Abstract: ON DECEMBER 5, 1986, AT APPROXIMATELY 9:45 AM, EMPLOYEES BEGAN HAVING HEADACHES AND NAUSEA. A CHECK FOR NATURAL GAS LEAKS WAS MADE. NONE WAS FOUND. MORE EMPLOYEES DEVELOPED HEADACHES AND NAUSEA, SO ALL EMPLOYEES WERE RELEASED EARLY FOR LUNCH. AT ABOUT 12:30 PM ONE EMPLOYEE BECAME WEAK AND WAS SENT TO THE HOSPITAL. SEVERAL OTHER EMPLOYEES WENT ALONG WITH HER. OTHER EMPLOYEES BEGAN TO EXPERIENCE WEAKNEES AND NAUSEA. ALL EMPLOYEES EXPERIENCING SYMPTOMS WERE SENT HOME AT 1 PM. AFTER THE PLANT WAS CLOSED THE CARBON MONOXIDE LEVELS WERE MEASURED. LEVELS ABOVE 50 PPM BUT LESS THAN 150 PPM WERE FOUND IN SOME AREAS OF THE PLANT. AN AIR MAKE-UP SYSTEM WAS TURNED ON. WHEN CARBON MONOXIDE LEVELS WERE REDUCED TO BELOW 50 PPM, THE SECOND SHIFT WAS ALLOWED TO GO TO WORK. THEY WORKED FROM 4:30 PM TO MIDNIGHT. PLANT OPERATIONS RETURNED TO NORMAL THE NEXT DAY. CARBON MONOXIDE LEVELS REMAINED BELOW 50 PPM TIME WEIGHTED AVERAGE. EIGHT EMPLOYEES WERE HOSPITALIZED. 103 EMPLOYEES WERE TREATED WITH OXYGEN IN THE HOSPITAL FOR 2-3 HOURS AND THEN RELEASED.