Lessons from gold mines

The deep old mine in South Africa, in the early 19th century, was a unique environmental occupational setting. The pioneering heat-acclimation procedures used to prevent fatalities and increase yield provide a template to dissect the evolution of ideas and physiology underling human heat acclimation, especially when the physiologic-genomic linkage is examined concomitantly.

The first references to heat acclimation are from the 18th century, coinciding with the emerging interests in colonization in tropical areas. There were deliberations of whether Europeans could adapt to live and work in the tropics. Luigi Westerna Sambon, in his address to the Royal Geographical Soc., London 1899, noted the possible deleterious effects of tropical infectious diseases and hygiene conditions, including “sunstroke … a condition that authors could prove a noxious influence of heat.” Although Dr Sambon thought that this is also an “infectious disease,” he clearly differentiated between fever and sunstroke (Siriasis—after Sirius that rises in the hottest month with the sun) and recognized that two important obstacles to acclimatization are heat (especially “if combined with moisture”) and diseases.¹ Physiological experimentation on acclimatization to heat, only began at the start of the 20th century.

In October 1926 and then in March–April 1927, there were fatal cases of heat stroke at City Deep mine, and Dr Aldo Dreosti was commissioned by Dr Orenstein, the chief medical officer of the Rand Gold Mines Company to conduct experiments to improve the adaptability of the workers to the underground environment of the mine. Dr Dreosti developed a heat tolerance test (HTT) using conditions that matched the underground heat and workload in the mine, and subsequently underground heat acclimation protocols to enhance heat tolerance corresponding with increased yield² were introduced. In the review by Dr Suzanne M. Schneider “Heat Acclimation: Gold Mines and Genes”³ she describes an occupational scenario using several approaches to acclimate tens of thousands of workers allowing them to perform in the extreme conditions of high temperatures and 100% relative humidity with few fatalities. Although Dr Schneider reviewed the period between the 1890s and 1940, before mechanized tools and artificial cooling were introduced to the mine, she used Dreosti's acclimation paradigms² as a template to analyze the evolution of ideas regarding cost-effective heat acclimation and HTT procedures, which are still used today. In 1953, following increased cases of fatal heat stroke with deepening of the mining area, the Chamber of Mines funded research by C. Wyndham to improve acclimation protocols. Wyndham and Strydom (1973) (see in ref. 4) modified Dreosti's program while using above-ground climatic chambers. Interestingly, the acclimation protocols used by both Dreosti and Wyndham were flexible and differentiated between heat-tolerant (adapted) and non-tolerant workers and thus assigned the new recruits to “personalized” acclimation protocols. Remarkably, the conclusion of both Dreosti's and Wyndham's protocols were that no differences in acclimation-performance capacity were noted among workers with different ethnicities or climatic origins. This conclusion fits with Nigel Taylor⁴who found no genetic differences in the ability to acclimate and attributed acclimatory changes to phenotypic adaptations. These findings also answered questions raised at the “Victorian debates” in 1899 of whether there are innate differences between the colonizers and those of the well-adapted natives. Given the 100% relative humidity in the mine, Dreosti only used Rectal Temperature (Tre) as a measure of tolerance, whereas Wyndham added cardiovascular and sweat rate parameters as strain criteria. To this end, HTT is used when a “return to duty verdict” in military troops is required. There are still debates regarding the best HTT protocol among troops.

In the section on “Heat acclimation today,” Dr Schneider³ comments on the various approaches currently used to induce heat acclimation in humans. Traditional human heat acclimation in artificial climatic chamber includes heat and exercise stimuli, lasts between 10 and 14 d, and leads to a reduced core temperature, reduced skin temperatures, reduced heart rate, and increased sweating for a given level of exercise and heat exposure. This “traditional heat acclimation protocol” has many practical benefits to individuals encountering specific heat stress and workload. According to Nigel Taylor,⁴ by using the “traditional acclimation protocol,” achieved by repeated challenges, the load decreases as acclimation progresses; hence, prolonged exposure to a fixed environmental load eventually leads to habituation.⁴ To Taylor, habituation may take years to develop and is best observed in the habitually active indigenous inhabitants of hot climates. This philosophy led Fox (see in ref 4) to develop a different acclimation approach in 1963, whereby the acclimatory load is adjusted progressively ensuring a constant acclimating stimulus. At the time, the benefits of each acclimation method described were only evaluated in terms of the capacity of the physiologic effectors. Understanding the molecular mechanisms of heat acclimation may highlight differences between acclimating approaches. Unfortunately, studies on the molecular mechanisms of heat acclimation, particularly in humans, are still in their infancy. Animal studies, using fixed passive heating protocols show that there is a continuum of molecular changes, modified by the initial physiologic strain; first to maintain DNA integrity, and then (when strain is relieved) to reprogram cytoprotective and metabolic genes, to produce an adequate transcript level and the optimal mRNA/protein ratio to maintain proteostasis and thermotolerance.⁵ Phases in the molecular profile reflect the molecular stress during acclimation. Along this line Gibson and team in 2015 measured the HSP72 transcript profile in human lymphocytes during 10 d of heat acclimation using three different approaches: exogenously controlled, fixed-intensity heat-acclimation, an endogenously controlled isothermic heat-acclimation method, and a progressive endogenous isothermic heat-acclimation method. No differences were found between the groups, suggesting similar molecular thresholds for all procedures. Translation was not measured; thus, HSP72 levels are unknown. In congruence with the finding that elevated HSP reserves are only noted when acclimatory homeostasis has been achieved,⁵ several studies on humans did not show increased HSP72 levels following short acclimation. The time required for molecular/physiologic acclimatory homeostasis (the author) is longer than that suggested in most human studies.

In her review,³ Dr Schneider, initially focuses on the scientific background leading to experimentation to improve performance in a harsh hot-humid environment and then elegantly develops the evolution of ideas in the field of heat acclimation. Dr Dreosti received the prestigious Gold Medal of the South African Institute of Mining and Metallurgy for his work on heat acclimation.