Abstract: Employee #1 was instructed to enter a 1.5 million gal water tank to take water samples to determine what was contaminating the tank water. The water tank measured 40 ft high and 80 ft in diameter. The smell of the tank prior to entry was similar to creosote or diesel. Employee #1 entered the tank, which was 1/2 to 2/3 full of water and had an inside temperature of 100 degrees F with high humidity, without appropriate pre-entry monitoring. After taking water samples, Employee #1 had difficulty ascending the interior fixed ladder. He experienced severe breathing difficulties and later that afternoon went to the emergency room of Hemet Valley Hospital. The attending physician diagnosed Employee #1 as suffering a myocardial infarction due to the breathing of petroleum vapors. Analysis of water samples taken from the tank revealed diesel contamination.