The MSDS lists information on the physical properties of the chemical.  Vapor density tells you whether the chemical vapors are heavier or lighter than air.  Those with vapor densities greater than one are heavier than air and will tend to sink to the ground in poorly ventilated areas.  Chemicals with vapor densities less than one are lighter than air and will tend to rise.  Vapor density helps in determining where chemical vapors will accumulate in poorly ventilated areas.  Vapor pressure tells you something about how volatile a chemical is.  The higher the vapor pressure the greater the tendency for the chemical to evaporate.  Chemicals with vapor pressures less than one are not very volatile.  You don’t have to worry as much about breathing in these chemicals.  The MSDS will also give you information on what the chemical looks and smells like.  That way you may be able to tell if a chemical has been released in your work area.  There chemical odor threshold may be listed.  The odor threshold is the airborne concentration at which most people can smell the chemical.  Don’t rely on your nose to detect chemicals with odor thresholds that are above the PELs or TLVs.  You could be overexposed before you smell the chemical.