2.1
Chemical and Physical Properties
In the
crystalline state, one silicon atom and four oxygen atoms are arranged
in an ordered, repetitive array of three-dimensional tetrahedrons. The
silicon atom is the center of the tetrahedron. Each of the four corners
consists of a shared oxygen atom.
Exposure
to changes in temperature and pressure, either natural or synthetic,
may cause the crystalline structure to change [Iler 1979; Klein and
Hurlbut 1993; Navrotsky 1994; Hemley et al. 1994; IARC 1997]. An example
of a naturally occurring pressure change is the transformation of alpha
quartz to coesite in a rock subjected
to the impact of a large meteorite [Iler 1979; Klein and Hurlbut 1993;
IARC 1997]. Alpha quartz and beta quartz are the respective designations
given to the low- and high-temperature crystal structures. Quartz changes
from the alpha to the beta form at 573 °C (1,063 °F) [Ampian
and Virta 1992; NIOSH 1983a; Virta 1993; Guthrie and Heaney 1995].
The solubility
of quartz in water at room temperature ranges from 6 to 11 micrograms
per cubic centimeter (µg/cm3) (6 to 11 parts per million
[ppm]) as SiO2 [Coyle 1982; Iler 1979]. Quartz is slightly
soluble in body fluids, where it forms silicic acid and is excreted
by the urinary system [IARC 1987]. The amount of silica dissolved depends
on various factors, including particle size, shape, and structure; solution
temperature; viscosity; pH; the proportion of dust to liquid; and the
presence of trace minerals [King and McGeorge 1938; King 1937; Iler
1979; Wiecek 1988; IARC 1997; Guthrie 1997]. However, the dissolution
of quartz does not contribute substantially to its clearance
or to changes in its biological activity [IARC 1997; Heppleston 1984;
Vigliani and Pernis 1958].
2.2
Number of Workers Potentially Exposed
NIOSH
[1991] estimates that at least 1.7 million U.S. workers are potentially
exposed to respirable crystalline silica.
This estimate is based on information from the National Occupational
Exposure Survey (NOES) [NIOSH 1983b] and the County Business Patterns
1986 [Bureau of the Census 1986]. Table 1
lists the nonmining industries (excluding agriculture) and mining industries
with the largest numbers of workers potentially exposed to respirable
crystalline silica. In addition, an undetermined portion of the
3.7 million U.S. agricultural workers [Bureau of the Census 1997] may
be exposed to dust containing a significant percentage of respirable
crystalline silica [Linch et al. 1998].
2.3
Dust-Generating Activities, Uses, and Potential Exposures
Crystalline
silica (quartz) is a component of nearly every mineral deposit [Greskevitch
et al. 1992]. Thus most crystalline silica exposures are to mixed dust
with variable silica content that must be measured by dust collection
and analysis [Wagner 1995; Donaldson and Borm 1998].
Workers
in a large variety of industries and occupations may be exposed to crystalline
silica because of its widespread natural occurrence and the wide uses
of the materials and products containing it. OSHA compliance officers
found respirable quartz in 6,779 personal samples (8-hr TWA) taken in
255 industries that were targeted for inspection (excluding mining and
agriculture). In 48% of the industries, average overall exposure exceeded
the PEL for respirable quartz [Freeman and Grossman 1995]. Linch et
al. [1998] applied an algorithm to OSHA compliance data from the period
1979-1995 and County Business Patterns 1993 data [Bureau of the
Census 1993] to estimate the percentage of workers by industry (excluding
mining and agriculture) exposed to defined concentrations
of respirable crystalline silica (e.g., >0.05 mg/m3)
in 1993. Area samples and samples involving complaints to OSHA were
excluded from the analysis. Although data limitations could have resulted
in underestimating or overestimating the number of workers exposed,
the authors found 5 three-digit standardized industrial classification
(SIC) codes in which an estimated number of workers were exposed to
concentrations at least 10 times the NIOSH REL:
SIC
|
No.
workers
|
174
|
Masonry
and plastering |
13,800
|
1.8%
|
162
|
Heavy
construction |
6,300
|
1.3%
|
172
|
Painting
and paper hanging |
3,000
|
1.9%
|
332
|
Iron
and steel foundries |
800
|
0.3%
|
347
|
Metal
services |
400
|
0.2%
|
Additional
three-digit SICs had a number of workers with crystalline silica exposures
that were two or five times higher than the NIOSH REL [Linch et al.
