A substance with a vapor density of 0.9 or less will:

A substance with a vapor density of 0.9 or less is lighter than air, which typically has a vapor density of about 1. This means that when released into the atmosphere, the substance will tend to rise rather than sink. The lower the vapor density relative to air, the more buoyant the substance is, allowing it to ascend into the atmosphere.

This characteristic is important in fire and hazardous materials situations, as it affects how the substance behaves when released. For example, if a gas with a vapor density of 0.9 or lower escapes, it can disperse quickly into the air, which might impact ventilation strategies and the potential for exposure during firefighting efforts. Understanding vapor density is crucial for assessing safety and planning responses in scenarios involving chemical spills or gas leaks.