-methylhexanoic acid tended to be perceived as less pleasant and induced lower sniff duration in women compared with men. As to the effects of HMHA on face perception (vs. control conditions), we found that gender identification and the associated response time were unaffected by HMHA. Attractiveness of the faces, however, increased in presence of HMHA, but not in a sex-specific manner and only for unattractive faces with ambiguous gender. In sum, this study found no evidence in favor of a possible role of this sexually dimorphic compound in intrasexual competition nor in intersexual attraction.

An electronic nose can identify humans by the smell of their ear

Terrestrial mammals identify conspecifics by body odor. Dogs can also identify humans by body odor, and in some instances, humans can identify other humans by body odor as well. Despite the potential for a powerful biometric tool, smell has not been systematically used for this purpose. A question arising in the application of smell to biometrics is which bodily odor source should we measure. Breath is an obvious candidate, but the associated humidity can challenge many sensing devices. The armpit is also a candidate source, but it is often doused in cosmetics. Here, we test the hypothesis that the ear may provide an effective source for odor-based biometrics. The inside of the ear has relatively constant humidity, cosmetics are not typically applied inside the ear, and critically, ears contain cerumen, a potent source of volatiles. We used an electronic nose to identify 12 individuals within and across days, using samples from the armpit, lower back, and ear. In an identification setting where chance was 8.33% (1 of 12), we found that we could identify a person by the smell of their ear within a day at up to ~87% accuracy (~10 of 12, binomial P < 10-5), and across days at up to ~22% accuracy (~3 of 12, binomial P < 0.012). We conclude that humans can indeed be identified from the smell of their ear, but the results did not imply a consistent advantage over other bodily odor sources.

Unraveling the potential of breath and sweat VOC capture devices for human disease detection: a systematic-like review of canine olfaction and GC-MS analysis

The development of disease screening methods using biomedical detection dogs relies on the collection and analysis of body odors, particularly volatile organic compounds (VOCs) present in body fluids. To capture and analyze odors produced by the human body, numerous protocols and materials are used in forensics or medical studies. This paper provides an overview of sampling devices used to collect VOCs from sweat and exhaled air, for medical diagnostic purposes using canine olfaction and/or Gas Chromatography-Mass spectrometry (GC-MS). Canine olfaction and GC-MS are regarded as complementary tools, holding immense promise for detecting cancers and infectious diseases. However, existing literature lacks guidelines for selecting materials suitable for both canine olfaction and GC-MS. Hence, this review aims to address this gap and pave the way for efficient body odor sampling materials. The first section of the paper describes the materials utilized in training sniffing dogs, while the second section delves into the details of sampling devices and extraction techniques employed for exhaled air and sweat analysis using GC-MS. Finally, the paper proposes the development of an ideal sampling device tailored for detection purposes in the field of odorology. By bridging the knowledge gap, this study seeks to advance disease detection methodologies, harnessing the unique abilities of both dogs and GC-MS analysis in biomedical research.

Intrinsic and extrinsic factors affecting axillary odor variation. A comprehensive review

Humans produce odorous secretions from multiple body sites according to the microbiomic profile of each area and the types of secretory glands present. Because the axilla is an active, odor-producing region that mediates social communication via the sense of smell, this article focuses on the biological mechanisms underlying the creation of axillary odor, as well as the intrinsic and extrinsic factors likely to impact the odor and determine individual differences. The list of intrinsic factors discussed includes sex, age, ethnicity, emotions, and personality, and extrinsic factors include dietary choices, diseases, climate, and hygienic habits. In addition, we also draw attention to gaps in our understanding of each factor, including, for example, topical areas such as the effect of climate on body odor variation. Fundamental challenges and emerging research opportunities are further outlined in the discussion. Finally, we suggest guidelines and best practices based on the factors reviewed herein for preparatory protocols of sweat collection, data analysis, and interpretation.

Past, Present, and Future of Human Chemical Communication Research

Although chemical signaling is an essential mode of communication in most vertebrates, it has long been viewed as having negligible effects in humans. However, a growing body of evidence shows that the sense of smell affects human behavior in social contexts ranging from affiliation and parenting to disease avoidance and social threat. This article aims to (a) introduce research on human chemical communication in the historical context of the behavioral sciences; (b) provide a balanced overview of recent advances that describe individual differences in the emission of semiochemicals and the neural mechanisms underpinning their perception, that together demonstrate communicative function; and (c) propose directions for future research toward unraveling the molecular principles involved and understanding the variability in the generation, transmission, and reception of chemical signals in increasingly ecologically valid conditions. Achieving these goals will enable us to address some important societal challenges but are within reach only with the aid of genuinely interdisciplinary approaches.

Human scent characterization: A review

Human scent has long been cited as a probable parameter that can be exploited as a biometric measure. Identifying the scent of individual persons using specially trained canines is a well-known forensic method which is frequently used in criminal investigations. To date there has been limited research on the chemical components present in human scent and their usefulness in distinguishing between people. This review delivers insight into studies which have dealt with human scent in forensics. Sample collection methods, sample preparation, instrumental analysis, compounds identified in human scent and data analysis techniques are discussed. Methods for sample collection and preparation are presented, but to date, there is no available validated method. Instrumental methods are presented and from the overview it is clear that gas chromatography combined with mass spectrometry is the method of choice. New developments such as two-dimensional gas chromatography offer exiting possibilities to collect more information. Given the amount and complexity of data, data processing is used to extract the relevant information to discriminate people. Finally, sensors offer new opportunities for the characterization of human scent.

An Identity Recognition Model Based on RF-RFE: Utilizing Eye-Movement Data

Can eyes tell the truth? Can the analysis of human eye-movement data reveal psychological activities and uncover hidden information? Lying is a prevalent phenomenon in human society, but research has shown that people's accuracy in identifying deceptive behavior is not significantly higher than chance-level probability. In this paper, simulated crime experiments were carried out to extract the eye-movement features of 83 participants while viewing crime-related pictures using an eye tracker, and the importance of eye-movement features through interpretable machine learning was analyzed. In the experiment, the participants were independently selected into three groups: innocent group, informed group, and crime group. In the test, the eye tracker was used to extract a total of five categories of eye-movement indexes within the area of interest (AOI), including the fixation time, fixation count, pupil diameter, saccade frequency, and blink frequency, and the differences in these indexes were analyzed. Building upon interpretable learning algorithms, further investigation was conducted to assess the contribution of these metrics. As a result, the RF-RFE suspect identification model was constructed, achieving a maximum accuracy rate of 91.7%. The experimental results further support the feasibility of utilizing eye-movement features to reveal inner psychological activities.

