Heat Exchange in Young and Older Men during Constant- and Variable-Intensity Work

Area of Science:

• Occupational Health
• Environmental Physiology
• Human Heat Balance

Background:

• Current occupational heat stress guidelines use time-weighted averaging for variable-intensity work.
• Variable-intensity work may impair heat loss, increasing heat strain, particularly in older adults.
• Direct calorimetry is needed to assess heat loss during variable work intensities.

Purpose of the Study:

• To evaluate if variable-intensity work causes decrements in whole-body heat loss compared to constant-intensity work.
• To determine if these decrements augment body heat storage and core temperature.
• To compare these effects in young and older men.

Main Methods:

• Eight young and eight older men underwent four 90-min direct calorimetry trials in 40°C heat.
• Work involved constant intensity (CON) or variable intensity (VAR 5:5, VAR 6:4, VAR 7:3).
• Metabolic heat production, total heat loss, heat storage, and core temperature were measured.

Main Results:

• Averaged over 90 minutes, metabolic heat production, total heat loss, and heat storage were similar between conditions and age groups.
• Peak core temperature was also similar across all trials and participants.
• No significant differences were observed in heat loss, storage, or core temperature based on work intensity partitioning.

Conclusions:

• Time-weighted averaging is appropriate for quantifying metabolic demands of variable-intensity work in occupational heat stress assessment.
• Work intensity partitioning did not significantly influence heat loss, storage, or core temperature in young or older men.
• Current guidelines appear adequate for managing heat strain from variable work patterns.