OSHA National Emphasis Program on heat: Water, Rest, Shade. Heat Index action levels: Below 80°F — basic precautions. 80-90°F (Caution): water available, shade breaks, buddy system, watch for symptoms. 91-103°F (Warning): enforce water breaks every 15-20 min, mandatory rest in shade, acclimatize new/returning workers over 7-14 days, pre-shift briefing. 104-115°F (Danger): rest 15 min every hour, mandatory water (1 qt/hr), continuous monitoring, cancel non-essential work. >115°F (Extreme): stop outdoor work except emergency. Acclimatization: new workers 20% exposure day 1, increase 20%/day. Returning after >7 days away: 50% day 1, +10%/day. Heat-related illnesses: heat cramps → heat exhaustion (heavy sweating, weakness, nausea) → heat stroke (confusion, no sweating, T>104°F — CALL 911). Risk factors: obesity, medications, previous heat illness, impermeable clothing, dehydration. California, Washington, Oregon, Minnesota, Colorado have state-specific heat standards exceeding federal.