Multivariate regression modelling for gender prediction using volatile organic compounds from hand odor profiles via HS-SPME-GC-MS

The efficacy of using human volatile organic compounds (VOCs) as a form of forensic evidence has been well demonstrated with canines for crime scene response, suspect identification, and location checking. Although the use of human scent evidence in the field is well established, the laboratory evaluation of human VOC profiles has been limited. This study used Headspace-Solid Phase Microextraction-Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS) to analyze human hand odor samples collected from 60 individuals (30 Females and 30 Males). The human volatiles collected from the palm surfaces of each subject were interpreted for classification and prediction of gender. The volatile organic compound (VOC) signatures from subjects' hand odor profiles were evaluated with supervised dimensional reduction techniques: Partial Least Squares-Discriminant Analysis (PLS-DA), Orthogonal-Projections to Latent Structures Discriminant Analysis (OPLS-DA), and Linear Discriminant Analysis (LDA). The PLS-DA 2D model demonstrated clustering amongst male and female subjects. The addition of a third component to the PLS-DA model revealed clustering and minimal separation of male and female subjects in the 3D PLS-DA model. The OPLS-DA model displayed discrimination and clustering amongst gender groups with leave one out cross validation (LOOCV) and 95% confidence regions surrounding clustered groups without overlap. The LDA had a 96.67% accuracy rate for female and male subjects. The culminating knowledge establishes a working model for the prediction of donor class characteristics using human scent hand odor profiles.

Gas-Phase Biosensors (Bio-Sniffers) for Measurement of 2-Nonenal, the Causative Volatile Molecule of Human Aging-Related Body Odor

The molecule 2-nonenal is renowned as the origin of unpleasant human aging-related body odor that can potentially indicate age-related metabolic changes. Most 2-nonenal measurements rely on chromatographic analytical systems, which pose challenges in terms of daily usage and the ability to track changes in concentration over time. In this study, we have developed liquid- and gas-phase biosensors (bio-sniffers) with the aim of enabling facile and continuous measurement of trans-2-nonenal vapor. Initially, we compared two types of nicotinamide adenine dinucleotide (phosphate) [NAD(P)]-dependent enzymes that have the catalytic ability of trans-2-nonenal: aldehyde dehydrogenase (ALDH) and enone reductase 1 (ER1). The developed sensor quantified the trans-2-nonanal concentration by measuring fluorescence (excitation: 340 nm, emission: 490 nm) emitted from NAD(P)H that was generated or consumed by ALDH or ER1. The ALDH biosensor reacted to a variety of aldehydes including trans-2-nonenal, whereas the ER1 biosensor showed high selectivity. In contrast, the ALDH bio-sniffer showed quantitative characteristics for trans-2-nonenal vapor at a concentration range of 0.4-7.5 ppm (with a theoretical limit of detection (LOD) and limit of quantification (LOQ) of 0.23 and 0.26 ppm, respectively), including a reported concentration (0.85-4.35 ppm), whereas the ER1 bio-sniffer detected only 0.4 and 0.8 ppm. Based on these findings, headspace gas of skin-wiped alcohol-absorbed cotton collected from study participants in their 20s and 50s was measured by the ALDH bio-sniffer. Consequently, age-related differences in signals were observed, suggesting the potential for measuring trans-2-nonenal vapor.

Soap application alters mosquito-host interactions

To find nutrients, mosquitoes use volatile organic compounds (VOCs) emitted by plants and animal hosts. These resources overlap in their chemical composition, and an important layer of information resides in VOCs' relative abundance in the headspace of each resource. In addition, a large majority of the human species regularly uses personal care products such as soaps and perfumes, which add plant-related VOCs to their olfactory signature. Using headspace sampling and gas chromatography-mass spectrometry, we quantified how human odor is modified by soap application. We showed that soaps alter mosquito host selection, with some soaps increasing the attractiveness of the host and some soaps reducing it. Analytical methods revealed the main chemicals associated with these changes. These results provide proof-of-concept that data on host-soap valences can be reverse-engineered to produce chemical blends for artificial baits or mosquito repellents, and evince the impact of personal care products on host selection processes.

Investigating the human chemical communication of positive emotions using a virtual reality-based mood induction

Humans can communicate their emotions to others via volatile emissions from their bodies. Although there is now solid evidence for human chemical communication of fear, stress and anxiety, investigations of positive emotions remain scarce. In a recent study, we found that women's heart rate and performance in creativity tasks were modulated by body odors of men sampled while they were in a positive vs. neutral mood. However, inducing positive emotions in laboratory settings remains challenging. Therefore, an important step to further investigate the human chemical communication of positive emotions is to develop new methods to induce positive moods. Here, we present a new mood induction procedure (MIP) based on virtual reality (VR), that we assumed to be more powerful than videos (used in our previous study) to induce positive emotions. We hypothesized that, consequently, given the more intense emotions created, this VR-based MIP would induce larger differences between the receivers' responses to the positive body odor versus a neutral control body odor, than the Video-based MIP. The results confirmed the higher efficacy of VR to induce positive emotions compared with videos. More specifically, VR had more repeatable effects between individuals. Although positive body odors had similar effects to those found in the previous video study, especially faster problem solving, these effects did not reach statistical significance. These outcomes are discussed as a function of the specificities of VR and of other methodological parameters, that may have prevented the observation of such subtle effects and that should be understood more in-depth for future studies on human chemical communication.

