What is the primary hazard of hydrogen cyanide (HCN) when exposed to humans?

The primary hazard of hydrogen cyanide (HCN) is its ability to cause chemical asphyxiation and severely hinder respiration. HCN interferes with the body’s ability to use oxygen by binding to the cytochrome c oxidase enzyme in the mitochondria, which is essential for cellular respiration. This disruption prevents cells from using oxygen effectively, leading to symptoms associated with asphyxiation such as shortness of breath, confusion, and ultimately, loss of consciousness if exposure continues.

Unlike skin burns or irritation, which are associated with direct contact or corrosive substances, the effects of HCN are primarily due to its inhalation or ingestion, contributing to systemic toxicity rather than localized harm. The potential for acute allergies and sensitization, as well as temporary blindness, are not primary effects attributed to hydrogen cyanide exposure. This reinforces why the correct answer emphasizes the respiratory dangers posed by HCN.