1998].
Table
2 lists the main industries around the world in which silica exposure
has been reported. Virtually any process that involves movement of earth
or disturbance of silica-containing products such as masonry and concrete
may expose a worker to silica (see Table 3
for uses of industrial silica sand and gravel). Table
4 presents, from selected States, the most frequently recorded occupations
of U.S. residents aged 15 or above whose death certificates list silicosis
as an underlying or contributory cause of death [NIOSH 1996a]. In addition,
Table 5 lists published case reports of
silicosis in workers from other industries and occupations.
2.4
Sampling and Analytical Methods
Historically,
several methods have been used to measure worker exposure to airborne
crystalline silica (quartz, cristobalite, or tridymite). These methods
differ primarily in the analytical technique employed, although they
all rely on a collection procedure that uses a cyclone for size-selective
sampling. Airborne samples are collected using a cyclone to remove nonrespirable
particles and an appropriate filter medium (e.g., polyvinyl chloride)
to retain the respirable dust fraction. Preparation of the sample for
crystalline silica determination differs depending on the type of analytical
technique used. One of three analytical techniques is typically used
for the quantitative determination of crystalline silica: X-ray diffraction
(XRD) spectrometry, infrared absorption (IR) spectrometry, or colorimetric
spectrophotometry. XRD and IR are the most common techniques used for
crystalline silica analyses. The quantitative limit of detection for
these methods ranges from 5 to 10 µg per sample; but the accuracy
is poor, particularly at the low filter loadings (<30 µg
per sample) that are typically collected when workplace concentrations
of airborne crystalline silica are near the NIOSH REL of 50 µg/m3
(or 0.05 mg/m3).
2.4.1
Sampling Methods
Current
sampling methods for crystalline silica involve the use of a cyclone
attached to a filter cassette to collect the respirable fraction of
the airborne particulate. To minimize measurement bias and variability,
these samplers should conform to the criteria of the International Organization
for Standardization (ISO), the European Standardization Committee (CEN),
and the American Conference of Governmental Industrial Hygienists (ACGIH)
for collecting particles of the appropriate size [ISO 1991; CEN 1992;
ACGIH 2001]. Also, the cyclone should exhibit sufficient conductivity
to minimize the electrostatic effects on particle collection. Cyclones
typically used for crystalline silica measurements include the Dorr-Oliver
10-mm nylon cyclone and the Higgins-Dewell conductive cyclone. These
cyclones have been evaluated for their compliance with the ISO/CEN/ACGIH
respirable aerosol sampling convention. Flow rates of 1.7 L/min for
the Dorr-Oliver cyclone and 2.2 L/min for the Higgins-Dewell cyclone
provide minimum bias for a wide range of particle size distributions
that are likely to occur in the workplace [Bartley et al. 1994]. The
Dorr-Oliver 10-mm cyclone is required by MSHA, and the Higgins- Dewell
cyclone is used in the United Kingdom. Recently, the GK2.69 cyclone
[Kenny and Gussman 1997] has become available with a sampling rate equal
to 4.2 L/min. The GK2.69 cyclone is expected to be at least as adequate
as the nylon cyclone for conforming to the ISO/ CEN/ACGIH respirable
aerosol sampling convention; and it may be preferable for silica sampling
since it is conductive, has well-defined dimensional characteristics,
and can be used at higher flow rates for better mass sensitivity. Because
each type of cyclone exhibits specific particle collection characteristics,
the use of a single cyclone type for each application would be advisable
until evidence becomes available indicating that bias among cyclone
types will not increase laboratory-to-laboratory variability.
Cyclones
and filter cassettes should be leak tested to avoid gross failure in
the field. The cyclones may be tested using a simple pressure- (or vacuum-)
holding test. The filter cassette should also be checked for leakage
while attached to the cyclone. Two approaches to testing the cassettes
have been used. A micromanometer has been used to measure the pressure
drop across a single type of cassette and compare it with the average
pressure drop across well-sealed cassettes [Van den Heever 1994]. An
alternative approach uses a particle counter to measure the penetration
of submicrometer ambient aerosol through the cassette, with the percentage
of penetration serving as an indicator of leakage [Baron 2001]. Measurement
of cassette leakage by several laboratories indicates that significant
leakage can occur in certain situations. Cassettes should be assembled
using a press, and they should be routinely checked for leakage.
