Which of the following gases can be absorbed through the skin?

Hydrogen cyanide (HCN) is known to penetrate the skin and can be absorbed into the bloodstream, which poses significant health risks. This absorption capability makes HCN particularly dangerous in situations where skin exposure might occur, such as in industrial settings or during certain fire scenarios.

In comparison, while hydrogen chloride (HCl), ammonia (NH3), and chlorine (Cl2) can also be hazardous, their primary risks are associated more with inhalation or direct corrosive effects on contact rather than systemic absorption through the skin. HCl can cause severe irritation to the skin and mucous membranes, but it does not typically penetrate the skin to the extent of hydrogen cyanide. Ammonia is highly irritating and can cause chemical burns but is less absorbed through the skin compared to HCN. Chlorine, a strong oxidizer, is similar in that it poses significant inhalation risks and can cause respiratory distress but does not absorb through the skin as effectively as hydrogen cyanide. Therefore, the properties of hydrogen cyanide highlight its unique danger regarding skin absorption.