Heat stress is a deadly occupational hazard that is projected to increase in severity with global warming. While upper limits for heat stress designed to protect all workers have been recommended by occupational safety institutes for some time, heat stress continues to compromise health and productivity. The continuing detrimental effects of heat stress may be partly explained by the inability of existing guidelines to consider individual factors (e.g., age, sex, health status, work duration) that can modify an individual’s tolerance to a given heat stress.