What is a dangerous heat index?
National weather service considers it a dangerous number when the index starts to reach 100° F. Heat index of 105° F or more, lasting for two or more days, is considered as dangerous and prompts an "excessive heat warning" from the weather service.
On July 8th 2003 a record heat index was recorded in Saudi Arabia, with a temperature of 178 degrees (81°C).
Note, it's important to remember that heat-index-temperature is not the same as an actual temperate. Refer to "What is heat index" for explanation of how heat index is measured.