Environmental Health, Safety and Risk Management
University of Alaska Fairbanks
A GUIDE TO WORKING WITH
SILICA
December 2010
1
Overview
 Silica: crystalline vs. amorphous
 Part I: Crystalline Silica
 Health Hazards
 Exposure Limits
 Exposure Monitoring
 Ways to Reduce Exposure
 Online Resources
2
Overview (cont.)
 Part II: Amorphous Silica
 Examples
 Exposure Limits
 Synthetic Vitreous Fibers
 Health Hazards
 Exposure Limits
 Engineering Controls
 Internet Resources
3
Silica
 Silica (SiO2, silicon dioxide) is
one of the most common
minerals in the earth’s crust
and is a major component of
sand, rock and mineral ores
 Silica is used in the
manufacturing of a variety of
products from kitty litter to
fiber optic cables to cosmetics
and food additives
4
Silica (cont.)
 Silica occurs as both
crystalline (structured)
and non-crystalline
(amorphous) forms.
Diagram source: http://www.osha.gov/SLTC/etools/silica/silicosis/silicosis.html
5
Part I: Crystalline Silica
6
Silica: Crystalline
 Has a diagnostic X-ray diffraction*
pattern
*analytical technique which reveals
information about the crystallographic
structure, chemical composition, and
physical properties of materials
 Most common types
(formed under different
pressures and temperatures)
 Quartz (alpha & beta)
 Cristobalite
 Tridymite
7
Silica, Crystalline,
Health Hazards,
Silicosis
 Silicosis is a disabling, nonreversible and
sometimes fatal lung disease caused by
overexposure to respirable* dust containing
crystalline silica
*respirable means deposited in the gas-exchange
region or deepest part of the lungs
 Overexposure to dust that contains respirable
crystalline silica can cause scar tissue to form
in the lungs, which reduces the lungs’ ability
to extract oxygen from the air
8
Silicosis (cont.)
 Inhalation of crystalline silica particles
has been associated with other
diseases, such as bronchitis,
emphysema, and tuberculosis
 Some studies also indicate an
association with lung cancer,
immunologic disorders, autoimmune
diseases, renal disease and stomach and
other cancers
9
Silicosis (cont.)
 There are 3 types of silicosis:
 CHRONIC silicosis, which usually occurs after
10 or more years of exposure at relatively low
concentrations. This is the most common
form of silicosis
 ACCELERATED silicosis, which develops 5 to
10 years after the first exposure
 ACUTE silicosis, which develops after exposure
to high concentrations of respirable crystalline
silica and results in symptoms within a period
ranging from a few weeks to 5 years after the
initial exposure
10
Silicosis (cont.)
 Common symptoms
 shortness of breath following physical exertion
 severe cough
 fatigue
 loss of appetite
 chest pains
 fever
 cyanosis (bluish skin)
11
Silicosis:
Who is at risk?
Do you work in:
 construction
 manufacturing and use of
abrasives (including sand
blasting denim to “age” it!)
foundry work
 dental laboratories
ceramics, clay, and pottery
 manufacturing of soaps
stone cutting
and detergents
glass manufacturing
agriculture
shipyards
railroad
 mining, quarrying







12
Silicosis, Special Emphasis
Program
 In 1996, OSHA established a Special
Emphasis Program (SEP) for Silicosis,
which provided guidance for
targeting inspections of worksites
with employees at risk of developing
silicosis
13
Crystalline Silica, National
Emphasis Program
 Effective January 24, 2008, OSHA
implemented a National Emphasis Program
(NEP) to identify, reduce, and eliminate the
health hazards associated with occupational
exposure to crystalline silica
 Why? Because silica-related illnesses and
fatalities continue to occur
14
National Emphasis Program
(cont.)
 The NEP is a written compliance directive
incorporating updated information and
policies and procedures adopted since 1996
 Expands 1996 Special Emphasis Program
 Provides updated research results on silica
exposure hazards
15
National Emphasis Program
(cont.)
 Details inspection procedures, including
follow-up inspections where overexposure
found
 Addresses targeting of worksites and
provides updated NAICS (North American
Industrial Classification System ) codes for
industries with worker exposure to
crystalline silica
16
National Emphasis Program
(cont.)
 Explains calculation of PELS (Permissible
Exposure Limits) in General Industry,
Construction, and Shipyard
 Establishes program evaluation procedures
 Provides for Regional and Area Office outreach
programs
17
OSHA Crystalline Silica Exposure
Standards
 General Industry: 29 CFR 1910.1000
 Construction: 29 CFR 1926.55(a)
 Shipyard: 29 CFR 1915.1000
18
Crystalline Silica Exposure
Limits, OSHA PELs
 OSHA regulates silica exposure using the
permissible exposure limit (PEL), which is the
maximum amount of airborne dust an
employee may be exposed to during a full work
shift.