Biological studies with comprehensive 2D-GC-HRMS screening: Exploring the human sweat volatilome

A key issue in GCxGC-HRMS data analysis is how to approach large-sample studies in an efficient and comprehensive way. We have developed a semi-automated data-driven workflow from identification to suspect screening, which allows highly selective monitoring of each identified chemical in a large-sample dataset. The example dataset used to illustrate the potential of the approach consisted of human sweat samples from 40 participants, including field blanks (80 samples). These samples have been collected in a Horizon 2020 project to investigate the capacity of body odour to communicate emotion and influence social behaviour. We used dynamic headspace extraction, which allows comprehensive extraction with high preconcentration capability, and has to date only been used for a few biological applications. We were able to detect a set of 326 compounds from a diverse range of chemical classes (278 identified compounds, 39 class unknowns, and 9 true unknowns). Unlike partitioning-based extraction methods, the developed method detects semi-polar (log P < 2) nitrogen and oxygen-containing compounds. However, it is unable to detect certain acids due to the pH conditions of unmodified sweat samples. We believe that our framework will open up the possibility of efficiently using GCxGC-HRMS for large-sample studies in a wide range of applications such as biological and environmental studies.

Sex-typing of ingested human blood meal in Anopheles stephensi mosquito based on the amelogenin gene

Identifying the sex of human hosts of insect disease vectors, using PCR amplification of the amelogenin gene (AMEL) from the ingested blood meal is an increasingly useful technique for epidemiological studies of vector-borne diseases, as well as within the criminal justice system. Detection of DNA from ingested blood is influenced by the choice of DNA extraction method, genomic target region, type and length of PCR, and rate of degradation in the DNA samples over time. Here, we have tested two types of PCR (i.e. conventional and nested), producing differently-sized PCR products, in time-course assays targeting the human AMEL gene in Anopheles stephensi mosquitoes that were fed on human male and female blood. The fed female mosquitoes were allowed to digest at 28 °C for times ranging from 0 to 120 h. Three AMEL primer pairs were used to amplify three sequences that were 977, 539, and 106 bp for the X chromosome and 790, 355, and 112 bp for Y. We found that time since feeding had a significant negative effect on the success of PCR amplification. The shortest fragments (106 and 112 bp) were amplified for the longest time after blood feeding (up to 60 h), whereas the medium and longest loci were not amplified by conventional PCR even at 0 h. However, the nested PCR protocol, targeting the medium sequence, could detect small amounts of human DNA up to 36 h (1.5 days) after the blood meal. The shortest PCR assay standardized herein successfully detected small amounts of human DNA in female mosquitoes up to 60 h after the blood meal. This assay represents a promising tool for identifying the sex of the human host from the blood meal in field-collected female mosquitoes.

Malodour classification with low-cost flexible electronics

Understanding body malodour in a measurable manner is essential for developing personal care products. Body malodour is the result of bodily secretion of a highly complex mixture of volatile organic compounds. Current body malodour measurement methods are manual, time consuming and costly, requiring an expert panel of assessors to assign a malodour score to each human test subject. This article proposes a technology-based solution to automate this task by developing a custom-designed malodour score classification system comprising an electronic nose sensor array, a sensor readout interface and a machine learning hardware fabricated on low-cost flexible substrates. The proposed flexible integrated smart system is to augment the expert panel by acting like a panel assessor but could ultimately replace the panel to reduce the test and measurement costs. We demonstrate that it can classify malodour scores as good as or even better than half of the assessors on the expert panel.

Exploring the interrelationship between the skin microbiome and skin volatiles: A pilot study

Unravelling the interplay between a human’s microbiome and physiology is a relevant task for understanding the principles underlying human health and disease. With regard to human chemical communication, it is of interest to elucidate the role of the microbiome in shaping or generating volatiles emitted from the human body. In this study, we characterized the microbiome and volatile organic compounds (VOCs) sampled from the neck and axilla of ten participants (five male, five female) on two sampling days, by applying different methodological approaches. Volatiles emitted from the respective skin site were collected for 20 min using textile sampling material and analyzed on two analytical columns with varying polarity of the stationary phase. Microbiome samples were analyzed by a culture approach coupled with MALDI-TOF-MS analysis and a 16S ribosomal RNA gene (16S RNA) sequencing approach. Statistical and advanced data analysis methods revealed that classification of body sites was possible by using VOC and microbiome data sets. Higher classification accuracy was achieved by combination of both data pools. Cutibacterium, Staphylococcus, Micrococcus, Streptococcus, Lawsonella, Anaerococcus, and Corynebacterium species were found to contribute to classification of the body sites by the microbiome. Alkanes, esters, ethers, ketones, aldehydes and cyclic structures were used by the classifier when VOC data were considered. The interdisciplinary methodological platform developed here will enable further investigations of skin microbiome and skin VOCs alterations in physiological and pathological conditions.

How priming with body odors affects decision speeds in consumer behavior

To date, odor research has primarily focused on the behavioral effects of common odors on consumer perception and choices. We report a study that examines, for the first time, the effects of human body odor cues on consumer purchase behaviors. The influence of human chemosignals produced in three conditions, namely happiness, fear, a relaxed condition (rest), and a control condition (no odor), were examined on willingness to pay (WTP) judgments across various products. We focused on the speed with which participants reached such decisions. The central finding revealed that participants exposed to human odors reached decisions significantly faster than the no odor control group. The main driving force is that human body odors activate the presence of others during decision-making. This, in turn, affects response speed. The broader implications of this finding for consumer behavior are discussed.

Human scent signature on cartridge case survives gun being fired: A preliminary study on a potential of scent residues as an identification tool

This paper focuses on a chemical analysis of human scent samples that were obtained from cartridge cases after being fired and their comparison with scent samples collected under laboratory conditions. Scent samples were analyzed by comprehensive two-dimensional gas chromatography coupled with the time-of-flight mass spectrometer. The results obtained from the chemical analyzes confirmed the desired stability of the human scent evidence and outlined the possible application for forensic purposes. The qualitative results of the study converge with the findings of previous studies on the composition of human scent and the chemical composition of human fingerprints. Furthermore, statistical analyzes were performed employing similarity algorithms such as Pearson's and Spearman's correlations, or Kendall's tau. The resulting comparison of the scent samples secured on fired cartridge cases compared with those samples collected under laboratory conditions yielded ten out of ten correct identifications of the scent inflictor.

