EN ISO 7933:2023 PDF

Overview

EN ISO 7933:2023 (ISO 7933:2023) specifies the Predicted Heat Strain (PHS) model for assessing heat stress in occupational and other hot environments. The standard provides an analytical method to predict physiological responses - primarily sweat (water) loss and rectal (core) temperature - for an average, healthy person performing specified work. It also defines how to determine maximum allowable exposure times within which physiological strain remains acceptable for 95% of the exposed population.

Key topics and technical requirements

• Description and principles of the PHS model, based on a detailed heat balance equation.
• Components of the heat balance addressed: metabolic rate (M), mechanical power (W), respiratory heat flows, conduction (K), convection (C), radiation (R) and evaporation (E).
• Calculation steps for required evaporative heat flow, skin wettedness, and required sweat rate.
• Interpretation criteria: stress and strain limits, reference values, and computation of allowable exposure time (Dlim) so that maximum tolerable rectal temperature and water loss are not exceeded by 95% of people.
• Annex material: data needed for computations, metabolic rate tables, clothing thermal characteristics, worked examples, and a computer programme for PHS calculations.
• Limitations and exclusions: not applicable when wearing special protective clothing (e.g., fully reflective suits, active cooling garments, impermeable coveralls). The model predicts group-average responses - it is not designed to predict an individual’s physiological response.
• Caution for conditions with unsteady environmental parameters, variable metabolic rates or changing clothing, where the model has limited validation.

Practical applications

• Quantitative heat stress assessment and risk evaluation in industries with hot processes (foundries, glassworks, heavy manufacturing), outdoor work, and emergency response planning.
• Determining safe work–rest schedules, hydration needs, and engineering or administrative controls to reduce heat strain.
• Using the model to identify which environmental or clothing parameters can be changed to reduce physiological risk (e.g., increased ventilation, reduced radiant load, adjusted workload).

Who should use this standard

• Ergonomists, industrial hygienists, occupational health and safety professionals, facility designers and safety managers who need a robust analytical method to evaluate occupational heat stress for groups of workers.

Related standards

• ISO 7243 (WBGT screening index) - screening tool complementary to PHS.
• ISO 8025 - recommendations on when and how to apply the PHS model.
• ISO 15265, ISO 9886, ISO 12894 - broader context for thermal risk assessment and physiological monitoring.

Keywords: EN ISO 7933:2023, ISO 7933, predicted heat strain, PHS model, heat stress assessment, occupational heat strain, allowable exposure time, sweat rate, rectal temperature.