Mining Project: Predicting Heat Strain in Underground Metal and Nonmetal Miners

Research Summary

Heat stress is a challenge in many industries including mining, and can lead to heat strain among workers. Heat stress refers to the total heat load placed on the body from external environmental sources and from physical exertion, whereas heat strain refers to the physical strain the body experiences as a result of heat stress. Heat strain can lead to adverse heat-related conditions of varying severity (heat syncope, heat rash, heat exhaustion, heat stroke) and can increase the risk of injury (e.g., sweaty palms, fogged safety glasses, dizziness, impaired reaction time). As mines expand into deeper, hotter environments, heat stress and strain among miners is likely to increase. Such increases could affect the health of mine workers and potentially lead to unsafe conditions. Therefore, a better understanding of factors that contribute to and that mitigate heat strain is needed.

That heat strain can progress to severe heat illness such as heat exhaustion or heat stroke is broadly understood. Less well recognized is the prospect that heat strain may also lead to decreased cognitive performance, potentially increasing the risk of accidents in a mining environment. Finding ways to evaluate the cognitive effects of heat strain and identifying predictive indicators that can alert workers to an imminent decrease in mental performance would fill an important gap in heat research. Although previous research has provided important insights into heat-related factors such as hydration and acclimatization, most research has not attempted to evaluate mental performance under heat stress. Furthermore, a standard heat stress index has not been developed for use in mining. Many heat stress indices are used throughout the world, but most incorporate only measured environmental conditions. Guidelines for acceptable thresholds of heart rate, body temperature, and environmental conditions (e.g. wet bulb globe temperature) for workers have been established, but these guidelines can be difficult to use in practice because physiologic parameters are not always easily measured and environmental measurements can be difficult to interpret. Neither the Occupational Safety and Health Administration (OSHA) nor the Mine Safety and Health Administration (MSHA) has promulgated a formal rule for work in hot environments. A better understanding of factors that predict when a worker has reached the limits of his or her ability to perform job tasks safely is a crucial step in managing heat stress in industrial settings. Cognitive deterioration may decrease a miner’s ability to recognize signs of imminent danger, or contribute to a relaxation of normally stringent safety standards, leading to accidents.

A better understanding of relevant factors will inform guidance for mitigating heat strain and will facilitate evaluations of the effectiveness of mitigation measures. Specifically, improved understanding of heat strain thresholds at which mental performance decreases, and a more comprehensive understanding of the many factors contributing to heat strain, will help determine appropriate strategies for monitoring the hazard. If feasible, these strategies could include real-time monitoring methods for miners. For example, a sensor system could collect information on physiologic parameters, environmental conditions, and personal factors to provide information needed to modify work/rest cycles, hydration frequency, and job tasks to prevent heat illness. The system could alert miners or managers when workers are approaching a “danger zone” with substantially increased risk of heat illness or injury. This could trigger ventilation-based mitigation measures, such as “ventilation-on-demand.” The Spokane Mining Research Division’s Ventilation Team may be able to use information from this project to design studies that evaluate those types of options.

This one-year pilot project identified that changes to the research methodology must be made to effectively characterize heat strain. This information was used to revise the methodologies and resulted in a follow-on project entitled "Predicting the Impact of Heat Strain on Cognitive Functions in Miners."