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. Toxic or flammable substances with high vapor pressures present increased risks since they will escape from open containers and create a hazardous environment. This is just a sample of the terms contained on MSDSs.