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WBGT Heat Stress Calculator — NIOSH Limits & Work/Rest

Calculate Wet Bulb Globe Temperature (WBGT) and heat stress risk. Instrument and simplified (temp + RH) modes, NIOSH REL/RAL limits, work/rest schedule and clothing adjustments.

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Disclaimer: This is a planning aid. Heat risk must be managed by a competent person. WBGT should be measured with calibrated instruments on site. Individual factors (fitness, medication, illness) affect heat tolerance significantly.

Simplified mode uses the Bureau of Meteorology approximation (WBGT = 0.567·Ta + 0.393·e + 3.94), which assumes moderately sunny conditions and light wind. It is an estimate and can over- or under-state the real value, so use instrument measurement where you can.

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Heat Stress: A Serious Occupational Hazard

Heat stress is a significant occupational health hazard in Australia, particularly in outdoor industries such as construction, agriculture, mining, landscaping, and emergency services. Heat-related illness ranges from heat cramps and heat exhaustion (treatable with rest and fluids) to heat stroke — a life-threatening medical emergency. Even sub-clinical heat strain impairs cognitive function, decision-making, and reaction time, increasing the risk of other workplace accidents.

The Wet Bulb Globe Temperature (WBGT) index is the most widely accepted measure of environmental heat stress for occupational settings, as set out in NIOSH's Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments. Unlike simple air temperature or the "feels like" heat index, WBGT incorporates humidity, radiant heat, and air movement — all of which affect the body's ability to lose heat through sweating and convection.

WBGT Formula

Outdoor (solar load):  WBGT = 0.7·Tnwb + 0.2·Tg + 0.1·Tdb
Indoor (no solar):     WBGT = 0.7·Tnwb + 0.3·Tg

Where:
  Tnwb = natural wet-bulb temperature (°C) — measured with aspirated thermometer
  Tg   = black globe temperature (°C) — 150 mm black globe thermometer
  Tdb  = dry-bulb (air) temperature (°C)

Note: The 70% weighting on Tnwb reflects the dominant role of
humidity in heat stress, since evaporative sweating is the primary
cooling mechanism at high ambient temperatures.

NIOSH Heat Exposure Limits

NIOSH sets two WBGT limits as functions of the metabolic rate M (in watts), as a 1-hour time-weighted average: the Recommended Exposure Limit (REL) for acclimatised workers and the lower Recommended Alert Limit (RAL) for unacclimatised workers. The acclimatised formula is the same as the ACGIH TLV, which ACGIH publishes under licence.

REL (acclimatised)    = 56.7 − 11.5 · log₁₀(M)   °C WBGT
RAL (unacclimatised)  = 59.9 − 14.1 · log₁₀(M)   °C WBGT
  • Rest (115 W): REL 33.0°C / RAL 30.8°C
  • Light work (180 W): REL 30.8°C / RAL 28.1°C
  • Moderate work (300 W): REL 28.2°C / RAL 25.0°C
  • Heavy work (415 W): REL 26.6°C / RAL 23.0°C
  • Very heavy work (520 W): REL 25.5°C / RAL 21.6°C

Work:Rest Schedules

When the WBGT is above the limit for the work being done, the calculator uses the NIOSH time-weighted approach. Rest is assumed to be taken in the same conditions at 115 W, and the highest time-weighted metabolic rate allowed at that WBGT decides how much of each hour can be worked. For example, moderate work by an acclimatised worker at a WBGT of 31°C allows a time-weighted rate of about 172 W, which is roughly 15 minutes of work and 45 minutes of rest per hour. If the WBGT is above the limit even at rest, work should not continue without controls or cooling.

Clothing Adjustment Factors

Clothing that limits sweat evaporation raises the effective WBGT. NIOSH (2016) gives these adjustments, added to the measured WBGT: work clothes 0°C; cloth (woven) coveralls 0°C; SMS polypropylene coveralls +0.5°C; polyolefin coveralls +1°C; double-layer woven clothing +3°C; limited-use vapour-barrier coveralls +11°C. For an acclimatised worker doing moderate work, vapour-barrier coveralls reduce the allowable measured WBGT from 28.2°C to 17.2°C.

