Core temperature and heart rate at the upper limit of the prescriptive zone

Area of Science:

• Occupational Health
• Environmental Physiology
• Human Factors Engineering

Background:

• The 1969 World Health Organization (WHO) report aimed to limit workplace heat exposure to a core temperature (T_c) of 38°C.
• This limit corresponds to the upper limit of the prescriptive zone (ULPZ), a critical threshold for physiological strain.

Purpose of the Study:

• To analyze the relationship between core temperature (T_c) and heart rate (HR) relative to metabolic rate (MR) at and below the ULPZ.
• To investigate the influence of sex, acclimatization, and clothing on physiological responses during heat stress exposure.

Main Methods:

• Utilized progressive heat stress protocol data from studies conducted at Penn State University (PSU) and the University of South Florida (USF).
• Employed regression models to determine the association of metabolic rate (MR) and sex with core temperature (T_c) and heart rate (HR).

Main Results:

• Women exhibited higher T_c and HR than men at the same MR within the ULPZ, suggesting greater relative physiological demands.
• Acclimatization state and clothing type did not demonstrate a significant effect on T_c or HR.
• Core temperature (T_c) remained relatively stable below the ULPZ, while heart rate (HR) increased as exposure approached the ULPZ.

Conclusions:

• The ULPZ represents a significant physiological threshold, with distinct responses observed between sexes.
• Findings highlight the need to consider sex-specific physiological differences in workplace heat stress management strategies.
• The established 38°C core temperature limit is approached by men at high metabolic rates (490 W), with lower temperatures at reduced metabolic loads.