Heat Stress Indices Reference Tool
A practical occupational hygiene guide to common heat stress indices, including WBGT, P4SR, Heat Stress Index, Required Sweat Rate, Predicted Heat Strain and Thermal Work Limit.
Different heat stress indices answer different occupational hygiene questions.
Heat stress assessment can range from simple workplace screening to detailed physiological modelling. This page helps users compare commonly referenced heat stress indices and understand when each may be useful, what input data is needed and what professional limitations apply.
Screen heat exposure
WBGT is widely used as a screening index for occupational heat stress and is often the practical starting point for workplace heat assessment.
Model heat strain
P4SR, HSI, Required Sweat Rate and PHS are more analytical indices that estimate sweat demand, evaporative requirement or physiological strain.
Plan sustainable work
TWL estimates the maximum sustainable metabolic rate for a given environment, clothing and acclimatisation condition.
A practical sequence before selecting a heat stress index.
The choice of index should be driven by the assessment objective, available measurements, worker activity, clothing, acclimatisation and the required level of technical interpretation.
Define the question
Decide whether you need screening, work-rest planning, physiological modelling or sustainable work-rate estimation.
Measure conditions
Collect suitable environmental measurements such as air temperature, humidity, radiant heat and air movement.
Characterise work
Estimate metabolic workload, clothing/PPE, acclimatisation status, work duration and recovery conditions.
Select the index
Use WBGT for screening, PHS or Required Sweat Rate for detailed modelling, or TWL for sustainable work-rate decisions.
Control exposure
Review engineering controls, work-rest regimes, hydration, acclimatisation, supervision and emergency response.
Heat Stress Indices Finder
Search and compare common heat stress indices used in occupational hygiene, ergonomics and thermal environment assessment.
| Index | Full Name | What It Estimates | Typical Inputs | Primary Use | Key Limitations | Reference |
|---|---|---|---|---|---|---|
| WBGT | Wet Bulb Globe Temperature | A screening index for environmental heat stress based on wet bulb temperature, globe temperature and air temperature. | Natural wet bulb temperature, globe temperature, dry bulb temperature, indoor/outdoor condition and exposure context. | Heat stress screening, work-rest planning and prioritising further assessment. | Does not fully model individual physiology, clothing effects, all PPE burden or individual susceptibility. | ISO 7243 |
| P4SR | Predicted 4-Hour Sweat Rate | Estimated sweat rate over a four-hour period under specified heat exposure and work conditions. | Air temperature, humidity, radiant heat, air movement, metabolic rate and clothing assumptions. | Historical heat stress comparison and more detailed thermal strain discussion. | Older index; less commonly used in routine workplace practice than WBGT or current PHS-based approaches. | Heat stress review |
| HSI | Heat Stress Index / Belding-Hatch Index | Ratio of required evaporative cooling to maximum evaporative cooling capacity. | Metabolic heat, convective/radiant heat exchange, evaporative requirement and maximum evaporative capacity. | Evaluating heat balance and evaporation demand in hot environments. | Conceptually useful but older; requires careful interpretation and may be less user-friendly for non-specialists. | Heat stress review |
| SWreq | Required Sweat Rate | The sweat rate required to maintain heat balance under the measured thermal environment and workload. | Thermal environment, metabolic rate, clothing, evaporative capacity and heat exchange parameters. | Detailed evaluation of sweat demand, dehydration risk and physiological feasibility. | Requires expert calculation and interpretation; now often discussed together with PHS and ISO 7933 approaches. | ISO 7933 |
| PHS | Predicted Heat Strain | Predicted physiological strain, including water loss and rectal temperature response for an average person. | Air temperature, humidity, radiant heat, air speed, metabolic rate, clothing, posture and exposure duration. | Determining acceptable exposure duration based on predicted water loss and core temperature constraints. | Model-based estimate for an average person; individual susceptibility, health status and acclimatisation still matter. | ISO 7933 |
| TWL | Thermal Work Limit | Maximum sustainable metabolic rate that workers can maintain in a specific thermal environment without exceeding accepted strain limits. | Dry bulb temperature, wet bulb temperature, globe temperature, wind speed, atmospheric pressure, clothing and acclimatisation assumptions. | Planning sustainable work intensity and restriction zones for hot work environments. | Requires correct input measurements and interpretation; especially useful where workers can self-pace and are adequately informed. | TWL assessment paper |
Choose an index based on your assessment purpose.
