safety-in-academic-chemistry-laboratories-students

14 students short index 11503 1245 pm page 15

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Unformatted text preview: numerical values of the flammable limits of a liquid only approximate the true values of the flammable limits under the actual existing laboratory conditions. 14 students short index 1/15/03 12:45 PM Page 15 Reactivity. Some chemicals react vigorously with other chemicals; some are self-reactive; and others are unstable and decompose vigorously if disturbed. Reactivity includes all these characteristics. A chemical’s reactivity is described in its MSDS. Short-term exposure limit (STEL). This number is the concentration in ppm or mg/m3 that should not be exceeded for more than a short period (usually 15 minutes). If measurements show that the concentration of the hazardous chemical exceeds this limit for more than the specified time period, the TWA (see below) is rendered useless and the PEL or TLV has been violated.7 Target organ. The name of an organ or organs (kidney, liver, skin, eyes, etc.) or system or systems (respiratory system, central nervous system, etc.) that are likely to be adversely affected by an overexposure to the chemical. Time-weighted average (TWA). In practice, worker exposures must be measured and averaged over an 8-hour day. If the TWA does not exceed the PEL or TLV for a worker, then he or she is not harmed. It is possible, of course, that for one or more short time periods during the working day the PEL or TLV limit could be exceeded even though the TWA is not exceeded; for such an event, see STEL, above. Threshold limit value (TLV). This number is a concentration limit. It is similar to the PEL, although it was established by the American Conference of Governmental Industrial Hygienists (ACGIH) instead of OSHA. ACGIH updates its TLV list annually, whereas the PEL list is rarely revised. The list of chemicals for which a TLV has been established includes all of the chemicals for which a PEL has been established, plus several more. Some of the TLV limits for the same chemicals differ in numerical value from the PEL limits. The PEL limit is a legal limit; the TLV limit is a voluntary, recommended limit. Because the TLV limits...
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This document was uploaded on 02/27/2014 for the course PHYS 1B at UCSD.

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