2.4.2
Analytical Methods
2.4.2.1
XRD Spectrometry
XRD methods
used for crystalline silica determination include NIOSH Method 7500
[NIOSH 1998], OSHA Method ID-142 [OSHA 1996], MSHA Method P-2 [MSHA
1999], and the Health and Safety Executive (HSE) Method for the Determination
of Hazardous Substances (MDHS) 51/2 [HSE 1988]. Details of these methods
are presented in Table 6. XRD is capable
of distinguishing the three prevalent polymorphs of crystalline silica
(quartz, cristobalite, and tridymite) and can simultaneously analyze
for each polymorph while correcting for interferences that may be present
on the sample [Madsen et al. 1995]. Although most samples collected
in industrial workplaces are relatively free of mineral interferences,
an XRD scan of some samples should be performed to ensure the absence
of interferences through confirmation of the correct peak ratios for
the three largest peaks.
2.4.2.2
IR Spectrometry
IR methods
used for crystalline silica determination include NIOSH Methods 7602
and 7603 [NIOSH 1994a,c], MSHA Method P-7 [MSHA 1994], and MDHS 37 and
38 [HSE 1987, 1984]. Details of these methods are presented in Table
7. Although IR is less specific than XRD (IR methods cannot readily
distinguish crystalline silica polymorphs), the technique is less expensive
and can be optimized for measuring quartz
in well-defined sample matrices [Madsen et al. 1995; Smith 1997; Hurst
et al. 1997]. Samples that contain other silicates (such as kaolinite)
and amorphous silica can present interferences in the analyses. Also,
a potential for bias exists when correcting for matrix absorption effects,
with an increasing risk of bias at lower quartz concentrations.
2.4.2.3
Colorimetric Spectrophotometry
The NIOSH
colorimetric method for crystalline silica (NIOSH Method 7601) [NIOSH
1994b] is significantly less precise than IR or XRD methods. The colorimetric
analytical method exhibits a nonlinear dependence on the mass of crystalline
silica present [Eller et al. 1999a]. The linear range of the method
is limited, and the blank values for samples can be high (20 µg
silica or higher) [Talvitie 1951, 1964; Talvitie and Hyslop 1958]. High
intra-laboratory variability of the method (up to twice that of IR or
XRD) has been noted in studies conducted in the Proficiency Analytical
Testing Program (PAT) [Shulman et al. 1992]. The colorimetric method
cannot distinguish between silica and silicates, since it is based on
the measurement of silicon.
2.4.2.4
Factors Affecting the Sensitivity and Accuracy of Analytical Techniques
Samples
prepared for XRD analyses are measured directly (MDHS 51/2) or are redeposited
onto 25-mm silver membrane filters (NIOSH Method 7500 and OSHA Method
ID-142). IR samples can be measured directly (MDHS 37), redeposited
on an acrylic copolymer membrane filter (NIOSH Method 7603 and MSHA
Method P-7 ), or incorporated into a potassium bromide (KBr) pellet
(NIOSH Method 7602 and MDHS 38). Techniques used for redepositing the
sample (both IR and XRD) are difficult to perform at low sample loadings
and require the laboratory analyst to demonstrate good intra-laboratory
reproducibility. However, these techniques can be optimized by preparing
multiple working standards from multiple suspensions of calibration
standards and by ensuring that the sample is redeposited evenly as a
thin layer on the filter. No statistically significant difference has
been observed between ashing the filter (muffle furnace and low-temperature
asher) and dissolving the filter by tetrahydrofuran before redepositing
the sample [Eller et al. 1999a].
The instrument
response of all three analytical techniques is influenced by the size
of the particles in the sample. With XRD, the diffraction intensity
(as measured by peak height) can vary considerably with particle size,
with smaller particles showing lower intensities [Bhaskar et al. 1994].
The sensitivity of IR analyses decreases with increasing particle size.
