WBGT Heat Stress Calculator

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.