Which asphyxiant carries the highest risk of secondary contamination?

Cyanide is known to carry the highest risk of secondary contamination among the listed asphyxiants due to its highly toxic nature and the potential for lingering effects in the environment. When cyanide is released, it can quickly contaminate surfaces, clothing, and equipment, posing a significant risk to responders and victims alike. The volatility of cyanide compounds means they can easily be absorbed by surfaces or accumulate in the air, leading to further exposure through inhalation or skin contact even after the initial exposure has occurred.

In contrast, while substances like carbon dioxide and nitrogen can cause asphyxiation by displacing oxygen, they are generally not toxic at the levels that would necessitate concern for secondary contamination. Chlorine, while also hazardous and able to cause respiratory issues, has a more distinct odor and is less likely to cause contamination spread compared to cyanide. Thus, the distinctive properties and hazards associated with cyanide establish it as the asphyxiant with the highest risk for secondary contamination.