Wet-bulb globe temperature from your three thermometer readings, the ACGIH screening TLV and action limit for your work/rest cycle, and why the tables stop short of turnout gear.
The formula. Outdoors in sunlight: WBGT = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Tdb. Indoors or in full shade: WBGT = 0.7 × Tnwb + 0.3 × Tg, because there is no direct solar load to weight. The wet bulb dominates for a reason — it carries the humidity, which is what actually determines whether sweat can evaporate. Worked example: outdoor readings of 24 °C wet bulb, 32 °C globe, 28 °C dry bulb give 0.7 × 24 + 0.2 × 32 + 0.1 × 28 = 16.8 + 6.4 + 2.8 = 26.0 °C.
The ACGIH thresholds. The Screening TLV applies to acclimatised, healthy workers; the Action Limit applies to unacclimatised workers, who are markedly more susceptible. Both are indexed by work demand across an hourly cycle and by the proportion of the hour spent working. For moderate work at 25–50 % duty, the TLV is 30.0 °C and the Action Limit 27.0 °C. For heavy work at 25–50 %, 29.0 °C and 25.5 °C. The very heavy column disappears entirely at high duty cycles, because the work itself is unsustainable regardless of the temperature.
Clothing adjustment factors. ACGIH adds a fixed number of degrees to the measured WBGT to account for what the worker is wearing: short sleeves −1, ordinary work clothes or cloth coveralls 0, SMS polypropylene coveralls +0.5, polyolefin coveralls +1, double-layer woven coveralls +3, and a hood on top of anything +1. These are additive to the WBGT and then compared against the same thresholds.
The honest problem. ACGIH states plainly that these factors must not be used for completely encapsulating chemical protective suits, and that factors for multiple layers must not simply be added together. NIOSH goes further: when an encapsulating or impermeable ensemble is worn, WBGT is not the appropriate heat stress index at all, and adjusted dry-bulb temperature with physiologic monitoring should be used instead above roughly 20 °C. Structural turnout gear plus SCBA sits in exactly that category — a vapour-barrier, moisture-impermeable ensemble with a facepiece. So this tool will show you a WBGT and the thresholds, but it cannot give you a defensible adjusted figure for a firefighter in full PPE, and any number it produces for that case should be treated as an indication that conditions are unsafe, not as a measurement of how much margin there is.
What actually governs rehab. Physiologic monitoring beats every index: heart rate above roughly 110–180 beats per minute depending on the protocol, core or oral temperature, blood pressure, dizziness, cramps, nausea, confusion, skin that has stopped sweating. Mandatory rehab after two cylinders of air or about twenty minutes of work in hot conditions is a widely used baseline, but your department's rehab SOP, NFPA 1584 and the rehab sector officer's judgement override anything on this page. Shade, air movement, active cooling, electrolyte replacement and re-entry accountability do the actual work — the index only tells you when to stop waiting for a number.
Disclaimer. An educational aid for rehab sector planning — not a medical device and not a substitute for NFPA 1584, your department's rehab SOPs, or on-scene EMS assessment. Sources: ACGIH Threshold Limit Values and Biological Exposure Indices, heat stress section · ISO 7243 · NIOSH Criteria for a Recommended Standard: Occupational Exposure to Hot Environments · NFPA 1584. Last reviewed: 2026-09-21.