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Kyle Steenland raises some interesting points in his commentary on silica  both on our papers reporting exposure assessment and mortality in the UK silica sand industry [2,3] and on the adverse effects of silica in general.
With the exception of one quarry, where other exposures such as polycyclic aromatic hydrocarbons could have occurred, no relationship was found with cumulative silica ex...
With the exception of one quarry, where other exposures such as polycyclic aromatic hydrocarbons could have occurred, no relationship was found with cumulative silica exposure in the UK silica sand study. Steenland points out that the estimated exposure levels were relatively low with only a few workers having a cumulative exposure over 1
mg/m3.years (in fact only 8% of the study population overall and 5 lung cancer cases had a cumulative exposure over 2 mg/m3.years). We agree that this may be why only 2 silicosis deaths were observed, although, as we point out in the paper, fibroses, including silicosis, are poorly recorded
on death certificates and therefore could not be accurately assessed.
Steenland draws attention to the current controversy over the relationships between silica, silicosis and lung cancer. The epidemiological literature is indeed inconsistent. OEM readers might be interested to know about a two day workshop organised by the European Association of Industrial Silica Producers (EUROSIL) that was held in New
York in August 2004, which brought together leading scientists from Europe, the USA, Canada, China, South Africa and Australia involved with the major studies in the industrial sand, diatomaceous earth, mining, heavy clay, granite, stone, pottery and brick industries  (Kyle
Steenland was invited but was unfortunately unable to attend). The aim of the workshop was to gain a clear understanding of the epidemiological work to date, and to prioritise future research needs. Following brief presentations summarising the results from these studies, break out groups evaluated the variations between studies in the design, definition and derivation of health outcomes, assessment of exposure, collection of confounding data and statistical methodology. The groups identified the knowledge gaps and discussed the feasibility and desirability of filling these.
The overwhelming conclusion from the workshop was that heterogeneity occurs across all aspects of both the actual nature of the industries in which silica exposure occurs and the design, conduct, analysis and interpretation of the studies that have been carried out.
Heterogeneity in one or more of the following might contribute towards the differing results:
Many of the above are, of course, inherent difficulties in occupational epidemiology in general. However, in the area of silica some of these differences may be so fundamental that it may be necessary to rethink the concept of the silica industry being a uniform entity. As the
IARC Working Group pointed out, given the wide range of populations and exposure circumstances some non-uniformity of results would be expected  and even within the same industry the results from different studies may vary. Some of these are highlighted below.
At the workshop the group discussing studies in the diatomaceous earth industry highlighted uncertainties in exposure assessment methodologies including the conversion factors used to convert total dust counts to respirable silica, extrapolation methods used for exposures prior to 1950 and lack of adjustment for calcining for exposure before 1930. They were also concerned about co-exposures to asbestos, lack of smoking data, and the difficulty of separating small round opacities from small irregular opacities when reading chest x-rays. Conversion factors
and extrapolation in exposure assessment were also of concern regarding the studies in the sand industry as was the origin and composition of the sand. For example, in the North American studies some sands were almost pure quartz, whilst others elsewhere were dune sand or feldspar sands.
There were also differences in the aluminium content. Workforces in mining were considered to have the advantages of large and stable workforces, good exposure measurement data and special health programmes and surveillance. Co-exposures, for example, to radon, asbestos and PAHs were
highlighted as particular problems, however, as was the variation in the quartz content of dusts from mines extracting tin, gold, or coal.
The group discussing the mining studies also drew attention to the need to investigate exposure metrics such as intensity and peaks of exposure as an alternative to cumulative exposure. Many of the above considerations were also identified as potential problems in the pottery, brick and granite industries. The researchers involved in the Vermont granite studies expressed interest in collaborating to explore the reasons why their studies had produced different results. This group drew attention to the issues of survivor populations and susceptibility. For example, in the studies in China, there is a high incidence of non-malignant respiratory disease at an early age that could impact on the results for lung cancer occurring at older ages.
One of the aims of the workshop was to try and target any future research so that key issues concerning adverse health effects of silica and uncertainties around current knowledge can be addressed. These include:
From the various presentations and discussions, eight priorities were
1. Effectively control workers’ dust exposure and implement proper
evaluation and prevention measures.
2. Harmonise sampling and analytical methods for future collection of
dust measurements and develop a standardised job/task industry wide Job
Exposure Matrix (JEM).
3. In parallel, collect information on the type and use of personal
protective equipment and develop the methodology for incorporating this
into the JEM for future exposure assessment.
4. Investigate the toxicological potency of different types of silica
using industry samples
5. Focus on industries with similar exposures and review the
differences that may have given rise to different estimates of risk.
6. Consider whether pooling of the data might be useful and
investigate what this might entail, e.g. development of a harmonised JEM
and exposure assessment methodology, bearing in mind that indiscriminate
pooling might give misleading and imprecise results.
7. Consider whether current cohorts might be able to re-analyse their
data to address the priority areas of concern and/or whether they can
collect supplementary data to assist with this.
8. Alternatively carry out a new study(ies) but ensure that there is
an agreed protocol and a design that ensures knowledge gaps will be
Some of the above are already being developed. For example, the
Industrial Minerals Association in Europe is developing a harmonised dust
monitoring strategy and encouraging all its members to implement this.
IARC have classified silica as a Group 1 carcinogen, but rather than
focusing on hazard identification, workshop participants felt that the
focus should be on reducing exposure to a level that would protect against
silicosis, and thus probably lung cancer, and be technically achievable
across the whole industry. The need for enforcement of current national
standards is illustrated by findings of high percentages of samples taken
by the US Occupational Safety and Health Administration (OSHA) still
exceeding the OSHA Permissible Exposure Limit (PEL) of 0.1 mg/m3 . For
example in 1999, 47% and 38% of samples in construction and manufacturing
groups, respectively, exceeded the OSHA PEL. The fact that silicosis (and
other potentially related diseases) still occurs is likely to be due to
the continuation of overexposure that is perhaps not reflected in some of
the industries investigated in much of the current epidemiological
literature, which tends to focus on producers rather than manufacturers or
1. Steenland K Silica: déjà vu all over again? Occup Env Med 2005; 62: 430-432
2. Brown TP, Rushton L Mortality in the UK Industrial Silica Sand
Industry: 1. Assessment of exposure to respirable crystalline silica.
Occup Env Med 2005; 62: 442-445
3. Brown TP, Rushton L Mortality in the UK Industrial Silica Sand
Industry: 2 a retrospective cohort study. Occup Env Med 2005; 62: 446-452
4. Eurosil Proceedings of expert workshop: Epidemiological
perspectives on silica and health. Brussels, Belgium, European Association
of Industrial Silica Producers, 2005
5. IARC Monographs on the evaluation of carcinogenic risk to human,
vol. 68: silica, some silicates, coal dust and para-aramid fibrils. Lyon,
International Agency for Research on Cancer, 1997
6. NIOSH Work-related lung disease surveillance report 2002 (DHHS
(NIOSH) Number 2003-111) Cincinnati, Ohio, National Institute for
Occupational Safety and Health, 2003