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Preventing Chemical Accidents:  Understanding MSDS’s & Assessing Chemical Hazards

Factsheet #11 continued

Section II: Hazardous Ingredients

This section is the key part of the MSDS.  It gives you the basic ingredients in the product and tells you the legal and recommended limits for workplace exposures.  Remember to get the exact spelling of the chemicals because many chemicals have similar names but different health effects.  The following explains some technical language you might find on data sheets related to exposure limits:

PEL (Permissible Exposure Limit): This is the maximum exposure established by OSHA.  It can be a time-weighted average (TWA) exposure limit, a “ceiling” exposure limit, or a “peak” exposure limit.  These are all legal standards.

TLV (Threshold Limit Value): This is a recommended average concentration over an 8-hour day.  This term is used to express the airborne concentration of a material to which nearly all persons supposedly can be exposed without adverse effects, day after day.  TLVs can be expressed in three different ways.

(TLVs are suggested, not legal, standards established by the American

Conference of Governmental Hygienists [ACGH], which is not a government agency.)

•  TLV-TWA (Time-Weighted Average): This is the concentration for a normal 8-hour workday or 40-hour workweek.  If the MSDS only lists TLV, it usually means a time-weighted average.

 TLV-STEL (Short-Term Exposure Limit): This is the maximum concentration for a 15-minute period (maximum of four such periods per day, with at least 60 minutes between exposure periods, provided that the daily TLV-TWA is not exceeded).  This is like the OSHA “ceiling” limit.

 TLV-C (Ceiling Exposure Limit): This is the concentration that should not be exceeded even for a split second.  This is like the OSHA “peak” limit.


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