Which type of gas is usually classified under toxic materials?

Chlorine is commonly classified as a toxic material due to its hazardous properties when inhaled or exposed to individuals. It is a greenish-yellow gas with a pungent odor, and even at low concentrations, it can irritate the respiratory system, eyes, and skin. Chlorine exposure can lead to serious health effects, such as pulmonary edema or damage to lung tissues, making it crucial to handle with caution in hazardous materials operations.

In contrast, helium is a noble gas that is non-toxic and generally safe for use, while carbon dioxide, though it can be harmful in high concentrations, is not classified as a toxic gas under standard conditions since it is a natural part of the atmosphere. Oxygen is essential for life and is not considered toxic under normal circumstances, though it can become dangerous at very high concentrations due to its reactive properties. Thus, chlorine is the correct choice for a gas typically identified as a toxic material.