The meeting discussed the implications for those who live and work
in the Built Environment of the health effects from exposure to
respirable silica dust. The impetus for the meeting was the re-evaluation by IARC (Lyon) of
respirable silica as a class 1 carcinogen. Previous meetings on silica at IARC have been
reported,
in their monographs of 1972 and 1987, in these volumes they reported that there was insufficient
evidence for regarding silica as a class 1 carcinogen but in their 1997 report they felt that
there was sufficient evidence.
Crystalline Silica: Occurrence and Use. (MA Moore)
Silica is the natural compound formed from two of the most abundant
elements on Earth - oxygen and silicon. It occurs in amorphous and crystalline forms, the most
common of which is quartz. Quartz accounts for about 12 wt% of the Earth's crust and is present
in all major rock types and top-soils. Crystalline silica-containing airborne dusts are ubiquitous
in the human environment coming from the unconsolidated materials on the Earth's surface,
from natural raw materials processed by industry and from many materials of construction.
The Importance of Silica to the Modern World. (Mike Lavender)
Silica is increasingly important in today's world.
It is a major constituent of the modern industrial base being used in diverse industries
from glass making to oil-well drilling: a full list would fill pages. As alpha-quartz silica
is found in virtually all natural mineral raw materials and represents the second most abundant
mineral species after the feldspars. The importance of silica cannot be disputed and it is
difficult to envisage a world which places major restrictions on it use.
Respirable Dust and Respirable Silica Concentrations from Construction Activities. (Jim Chisholm)
This paper presented the results of a survey completed last year (1998) in which respirable dust
and respirable crystalline silica concentrations were
measured for a range of activities including tunnelling, drilling, chasing, grinding, cutting,
general clearing and rubble removal and concrete crushing. The survey covered all sorts of
building materials such as rock, cement, concrete, paving curbs, brick, breeze block and
plasterboard; and all sorts of sites and working conditions. It was an addition to HSE's database
of results on dust producing activities in the construction industry. This is intended to
target areas of greatest risk and to provide guidance for HSE inspectors and for employers.
The Quartz Hazard in the Construction Industry. (PJA Borm, K Donaldson)
The re-evaluation of crystalline silica as a Class 1
carcinogen was based mainly on epidemiological evidence published since 1987.
In the final IARC monograph (no. 68) the following sentences were inserted: 'In making
the overall evaluation, the Working Group noted that carcinogenicity to humans was
not detected in all industrial circumstances studied. Carcinogenicity may be dependent
on inherent characterisitics of the crystalline silica or on external factors affecting
its biological activity or distribution of its polymorphs.' Agreement on the
classification was not unanimous. The Working Group disagreed on a number of issues
which will be discussed here in the light of mechanistic studies. The proposal will
be made that the hazard posed by quartz is not a constant entity, but one that may
vary dramatically depending on the origin of the silica sample or its contact with
other chemical/minerals within its complex constitution.
Regulation of Crystalline Silica: Where Next. (RC Brown)
The recent IARC classification of crystalline silica as a carcinogen
was more complicated than is normal. Sufficient evidence was claimed for carcinogenicity, but a
proviso added that this might be dependent on "inherent or external factors". In the same monograph
it was found that there is "inadequate evidence" for the carcinogenicity of coal dusts although
these might contain substantial proportions of crystalline silica. Were silica like other
carcinogens this could not be true! However, as the IARC decision trickles down to national or
regional regulation the distinctions will probably be forgotten. The simple notion that
crystalline silica is "a carcinogen" will become dogma. Unfortunately, treating a ubiquitous
substance such as quartz in the same way as other carcinogens will bring the whole classification
and labelling exercise into disrepute. Most commercial forms of fine quartz do not produce any
marked biological response in experimental studies and probably pose little risk to man. It will
be suggested that a better approach to control of risk from materials such as silica is through
the continued control of exposure.
