The CDC's review was intended to test its own NIOSH recommended occupational exposure limits, which take factors like levels of heat acclimation, the type of work clothing and worker predisposition to heat illness into consideration in a real-world setting. Of the 25 cases being analyzed, six fatalities occurred when the heat index was less than 91 F, and four fatalities occurred when the heat index was between 85 F and 90 F. Heat index screening should be used when the wet bulb globe temperature method is unavailable. The wet bulb globe temperature method measures the heat stress in direct sunlight via temperature, wind speed, humidity, sun and cloud cover.