WBGT Heat Stress Calculator: Wet-Bulb Globe Temperature and Work/Rest Limits

How it works

Outdoor WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Ta · Indoor WBGT = 0.7 Tnwb + 0.3 Tg · Effective WBGT = WBGT + clothing adjustment · NIOSH REL = 56.7 − 11.5 log₁₀ M, RAL = 59.9 − 14.1 log₁₀ M (M in W) · work share = (M_allowed − 117) ÷ (M_work − 117), where M_allowed is the 1-hour average metabolic rate the limit allows at your WBGT

Wet-bulb globe temperature (WBGT) is the heat-stress index used by OSHA, NIOSH, ACGIH, the military and most sports bodies. It combines air temperature, humidity (through the natural wet-bulb Tnwb, a wet wick exposed to the air), radiant heat from the sun or hot surfaces (the 15 cm black globe Tg) and wind, which affects both. If you have a WBGT meter, choose “Measured” and enter its three readings. Otherwise the calculator estimates Tg and Tnwb from weather data with the Liljegren et al. (2008) heat- and mass-balance model, the method behind the OSHA outdoor WBGT calculator and the US National Weather Service WBGT forecasts: clear-sky solar radiation comes from the sun height (Haurwitz model), overcast and partly cloudy skies use about 25% and 60% of it, and you can type a measured value instead. The result is then compared with three sets of limits. The ACGIH screening table pairs WBGT with workload (light about 180 W, moderate 300 W, heavy 415 W, very heavy 520 W) and gives the share of each hour that can be spent working: the TLV applies to acclimatised workers and the lower Action Limit to unacclimatised ones. NIOSH’s REL and RAL curves give the same kind of limit as a formula of metabolic rate, which the calculator uses to estimate minutes of work per hour. Clothing that blocks sweat evaporation adds to the WBGT before comparison (ACGIH clothing adjustment factors). The US Army TB MED 507 categories (white to black flag) are shown for reference. Limits assume healthy, hydrated workers; illness, medication, obesity, age and prior heat illness lower tolerance.

Worked example

Default inputs: air 31 °C (87.8 °F), 55% humidity, 3.5 mph (1.56 m/s) wind, full sun with the sun 60° high, sea-level pressure. Clear-sky radiation is about 888 W/m², 82% of it direct beam. The Liljegren model then gives a globe temperature of 47.8 °C and a natural wet bulb of 26.1 °C, so WBGT = 0.7 × 26.06 + 0.2 × 47.76 + 0.1 × 31 = 30.9 °C (87.6 °F). In the shade the same weather gives 25.7 °C. For moderate work by an acclimatised worker in ordinary clothes, 30.9 °C is above the 30.0 °C TLV for 25–50% work but under the 31.5 °C limit for 0–25% work, so work should be limited to about 15 minutes per hour with rest in the shade. The NIOSH REL allows continuous moderate work only up to 28.2 °C; at 30.9 °C it allows a 1-hour average of 175 W, which is (175 − 117) ÷ (300 − 117) = 32% work, about 19 minutes per hour. On the US Army scale it is Category 3, a yellow flag.

Sources: OSHA: Outdoor WBGT calculator (Liljegren method) · OSHA Technical Manual, Section III Chapter 4: Heat Stress · NIOSH Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (2016-106) · pythermalcomfort: NIOSH work-capacity model (REL equation) · Emory University EHS heat stress program (ACGIH screening tables)

Frequently asked questions