The colorimetric method requires the use of a precisely timed heating
step with phosphoric acid to digest amorphous silica and silicates during
sample preparation, causing a possible loss of some small crystalline
silica particles [Eller et al. 1999a]. Since particle size affects the
sensitivity of all three analytical techniques, the particle size distribution
of the calibration standard should closely match the size of the particles
retained on the collected sample.
For all
analytical techniques, strict adherence to standardized procedures is
necessary to produce accurate results. Specifically, appropriate calibration
of the technique has been shown to be critical in the accurate measurement
of crystalline silica [Eller et al. 1999b]. Also, it is essential that
only standard reference materials from the National Institute of Standards
and Technology (NIST) (for which particle size and phase purity has
been established) be used to prepare calibration curves for quartz (1878a)
and cristobalite (1879a) [Eller et al. 1999a]. No standard reference
material for tridymite is available, since this silica polymorph rarely
exists in the workplace. However, a well-characterized sample of tridymite
of the appropriate particle size is available from the U.S. Geological
Survey* and can be used as a reference standard.
Direct-on-filter
techniques are used by the United Kingdom, the European Union, and Australia
[Madsen et al. 1995]. These techniques require less time and labor than
others and are amenable to both XRD and IR analyses [Lorberau et al.
1990]. However, direct-on-filter techniques are affected by the manner
in which the particles are deposited on the filter sample (particle
deposition may be nonuniform). Thus care must be taken when choosing
the area of the filter to measure so that results can be compared with
other methods. Sample overloading is possible for a sample collected
over a full work shift.
*Tridymite
reference material may be obtained from Dr. Stephen A. Wilson, U.S.
Geological Survey, Box 25046, MS 973, Denver, CO 80225 (telephone: 303-236-2454;
FAX: 303-236-3200; e-mail: swilson
@usgs.gov; Web site: http://minerals.cr.usgs.gov/geochem).
2.4.3
Feasibility of Measuring Crystalline Silica at Various Concentrations
The efficacy
of sampling and analytical methods for measuring concentrations
of hazardous materials may be established using the NIOSH accuracy criterion
[NIOSH 1995b], which requires better than 25% accuracy at concentrations
of expected method application. Accuracy, as a percentage of true concentration
values, is defined in terms of an interval expected to contain 95% of
(future) measurements. To account for uncertainty in method evaluations,
the upper 95% confidence limit on
the accuracy is measured and used in the criterion. Generally, the accuracy
of a method is measured over a range of concentrations bracketing the
OSHA PEL. Use of a range of measurements means that accuracy is assured
both at concentrations below the PEL (for possible use in action level
determinations) and, more significantly, at the PEL (where method results
must be legally defensible).
NIOSH
has evaluated both the XRD silica method (NIOSH Method P&CAM 259,
the forerunner to NIOSH Method 7500) [NIOSH 1979b] and an IR silica
method (MSHA Method P-7, equivalent to NIOSH Method 7603) in a collaborative
test among several laboratories [NIOSH, BOM 1983]. One result of the
test was that the accuracy of the methods was estimated by evaluating
the intralaboratory variability at various filter loadings. The concentrations
to which these filter loadings correspond depend on the flow rate of
the pre-sampler used. Experimental conditions and results relevant to
the derivation of these estimates are summarized in Tables
8 and 9. The results of
the collaborative tests indicate that both the XRD and IR methods tested
meet the NIOSH accuracy criterion [NIOSH 1995b] over the range of filter
loadings measured. Currently, OSHA uses the 10-mm nylon cyclone at a
sampling rate of 1.7 L/min for sampling crystalline
silica. The concentrations relevant to the collaborative test conditions
are listed in Tables 10 and
11 and assume an 8-hr sampling
period. As indicated in Tables
10 and 11, the traditional
nylon cyclone meets the accuracy criterion over a range of concentrations
bracketing 100 µg/m3.
Since
the GK2.69 cyclone is expected to conform to the ISO/CEN/ACGIH respirable
aerosol sampling convention, the NIOSH intralaboratory collaborative
tests can be used to establish confidence
limits on its accuracy. The results of the collaborative tests indicate
that the GK2.69 cyclone meets the accuracy criterion over a range of
concentrations bracketing 50 µg/m3, as illustrated
in Tables 10 and 11.
|