The weight of a vapor or gas compared to the weight of an equal volume of air is an expression of the density of the vapor or gas. Materials lighter than air have vapor densities less than 1.0. Some examples are acetylene, methane, hydrogen. Materials heavier than air, such as propane, hydrogen sulfide, ethane, butane, chlorine, and sulfur dioxide, have vapor densities greater than 1.0. All vapors and gases will mix with air, but the lighter materials will tend to rise and dissipate unless confined. Heavier vapors and gases are likely to concentrate in low places-along or under floors, in sumps, sewers, and manholes, in trenches and ditches where they may create fire or health hazards.