What is a common characteristic of a toxic gas in confined spaces?

A common characteristic of toxic gases in confined spaces is that they can cause rapid unconsciousness without warning. This is particularly important for safety awareness because many toxic gases can be odorless and colorless, making them undetectable by smell or sight. When working in confined spaces, it is crucial to recognize that exposure to these gases can quickly lead to a dangerous situation, including loss of consciousness, which increases the risk of injury or death for individuals present. This characteristic emphasizes the importance of continuous monitoring for toxic atmospheres and the need for proper personal protective equipment and safety protocols when entering such environments.

The other options do not reliably characterize toxic gases. For instance, some toxic gases can be lighter than air, making the assertion that they are always heavier than air misleading. Additionally, many gases that are toxic can have explosive potential, and relying solely on smell for detection is not safe, as some toxic gases do not produce detectable odors.