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WBGT Heat Stress Calculator — ACGIH TLV Work/Rest Ratios

Calculate Wet Bulb Globe Temperature (WBGT) and heat stress risk. ACGIH TLV action limits, work/rest schedule. Acclimatized and clothing adjustments included.

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 Stull (2011) wet-bulb approximation. Result is labelled as an estimate — instrument measurement is more accurate.

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 medical emergency with a fatality rate of up to 80% if untreated. 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. 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.

ACGIH TLV Action Limits

The ACGIH Threshold Limit Values provide WBGT limits by work intensity category. Two sets of limits apply — one for acclimatised workers and one for unacclimatised workers. The difference (typically 2–3°C) reflects the physiological protection that acclimatisation confers.

  • Rest: 32.5°C (both acclimatised and unacclimatised)
  • Light work: Acclimatised 30.0°C / Unacclimatised 28.0°C
  • Moderate work: Acclimatised 27.5°C / Unacclimatised 25.0°C
  • Heavy work: Acclimatised 25.0°C / Unacclimatised 22.5°C
  • Very heavy work: Acclimatised 23.0°C / Unacclimatised 20.0°C

Work:Rest Schedules

When WBGT exceeds the TLV, work:rest regimens reduce heat strain by allowing the body to dissipate accumulated heat. The standard 60-minute cycle approach:

  • WBGT within TLV: continuous work or 45 min work / 15 min rest
  • 0–2°C above TLV: 30 min work / 30 min rest
  • 2–4°C above TLV: 15 min work / 45 min rest
  • More than 4°C above TLV: work must stop until temperature reduces or controls are implemented

Clothing Adjustment Factors

Standard work clothing has no WBGT adjustment. However, protective clothing that impairs evaporative cooling requires an upward adjustment to the effective WBGT before comparison with TLVs: vapour-barrier or impermeable clothing +10°C; fully encapsulated suit +12°C. This means the allowable working WBGT for someone in a vapour-barrier suit at moderate work (acclimatised) is effectively 27.5 − 10 = 17.5°C — significantly cooler conditions than would be required for standard clothing.

Hydration and Heat Illness Prevention

Dehydration significantly worsens heat tolerance. Workers should drink approximately 500 mL of cool water per hour during moderate work in hot conditions, regardless of thirst. High-sugar or high-caffeine drinks impair thermoregulation. 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 internationally recognised heat stress index that accounts for all four environmental heat load factors: air temperature, humidity (via natural wet-bulb temperature), radiant heat (via black globe temperature), and air movement. A WBGT of 28°C means conditions are significantly more stressful than 28°C dry-bulb air temperature alone. It is used in ISO 7243, ACGIH TLV standards, and Australian heat stress guidelines.
How is WBGT different from the heat index?
The heat index (HI or 'feels like' temperature) accounts only for temperature and humidity — it ignores solar radiation and air movement. WBGT additionally incorporates radiant heat load (black globe) and is therefore a much more accurate predictor of physiological heat strain in outdoor work environments. WBGT is the standard used for occupational health and safety assessment; the heat index is primarily a public weather communication tool.
What are ACGIH TLVs for heat stress?
The ACGIH (American Conference of Governmental Industrial Hygienists) Threshold Limit Values (TLVs) for heat stress are WBGT action limits by work intensity. For moderate work: acclimatised workers can safely work continuously at WBGT up to 27.5°C, but unacclimatised workers require additional controls above 25.0°C. These limits apply to standard work clothing. Vapour-barrier clothing requires adding 10°C to the measured WBGT before comparing to TLVs.
What does 'acclimatised' mean?
Acclimatisation is the physiological adaptation that occurs when workers are gradually exposed to hot conditions over 1–2 weeks. Acclimatised workers develop increased plasma volume, earlier onset of sweating, increased sweat rate, and lower core body temperature responses to the same heat load. Acclimatisation allows about 2–5°C higher WBGT tolerance. Workers returning from absence of 3+ days, new workers, and workers moved from cool to hot environments are considered unacclimatised.
How does clothing affect heat stress?
Standard clothing (single-layer work clothes) has no WBGT adjustment. However, vapour-barrier clothing (impermeable suits, PVC protective gear) significantly impairs the body's ability to evaporate sweat, dramatically increasing physiological heat load. ACGIH recommends adding 10°C to measured WBGT when assessing workers in vapour-barrier clothing, and 12°C for fully encapsulated suits. This means the effective working WBGT limit is reduced by that amount.
When should work stop due to heat stress?
Work should be suspended (or stopped and rescheduled) when WBGT exceeds the TLV by more than 4°C above the action limit for the given work intensity after applying all available controls (rest, shade, hydration, cooling). Signs of heat stroke — confusion, loss of consciousness, hot/dry skin, core temperature >40°C — require immediate medical emergency response. The WBGT limit is a prevention threshold, not a 'safe until exceeded' boundary; individual factors (fitness, medication, age) significantly affect heat tolerance.

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