Different armpits under my new nose: Olfactory sex but not gender affects implicit measures of embodiment

Conflicting multisensory signals may alter embodiment to produce self-identification with a foreign body, but the role of olfaction in this process has been overlooked. We studied in healthy participants how sex (male and female sweat odors) and gender (male and female cosmetic scents) olfactory stimuli contribute to embodiment. Participants saw, on a head mounted display, the first-person perspective of a sex mismatching person. Synchronous visuotactile stimulation was applied to enhance illusory embodiment. Simultaneously, they smelled either sex- or gender- congruent or incongruent stimuli. We assessed implicit (skin conductance responses to visual threats) and explicit (questionnaire) measures of embodiment. Stronger responses to threat were found when participants smelled the sex-congruent compared to the sex-incongruent odor, while no such differences were found for the cosmetic scents. According to the questionnaire, embodiment did not differ between conditions. Post-experimental assessment of the presented cues, suggest that while both sweat odors were considered generally male, cosmetic scents were not. The presented scents were generally not associated with the embodied body. Our results suggest that sex-related body odors influence implicit but not explicit aspects of embodiment and are in line with unique characteristics of olfaction in other aspects of cognition.

Chemical Characterization of the Marking Fluid of Breeding and Non-Breeding Male Cheetahs

Simple Summary

The study aimed at chemically characterizing the marking fluid of both breeding and non-breeding male cheetahs (Acinonyx jubatus). Specifically, it focused on identifying potential differences in pheromones related to sexual behavior/attraction in this species. Furthermore, it aimed at providing more information as a basis for future studies, such as the investigation of specific semiochemicals in the reproductive behavior of cheetahs. The results of this study support the hypothesis of differences in the relative concentration of volatile organic chemicals between male cheetahs; however, they highlight the importance of diet and age on the presence of volatile organic chemicals in the marking fluid.

Abstract

Scent is known to play an important role in the reproduction of cheetahs and other felids. In fact, the presence/odor of a male cheetah has been noted to trigger the estrous cycle in females. The objective of this study was to analyze the marking fluid (MF) of male cheetahs from different breeding groups to determine the composition of volatile organic compounds (VOCs) present, with the aim of identifying potential pheromones relating to sexual behavior/attraction in this species. Four breeding (B; age: 8.9 ± 1.3 years old) and four non-breeding (NB; age: 5.5 ± 0.8 years old) males were selected for this study. Samples were collected into a glass beaker, transferred immediately into a 20 mL glass screw-cap vial with a polytetrafluoroethylene (PTFE) coated silicone septum, and stored until analyzed by headspace solid-phase microextraction (HS-SPME) using gas chromatography–mass spectrometry. A contingency test with Fisher’s exact test, using the frequency (FREQ) procedure of SAS 9.4, was conducted to determine the difference between the number of VOCs identified per breeding group; furthermore, differences in relative concentration (RC) of the identified VOCs between breeding groups were analyzed using ANOVA for repeated measures with the GLIMMIX procedure. From the 13 MF samples analyzed, 53 VOCs were identified, and 12 were identified in all the samples. Five of these (dimethyl disulfide, benzaldehyde, acetophenone, phenol, and indole) are known to be involved in attraction/sexual behavior in mammals. Between the two groups, the RC of indole was significantly higher in the NB group, whereas the RC of dodecanoic acid was significantly higher in the B group. Although not significant, the RC of benzaldehyde was higher in the B versus the NB group. The results of this study do support the hypothesis of differences in VOCs’ between B and NB male cheetahs. However, the overlapping of age and breeding status and the diet differences could not be controlled. Still, the evidence of changes in MF composition in male cheetahs necessitates further studies on possible strategies to improve reproduction in captivity.

In Vitro and In Vivo Human Body Odor Analysis Method Using GO:PANI/ZNRs/ZIF−8 Adsorbent Followed by GC/MS

Among various volatile organic compounds (VOCs) emitted from human skin, trans-2-nonenal, benzothiazole, hexyl salicylate, α-hexyl cinnamaldehyde, and isopropyl palmitate are key indicators associated with the degrees of aging. In our study, extraction and determination methods of human body odor are newly developed using headspace-in needle microextraction (HS-INME). The adsorbent was synthesized with graphene oxide:polyaniline/zinc nanorods/zeolitic imidazolate framework-8 (GO:PANI/ZNRs/ZIF−8). Then, a wire coated with the adsorbent was placed into the adsorption kit to be directly exposed to human skin as in vivo sampling and inserted into the needle so that it was able to be desorbed at the GC injector. The adsorption kit was made in-house with a 3D printer. For the in vitro method, the wire coated with the adsorbent was inserted into the needle and exposed to the headspace of the vial. When a cotton T-shirt containing body odor was transferred to a vial, the headspace of the vial was saturated with body odor VOCs. After volatile organic compounds were adsorbed in the dynamic mode, the needle was transferred to the injector for analysis of the volatile organic compounds by gas chromatography/mass spectrometry (GC/MS). The conditions of adsorbent fabrication and extraction for body odor compounds were optimized by response surface methodology (RSM). In conclusion, it was able to synthesize GO:PANI/ZNRs/ZIF−8 at the optimal condition and applicable to both in vivo and in vitro methods for body odor VOCs analysis.

