Subject: Common MSDS Terminology Defined

Evaluating the hazards of a material based on the information on the Material Safety Data Sheet (MSDS) can be a confusing experience. Some of the terminology used on an MSDS is not universally understood. The following is a list of some definitions that you may find useful in determining the physical hazards of a substance according to a MSDS you may be working with.

Boiling Point: This is the temperature at which a substance will start to boil. Flammable or toxic substances with low boiling points present greater fire or toxic exposure hazards than substances with high boiling points.

Flash Point: This is a measure of the flammability of a liquid. Flash point is the temperature at which a flammable liquid will generate enough vapors to burn. Liquids with low flash points are greater fire hazards than liquids with high flash points.

LD/50 or LC/50: These abbreviations stand for the lethal dose or lethal concentration for 50 percent of the test population. 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.

LEL or LFL: These synonymous acronyms stand for the "lower explosive limit" or "lower flammable limit." The LEL/LFL is the concentration of a flammable vapor or gas that will burn or explode if an ignition source is present. Concentrations below the LEL/LFL will not ignite.

Molecular Weight (MW): Normal air has a molecular weight of about 29. Gases with MW's less than 29 will rise in the air and dissipate. Gases with MW's greater than 29 will sink or "pool" in low areas. 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.

Specific Gravity: This is a measure of a material's weight as compared to water. 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.

UEL or UFL: These interchangeable acronyms stand for the "upper explosive limit" or "upper flammable limit." Concentrations above the UEL/UFL will not ignite.

Vapor Density: This is a measure of a vapor's weight as compared to ambient air. Ambient air has a vapor density of one. Vapors with densities greater than one will sink; vapors with densities less than one will raise. Vapors with densities greater than one present an increased fire/toxic hazard in confined spaces.

Vapor Pressure: This is a measure of a liquid's ability to evaporate. Vapor pressure is expressed in either millimeters of mercury (mmHg) or atmospheres (one atmosphere = 760 mmHg). 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.

NOTE: This is just a sample of the terms contained on MSDSs. For additional or specific guidance on the hazards of the substance you're working with; contact the manufacturer of the material. You may call Lab Safety Supply's Safety TechLine Technical Support at (800)356-2501, or e-mail your questions to: techsvc@labsafety.com.

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