Abstract: Between 5:25 a.m. and 8:45 a.m., Employee #1, a grounds maintenance worker at a golf course, was using a mobile applicator with booms to spray growth regulator on the course's greens. The pesticide being used was considered "slightly toxic" if absorbed through the skin, and had a dermal LD50 of 2020 mg/kg. Although the pesticide's label said it required no protective clothing beyond a long-sleeved shirt and pants, Employee #1 wore a respirator and a coverall suit made of Tyvek 1422 material, which inhibits evaporative cooling. Employee #1 began to feel ill at 8:45 a.m., left work, and reported to an emergency room later that evening. It was eventually determined that a heat-related condition had triggered a heart attack, and Employee #1 underwent coronary bypass surgery. The cause of the heat stress was determined to be the Tyvek clothing. ACGIH and WBGT heat stress calculations for the weather conditions, when corrected for Employee #1's clothing, indicate that the TLV was exceeded by his work activities. Employee #1's underlying condition of partial coronary artery blockage was unknown to him, and therefore not reported on the employer's medical screening questionnaire.