Guides
Read the instrument, not just the number
Quick answer: the heat index is a shade “feels like” number from temperature and humidity; WBGT adds sun and wind and needs a real meter to measure, not just estimate; OSHA’s 80°F/90°F triggers come from a rule that is still proposed, not final.
What is the heat index?
The heat index is the National Weather Service’s calculated “how hot it feels” temperature. It combines air temperature and relative humidity — because humidity slows how efficiently sweat evaporates — into a single number the NWS assigns to a risk band (Caution, Extreme Caution, Danger, Extreme Danger).
Read the guide →Heat index vs. WBGT
Heat index uses only air temperature and humidity, and assumes shade. WBGT (wet-bulb globe temperature) adds solar radiation and wind, so it reads higher in direct sun. A phone can only produce a WBGT estimate from available weather data — a governing body that requires WBGT compliance wants a physical black-globe meter reading, not an estimate.
Read the guide →OSHA’s proposed heat triggers: 80°F and 90°F
OSHA’s proposed federal heat rule (not yet final) sets two heat-index trigger points: 80°F, an “initial heat trigger” requiring water, shade or cooling, and rest breaks; and 90°F, a “high-heat trigger” adding more frequent breaks and closer monitoring. As of this writing OSHA has set no target date for a final rule.
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