Those who work with chemicals will need to know the following terms:
Solubility (Sol) Solubility is usually given in water and is measured in percent by weight. It is a measure of how well something dissolves
Specific Gravity (SG) The density of a chemical compared to water. Water = a specific gravity of 1. (Water weighs 8.34 pounds/gallon)
Relative Density of Gases (RDG) The density of gas compared to air.
Vapor Density (VD) The density of a vapor compare to air.
Vapor Pressure (VP) The pressure a liquid exerts at 68 degrees F. It is measured in mmHg. The lower the Vapor Pressure, the more rapidly the chemical will volatilize (evaporate) into the air.
Pyrophoric A substance that burns spontaneously in air at a temperature of 130 degrees F or below.
Autoignition Temperature The lowest temperature at which a liquid will give off enough flammable vapors and heat energy to ignite spontaneously and maintain combustion.
Upper Explosive Limit (UEL) The upper explosive limit in air measured by percent by volume at room temperature. The maximum concentration of gas or vapor above which the substance will not burn or explode. Above this concentration, the mixture is too "rich" to ignite or explode. (Like a carburator that "floods")
Lower Explosive Limit (LEL) The lower explosive limit in air measured in percent by volume at room temperature. The minimum concentration of gas or vapor in the air below which the substance will not burn or explode. Below this concentration, the mixture is too "lean" to ignite or explode.
Flash Point (FP) The temperature at which the liquid phase gives off enough vapor to flash when exposed to an external ignition source.
Fire Triangle (FT) In order to have a fire or explosion the following elements are needed: Fuel, oxygen, ignition. The fuel and oxygen must be blended in the right combination. That is, the concentration of the chemical in the sir must be between the LEL and UEL and the oxygen concentration must be at or near 20.9%

When a substance is in the air in a concentration above its LEL and below the UEL, extreme danger of sudden explosion or flash-over is present. [Link to First Responder Awareness button] Instruments such as Photo-ionization detectors have been developed that will read LEL levels. Generally, any level above 25% of the LEL in the open air is considered a hazardous situation. In a confined space, that safe level drops to 10% of the LEL. Some instruments have alarms that will sound when the concentration of gases or vapors meet the 10-25% of the LEL. Example: Gasoline has an LEL of 1.4%, a UEL of 7.6% and a flash point of -45 degrees F. In a confined area, above -45 degrees F, gasoline fumes that exceeded .14% of the atmosphere would be considered a hazardous situation and all personnel should be immediately removed. If the gasoline fumes are between 1.4% (14,000 ppm) and 7.6% (76,000 ppm) then any sort of ignition source, i.e. spark of some kind, could cause the atmosphere to ignite or explode.

TO FIND OUT A CHEMICALS PHYSICAL AND HEALTH HAZARDS, LOOK UP THE CHEMICAL ALPHABETICALLY IN THE National Institute for Occupational Safety and Health (NIOSH)'s "Pocket Guide to Chemical Hazards." [LINK http://www.cdc.gov/niosh/npg/npg.html ]