Analyses of volatile organic compounds from human skin.

Skin is the largest human organ, accounting for approximately 12-15% of body weight.1 Volatile organic compounds (VOCs) emanating from skin contribute to a person’s body odour, and may convey important information about metabolic processes. VOCs from skin derive from eccrine, sebaceous and apocrine gland secretions and their interactions with resident skin bacteria.2,3 These glands are distributed differently across the body; hence different regions of the body have different VOC profiles, and thus different odours. Eccrine glands are found throughout the skin, but are especially concentrated in palms of hands, soles of feet, and the forehead. Eccrine sweat is mostly water, but contains glycoproteins (notably interleukin 1), lactic acid, sugars, amino acids and electrolytes.4 Sebaceous glands are concentrated on the upper part of the body.3 The upper chest, back, scalp, face and forehead may have as many as 400-900 sebaceous glands cm-2. Sebaceous gland secretions are rich in lipid materials such as cholesterol, cholesterol esters, long-chain fatty acids, squalene and triglycerides.3 These lipids provide substrate for growth and metabolism of skin microorganisms. Apocrine glands are concentrated in the axillae, pubic area and areolas.2,4 Apocrine secretions are the chief source of underarm odorants (commonly known as ‘body odour’) and play a role in chemical signalling (for a review see Wysocki and Preti.5) Many previous studies have focused on VOCs emanating from the axillae, which reflect some contribution from all skin glands located in the axillae.6-10 VOCs from nonaxillary skin secretions have been studied as potential mosquito attractants,11-13 indicators of seasonal changes14 and ageing,15 and moderators of fragrances.16-19 It was recently demonstrated that skin emanations could be collected via rolling a stir-bar coated with polydimethylsiloxane across the arm with subsequent desorption and analysis by gas chromatography/mass spectrometry (GC/MS).20 Bernier et al.11-13 reported that hundreds of compounds canbe volatilized from skin secretions collected from the palms and backs of hands. Most of these compounds have been documented to be organic acids ranging in carbon size from C2 to C20. However, the most abundant (75-80%) organic acids found on skin are C16 and C18 saturated, monounsaturated and diunsaturated acids,12 which are not volatile at body temperatures. In contrast, collection of skin VOCs using solid-phase microextraction (SPME) will collect low molecular weight compounds that are volatile at body temperature. SPME-GC/MS analyses of hand/wrist VOCs sampled in both winter and spring revealed 35 organic compounds.14 VOCs were reported to be more abundant in winter samples; however, the relative ratios of many (but not all) of the compounds did not vary between seasons. This observation led the authors to speculate that the moist spring air allowed the skin to harbour more bacteria that hydrolysed and decomposed some of the VOCs. A study of male Japanese subjects used T-shirts worn for 3 days to collect skin odours. VOCs emanating from rectangular pieces cut from the backs of these T-shirts were studied.15 The authors suggested that skin secretions in men older than age 39 years contain larger amounts of unsaturated aldehydes than secretions from younger men. These compounds, particularly 2-nonenal, were reported to impart an unpleasant ‘ageing odour’ to older Japanese men. Most acids, alcohols and aldehydes found in skin secretions apparently originate from the interactions between sebaceous gland secretions and cutaneous bacteria.12,21 Anaerobic bacteria living in the hair follicle/sebaceous gland duct use lipases to liberate long-chain acids from triglycerides, which are further metabolized by aerobic bacteria into longer, saturated and unsaturated acids and smaller volatile acids, aldehydes and alcohols.21 Other potential sources of skin volatiles are putative odorants carried by the apocrine secretion odour-binding protein (ASOB) 1.22 This protein might bind odorous acids that can be liberated by interaction with cutaneous bacteria. A previous study demonstrated that this protein does carry odorants in the underarm, and is present in sebaceous-rich perspiration from the forehead.22 Tracking dogs trained to recognize an individual’s scent on garments from a particular body part (e.g. chest, arm, leg) are not reliably able to generalize the individual’s odour to other body parts, suggesting that a different odorant milieu may characterize the back and forearm (for a review see Jenkins23). In this study, we performed a comprehensive analysis of VOCs from the upper back and forearm of 25 healthy subjects using two complementary sampling techniques, SPME and solvent extraction. Skin secretions from the back of each subject also were analysed for the presence of the potential VOC carrier ASOB1. The goals of the study were: (i) to identify VOCs emanating from the upper back and forearm skin; (ii) to determine whether VOCs vary in a qualitative or quantitative manner; and (iii) to determine if VOCs vary in relative abundance with gender, age or locus, i.e. back and forearm. The upper back and forearm were chosen because they are easily accessible with minimal inconvenience to subjects. In addition, these body regions differ with respect to density of sebaceous glands (upper back > forearm) and skin bacteria, as well as exposure to sun, environment and consumer products.3,21