Which of the following best describes carbon monoxide?

Carbon monoxide (CO) is accurately described as odorless, slightly lighter than air, and flammable in high concentrations. This means that it cannot be detected by smell, which is one of the reasons it is particularly dangerous, as individuals can be exposed to it without realizing it. Its slightly lighter-than-air property suggests that may not accumulate at ground level, but rather can disperse in the air, making it difficult to identify concentrations without detection equipment.

The fact that it is flammable in high concentrations emphasizes the risk it poses in environments where combustion might occur. Unlike other gases, if carbon monoxide reaches certain concentration levels, it can ignite, leading to potential explosions or fires. This aspect underscores the importance of proper ventilation and monitoring in areas where gas appliances are used, making awareness of its properties crucial for safety and health.

This understanding illustrates why the other descriptions are not accurate—the gas is indeed flammable and poses significant risks, particularly in poorly ventilated spaces.