How is the density of vapor or gas usually compared?

The density of vapor or gas is commonly compared to the density of air because this comparison is practical and relevant for many real-world applications, particularly in firefighting and safety scenarios. Air serves as a baseline for understanding how gases will behave in an environment. For instance, gases that are lighter than air will rise, while those that are heavier than air will settle, which is critical information for firefighters when assessing the behavior of smoke, fumes, or hazardous gases during a fire.

By knowing the relative density of a gas compared to air, first responders can make informed decisions about ventilation strategies, the potential movement of gases in a space, and safety precautions needed while operating in environments where these gases are present. Other comparisons, such as that to the density of water or solids, do not provide the same relevant insight for safety and operational procedures in scenarios firefighters frequently encounter.