Acute Effect

Acute Effects:

* Cyanide is extremely toxic to humans. Acute (short-term) inhalation exposure to 100 milligrams per cubic meter (mg/m3) or more of hydrogen cyanide will cause death in humans. (1,2)

* Acute exposure to lower concentrations (6 to 49 mg/m3) of hydrogen cyanide will cause a variety of effects in humans, such as weakness, headache, nausea, increased rate of respiration, and eye and skin irritation. (1,2)

* Tests involving acute exposure of rats and mice have shown hydrogen cyanide to have extreme acute toxicity from inhalation exposure. (3)

Chronic Effects (Noncancer):

* Chronic exposure to cyanide in humans via inhalation results in effects on the CNS, such as headaches, dizziness, numbness, tremor, and loss of visual acuity. (1,2)

* Other effects in humans include cardiovascular and respiratory effects, an enlarged thyroid gland, and irritation to the eyes and skin. (1,5)

* Animal studies have reported effects on the nervous, cardiovascular, and respiratory systems. (1,4)

* The Reference Concentration (RfC) for hydrogen cyanide is 0.003 mg/m3 based on CNS symptoms and thyroid effects in humans. The RfC is an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups), that is likely to be without appreciable risk of deleterious noncancer effects during a lifetime. It is not a direct estimator of risk but rather a reference point to gauge the potential effects. At exposures increasingly greater than the RfC, the potential for adverse health effects increases. Lifetime exposure above the RfC does not imply that an adverse health effect would necessarily occur. (5)

* EPA has low to medium confidence in the study on which the RfC was based because the symptomatology, although consistent with other epidemiological studies, was mostly subjective and self-reported among a small group of workers; low confidence in the database as...