Introduction: Single sample TWA samples collected over an 8-hour shift have the
potential to mask elevated exposures, excursions or “peaks” that may have occurred
thus permitting situations where workers are over-exposed or indeed over-dosed. The
objectives of this study, undertaken in a small acrylic sheet manufacturing plant, are
therefore to identify tasks that have the potential to exceed short-term occupational
exposure levels and then simultaneously monitor employees undertaking these tasks for
8-hour TWA and Short-Term exposure concentrations. The results obtained from this
sampling are then compared to their respective legal limits and then finally correlated to
establish their statistical significance.
Materials and Methods: The study setting comprises a syrup room wherein two
employees are assigned per shift. Employees in this setting manufacture an acrylic
“syrup” which is achieved by dosing raw materials into any one of 13 mixing vessels.
Whilst mixing, these vessels also heat the ingredients until the required viscosity is
reached. This “syrup” comprising mostly of liquid methyl methacrylate, is then
decanted into a pressure vessel from where it is pipe-fed into a casting chamber and
finally poured between two glass sheets. When cured, the final product is stored and
sold as a clear or tinted acrylic sheet. All operations with this area are therefore
associated with facilitating the syrup manufacture. Personal 8-hour TWA and Task-
Based measurements of methyl methacrylate vapour were simultaneously obtained from
the breathing zones of six employees over five separate shifts. These employees
routinely work within the setting and also undertake tasks that have the potential to
exceed the Short-Term Occupational Exposure Limit (ST-OEL) for methyl
methacrylate vapour. Tasks were studied and those selected for quantitative monitoring
were captured using a qualitative risk assessment tool. These selections were based on
studying each task to establish the employee’s exposure probability and severity i.e.
whether performing the task could indeed lead to excessive Short-Term exposures.
Eight-hour TWA monitoring was undertaken using activated carbon 3M 3500 passive
monitoring badges which were attached to each of the subject’s breathing zone and left
over 80 % of the shift. The task-based measurements were obtained by using a Drager
PAC III electro-chemical monitoring instrument, which was also placed in each
each
4
subject’s breathing zone, and provided real-time exposure data whilst the employees
were undertaking the various tasks.
Results: All measurements (N = 116) were obtained over a series of 5 full-shift
monitoring periods. When analysed, 8 of the 10 of the TWA samples returned results
that were below the 8-hour TWA OEL. Of the 106 task-based measurements obtained
for the nine identified tasks, when averaged, 89.1 % of results exceeded the ST-OEL.
When the TWA and ST measurements were correlated, only one of the nine tasks were
statistically significant in their correlation. This correlation coefficient was however
highly statistically significant (r = 0.339, p = 0.032 and r = 0.337, p = 0.022
respectively). Both negative and positive correlations were obtained however these were
statistically insignificant.
Discussion: A significantly higher proportion of the sample results were above the STOEL
than the 8-hour TWA OEL concentrations contributing to the argument that ST
exposure monitoring may add additional insight to employees’ exposure profiles. A
major limitation of the study is however the small sample size, which makes it difficult,
due to inter-worker variability amongst other factors, to extrapolate the results and their
corresponding interpretations to larger, more generalised occupational hygiene
monitoring scenarios.
Conclusion: The results obtained therefore support the assertion that the inclusion of
short-term monitoring is important in characterising employee exposures in situations
where these tasks are themselves potential sources of significant chemical exposures.
Recommendations: As a basis for undertaking any form of monitoring and particularly
in settings where short-term, task-based exposures may exist, the importance of
undertaking a systematic approach to hazard identification and risk profiling via the use
of a known risk assessment tool to compile a air sampling programme, has been
demonstrated in the results of this research. Further research that specifically addresses
the problem of characterising workplace exposures would be useful in larger study
populations as well as occupational settings which expose employees to the various
types of airborne contaminant e.g. fume, mists, particulates and gases.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/7007 |
| Date | 01 June 2009 |
| Creators | Chester, Sean John |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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