Thesis (Ph.D.(Public Health))--University of the Witwatersrand, Faculty of Health Sciences, 2012 / Introduction
Personal exposure to respirable crystalline silica (commonly in the form of quartz) causes
serious adverse health effects and has been well documented in industries formally known as
the ‘dusty trades’. Very little information exists on personal quartz exposure in agricultural
settings. Silica exposure is an important public health issue particularly in settings of high
tuberculosis (TB) and Human Immunodeficiency Virus (HIV) rates. The mineral has
contributed to serious epidemics of TB in southern Africa and other low and middle - income
regions of the world because of the increased risk of pulmonary tuberculosis in silica exposed
workers. It is well known that HIV infection increases the incidence of TB and the risk of
tuberculosis in individuals with both HIV and silicosis is larger than the sum of each factor.
Globally (especially in resource poor countries), agriculture employs a very large population
that may be affected.
Objectives
The objectives of the PhD were to review the published literature on respirable quartz
exposure and associated disease in agricultural related settings systematically; to measure
personal time weighted average respirable dust and quartz on sandy, sandy loam and clay soil
farms in the Free State and North - West provinces of South Africa; to ascertain whether soil
type is a determinant of exposure to respirable quartz; to identify additional determinants of quartz exposure in farming; to estimate annual cumulative respirable quartz exposure; and to
discuss some occupational health implications and research needs.
Methodology
Published studies on exposure to silica and quartz in agriculture and related settings as well
as silica - associated disease in farming were searched systematically through “PubMed”;
and, critiqued. Three farms, located in the Free State and North West provinces of South
Africa, had their soil type confirmed as sandy, sandy loam and clay; and, from these, a total
of 298 respirable dust and respirable quartz measurements were collected between July 2006
- November 2009 during periods of major farming operations. These measurements were
collected using standard international measurement and analytical methods. Quartz
determinations were done using X - ray diffraction by a quality - assured, accredited
laboratory, and were verified by a leading external agency.
Respirable quartz values below the limit of detection (22 μg.m-3) were estimated using
multiple imputation. Non - parametric tests were used to compare quartz exposure from the
three different soil types. Variables such as soil type, commodity farmed, activity (nature of
work done), process (manual vs mechanical), quartz % and weather variables (e.g. wind and
humidity during the week prior of and during sampling) were used in bivariate and multiple
regression modeling to identify determinants of respirable quartz exposure. Logistic
regression was done aiming to identify determinants of respirable quartz greater than 50
μg.m-3 and to deal with data points below the detection limit without including multiple
imputation methods. The annual cumulative exposure was estimated for a typical farm worker on the sandy soil farm using activity - specific measurements and duration of each
activity in a year. Particle sizes of respirable dust fraction were determined using laser light
scattering.
Results
Literature review
In total, 17 studies were identified: 11 investigated respirable dust and quartz exposure on
farms and six quartz related disease in agricultural settings. They provided convincing
evidence of a respirable quartz risk on sandy soil farms but scant evidence of associated
disease.
Exposure to respirable dust and quartz
Respirable quartz measurements from the three South African farms ranged from not
detectable to 626 μg.m-3 and confirmed the quartz hazard: some concentrations exceeded
generally accepted occupational exposure limits in all activities evaluated, even though 278
(93.3 %) of the respirable dust concentrations were well below a commonly used
occupational exposure limit of 2 mg.m-3. Fifty seven percent, 59% and 81% of the respirable
quartz measurements on the sandy soil, sandy loam soil and clay soil farm respectively
exceeded the American Conference of Governmental Industrial Hygienists (ACGIH)
Threshold Limit Value (TLV) of 25 μg.m-3. Twelve percent and 13% of respirable quartz
concentrations exceeded 100 μg.m-3 on the sandy soil and sandy loam soil farms respectively,
but none exceeded this level on the clay soil farm. Determinants of exposure
The proportions of measurements greater than 100 μg.m-3 were not significantly different
between the sandy and sandy loam soil farms (prop.test; p = 0.65), but both were significantly
larger than for the clay soil farm (prop.test; p= 0.001). The percentage of quartz in respirable
dust was determined for all three farms using samples of a size such that all measurements
were well above the analytical detection limit. Percentages ranged from 0.5 - 94.4% with no
significant difference in the median quartz percentages across the three farms (Kruskal -
Wallis test; p = 0.91).
