Variation in volatile organic compounds in the breath of normal humans

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Abstract

We studied the variation in volatile organic compounds (VOCs) in the breath of 50 normal humans, using gas chromatography and mass spectroscopy. An average breath sample contained 204.2 VOCs (SD=19.8, range 157–241).The alveolar gradient of each VOC (abundance in breath minus abundance in air) varied with rate of synthesis minus rate of clearance. A total of 3481 different VOCs were observed: 1753 with positive alveolar gradients, 1728 with negative alveolar gradients. Twenty-seven VOCs were observed in all fifty subjects. This study confirmed previous reports of wide inter-individual variations. Two new findings were the comparatively small variation in total number of breath VOCs, and the presence of a ‘common core’ of breath VOCs in all subjects.

Introduction

Alveolar breath is a distinctive gas whose chemical composition differs markedly from inspired air. Volatile organic compounds (VOCs) are either subtracted from inspired air (by degradation and/or excretion in the body) or added to alveolar breath as products of metabolism. Some features of this transformation have been well understood for many years: e.g. oxygen is subtracted and carbon dioxide is added by the oxidative metabolism of glucose [1]. Pauling et al., in 1971, employed cold trapping to concentrate the VOCs in breath and found that normal human breath contained several hundred different VOCs in low concentrations [2]. This observation has been subsequently confirmed in many different laboratories, employing progressively more sophisticated and sensitive assays. More than a thousand different VOCs have been observed in low concentrations in normal human breath [3].

Analysis of VOCs in inspired air and alveolar breath is a useful research tool with potential applications in clinical medicine. Breath analysis opens a non-invasive window on to normal metabolic pathways, and also illustrates how these pathways are altered in disease. For example, breath pentane is a marker of increased oxygen free radical (OFR) activity in several diseases. Pentane is a product of OFR-mediated lipid peroxidation of n-6 polyunsaturated fatty acids [4], [5] and it is subsequently degraded by cytochrome P450 enzymes [6], [7]. Breath pentane is increased in a number of disorders including breast cancer [8], heart transplant rejection [9], acute myocardial infarction [10], schizophrenia [11] and rheumatoid arthritis [12].

Despite numerous studies of pentane and several other breath VOCs, the range of composition of VOCs in normal human breath has not been well defined. Early studies reported substantial quantitative and qualitative differences amongst small groups of normal humans: concentrations of breath VOCs varied widely, and a number of VOCs were detectable in the breath of some subjects but not in others [13], [14]. We undertook this study in order to better define the range of inter-individual variation in breath VOCs in normal humans. We employed a portable breath collection apparatus BCA to study a group of normal volunteers; breath VOCs were collected onto sorbent traps which were assayed by gas chromatography–mass spectroscopy (GC–MS) [3].

Section snippets

Breath collection apparatus

This device has been described [3]. In summary, the BCA is a portable, microprocessor-controlled device with a heated breath reservoir which prevents condensation of water. Alveolar breath is pumped from the breath reservoir through a sorbent tube which captures the VOCs on activated carbon. In this study, modified sorbent tubes were employed containing 200 mg Carbotrap C (20/40 mesh) and 200 mg Carbopack B (60/80 mesh) (Supelco, Bellefonte, PA, USA) The volume of the breath sample can be

Human subjects

There were 50 subjects studied, comprising 27 males (mean age 38.8 years, SD=12.8) and 23 females (mean age 38.65 years, SD=11.4). Typical chromatograms of breath and air are shown in Fig. 5, with a subtraction chromatogram of the difference between the two.

Inter-individual variation in number of VOCs

The number of VOCs detected in each breath sample ranged from 157 to 241 (mean=204.2, SD=19.8, CV=9.7%) (Fig. 1). Different VOCs totaling 3481 were observed at least once, 1753 with positive alveolar gradients and 1728 with negative alveolar

Discussion

More than 200 different VOCs were observed in most breath samples, and more than 3000 different VOCs were observed at least once. These numbers probably represent an underestimate of the total number of VOCs in normal human breath, since the assay was limited to C4 to C20 VOCs within the trapping range of the sorbent traps. The majority of these VOCs were observed only once. The number of breath VOCs observed in more than one subject fell rapidly as the size of the group increased, and only a

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