Protein profiles from used nesting material, saliva, and urine correspond with social behavior in group housed male mice, Mus musculus

Current understanding of how odors impact intra-sex social behavior is based on those that increase intermale aggression. Yet, odors are often promoted to reduce fighting among male laboratory mice. It has been shown that a cage of male mice contains many proteins used for identification purposes. However, it is unknown if these proteins relate to social behavior or if they are uniformly produced across strains. This study aimed to compare proteomes from used nesting material and three sources (sweat, saliva, and urine) from three strains and compare levels of known protein odors with rates of social behavior. Used nesting material samples from each cage were analyzed using LC-MS/MS. Sweat, saliva, and urine samples from each cage's dominant and subordinate mouse were also analyzed. Proteomes were assessed using principal component analyses and compared to behavior by calculating correlation coefficients between PC scores and behavior proportions. Twenty-one proteins from nesting material either correlated with affiliative behavior or negatively correlated with aggression. Notably, proteins from the major urinary protein family, odorant binding protein family, and secretoglobin family displayed at least one of these patterns, making them candidates for future work. These findings provide preliminary information about how proteins can influence male mouse behavior. SIGNIFICANCE: Research on how olfactory signals influence same sex social behavior is primarily limited to those that promote intermale aggression. However, exploring how olfaction modulates a more diverse behavioral repertoire will improve our foundational understanding of this sensory modality. In this proteome analysis we identified a short list of protein signals that correspond to lower rates of aggression and higher rates of socio-positive behavior. While this study is only correlational, it sets a foundation for future work that can identify protein signals that directly influence social behavior and potentially identify new murine pheromones.

The Attractivity of the Head Louse, Pediculus humanus capitis (Pediculidae: Anoplura) to Isolated Compounds and Blends of Human Skin

Human head lice Pediculus humanus capitis De Geer (Phthiraptera: Pediculidae) are ectoparasites that cause pediculosis, a global scale disease mainly found in school-age children. Previous works from our laboratory found nonanal, sulcatone, and geranylacetone as the main human scalp volatile components, and individually evaluated their attraction to head lice using an olfactometer. In this work, we compared how their blends at different concentrations attract head lice, and how their blended effect compares to the effect of isolated compounds. At the concentrations evaluated, individual components did not show attraction towards head lice, but a ternary mixture of them was attractive. Moreover, a solvent extract from the human head scalp was analyzed by GC-MS, finding that tetradecanoic acid, palmitic acid, oleic acid, palmitoleic acid, and squalene are the most abundant components. Attraction to these individual compounds at natural concentrations was tested by bioassays in a circular experimental arena. No attraction was observed when the components were tested individually, but when they were evaluated as a blend they attracted head lice. This work presents new information about how chemical signals are attractive at certain concentrations and proportions. This information could be used to better understand communication mechanisms in head lice and for the development of louse repellents.

Feather chemicals contain information about the major histocompatibility complex in a highly scented seabird

Mate choice informed by the immune genes of the major histocompatibility complex (MHC) may provide fitness benefits including offspring with increased immunocompetence. Olfactory cues are considered the primary mechanism organisms use to evaluate the MHC of potential mates, yet this idea has received limited attention in birds. Motivated by a finding of MHC-dependent mate choice in the Leach's storm-petrel (Oceanodroma leucorhoa), we examined whether the chemical profiles of this highly scented seabird contain information about MHC genes. Whereas previous studies in birds examined non-volatile compounds, we used gas chromatography-mass spectrometry to measure the volatile compounds emitted from feathers that potentially serve as olfactory infochemicals about MHC and coupled this with locus-specific genotyping of MHC IIB genes. We found that feather chemicals reflected individual MHC diversity through interactions with sex and breeding status. Furthermore, similarity in MHC genotype was correlated with similarity in chemical profiles within female-female and male-female dyads. We provide the first evidence that volatile chemicals from bird feathers can encode information about the MHC. Our findings suggest that olfaction likely aids MHC-based mate choice in this species and highlight a role for chemicals in mediating genetic mate choice in birds where this mode of communication has been largely overlooked.

Volatile Organic Compounds Analysis as a Potential Novel Screening Tool for Breast Cancer: A Systematic Review

Introduction

An early diagnosis is crucial in reducing mortality among people who have breast cancer (BC). There is a shortfall of characteristic early clinical symptoms in BC patients, highlighting the importance of investigating new methods for its early detection. A promising novel approach is the analysis of volatile organic compounds (VOCs) produced and emitted through the metabolism of cancer cells.

Methods

The purpose of this systematic review is to outline the published research regarding BC-associated VOCs. For this, headspace analysis of VOCs was explored in patient-derived body fluids, animal model-derived fluids, and BC cell lines to identify BC-specific VOCs. A systematic search in PubMed and Web of Science databases was conducted according to the PRISMA guidelines.

Results

Thirty-two studies met the criteria for inclusion in this review. Results highlight that VOC analysis can be promising as a potential novel screening tool. However, results of in vivo, in vitro and case-control studies have delivered inconsistent results leading to a lack of inter-matrix consensus between different VOC sampling methods.

Discussion

Discrepant VOC results among BC studies have been obtained, highly due to methodological discrepancies. Therefore, methodological issues leading to disparities have been reviewed and recommendations have been made on the standardisation of VOC collection and analysis methods for BC screening, thereby improving future VOC clinical validation studies.

Skin microbiome alters attractiveness to Anopheles mosquitoes

BACKGROUND

Some people produce specific body odours that make them more attractive than others to mosquitoes, and consequently are at higher risk of contracting vector-borne diseases. The skin microbiome can break down carbohydrates, fatty acids and peptides on the skin into volatiles that mosquitoes can differentiate.

RESULTS

Here, we examined how skin microbiome composition of women differs in relation to level of attractiveness to Anopheles coluzzii mosquitoes, to identify volatiles in body odour and metabolic pathways associated with individuals that tend to be poorly-attractive to mosquitoes. We used behavioural assays to measure attractiveness of participants to An. coluzzii mosquitoes, 16S rRNA amplicon sequencing of the bacteria sampled from the skin and gas chromatography of volatiles in body odour. We found differences in skin microbiome composition between the poorly- and highly-attractive groups, particularly eight Amplicon Sequence Variants (ASVs) belonging to the Proteobacteria, Actinobacteria and Firmicutes phyla. Staphylococcus 2 ASVs are four times as abundant in the highly-attractive compared to poorly-attractive group. Associations were found between these ASVs and volatiles known to be attractive to Anopheles mosquitoes. Propanoic pathways are enriched in the poorly-attractive participants compared to those found to be highly-attractive.

CONCLUSIONS

Our findings suggest that variation in attractiveness of people to mosquitoes is related to the composition of the skin microbiota, knowledge that could improve odour-baited traps or other next generation vector control tools.

