The natural history of insensible perspiration: a forgotten doctrine of health and disease

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

Wearable Sweat Loss Measuring Devices: From the Role of Sweat Loss to Advanced Mechanisms and Designs

Wearable sweat sensors have received significant research interest and have become popular as sweat contains considerable health information about physiological and psychological states. However, measured biomarker concentrations vary with sweat rates, which has a significant effect on the accuracy and reliability of sweat biosensors. Wearable sweat loss measuring devices (SLMDs) have recently been proposed to overcome the limitations of biomarker tracking and reduce inter- and intraindividual variability. In addition, they offer substantial potential for monitoring human body homeostasis, because sweat loss plays an indispensable role in thermoregulation and skin hydration. Previous studies have not carried out a comprehensive and systematic review of the principles, importance, and development of wearable SLMDs. This paper reviews wearable SLMDs with a new health perspective from the role of sweat loss to advanced mechanisms and designs. Two types of sweat and their measurement significance for practical applications are highlighted. Then, a comprehensive review of advances in different wearable SLMDs based on hygrometers, absorbent materials, and microfluidics is presented by describing their respective device architectures, present situations, and future directions. Finally, concluding remarks on opportunities for future application fields and challenges for future sweat sensing are presented.

'To preserve the skin in health': drainage, bodily control and the visual definition of healthy skin 1835-1900

The concept of a healthy skin penetrated the lives of many people in late-nineteenth-century Britain. Popular writings on skin and soap advertisements are significant for pointing to the notions of the skin as a symbolic surface: a visual moral ideal. Popular health publications reveal how much contemporary understanding of skin defined and connected ideas of cleanliness and the visual ideals of the healthy body in Victorian Britain. Characterised as a 'sanitary commissioner' of the body, skin represented the organ of drainage for body and society. The importance of keeping the skin clean and purging it of waste materials such as sweat and dirt resonated in a Britain that embraced city sanitation developments, female beauty practices, racial identities and moral reform. By focusing on the popular work by British surgeon and dermatologist Erasmus Wilson (1809-84), this article offers a history of skin through the lens of the sanitary movement and developments in the struggle for control over healthy skin still in place today.

Regional variations in transepidermal water loss, eccrine sweat gland density, sweat secretion rates and electrolyte composition in resting and exercising humans

Literature from the past 168 years has been filtered to provide a unified summary of the regional distribution of cutaneous water and electrolyte losses. The former occurs via transepidermal water vapour diffusion and secretion from the eccrine sweat glands. Daily insensible water losses for a standardised individual (surface area 1.8 m2) will be 0.6-2.3 L, with the hands (80-160 g.h-1) and feet (50-150 g.h-1) losing the most, the head and neck losing intermediate amounts (40-75 g.h-1) and all remaining sites losing 15-60 g.h-1. Whilst sweat gland densities vary widely across the skin surface, this same individual would possess some 2.03 million functional glands, with the highest density on the volar surfaces of the fingers (530 glands.cm-2) and the lowest on the upper lip (16 glands.cm-2). During passive heating that results in a resting whole-body sweat rate of approximately 0.4 L.min-1, the forehead (0.99 mg.cm-2.min-1), dorsal fingers (0.62 mg.cm-2.min-1) and upper back (0.59 mg.cm-2.min-1) would display the highest sweat flows, whilst the medial thighs and anterior legs will secrete the least (both 0.12 mg.cm-2.min-1). Since sweat glands selectively reabsorb electrolytes, the sodium and chloride composition of discharged sweat varies with secretion rate. Across whole-body sweat rates from 0.72 to 3.65 mg.cm-2.min-1, sodium losses of 26.5-49.7 mmol.L-1 could be expected, with the corresponding chloride loss being 26.8-36.7 mmol.L-1. Nevertheless, there can be threefold differences in electrolyte losses across skin regions. When exercising in the heat, local sweat rates increase dramatically, with regional glandular flows becoming more homogeneous. However, intra-regional evaporative potential remains proportional to each local surface area. Thus, there is little evidence that regional sudomotor variations reflect an hierarchical distribution of sweating either at rest or during exercise.

Medicine, 1450-1620, and the history of science

History of science and history of medicine are today largely organized as distinct disciplines, though ones widely recognized as interrelated. Attempts to evaluate the extent and nature of their relation have reached varying conclusions, depending in part on the historical period under consideration. This essay examines some characteristics of European medicine from the fifteenth to the early seventeenth century and considers their relevance for the history of science. Attention is given to the range of interests and activities of individuals trained in or practicing medicine, to the impact of changes in natural philosophy, to the role of observation, description, and accumulation of information, and to the exchange of knowledge among the medical community.

Balancing acts: Picturing perspiration in the long eighteenth century

This essay examines the historical fortunes of an image that throughout the seventeenth and eighteenth centuries became a landmark of the medical doctrine and practice of static medicine advanced by the physician Santorio Santorio (1561-1636). The image depicted a man sitting on a large Roman steelyard, which allowed the weighing of bodily discharges and gave guidance on the intake of food. Well into the eighteenth century, the image of the weight-watching man accompanied Santorio's work on the art of static medicine and, most likely, contributed to its success. It appeared in a variety of medical works and navigated across competing medical theories and different medical genres, while remaining largely unscathed. This essay explores the success and the historical agency of this image. Focusing on the history of its copies and variants, it investigates how the image came to symbolize the attempt to transform dietetics into an experimental practice, and accordingly preserve its pivotal significance in the medical world.

Causal (mis)understanding and the search for scientific explanations: a case study from the history of medicine

In 1747, James Lind carried out an experiment which proved the usefulness of citrus fruit as a cure for scurvy. Nonetheless, he rejected the earlier hypothesis of Bachstrom that the absence of fresh fruit and vegetables was the only cause of the disease. I explain why it was rational for James Lind not to accept Bachstrom's explanation. I argue that it was the urge for scientific understanding that guided Lind in his rejection and in the development of his alternative theory that humidity was the primary cause of the disease. Central in this process was the search for causal mechanisms which could provide understanding of how the disease developed and which fitted in with the knowledge of the time. Given that the relevant background knowledge and statistical methods were not yet available to Lind, he was right to prefer his own explanation to that of Bachstrom. Although his explanation turned out to be wrong, and Bachstrom's right, from a historical point of view it offered deeper causal understanding of both the development of the disease and the preventive and curative effects of fresh vegetable food. This case study illustrates how the search for causal mechanisms can not only be enlightening, but also very misleading.