Response to Simpson (2024): Standard heat stress indices may not be appropriate for assessing marathons

We thank Simpson1 for his insightful commentary on our research paper2, which critically examines the appropriateness of a standard heat stress index for a specific event. In our research, we chose the Universal Thermal Comfort Index (UTCI) to evaluate heat stress during a major ultra -marathon event. We acknowledge that while UTCI provides a broad overview of heat responses in the average person, and may not capture the specific physiological reactions and microclimatic conditions experienced by each athlete, this limitation is inherent to all heat-related stress indices.3 We argue that the UTCI is the appropriate index to use, as it is based on the most recent scientific progress in both thermophysiology and heat exchange theory.4,5 The validation of the physiological equivalent temperature (PET) vs UTCI (or other indices) was beyond the scope of our paper, because it has been done numerous times.6,7 Our conclusions, as we set out in our objectives and data analysis, remain valid and are supported by Simpson's own calculation of the UTCI, despite his criticism of the index - which will hold true many other comparisons as well.3,8