Heat index is useful because crews can get it quickly from temperature and humidity. OSHA and NIOSH also warn that heat index is only a screening metric. It is based on shade and light-wind assumptions, while real worksite heat stress also depends on radiant heat, air movement, clothing/PPE, metabolic workload, rest-area temperature, worker acclimatization, medical factors, and emergency controls.
This guide explains how to use heat index without over-trusting it. Use the ToolGrit screen for an initial NOAA/NWS heat-index value, OSHA/NIOSH risk-tier prompt, full-sun warning, acclimatization reminder, and source checklist. Use WBGT, current OSHA/NIOSH/state-plan guidance, employer procedures, and qualified occupational-safety or industrial-hygiene review before setting work/rest cycles.
Measuring Heat Stress: Heat Index vs. WBGT
The heat index combines air temperature and relative humidity into an apparent temperature. NOAA/NWS publishes the equation path used by the ToolGrit app: a low-range simple branch, Rothfusz regression, and two humidity corrections. OSHA/NIOSH use heat index in their heat-safety tool, but they describe it as a screening tool rather than the most accurate worksite heat-stress measurement.
WBGT is the occupational heat-stress metric OSHA/NIOSH point users toward when available. WBGT accounts for radiant heat and air movement in a way heat index does not. That matters outdoors in full sun and indoors near furnaces, ovens, steam lines, hot equipment, process heat, or poor air movement.
Heat-index risk tiers can help decide when to ask more questions. They do not prove a task is safe, and they do not set a legal work/rest cycle by themselves.
Heat index is a shade/light-wind screen. OSHA/NIOSH warn that full sun can increase heat-index exposure by up to 15 degrees F. Use WBGT or qualified site measurements for radiant heat, hot equipment, heavy PPE, or final work/rest decisions.
Heat Index & Work/Rest Schedule Calculator
Calculate heat index and OSHA heat stress work/rest schedule with water intake recommendations.
Work/Rest Cycles Need More Than Heat Index
Work/rest cycles are not determined by heat index alone. Current NIOSH and ACGIH heat-stress methods use measured heat stress, workload, acclimatization, clothing adjustment, and recovery conditions. OSHA also emphasizes water, rest, shade, training, monitoring, acclimatization, and emergency response as part of a broader heat program.
The ToolGrit app shows local planning prompts such as more frequent cool-down periods or rescheduling non-essential heavy work. Those prompts are intentionally labeled as source gaps. They are not OSHA rules, ACGIH TLVs, NIOSH REL/RAL limits, or medical clearances.
When the work is heavy, workers wear impermeable or heat-retaining clothing, the rest area is hot, or radiant heat is present, get a WBGT measurement and have a qualified safety or industrial-hygiene professional review the controls.
Do not copy heat-index planning prompts into a work/rest policy as if they were ACGIH TLV tables. Use authorized current ACGIH/NIOSH guidance, measured WBGT, clothing/PPE adjustments, workload, rest-area conditions, and qualified review.
Acclimatization: New and Returning Workers
OSHA and NIOSH warn that new and returning workers are at increased heat-illness risk because heat tolerance is built gradually. Workers may also lose heat tolerance after time away, illness, cooler seasons, or a sudden jump in weather.
The OSHA/NIOSH Rule of 20 Percent is the key planning reminder: new workers should start with a reduced heat workload and increase duration gradually. Returning workers may also need temporary added protections. The exact plan depends on the worker, task, employer procedure, and qualified supervision.
The app does not increase the heat-index value to simulate acclimatization. That would hide the issue. Instead, it flags the condition and keeps the acclimatization source warning visible.
Use current OSHA/NIOSH acclimatization guidance, supervisor monitoring, buddy checks, rest opportunities, and worker reporting. Do not rely on a heat-index number alone for a new hire or a worker returning after time away.
Water, Rest, Shade, and First Aid
OSHA frames heat prevention around cool water, rest, shade or cooler recovery areas, training, monitoring, and emergency response. Workers should not rely on thirst alone. For longer jobs, electrolyte-containing fluids may be needed in addition to cool water.
First-aid recognition belongs in the same system. Heat exhaustion can include headache, nausea, dizziness, weakness, irritability, thirst, heavy sweating, and fast heartbeat. Heat stroke is a medical emergency; confusion, loss of consciousness, seizures, or inability to regulate body temperature require immediate emergency response and rapid cooling while help is on the way.
ToolGrit first-aid notes are reference prompts only. They do not replace trained responders, site emergency procedures, EMS, or medical care.
Call 911 immediately, move the worker to a cool area, begin rapid cooling if available, and do not give fluids to an unconscious worker. Follow site emergency procedures and medical direction.