Bivariate analyses showed that commodity (Kruskal - Wallis test; p = 0.001), activity
(Kruskal-Wallis test; p = 0.001) and process (Wilcoxon test; p = 0.003) were associated with
respirable quartz concentrations. Multiple linear regression showed that soil type, season,
commodity, activity, process, quartz percentage, humidity on the morning of measurement
and interactions between activity, respirable dust exposure and quartz percentage were
significantly related to respitable quartz exposure. Logistic regression showed that, during
univariate analyses, cereal planter operator, increased quartz % in respirable dust, decreased
humidity on the day of measurement and increased respirable dust concentration were all
significantly associated with quartz levels above 50 μg.m-3. Multivariate analyses showed
that cereal planter operator remained a strong determinant of higher level of exposure relative
to other activity (OR 3.76 95% CI 1.64 - 8.63). Increased levels of quartz % in respirable
dust increased the ORs for exposure above 50 μg.m-3. Annual cumulative exposure
The estimated annual cumulative exposure of a farm worker exposed at the highest
concentration measured for each task exceeded the cumulative exposure had the worker been
exposed at 100 μg.m-3 by 1.46 times. At median quartz exposures measured for each of the
activities done during the year, the chances of exceeding the cumulative exposure if exposed
at the South African OEL are small: the ratio of the estimated cumulative exposure indices to
occupational exposure limit cumulative exposure = 0.29.
Particle size
The median D50 particle size of the 30 respirable dust samples measured in this study was
5.53 μm with the smallest D50 value reported by the laboratory being 3.39 μm.
Conclusion
Despite its ubiquity, little is known about quartz exposure in the agricultural industry. This
thesis not only demonstrates significant potential for overexposure in some settings, it also
may be the biggest study of its kind: it is the first study to have measured a large number
(298) of personal respirable quartz concentrations across various activities on three farms
known to have different soil types. It is also the first study in farming that attempted to relate
personal respirable quartz exposures to possible determinants, to consider potential modifiers
of quartz exposure and to estimate annual cumulative quartz exposure. The study adds
considerably to the evidence that over - exposure to quartz may be a risk in farming and has identified the need for practical interventions and research gaps. In addition, this study
makes it possible to use the detailed analyses of silica exposure determinants in farming in
other settings with similar conditions.
Soil type may determine whether exposure is greater than 100 μg.m-3, but the job type and
the manner in which the task is performed (e.g. mechanical or manual) may be important
determinants of exposure. Identifying quartz exposure determinants (e.g. type of job) and
modifiers will be of value to focus implementation of controls, which is of particular
importance in developing countries. The nature of farming and weather variables may add
information about exposure risk and should be included with soil type in future farming
exposure assessment studies. Given the large numbers of farm workers possibly exposed to
silica and the seriousness of silica - associated diseases, some dust - related occupational
health interventions should be considered in South African farming, particularly because
during the course of the studies none of the South African farms had medical surveillance
programmes in place and few, if any, dust control measures were observed. Practitioners
working in farming areas should be aware that silica - associated diseases may occur in farm
workers; therefore despite limited evidence for silica associated disease, occupational health
interventions appear justified.
No study previously has defined the burden of silica – associated diseases in farming. A
starting point may be to perform radiological surveys on long service farm workers (possibly
more than 20 years service and performing dusty activities) on a sandy soil farm, as sandy
soils are likely to produce the highest silica levels. An effort should also be made to define
the duration and intensity of silica exposure in farming over the annual cycle. Lastly, cost - effective methods to reduce respirable dust and silica exposure in farming should be
identified.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/12491 |
| Date | 05 March 2013 |
| Creators | Swanepoel, Andrew Johnstone |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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