ISO 7933:2023

Overview - ISO 7933:2023 (Predicted Heat Strain, PHS)

ISO 7933:2023 defines the Predicted Heat Strain (PHS) model for assessing heat stress in the thermal environment. The standard describes an analytical method to predict core (rectal) temperature, sweat/water loss and to calculate maximum allowable exposure times within which physiological strain remains acceptable for 95% of an average, healthy working population. This third edition (2023) updates the heat-balance–based model and clarifies limits (not applicable to special protective clothing or individual medical responses).

Key topics and technical requirements

• PHS model principles: heat-balance equation accounting for metabolic heat, convection, radiation, conduction, respiratory heat flows, evaporation and heat storage.
• Primary outputs: predicted evaporative heat flow, required sweat rate, skin wettedness, rectal temperature evolution, and allowable exposure time (D) for heat storage and water loss.
• Interpretation criteria: stress and strain criteria, reference values, and methods to determine allowable exposure time so physiological limits are not exceeded for 95% of the population.
• Scope limits: intended for average persons in good health; not applicable for special protective clothing (e.g., fully reflective suits, active cooling or impermeable coveralls).
• Supporting data and tools: annexes provide required data sets (clothing thermal characteristics, metabolic rates), example computations and a computer program for PHS calculations.
• Caveats: model is less validated for rapidly changing (unsteady) environmental parameters, clothing or metabolic rates.

Practical applications and users

• Who uses it: ergonomists, industrial hygienists, occupational health professionals, safety engineers and risk assessors.
• Use cases:
  • Detailed assessment of hot work environments (beyond screening indices like WBGT).
  • Designing work/rest schedules and allowable exposure durations.
  • Evaluating the effect of environmental changes, clothing or workload on heat strain.
  • Optimizing engineering controls (ventilation, shading), administrative controls and hydration policies.
  • Supporting emergency planning and medical surveillance strategies (in conjunction with ISO 9886 and ISO 12894).
• Benefits: enables targeted interventions by showing which environmental or clothing parameters most influence heat strain.

Related standards

• ISO 8025 (use/recommendations for hot environments)
• ISO 7243 (WBGT screening index)
• ISO 15265, ISO 13731 (vocabulary/symbols)
• ISO 9886, ISO 12894 (physiological monitoring and medical supervision)

Keywords: ISO 7933:2023, predicted heat strain, PHS model, heat stress, thermal environment, allowable exposure time, sweat rate, ergonomics, industrial hygiene.