1.1 The 8-hour Time Weighted Average exposure standard
The above exposure standard is based on an 8-hour working day and 5-day working week formula. In cases where the working week exceeds 40 hours, the TWA exposure standard should be revised to compensate for longer work shift exposure.
1.2 Peak limitation or Short-Term Exposure Limit (STEL) for specific substances
These exposure standards can not be regulated since the negative effects of the over exposure are already accounted for.
1.3 Short exposure periods
It is not recommended raise the 8-hour TWA exposure standard for a shorter exposure shift for example, increasing the TWA for 1 hour by eight times and no exposure for the remaining shift, main reason being that health implications for high exposures and short periods are not fully understood.
1.4 Models for regulating 8-hour TWA exposure standards
For prolonged work shifts, a range of mathematical models may be utilised to regulate the exposure standard. Some of these are:
- The ‘Brief and Scala Model’ 13,
- The US ‘Occupational Safety and Health Administration (OSHA) Model’ 14,
- The ‘Pharmacokinetic Model’ of Hickey and Reist15 and
- The Quebec Model16
All the above models provide valid methods to regulate the exposure standard but the main distinction is the level of conservatism. The values of exposure standards do not differentiate between safe and unsafe exposures and hence it is not advisable to apply precise adjustments.
Exposure standard involves exposure via inhalation of substances however risks linked with other exposure routes such as ingestion and absorption through skin must also be examined to understand the overall health effect on a worker.
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