After an analysis of 25 OSHA heat-related illnesses — 14 fatal and 11 nonfatal — the Centers for Disease Control suggested that employers start screening their workers for heat stress when the heat index reaches 85 degrees Fahrenheit rather than the 91 F OSHA currently recommends. Heat stress covers a wide variety of potential illnesses, including life-threatening heat stroke.

Full article: CDC, OSHA: Heat stress risk starts at 85 degrees Fahrenheit
Source: Safety Best Practices