I just checked in the book "OverExposure: Health Hazards in Photography" by Susan Shaw, 1983, and here is what is listed for potassium ferricyanide (Farmer's reducer):
Potassium ferrocyanide (carbro printing process, reducer) also has the same description.Potassium ferricyanide is only slightly toxic by skin contact, inhalation or ingestion. Exposing potassium ferricyanide to intense heat, hot acid or strong ultraviolet light can cause decomposition and release of highly toxic hydrogen cyanide gas.
The book also has sections on various processes including cyanotypes. It says "Potassium ferricyanide is only slightly hazardous by itself." Among the precautions is to "avoid using carbon arcs for printing" and saying that using "sunlight, a sunlamp or another light source" is preferable.
-Darren
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