The numbers are obtained by administering increasing concentrations of chemical to test animals until a 50 percent kill rate is noted. Figures can be used to compare different substances to determine which substances are the most toxic. Normal air has a molecular weight of about 29. Gases with MW's less than 29 will rise in the air and dissipate. If a gas or vapor is toxic or flammable and has a high MW, it will present a much greater hazard by collecting in low-lying areas or confined spaces.
Water has a specific gravity of one.
Substances that have a specific gravity of greater than one will sink. Substances with specific gravity's lower than one will float. Ambient air has a vapor density of one. Under normal conditions, water has a vapor pressure of about 24 mmHg. Substances with vapor pressures greater than 24 mmHg will evaporate faster than water.