What's the most reliable indicator you should use when determining if you should modify activity for heat?

Heath index chart is the best indicator you should use when determining if you should modify activity for heat. The "heat index" is a single value that looks for both temperature and humidity. The higher the heat index, the weather will feel the hotter since sweat does not immediately evaporate and cool the skin. The heat index used is much better than air temperature alone for estimating the risk to workers from environmental heat sources.