Abstract: At about 1:50 p.m. on October 15, 1994, employees of Fluor Daniels (F/D), the maintenance contractor for Rohm and Haas, were removing an I-beam used as a monorail for a hoist in preparation for replacing a 100% hydrogen cyanide (HCN) tank. It had been reported that the N-5 unit was shut down, but it later was found that the 2nd stage was recirculating while waiting for crude ACH from the N-3 unit. The 100% HCN tank, next to which the F/D employees had been working, had been washed 11 times since the Thursday before the incident. At about 1:50 p.m., a F/D pipefitter read 13 ppm on his personal monitox (cyanide monitor), so he went to the control room, where an operator checked the area with a Drager HCN detector tube. Hydrogen cyanide was not detected and another monitox was given to the pipefitter. Employees were told that they could continue working. At about this time a pipefitter helper began feeling dizzy. About 20 minutes later, the 2nd monitox indicated 161 ppm cyanide, and the pipefitter took it back to the control room while the other F/D employees left the area. At this point, the pipefitter began feeling very ill, coughing up blood and experiencing difficulty breathing. Rohm and Haas used detector tubes to check the area again for HCN, but it was not detected. The alarm was never sounded because detector tubes and the perimeter monitoring system indicated non-detection during the incident. The pipefitter was showered in the emergency shower and taken by ambulance to Southmore Hospital. His coworkers then went to the change house for showers. There, two of the coworkers became ill. Later, other F/D employees became ill. Belmont Construction Services employees had been working primarily in the KP area, which was shut down so there was no methyl methacrylate present. By 6:00 p.m. some of their employees had become ill and passed out; some of them became ill on the bus that took them from the Belmont area to the contractors' gate. Fifteen Belmont employees were taken to the hospital and 13 stayed overnight for observation. In addition, one Rohm and Haas employee and one Laidlaw employee, who had been working in the West Waste Treatment Plant, were hospitalized. Overall, 34 employees from the four different companies were affected. Medical tests for Belmont, Laidlaw, F/D, and Rohm and Haas employees all returned normal. The employees who left the plant site and were taken directly to the hospital were reportedly tested for cyanide in the blood within 20 minutes of the onset of known symptoms. Later tests could not substantiate a cyanide exposure, but some employees' doctors stated that the employees had been exposed. The weather that day consisted of heavy rain and 100% humidity. The monitoxes were sent to the manufacturer, who said that water was inside them and that they could not find traces of HCN on the monitoxes. An outside operator stated that she had had her monitox fail high after getting it wet or dropping it. The manufacturer's operating manual specifies operating limits of 20 to 95% relative humidity. The Drager detector tubes also have a relative humidity range of less than 80%. Employee interviews indicate that no one smelled or tasted cyanide or anything else, except that the union representative stated that some of the Rohm and Haas employees could smell 81R from the semi-works next to N-5.