Ceiling limit, defined

TLV-C is defined as the tlireshold limit concentration value ceiling that should not be c.xccedcd during any part of the working e.xposure. This ceiling limit places a definitive boundtiry on concentrations of toxic or otlicrwisc hazardous substances tliat should not be exceeded. 

Recommended Exposure Limit (REL) was developed by the National Institute for Occupational Safety and Health (NIOSH) and defines the highest allowable airborne concentration that is not expected to injure a worker expressed as a ceiling limit of time-weighted average for an eight- to ten-hour work day. 

Predictable Peak Exposures. Ceiling limits, as defined, should be applied only to situations where the periods of maximum exposure can be prospectively identified. If the highest exposure can be anticipated, then its measurement is a physical problem, not a statistical one. All pertinent information related to the operation should be considered so that only air samples which are likely to represent peak air concentrations are collected. Consider, for instance, exposures of workers in a chemical plant to hydrogen sulfide (a reactant in a process). If analysis of the operation shows that the highest exposure should occur when a valve is opened to vent the reaction vessel, then only this period needs to be sampled to determine the peak exposure. 

Similarly, the Revisions to UFC Article 80 required that a continuous gas-detection system shall be provided to detect the presence of gas at or below the permissible exposure limit or ceiling limit. The detection system shall initiate a local alarm and transmit a signal to a constantly attended control station. The alarm shall be both visual and audible and shall be designed to provide warning both inside and outside of the storage area. The audible alarm shall be distinct from all other alarms. It defined a continuous gas detection system as a gas-detection system where the analytical instrument is maintained in continuous operation and sampling is performed without interruption. Analysis may be performed on a cyclical basis at a frequency not to exceed 30 minutes. P1... 

Define what Ceiling, Short term limit, and time weighted average values are. 

These systems can be inside large halls and may have no fixed limits for their influence, except for some parts of the system (inlet device surface, etc.) They can also be situated inside small rooms, where walls, floors, and ceilings are the natural boundaries. The systems usually consist of one exhaust hood and one supply inlet, which interact. There are also special combinations, as two or more inlets and one exhaust hood, or one supply inlet and two or more exhausts. All of these combinations need careful design and an accurate relation between supply and exhaust flow rates and velocities. Some systems also need stable temperature conditions to function properly. All combinations are dependent on having a defined contaminant concentration in the inlet air. This usually implies clean supply air, but some systems may use recirculated air with or without cleaning. 

EPA has established that hydrogen sulfide is a regulated toxic substance and is a hazardous substance as defined under the Federal Water Pollution Control Act. OSH A has established an acceptable ceiling concentration of 20 parts per million (ppm) for hydrogen sulfide in the workplace, with a maximum level of 50 ppm allowed for 10 minutes maximum duration if no other measurable exposure occurs. NIOSH has set a maximum Recommended Exposure Limit (REL) ceiling value (10 minutes) of 10 ppm. A complete listing of federal and state regulations and recommendations are found in Chapter 7. 

The TLV is not a single entity. The ACGIH defines three categories of TLV, namely, the time-weighted average (TLV-TWA), the short-term exposure limit (TLV-STEL), and the ceiling. Because these are proprietary terms, the U.S. Occupational Safety and Health Administration (OSHA) has adopted different terminology for essentially the same concepts, as follows ... 

This physical limitation is the result of mass and heat transfer limitations, which are stoichiometrically related to product formation. The vertieal dotted line in Figure 11.1 symbolizes the limitation which is a conseqnence of the faet that the eoneentration of the biocatalyst is bound to certain defined limits, for instanee solnbihty in case of isolated enzymes and space in case of suspended eells. Fignre 11.1 also shows that the biocatalyst should have a minimum speeifie aetivity to be able to operate the bioreactor close to its physical ceiling. 

Occupational exposure to phosgene" has been defined by NIOSH [1484] as exposure above half of the recommended time-weighted average (TWA) environmental limit whilst "overexposure" is defined as known or suspected exposure above either the TWA or ceiling concentration, or any exposure which results in the development of pulmonary symptoms. 

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