Heat Stress Calculator | Exactus Labs

How to use — WBGT

Enter WBGT directly if you have a WBGT meter reading. Measured mode computes WBGT from natural wet bulb (Tnwb), globe (Tg), and dry bulb (Tdb) thermometer readings. Estimate from met data derives WBGT from air temperature and relative humidity — shade uses psychrometric wet bulb; sun uses the Liljegren solar geometry model with date, time, and GPS coordinates.

Select the clothing type to apply the ACGIH Clothing Adjustment Value (CAV). Set your task’s metabolic rate, work allocation, and acclimatisation status. The tool compares WBGTeff (WBGT + CAV) against the ACGIH TLV-H and Action Limit from Table 3, and shows the full Table 3 screening matrix.

Input method

Measured WBGT

WBGT (°C)

Enter the WBGT as read from your instrument. The instrument measurement already incorporates any solar radiation load — no indoor/outdoor correction is needed. Clothing adjustment (CAV) is applied below.

Clothing (ACGIH CAV — Table 2)

Add hood (+1.0°C)

CAVs are not additive across layers. Hood is additive for all clothing except vapor barrier coveralls (already includes hood). Encapsulating suits (OSHA Level A) are outside TLV scope.

Work parameters

Metabolic rate

Work in cycle

Acclimatisation

Results

WBGTeff

—

°C (WBGT + CAV)

TLV

—

°C (Table 3)

Action Limit

—

°C (AL formula)

Thermal stress gauge

Table 3 screening — WBGTeff thresholds (°C) · highlighted = current selection

TLV & AL (Table 3 / ACGIH): TLV thresholds from ACGIH Table 3 by work allocation, metabolic category, and acclimatisation status. Action Limit = 60.0 − 14.1 × log₁₀M (formula), capped at TLV. WBGTeff = WBGT + CAV. Table 3 thresholds use representative metabolic rates at the upper bound of each work-allocation band; red = current WBGTeff exceeds that threshold; dashes = combination not applicable under Table 3. ACGIH TLV assumes healthy, hydrated workers with complete recovery between exposures.

Understanding your results

WBGTeff (Effective WBGT) is WBGT plus the ACGIH Clothing Adjustment Value (CAV). CAVs account for the impaired evaporative cooling caused by clothing covering the skin surface. WBGTeff — not raw WBGT — is compared against the TLV and Action Limit.

TLV (Threshold Limit Value) is the ACGIH Table 3 ceiling for the selected work allocation, metabolic rate, and acclimatisation status. At or above TLV, the combined metabolic and environmental heat burden exceeds safe physiological limits. An HSMP with General and Job-Specific Controls is required; work should stop until WBGTeff is reduced.

Action Limit (AL) is the lower warning threshold. Between AL and TLV, heat stress is significant but manageable with a documented HSMP, General Controls (hydration, monitoring, rest areas, work-rest scheduling), and a heat acclimatisation program. Operating in this band without a functioning HSMP is not acceptable.

Table 3 Screening shows WBGTeff thresholds across all work allocations and metabolic categories for the selected acclimatisation status. Red cells mean WBGTeff exceeds that cell’s limit. Your current selection is highlighted. Dashes indicate the combination is not defined under Table 3 for the selected acclimatisation status. Rest (0% work) does not appear in Table 3 because the table is designed around active work scenarios where metabolic heat generation drives strain; at 100% rest, metabolic contribution is minimal and no formal WBGT ceiling applies under this method. For rest and recovery periods, ensure access to cool, shaded rest areas and adequate hydration rather than applying a WBGT work-limit.

Enter WBGT data above to see results.

How to use — Heat Index

Enter air temperature (select °C or °F) and relative humidity. The NOAA/NWS Rothfusz (1990) regression is applied automatically. For apparent temperatures below 80°F (27°C), Steadman’s simpler formula is used. Low-RH and high-RH adjustment factors are applied where applicable.

The Heat Index is a public heat-safety index calibrated for shaded, light-wind outdoor conditions. It is not an occupational exposure standard and does not account for work rate, clothing, or acclimatisation. Use the WBGT tab for workplace regulatory compliance and the TWL tab for physiological work capacity.

Inputs

Air temperature

Relative humidity (%)

Enter temperature and humidity above to see results.

How to use — Thermal Work Limit

Enter dry bulb temperature (Ta), psychrometric wet bulb temperature (Twb), globe temperature (Tg), wind speed, and barometric pressure. Select the clothing ensemble. The Brake & Bates (2002) iterative model finds the equilibrium skin temperature and computes the maximum sustainable metabolic rate (TWL, in W/m²).

Compare the computed TWL against the task’s actual metabolic demand. If the task exceeds the TWL, the thermal load is unsustainable without engineering controls or work-rest scheduling. Zone boundaries — Unrestricted, Acclimatisation, Buffer, and Withdrawal — directly prescribe the level of heat hygiene controls required.

Environmental inputs

Dry bulb Ta (°C)

Wet bulb Twb (°C)

Globe Tg (°C)

Wind speed (m/s)

Pressure (kPa)

Clothing

I_cl = thermal insulation (clo). i_cl = clothing vapour permeation efficiency. Values from Brake & Bates (2002) Table I. Not valid for encapsulating suits.

Results

Thermal Work Limit

—

W/m²

Zone

—

Skin temp. (est.)

—

°C at equilibrium

TWL zone gauge

Thermal Work Limit from the iterative physiological heat balance model of Brake & Bates (2002), Applied Occupational and Environmental Hygiene 17(3):176–186. Limiting core temperature = 38.2°C. Wind clamped 0.2–4.0 m/s. Globe temperature used as proxy for mean radiant temperature. Ambient vapour pressure derived from wet bulb via psychrometric equation. Respiration correction applied analytically. Zones: >220 W/m² unrestricted; 140–220 acclimatisation; 115–140 buffer; <115 withdrawal. Additional withdrawal triggers: Ta > 44°C or Twb > 32°C.

Understanding your results

Thermal Work Limit (TWL) is the maximum sustained metabolic rate (W/m²) an acclimatised worker in the specified clothing can maintain without core temperature exceeding 38.2°C. Compare it against the task’s metabolic demand: light manual work ≈ 130–180 W/m², moderate ≈ 200–300 W/m², heavy ≈ 300–415 W/m². If task demand exceeds TWL, the work is thermally unsustainable without intervention.

Zone classification (Brake & Bates, Table II): Unrestricted (>220 W/m²) — normal scheduling; standard heat hygiene practices. Acclimatisation (140–220 W/m²) — acclimatised workers only; buddy system; heat hygiene program active. Buffer (115–140 W/m²) — acclimatised workers only; rectify ventilation; work-rest cycling required; minimum 1 L/hr fluid intake; no solo work. Withdrawal (<115 W/m²) — no ordinary work; essential or rescue tasks only under permit; 20 min work / 40 min rest minimum; no unacclimatised workers; no solo work.

Equilibrium skin temperature is where the heat balance equation is satisfied. Values approaching 37.5°C indicate very limited thermal tolerance margin — the body is near its physiological ceiling. It is a model output, not a directly measurable temperature.

Input quality notes: Wet bulb should be measured with an aspirated psychrometer for best accuracy. Globe temperature requires a 150 mm black globe thermometer. Wind speed should be measured at body height in the work zone. The model is not validated for Ta > 44°C or Twb > 32°C — withdrawal is unconditional at these values regardless of computed TWL.

Enter environmental data above to see results.