What attribute of chlorine gas poses a safety risk?

Chlorine gas presents a safety risk primarily because it is heavier than air. This characteristic means that when chlorine gas leaks, it tends to settle in low-lying areas rather than dissipating quickly into the atmosphere. The concentrated presence of chlorine gas near the ground can pose significant health hazards to personnel who are working in or near affected areas, potentially leading to exposure at high levels where respiratory issues and other health emergencies may occur.

While being colorless, odorless, and capable of combustion are notable safety considerations, the fact that it is heavier than air is particularly critical in terms of safety management and emergency response. This attribute necessitates that proper ventilation and monitoring are implemented in facilities that utilize chlorine to mitigate potential risks associated with its presence.