A headspace collection chamber for whole body volatilomics

The human body secretes a complex blend of volatile organic compounds (VOCs) via the skin, breath and bodily fluids. In this study, we have developed a headspace collection chamber for whole body volatilome profiling.

Breath odor-based individual authentication by an artificial olfactory sensor system and machine learning

The potential feasibility of breath odor sensing-based individual authentication was demonstrated by a 16-channel chemiresistive sensor array and machine learning.

The role of fragrance and self-esteem in perception of body odors and impressions of others

Human sweat odor serves as social communication signal for a person’s traits and emotional states. This study explored whether body odors can also communicate information about one’s self-esteem, and the role of applied fragrance in this relationship. Female participants were asked to rate self-esteem and attractiveness of different male contestants of a dating show, while being exposed to male participant’s body odors differing in self-esteem. High self-esteem sweat was rated more pleasant and less intense than low self-esteem sweat. However, there was no difference in perceived self-esteem and attractiveness of male contestants in videos, hence explicit differences in body odor did not transfer to judgments of related person characteristics. When the body odor was fragranced using a fragranced body spray, male contestants were rated as having higher self-esteem and being more attractive. The finding that body odors from male participants differing in self-esteem are rated differently and can be discriminated suggests self-esteem has distinct perceivable olfactory features, but the remaining findings imply that only fragrance affect the psychological impression someone makes. These findings are discussed in the context of the role of body odor and fragrance in human perception and social communication.

Understanding the Role of Semiochemicals on the Reproductive Behaviour of Cheetahs (Acinonyx jubatus)-A Review

Simple Summary

This review aims to provide an in-depth overview of the reproductive physiology and behaviour of cheetahs (Acinonyx jubatus). Specifically, it focuses on the role that pheromones (a class of semiochemicals) play by directly affecting the reproductive (e.g., precopulatory and copulatory) behaviour. Furthermore, it aims to critically analyze current research and provide new insights on study areas needing further investigation. It is clear, for instance, that further research is necessary to investigate the role of semiochemicals in the reproductive behaviour of cheetahs in order to rectify the current behavioural difficulties experienced when breeding younger females. This, in turn, would aid in improving captive breeding and the prevention of asymmetric reproductive aging.

Abstract

The cheetah species (Acinonyx jubatus) is currently listed as vulnerable according to the International Union for Conservation of Nature (IUCN). Captive breeding has long since been used as a method of conservation of the species, with the aim to produce a healthy, strong population of cheetahs with an increased genetic variety when compared to their wild counterparts. This would then increase the likelihood of survivability once released into protected areas. Unfortunately, breeding females have been reported to be difficult due to the age of these animals. Older females are less fertile, have more difficult parturition, and are susceptible to asymmetric reproductive aging whereas younger females tend to show a significantly lower frequency of mating behaviour than that of older females, which negatively affects breeding introductions, and therefore mating. Nonetheless, the experience from breeding methods used in some breeding centres in South Africa and the Netherlands, which also rely on the role that semiochemicals play in breeding, proves that cheetahs can be bred successfully in captivity. This review aims to give the reader an in-depth overview of cheetahs’ reproductive physiology and behaviour, focusing on the role that pheromones play in this species. Furthermore, it aims to provide new insight into the use of semiochemicals to improve conservation strategies through captive breeding.

The Importance of Intact Senses in Mating and Social Assessments Made by Deaf Individuals

Social perception is a multimodal process involving vision and audition as central input sources for human social cognitive processes. However, it remains unclear how profoundly deaf people assess others in the context of mating and social interaction. The current study explored the relative importance of different sensory modalities (vision, smell, and touch) in assessments of opposite- and same-sex strangers. We focused on potential sensory compensation processes in mate selection (i.e., increased importance of the intact senses in forming impressions of an opposite-sex stranger as a potential partner). A total of 74 deaf individuals and 100 normally hearing controls were included in the study sample. We found diminished importance of vision and smell in deaf participants compared with controls for opposite- and same-sex strangers, and increased importance of touch for the assessment of same-sex strangers. The results suggested that deaf people rely less on visual and olfactory cues in mating and social assessments, highlighting a possible role of sign language in shaping interpersonal tactile experience in non-romantic relationships.

Olfaction in the Multisensory Processing of Faces: A Narrative Review of the Influence of Human Body Odors

A recent body of research has emerged regarding the interactions between olfaction and other sensory channels to process social information. The current review examines the influence of body odors on face perception, a core component of human social cognition. First, we review studies reporting how body odors interact with the perception of invariant facial information (i.e., identity, sex, attractiveness, trustworthiness, and dominance). Although we mainly focus on the influence of body odors based on axillary odor, we also review findings about specific steroids present in axillary sweat (i.e., androstenone, androstenol, androstadienone, and estratetraenol). We next survey the literature showing body odor influences on the perception of transient face properties, notably in discussing the role of body odors in facilitating or hindering the perception of emotional facial expression, in relation to competing frameworks of emotions. Finally, we discuss the developmental origins of these olfaction-to-vision influences, as an emerging literature indicates that odor cues strongly influence face perception in infants. Body odors with a high social relevance such as the odor emanating from the mother have a widespread influence on various aspects of face perception in infancy, including categorization of faces among other objects, face scanning behavior, or facial expression perception. We conclude by suggesting that the weight of olfaction might be especially strong in infancy, shaping social perception, especially in slow-maturing senses such as vision, and that this early tutoring function of olfaction spans all developmental stages to disambiguate a complex social environment by conveying key information for social interactions until adulthood.

Scent of a Woman-Or Man: Odors Influence Person Knowledge

First impressions of social traits are regularly, rapidly, and readily determined from limited information about another individual. Relatively little is known about the way that olfactory information, particularly from scents that are not body odors, alters a first impression. Can the attributes of an odorant be conferred onto a person associated with that scent? To explore this, 101 participants were asked to form an impression of a hypothetical person based on the following stimuli: A gender-neutral silhouette, a list of six personal characteristics, and one of five odorants. Participants then rated the likelihood that the hypothetical person possessed each of 51 personality traits that were determined a priori as falling into six attribute categories. Participants also directly rated all odorants for the six categories and intensity. A T-test showed that ratings of the hypothetical person were less disparate from the odor that was presented during impression formation than from other odors. ANOVA revealed that the effects were heterogeneous, with odorants varying in their effectiveness in associating the hypothetical person with categories. The present data suggest that a hypothetical person can be imbued with the specific attributes of an odor and that some odors are better at contributing to impressions than others.