This simple selector gives a practical starting point. It does not replace professional judgement, and more than one index may be appropriate for a complete heat stress assessment.
Assessment context
WBGT is usually the practical first-line screening index for workplace heat stress, especially where the objective is initial assessment or work-rest planning.
For detailed physiological modelling, consider PHS or Required Sweat Rate. For sustainable work-rate decisions, consider TWL.
Parameters typically required
- Natural wet bulb temperature
- Globe temperature
- Dry bulb temperature
- Indoor / outdoor condition
- Workload estimate
- Clothing / PPE context
Enter field parameters to calculate simple heat stress indices.
This calculator is intentionally limited to indices that can be calculated transparently from a small number of inputs. WBGT can be calculated directly from wet bulb, globe and dry bulb temperatures. HSI and Required Sweat Rate are calculated from heat-balance values entered by the user. P4SR, PHS and TWL require detailed models or standards-based calculations and are therefore handled as parameter readiness checks rather than simplified numeric outputs.
WBGT Calculator
Calculate WBGT using indoor/no-solar or outdoor/solar weighted temperature formulas.
Heat Stress Index Calculator
Calculate HSI from required evaporative heat loss and maximum evaporative capacity.
Required Sweat Rate Calculator
Estimate required sweat rate from required evaporative heat loss. Use consistent units and professional interpretation.
P4SR / PHS / TWL Readiness Check
These indices require more detailed modelling. Use this checklist to confirm whether you have the main parameters needed before conducting a proper calculation.
For indoor or outdoor conditions without solar load, WBGT = 0.7 × natural wet bulb temperature + 0.3 × globe temperature.
Interpretation should consider metabolic workload, clothing/PPE, acclimatisation and work-rest arrangements.
Screening result onlySimple indices are easier to use, but advanced indices can answer deeper questions.
The practical challenge is not only calculating a number. The value must be interpreted with workload, clothing, acclimatisation, hydration, worker health, controls and emergency response arrangements.
Screening indices
- Useful for quick workplace assessment and prioritisation.
- Can support work-rest and control discussions.
- Generally easier to communicate to managers and workers.
- May not fully account for clothing, PPE, individual susceptibility or complex work patterns.
Analytical and physiological indices
- Can model sweat demand, water loss, core temperature or sustainable metabolic rate.
- Useful for complex, high-risk or technically demanding heat stress assessments.
- Require more detailed input data and specialist interpretation.
- Should be used carefully when individual health status and acclimatisation vary.
Heat stress assessment should be more rigorous when risk is high or controls are uncertain.
Professional occupational hygiene assessment is recommended when workers perform heavy work in hot environments, wear heat-retaining PPE, work near radiant heat sources or have a history of heat-related symptoms or incidents.
Outdoor and hot weather work
Construction, logistics, landscaping, maintenance and other outdoor work may need heat stress planning during hot periods.
Hot process areas
Foundries, kitchens, laundries, boiler rooms and process plants may involve radiant heat, high humidity or limited ventilation.
PPE burden
Chemical protective clothing, respirators and impermeable PPE can reduce heat loss and increase physiological strain.
Vulnerable workers
New, unacclimatised, medically vulnerable or returning workers may require closer supervision and gradual acclimatisation.
Primary references used for this page.
This IEH page summarises common heat stress indices for practical occupational hygiene orientation. It should be read together with current standards, guidance and competent professional interpretation.
ISO 7243: Ergonomics of the thermal environment — Assessment of heat stress using the WBGT index. Open ISO 7243 overview.
ISO 7933: Ergonomics of the thermal environment — Analytical determination and interpretation of heat stress using calculation of the predicted heat strain. Open ISO 7933 overview.
P4SR and HSI: P4SR and the Belding-Hatch Heat Stress Index are historically important analytical heat stress indices that remain useful for understanding heat balance concepts. Open heat stress measurement review.
Thermal Work Limit: TWL is a heat-balance based method for assessing sustainable heat stress exposure and work capacity in hot environments. Open TWL assessment paper.
Professional note: This page does not replace field measurement, current standards, medical advice, emergency planning or competent occupational hygiene assessment.
Need help assessing heat stress at work?
IEH can support workplace heat stress assessment, WBGT measurement, thermal environment review, work-rest planning, heat stress control recommendations and worker protection programmes.