When reading Material Safety Data Sheets (MSDS), you will frequently encounter strange abbreviations that can make understanding the data sheet difficult. While these abbreviations may look confusing at first, it is important that the data
sheets provide the correct technical information. In order to take the correct precautions when using a specific chemical you must know the hazards that exposure to it may present. MSDS’ are the common tool to educate you on the hazards
and assist you in selecting personal protective equipment. But all of this information is based on exposure limits.
This article will provide you some basic term definitions regarding chemical exposures. These abbreviations are not difficult to understand, only confusing until you become familiar with them. Exposure limits, where these abbreviations
are usually found in Part II, of the MSDS, entitled “Hazardous Ingredients.”
TLV-TWA - Threshold Limit Value—Time Weighted Average
This is a level at which the “average” worker can be safely exposed repeatedly, day after day, without adverse effect.The TLV on the form is normally based on an exposure period of 8 hours-per-day and a 40 hour-per-workweek schedule. While your exposure may be over the TLV-TWA levels at points within the workday or workweek, if the time weighted average over the 8 hour day is at or below the listed number the average person is not exposed to an adverse hazard.
TLV-STEL - Threshold Limit Value—Short Term Exposure Limit
This is the concentration that has been determined that you can be exposed to continuously for short periods of time (not to exceed 15 minutes). Being exposed to a STEL should not be repeated more than 4 times per day. Plus, there should be at least 60 minutes between successive exposure periods. For some toxic materials, the MSDS may dictate other maximum periods other than 15 minutes, although 15 minutes is the most common period of time.
TLV-C - Threshold Limit Value—Ceiling
This concentration should not be exceeded at any time. This is the top limit of allowable exposure for any period of time. The TLV-TWA is assumed to be a level that is safe. However, each person may have different tolerances, susceptibilities
and sensitivities to various chemicals. Some chemicals cause a person to be permanently sensitized, so that even minute subsequent exposures may cause discomfort. Therefore, the levels noted may not be appropriate for you personally.
Whenever you work with chemicals of any kind, here at work or at home, caution should be the byword. If you have symptoms at working with a chemical, such as redness of the skin, numbness, swelling of the mucous membranes, headaches, etc., report it immediately to your supervisor, evaluate ways to reduce your exposure to the chemical or seriously consider upgrading your personal protective equipment to a higher level of protection.Read the Material Safety Data Sheet for the chemicals you use carefully so you will be able to protect yourself with knowledge and appropriate precautions.