MMWR (Morbidity and Mortality Weekly Report) of Centers for Disease Control and Prevention (CDC) have posted "Evaluation of Occupational Exposure Limits for Heat Stress in Outdoor Workers — United States, 2011–2016."
Even though National Institute for Occupational Safety and Health (NIOSH) and American Conference of Governmental Industrial Hygienists (ACGIH) published occupational exposure limit for heat stress, these limits were derived from experimental condition, not real life outdoor environment. So, To evaluate outdoor heat exposure threshold, CDC reviewed 25 outdoor occupational heal-related illness, and concluded: "A Heat Index of 85°F (29.4°C) could be used as a screening threshold to prevent heat-related illness."
The link of the original article is below
Tustin AW, Lamson GE, Jacklitsch BL, et al. Evaluation of Occupational Exposure Limits for Heat Stress in Outdoor Workers — United States, 2011–2016. MMWR Morb Mortal Wkly Rep 2018;67:733–737.