Novel sampling methods for monitoring Anopheles arabiensis from Eritrea

Background

Studies comparing novel collection methods for host seeking and resting mosquitoes A. arabiensis were undertaken in a village in Eritrea. Techniques included an odor baited trap, a novel tent-trap, human landing collection and three methods of resting collection. A technique for the collection of mosquitoes exiting vegetation is also described. Pre-gravid rates were determined by dissection of host seeking insects and post-prandial egg development among insects collected resting.

Results

Overall 5,382 host-seeking, 2,296 resting and 357 A. arabiensis exiting vegetation were collected. The Furvela tent-trap was the most efficient, risk-free method for the collection of outdoor host-seeking insects, whilst the Suna trap was the least effective method. Mechanical aspirators (the CDC backpack or the Prokopack aspirator) were superior to manual aspiration in a dark shelter but there was no advantage over manual aspiration in a well-lit one. An estimated two-thirds of newly-emerged mosquitoes went through a pre-gravid phase, feeding twice before producing eggs. Mosquitoes completed gonotrophic development in a dark shelter but left a well-lit shelter soon after feeding. One blood-fed female marked in the village was recaptured 2 days after release exiting vegetation close to the oviposition site and another, shortly after oviposition, attempting to feed on a human host 3 days after release. Exit rates of males from vegetation peaked 3 min after the initial male had left. Unfed and gravid females exited approximately 6 min after the first males.

Conclusions

Furvela tent-traps are suitable for the collection of outdoor biting A. arabiensis in Eritrea whilst the Prokopack sampler is the method of choice for the collection of resting insects. Constructing well-lit, rather than dark, animal shelters, may encourage otherwise endophilic mosquitoes to leave and so reduce their survival and hence their vectorial capacity.

Molecularly imprinted electrospun fiber membrane for colorimetric detection of hexanoic acid

An imprinted electrospun fiber membrane was developed for the detection of volatile organic acids, which are key components of human body odor. In this study, hexanoic acid (HA) was selected as the target, polymethyl methacrylate (PMMA) was used as the substrate, and colorimetric detection of HA was achieved by a bromocresol purple (BCP) chromogenic agent. The results showed that the morphology of the fiber membrane was uniform and continuous, and it showed excellent selectivity and specificity to HA. Photographs of the color changes before and after fiber membrane adsorption were recorded by a camera and quantified by ImageJ software by the difference in gray value (ΔGray). This method is simple, intuitive, and low cost and has great potential for application in human odor analysis.

Compounds from plantar foot sweat, nesting material, and urine show strain patterns associated with agonistic and affiliative behaviors in group housed male mice, Mus musculus

Excessive home cage aggression often results in severe injury and subsequent premature euthanasia of male laboratory mice. Aggression can be reduced by transferring used nesting material during cage cleaning, which is thought to contain aggression appeasing odors from the plantar sweat glands. However, neither the composition of plantar sweat nor the deposits on used nesting material have been evaluated. The aims of this study were to (1) identify and quantify volatile compounds deposited in the nest site and (2) determine if nest and sweat compounds correlate with social behavior. Home cage aggression and affiliative behavior were evaluated in 3 strains: SJL, C57BL/6N, and A/J. Individual social rank was assessed via the tube test, because ranking may influence compound levels. Sweat and urine from the dominant and subordinate mouse in each cage, plus cage level nest samples were analyzed for volatile compound content using gas chromatography-mass spectrometry. Behavior data and odors from the nest, sweat, and urine were statistically analyzed with separate principal component analyses (PCA). Significant components, from each sample analysis, and strain were run in mixed models to test if odors were associated with behavior. Aggressive and affiliative behaviors were primarily impacted by strain. However, compound PCs were also impacted by strain, showing that strain accounts for any relationship between odors and behavior. C57BL/6N cages displayed the most allo-grooming behavior and had high scores on sweat PC1. SJL cages displayed the most aggression, with high scores on urine PC2 and low scores on nest PC1. These data show that certain compounds in nesting material, urine, and sweat display strain specific patterns which match strain specific behavior patterns. These results provide preliminary information about the connection between home cage compounds and behavior. Salient compounds will be candidates for future controlled studies to determine their direct effect on mouse social behavior.

A literature survey of all volatiles from healthy human breath and bodily fluids: the human volatilome

This paper comprises an updated version of the 2014 review which reported 1846 volatile organic compounds (VOCs) identified from healthy humans. In total over 900 additional VOCs have been reported since the 2014 review and the VOCs from semen have been added. The numbers of VOCs found in breath and the other bodily fluids are: blood 379, breath 1488, faeces 443, milk 290, saliva 549, semen 196, skin 623 and urine 444. Compounds were assigned CAS registry numbers and named according to a common convention where possible. The compounds have been included in a single table with the source reference(s) for each VOC, an update on our 2014 paper. VOCs have also been grouped into tables according to their chemical class or functionality to permit easy comparison. Careful use of the database is needed, as a number of the identified VOCs only have level 2-putative assignment, and only a small fraction of the reported VOCs have been validated by standards. Some clear differences are observed, for instance, a lack of esters in urine with a high number in faeces and breath. However, the lack of compounds from matrices such a semen and milk compared to breath for example could be due to the techniques used or reflect the intensity of effort e.g. there are few publications on VOCs from milk and semen compared to a large number for breath. The large number of volatiles reported from skin is partly due to the methodologies used, e.g. by collecting skin sebum (with dissolved VOCs and semi VOCs) onto glass beads or cotton pads and then heating to a high temperature to desorb VOCs. All compounds have been included as reported (unless there was a clear discrepancy between name and chemical structure), but there may be some mistaken assignations arising from the original publications, particularly for isomers. It is the authors' intention that this work will not only be a useful database of VOCs listed in the literature but will stimulate further study of VOCs from healthy individuals; for example more work is required to confirm the identification of these VOCs adhering to the principles outlined in the metabolomics standards initiative. Establishing a list of volatiles emanating from healthy individuals and increased understanding of VOC metabolic pathways is an important step for differentiating between diseases using VOCs.

