Abstract: At 6:45 a.m., at the beginning of his Monday morning shift, Employee #1, who had worked the day shift on Sunday, had climbed 12 ft to the second level of a carbon black unit baghouse where bag filters were being removed, cleaned, and replaced. He then collapsed. Employee #5, who was working on the floor level, saw Employee #1 and ran for help. Employee #2 arrived at the scene and climbed the ladder. He reached the second level to check Employee #1's pulse but passed out and fell to the floor. Employee #3 also arrived and climbed the ladder, but passed out and fell off the upper part of the ladder; he was caught by Employee #4. Employee #1 died. Employees #2 and #3 were hospitalized. The baghouse compartment had been isolated by a double-block-and-bleed-type valve and gas tested. However, carbon monoxide gas had leaked through the valve into the top of the compartment, and testing had been done only inside the doorway at the floor level, not in the actual work area. An entry permit had been issued for 12 days, the projected duration of the job.