Simplified Mode and the Bureau of Meteorology Estimate

When you only know air temperature and humidity, the calculator uses the Bureau of Meteorology approximation WBGT = 0.567 × Ta + 0.393 × e + 3.94, where e is the water vapour pressure in hPa. It assumes moderately sunny conditions and light wind, so it is an estimate only: at 35°C and 60% humidity it gives 37.0°C. A calibrated WBGT meter should be used for compliance decisions.

Hydration and Heat Illness Prevention

Dehydration significantly worsens heat tolerance. For moderate work in heat lasting under two hours, NIOSH advises about 1 cup (237 mL) of cool water every 15–20 minutes, roughly 700–950 mL an hour, without waiting for thirst, and no more than 6 cups (about 1.4 L) an hour. For longer periods of heavy sweating, a drink with electrolytes is advised. High-sugar or high-caffeine drinks are best avoided. Acclimatisation, buddy systems, scheduled rest breaks in cool/shaded areas, and providing cool drinking water are the primary administrative controls for heat stress management.

Frequently asked questions

What is WBGT?
WBGT (Wet Bulb Globe Temperature) is the heat stress index used in occupational health. It combines air temperature, humidity (through the natural wet-bulb temperature), radiant heat (through a black globe thermometer) and, indirectly, air movement. It is the basis of ISO 7243, the NIOSH 2016 heat criteria and the ACGIH heat stress limits, and the Bureau of Meteorology publishes an approximation of it.
How is WBGT different from the heat index?
The heat index (or 'feels like' temperature) accounts only for temperature and humidity. It ignores solar radiation and air movement. WBGT also includes radiant heat from the black globe, so it predicts heat strain far better for outdoor work, which is why occupational standards use it.
What are the NIOSH heat exposure limits?
NIOSH (2016) sets a Recommended Exposure Limit (REL) for acclimatised workers, REL = 56.7 − 11.5 × log10(M), and a Recommended Alert Limit (RAL) for unacclimatised workers, RAL = 59.9 − 14.1 × log10(M), where M is the metabolic rate in watts and the result is WBGT in °C as a 1-hour average. For moderate work (300 W) that is 28.2 °C for acclimatised workers and 25.0 °C for unacclimatised workers. The acclimatised formula is the same as the ACGIH TLV, which is published under licence by ACGIH.
What does 'acclimatised' mean?
Acclimatisation is the physiological adaptation that develops when workers are gradually exposed to heat over 1 to 2 weeks: more plasma volume, earlier sweating and a lower core temperature response. New workers, workers returning after an absence of several days, and workers moved from a cool to a hot environment should be treated as unacclimatised.
How does clothing affect heat stress?
Clothing that limits sweat evaporation raises the effective WBGT. NIOSH (2016) gives these clothing adjustment factors, added to the measured WBGT: work clothes 0 °C, cloth (woven) coveralls 0 °C, SMS polypropylene coveralls +0.5 °C, polyolefin coveralls +1 °C, double-layer woven clothing +3 °C and limited-use vapour-barrier coveralls +11 °C. For example, an acclimatised worker doing moderate work in vapour-barrier coveralls has an effective limit of 28.2 − 11 = 17.2 °C measured WBGT.
How is the simplified (temperature and humidity) WBGT calculated?
Simplified mode uses the Bureau of Meteorology approximation WBGT = 0.567 × Ta + 0.393 × e + 3.94, where Ta is air temperature in °C and e is the water vapour pressure in hPa calculated from relative humidity. It assumes moderately sunny conditions and light wind, so it is an estimate: at 35 °C and 60% humidity it gives 37.0 °C. Measure WBGT with a calibrated instrument for any compliance decision.
How is the work/rest schedule calculated?
The calculator uses the NIOSH time-weighted approach. Rest is taken in the same conditions at 115 W, and the highest time-weighted metabolic rate allowed at the measured WBGT sets how much of each hour can be worked. For example, moderate work (300 W) by an acclimatised worker at a WBGT of 31 °C allows a time-weighted rate of about 172 W, which is roughly 15 minutes of work and 45 minutes of rest per hour. Resting in a cool area allows more work.
When should work stop due to heat stress?
Work should stop or be redesigned when the WBGT is above the limit even at rest, or when workers show signs of heat illness. Heat stroke (confusion, loss of consciousness, hot dry skin, core temperature above 40 °C) is a medical emergency that needs immediate emergency care. Limits are a prevention threshold, and fitness, medication, illness and age all change an individual's heat tolerance.

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