Crystalline Silica and Lung Cancer: the Problem of Conflicting Evidence. (Corbett McDonald,
Nicola Cherry)
The IARC working group, in 1996, had considerable difficulty
in reaching a decision and might well not have done so had it not been made clear that we
were concerned with hazard identification, not risk. Ken Donaldson & Paul Borm have
underlined problems with the mechanistic findings, we shall do so with the epidemiological.
Of the many studies reviewed by the working group, nine were identified as providing the
least confounded evidence. The five studies which were positive included two of refractory
brick workers, our own and one other of pottery workers, and finally one in the diatomite
industry; the four which were negative or equivocal included studies of South Dakota gold
miners, Danish stone workers, US stone workers and US granite workers. This further example
that the truth is seldom pure and never simple will be discussed.
Methodological Issues Related to Studies of Workers in the Diatomaceous Earth Industry.
(PA Hessel)
The International Agency for Research on Cancer (IARC) recently reviewed the human and animal
evidence related to the carcinogenicity of silica, finding that there was sufficient evidence
to conclude that crystalline silica in the form of quartz or cristobalite is a human carcinogen.
A series of studies examining mortality of workers at several facilities processing
diatomaceous earth was considered by the IARC committee to have provided strong evidence
favouring their decision. A number of factors raise concerns about the results of these
studies. These factors include modestly elevated risk estimates, the lack of adequate
smoking information, bias in the estimates of exposures to crystalline silica and asbestos,
and post hoc selection of historical exposure weightings and lag periods. Most of these
shortcomings typify the literature available for the previous IARC review (1987) which
concluded that there was only "limited" evidence for the carcinogenicity of silica in humans,
and suggested that studies be undertaken in populations of workers not exposed to known
carcinogens. The limitations of the series of analyses of diatomaceous earth workers
preclude them from providing convincing support for the hypothesis that silica or cristobalite
are human carcinogens.
Smoking and Occupational Lung disease Epidemiology. (J Bernard L Gee)
Epidemiology studies of lung cancer which may be the result of work place exposure contrast
the Risk Ratios or SMRs of the exposed population with a control group. If a dose response
relationship can be detected then this suggests a causal relationship between the exposure
and its effect. Smoking is clearly a substantial confounding factor but unfortunately
adjustment for this is often limited by lack of accurate data. Most commonly only 'ever'
versus 'never' smokers numbers are available and even then this data may only be available
for some of the cohort. In the various studies of silica and lung cancer it is important that
the smoking history of both silica exposed and control groups is known. If the prevalence of
smoking was similar in the two groups this would remove the confounding but differences in the
groups are usually present. Since the SMRs from a number of studies of silica and lung cancer
are about 1.5 it is suggested that these might be close to smoking adjusted SMRs. By contrast,
where the SMR is >2.0, smoking adjustments are likely to affect the magnitude but not the
statistical significance of the result.
Research and Policy Implications of IARC's Classification of Silica as a
Type 1 Carcinogen. (DF Goldsmith)
In 1997 IARC classified silica dust exposure as a known
human carcinogen, Type 1. In Western countries this change in status has not resulted in
any immediate shift to reduce workplace exposure, adopt new occupational standards, or to
marked changes in environmental hazard assessments. The one place where silica has been
treated as an environmental carcinogen is under California's Proposition 65, which has
required since 1989 quantitative risk assessment to determine if exposures to carcinogens
could result in greater than 1 in 100,000 excess cancer risk. There have been several
peer-reviewed cancer and non-cancer risk assessments for silica based on animal, human
and threshold extrapolations. This presentation will review and contrast those
determinations, discuss the impact of IARC's reassessment and suggest where future
policy may lead.
The Impact of Funding, Purchase and Lease Agreements on the use of
Construction Materials. (CA Holwell)
This paper identifies what steps should be taken, and whether by scientists or businessmen,
to ensure that changes to industry practice identified as desirable by scientific research,
are properly implemented. It identifies the parties normally interested in and influential
in relation to, a construction project and the nature of their interests. The paper identifies
how the legal documents relating to construction projects deal with the
question of "problem" materials. It also warns of the dangers of creating "scares" about
particular materials and the ease with which that can be done.