Limit, short term work practices

OSHA requires employers of workers who are occupationally exposed to formaldehyde to institute engineering controls and work practices to reduce and maintain employee exposure at or below permissible exposure limits (PELs). The employer must use controls and practices, if feasible, to reduce exposure to or below an 8-hour time-weighted average (TWA) of 0.75 ppm. The 15-minute, short-term exposure limit (STEL) for formaldehyde is 2 ppm (OSHA 1992). 

In the U.K., the Health and Safety Executive (HSE) publish recommended levels for occupational exposure as maximum exposure limits (MEL), which are reasonably practicable for all work activities and occupational exposure standards (OES) which cover realistic plant design, engineering and control of exposure and can be used to help in the selection of personal protective equipment. Further subdivision of limits occurs into long-term (usually 8-hour) and short-term (10-minute). Short-term limits are of use where brief exposure may cause acute effects. Both levels are expressed as time-weighted average concentrations (TWAs) over the specified period. Studies of OELs in other countries reveal that there are some differences, but generally they fall close to each other in industrial societies. ... 

The ideal (bio)chemical sensor should operate reversibly and respond like a physical sensor (e.g. a thermometer), i.e. it should be responsive to both high and low analyte concentrations and provide a nil response in its absence. One typical example is the pH electrode. In short, a reversible (bio)chemical sensor provides a response consistent with the actual variation in the analyte concentration in the sample and is not limited by any change or disruption in practical terms, responsiveness is inherent in reversibility. An irreversible-non-regenerable (bio)chemical sensor only responds to increases in the analyte concentration and can readily become saturated only those (bio)chemical sensors of this type intended for a single service (disposable or single-use sensors) are of practical interest. On the other hand, an irreversible-reusable sensor produces a response similar to that from an irreversible sensor but does not work in a continuous fashion as it requires two steps (measurement and renewal) to be rendered reusable. Figures 1.12 and 1.13 show the typical responses provided by this type of sensor. Note... 

Control limits are based on Regulations, Approved Codes of Practice, European Community directives, or Health and Safety Commission instructions, and are limits which should not be exceeded. Failure to comply with a control limit may result in enforcement action by a Health and Safety Executive Inspector. With some controlled substances (i.e. those with no minimum threshold below which adverse effects do not occur, or where short exposure to high concentrations cause injury) exposures may need to be reduced to the lowest levels justifiable on a cost/risk basis. The present strategy behind the application of control limits for exposure to relatively few substances, some of which are not highly toxic, is based on the recognition that these substances are very commonly used at work. Until a few years ago, a great many were used without any real control of exposure (e.g. asbestos, vinyl chloride and lead compounds). Some, such as methylene chloride, formaldehyde and styrene, have been shown more recently to be potentially very harmful, in both the short and the long term, at levels of exposure just above the limits set, and they are included because of their widespread use at work. 

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