Olfactory signals and fertility in olive baboons

Female primates signal impending ovulation with a suite of sexual signals. Studies of these signals have focussed on visual, and to a lesser extent, acoustic signals, neglecting olfactory signals. We aimed to investigate the information content of female olfactory signals in captive olive baboons (Papio anubis) and relate these to the female fertile period. We studied eight adult females living in four groups at the CNRS Station de Primatologie, Rousset-sur-Arc, France. We used vaginal cytology to detect ovulation. We investigated the volatile component of odour signals using solid-phase microextraction and gas chromatography-mass spectrometry. We found a total of 74 volatile compounds, of which we tentatively identified 25, including several ketones, alcohols, aldehydes, terpenes, volatile fatty acids and hydrocarbons that have been identified in odour profiles of other primates. Our results show that vaginal odour intensity differs with sexual cycle stage suggesting that odour might play a role in signalling female baboon fertility. We found differences in vaginal odour between females living in all-female and in mixed sex groups but we could not distinguish the effects of group composition, female age and identity. This study of olfactory signalling improves our understanding of how female primates advertise their sexual receptivity.

Comparative dissection of the peripheral olfactory system of the Chagas disease vectors Rhodnius prolixus and Rhodnius brethesi

American trypanosomiasis, or Chagas disease, is transmitted by both domestic and sylvatic species of Triatominae which use sensory cues to locate their vertebrate hosts. Among them, odorants have been shown to play a key role. Previous work revealed morphological differences in the sensory apparatus of different species of Triatomines, but to date a comparative functional study of the olfactory system is lacking. After examining the antennal sensilla with scanning electronic microscopy (SEM), we compared olfactory responses of Rhodnius prolixus and the sylvatic Rhodnius brethesi using an electrophysiological approach. In electroantennogram (EAG) recordings, we first showed that the antenna of R. prolixus is highly responsive to carboxylic acids, compounds found in their habitat and the headspace of their vertebrate hosts. We then compared responses from olfactory sensory neurons (OSNs) housed in the grooved peg sensilla of both species, as these are tuned to these compounds using single-sensillum recordings (SSRs). In R. prolixus, the SSR responses revealed a narrower tuning breath than its sylvatic sibling, with the latter showing responses to a broader range of chemical classes. Additionally, we observed significant differences between these two species in their response to particular volatiles, such as amyl acetate and butyryl chloride. In summary, the closely related, but ecologically differentiated R. prolixus and R. brethesi display distinct differences in their olfactory functions. Considering the ongoing rapid destruction of the natural habitat of sylvatic species and the likely shift towards environments shaped by humans, we expect that our results will contribute to the design of efficient vector control strategies in the future.

Investigation of the relationship between skin-emitted volatile fatty acids and skin surface acidity in healthy participants - a pilot study

Volatile organic compounds (VOCs) emitted from human skin are of great interest in general in research fields including disease diagnostics and comprise various compound classes including acids, alcohols, ketones and aldehydes. The objective of this research is to investigate the volatile fatty acid (VFA) emission as recovered from healthy participant skin VOC samples and to characterise its association with skin surface acidity. VOC sampling was performed via headspace-solid phase microextraction (HS-SPME) with analysis via gas chromatography-mass spectrometry (GC-MS). Several VFAs were recovered from participants, grouped based on gender and site (female forehead, female forearm, male forearm). Saturated VFAs (C9, C12, C14, C15, C16) and the unsaturated VFA C16:1 (recovered only from the female forehead) were considered for this study. VFA compositions and abundances are discussed in the context of body site and corresponding gland type and distribution, and their quantitative association with skin acidity investigated. Normalised chromatographic peak areas of the recovered VFAs were found to linearly correlate with hydrogen ion concentration measured at each of the different sites considered and is the first report to our knowledge to demonstrate such an association. Our observations are explained in terms of the free fatty acid (FFA) content at the skin surface which is well-established as being a major contributor to skin surface acidity. Furthermore, it is interesting to consider that these VFA emissions from skin, governed by equilibrium vapour pressures exhibited at the skin surface, will be dependent on skin pH. It is proposed that these pH-modulated equilibrium vapour pressures of the acids could be resulting in an enhanced VFA emission sensitivity with respect to skin surface pH. To translate our observations made here for future wearable biodiagnostic applications, the measurement of skin surface pH based on the volatile emission was demonstrated using a pH indicator dye in the form of a planar colorimetric sensor, which was incorporated into a wearable platform and worn above the palm surface. As acidic skin surface pH is required for optimal skin barrier function and cutaneous antimicrobial defence, it is envisaged that these colorimetric volatile acid sensors could be deployed in robust wearable formats for monitoring health and disease applications in the future.

Induced volatolomics of pathologies

Volatolomics allows us to elucidate cell metabolic processes in real time. In particular, a volatile organic compound (VOC) excreted from our bodies may be specific for a certain disease, such that measuring this VOC may afford a simple, fast, accessible and safe diagnostic approach. Yet, finding the optimal endogenous volatile marker specific to a pathology is non-trivial because of interlaboratory disparities in sample preparation and analysis, as well as high interindividual variability. These limit the sensitivity and specificity of volatolomics and its applications in biological and clinical fields but have motivated the development of induced volatolomics. This approach aims to overcome issues by measuring VOCs that result not from an endogenous metabolite but, rather, from the pathogen-specific or metabolic-specific enzymatic metabolism of an exogenous biological or chemical probe. In this Review, we introduce volatile-compound-based probes and discuss how they can be exploited to detect and discriminate pathogenic infections, to assess organ function and to diagnose and monitor cancers in real time. We focus on cases in which labelled probes have informed us about metabolic processes and consider the potential and drawbacks of the probes for clinical trials. Beyond diagnostics, VOC-based probes may also be effective tools to explore biological processes more generally.