Overview
ISO 7243:2017, "Ergonomics of the thermal environment - Assessment of heat stress using the WBGT (wet bulb globe temperature) index", is an international standard that defines a screening method to evaluate occupational heat stress. It specifies how to measure and interpret the WBGT (wet bulb globe temperature) index for exposures up to 8 hours, and how to account for activity (metabolic rate) and clothing. The standard applies to indoor and outdoor environments and to male and female adults who are fit for work. It is intended as a first-line assessment to establish the presence or absence of heat stress; where WBGT exceeds reference limits, a detailed analysis (e.g., ISO 7933) is recommended.
Key Topics and Requirements
- WBGT calculation - Two formulae are provided:
- Without solar load: WBGT = 0.7 t_nw + 0.3 t_g
- With solar load: WBGT = 0.7 t_nw + 0.2 t_g + 0.1 t_a
(t_nw = natural wet-bulb temperature, t_g = globe temperature, t_a = air temperature)
- Effective WBGT and Clothing Adjustment (CAV) - Adjust measured WBGT for non‑standard clothing by adding a Clothing Adjustment Value (CAV) to produce an effective WBGT (WBGT_eff).
- Metabolic rate estimation - Classifies work intensity (resting to very high) and requires time-weighted averaging for intermittent work; Annex E gives guidance.
- Measurement and instrumentation - Sensor specifications and measurement procedures are detailed (Annex B), with notes on alternative globe thermometers (Annex C) and predicting natural wet-bulb temperature (Annex D).
- Timing and averaging - Guidance on timing, duration and spatial/temporal variability of measurements to represent up to 8‑hour exposures.
- Reference exposure limits - Annex A provides WBGT reference values (exposure limits) used to judge risk and trigger controls or further analysis.
Applications and Who Uses It
ISO 7243 is used for practical heat stress assessment in:
- Occupational health & safety programs (construction, mining, manufacturing, agriculture)
- Industrial hygienists and ergonomists conducting workplace risk assessments
- Safety managers, plant engineers, and regulators setting work–rest schedules and controls
- Event planners and sport/military planners assessing prolonged heat exposure
Practical uses include screening for heat-related illness risk, designing controls (ventilation, scheduling, PPE), and deciding when to perform detailed thermal-strain analyses.
Related Standards
- ISO 7933 - Analytical determination/interpretation of heat stress (detailed analysis)
- ISO 13731 - Vocabulary and symbols (thermal environment)
- ISO 8996 - Metabolic rate determination
- ISO 9920 - Thermal properties of clothing
- ISO 7726 - Measurement of physical quantities for thermal environments
Keywords: ISO 7243:2017, WBGT, wet bulb globe temperature, heat stress assessment, occupational heat, ergonomics, clothing adjustment value, metabolic rate, workplace heat exposure.