How it's worked out
This uses the National Weather Service's method. It starts with Steadman's simple formula. When that comes out at 80°F or more, it switches to the Rothfusz regression, with the NWS adjustments for very dry and very humid air. The equation is published by the NWS Weather Prediction Center.
The categories
- Caution, 80–89°F: fatigue is possible with long exposure and activity.
- Extreme caution, 90–102°F: heat cramps, heat exhaustion and heat stroke are possible with long exposure and activity.
- Danger, 103–124°F: heat cramps and heat exhaustion are likely, and heat stroke is possible.
- Extreme danger, 125°F and up: heat stroke is highly likely.
Those are the National Weather Service's general public categories. What each level means on your site, like how much water, rest and shade, and how new workers are brought up to speed, belongs in your heat illness prevention plan.
What it doesn't account for
The heat index is a shade number for a person at rest. It doesn't include full sun, heavy work, radiant heat off steel or concrete, or the clothing and PPE your crew is wearing, and all of them push the real strain higher. New and returning workers are at the most risk in their first week back in the heat.