EMPLOYEE HAD RETURNED TO WORK AREA ON TOP OF WASHER TANK AFTER LUNCH BREAK WHERE HE HAD BEEN WORKING FOR SEVERAL DAYS. HE WENT TO GET A DRINK OF WATER AS HE FELT WEAK & NAUSEATED. A CO-WORKER NOTICED HIM SITTING DOWN & CHECKED ON HIM. HE STATED HE DID NOT FEEL GOOD. HELP WAS CALLED FOR. THE RADIATED HEAT FROMT HE TANK SURFACE WAS 200 DEGREES. THE HEAT INDEX WAS 100 DEGREES.