The PEL is dependent on
 the % silica content of the dust from a sample
collected over an 8 hour time period
 whether it is respirable or not
 the composition (quartz, cristobalite or tridymite)
19
Crystalline Silica Exposure
Limits: Other Guidelines
 ACGIH (American Conference of
Governmental Industrial Hygienists)
Threshold Limit Value (TLV) for respirable
quartz (alpha) and cristobalite:
0.025 mg/m3
 NIOSH (National Institute for Occupational
Safety and Health) Recommended Exposure
limit (REL) for respirable crystalline silica:
0.05 mg/m3
20
Crystalline Silica
“Bottom Line” Message
The more crystalline silica there is in the dust,
the less of the dust you should breathe
21
Crystalline Silica: Exposure
Monitoring
 Exposure monitoring (air sampling)
 Measures worker exposures to respirable crystalline
silica and helps select appropriate engineering
controls and respiratory protection
 Monitors effectiveness of the controls
 Determines if exposures are in excess of permissible
exposure levels (PELs)
 Performed when an employee is experiencing
symptoms or health effects that may be attributable
to crystalline silica
 If you believe you need to have an exposure
assessment conducted, contact EHS&RM at
474-6771 or 474-5197
22
Ways to Reduce Exposure to
Crystalline Silica:
Engineering Controls
 Substitute materials that have no crystalline
silica
 Locate employees as far as possible from
dust-generation source
 Isolate employees OR the source
 Control rooms
 Enclosures
 Barriers
23
Ways to Reduce Crystalline
Silica Exposure: Engineering
Controls (cont’d)
 Use local exhaust ventilation (LEV systems)
 Use tools with dust-collecting systems
 Use wet methods





Cutting
Chipping
Drilling
Sawing
Grinding
 Clean surfaces with HEPA vacuums or wet
sweeping—no compressed air!
24
Ways to Reduce Crystalline
Silica Exposure: Respiratory
Protection
 And if other controls are not sufficient—
Use Proper Respiratory Protection
Source: NIOSH Publication No. 2004-108: Silicosis: Learn the Facts!
25
Ways to Reduce Exposure:
Respiratory Protection (cont’d)
Source: NIOSH Publication No. 2004-108: Silicosis: Learn the Facts!
26
Ways to Reduce Exposure:
Respiratory Protection
(cont’d)
 When respirators are used, the employer
must establish a comprehensive respiratory
protection program, required in the OSHA
respiratory protection standard (29 CFR
1910.134 and 1926.103) and as outlined in
the NIOSH Guide to Industrial Respiratory
Protection (NIOSH 1987a)
 The respiratory protection program should be
evaluated regularly by the employer
27
Ways to Reduce Exposure:
Respiratory Protection
(cont’d)
 Important elements of this standard are
 periodic environmental monitoring
 regular training of personnel
 selection of proper NIOSH-approved respirators
 an evaluation of the worker's ability to perform
the work while wearing a respirator
 respirator fit testing
 maintenance, inspection, cleaning, and storage of
respiratory protection equipment.
28
OSHA Internet Information on
Crystalline Silica
http://www.osha.gov/pls/oshaweb/owadisp.show_d
ocument?p_table=DIRECTIVES&p_id=3790
http://www.osha.gov/SLTC/silicacrystalline/index.html
http://www.osha.gov/dte/library/materials_library.html#silica
www.cdc.gov/niosh/topics/silica
www.cdc.gov/niosh/docs/2005-100/default.html
29
Part II: Amorphous Silica
30
Silica: Amorphous
 Cannot distinguish by
X-ray diffraction
 Under certain
conditions, heating
amorphous silica results
in conversion to the
crystalline form
31
Silica: Amorphous (cont.)
 Common examples of naturally
occurring amorphous silica:
 Diatom cell walls (a prolific group
of algae that live in oceans,
freshwater and soils)
 Volcanic glass
32
Silica: Amorphous (cont.)
 Common examples of products
containing amorphous silica:
 Diatomaceous earth
 Window glass
 Synthetic Vitreous Fibers such as
fiberglass
33
Silica: Amorphous, Health
Hazards
 Studies have found amorphous silica to be
biologically inert when ingested and inhaled,
with the exception of certain synthetic
vitreous fibers (to be discussed later)
 Studies have found that exposure to
amorphous silica is not associated with the
development of silicosis
34
Amorphous Silica
Exposure Limits, Regulated
 The PELs (permissible exposure limits) for
amorphous silica is found in the following
regulations:
 General Industry: 29 CFR 1910.1000 Z-3
 Construction: 29 CFR 1926.55 Appendix A
 Shipyard : 29 CFR1915.1000
35
Amorphous Silica
Exposure Limits, Regulated: PEL
 The PEL for amorphous silica, including natural
diatomaceous earth
80 mg/m3 as % silica content of the dust from a
sample collected over an 8 hour time period
 The PEL applies to General Industry, Construction
and Shipyard Standards
36
Amorphous Silica: Exposure
Limits, Other Guidelines,
REL & TLV
 The NIOSH REL for amorphous silica is
6 mg/m3
 ACGIH has withdrawn the TLV for amorphous
silica largely because the TLV was derived
from studies on diatomaceous earth , and did
not take into account the crystalline silica
content of the diatomaceous earth
37
Amorphous Silica:
Synthetic Vitreous Fibers
 Synthetic vitreous fibers (SVF) are a
group of fibrous inorganic materials
that contain aluminum or calcium
silicates and other trace oxides and
metal
 SVFs are made from rock, slag, clay,
or glass
 They are sometimes called
Synthetic Mineral Fibers (SMF)
38
Amorphous Silica:
Synthetic Vitreous Fibers
(cont.)
Source: “Toxicological Profile for Synthetic Vitreous Fibers”, U.S.
Department of Health and Human Services, Agency for Toxic
Substances and Disease Registry, September 2004
39
Amorphous Silica:
Synthetic Vitreous Fibers(cont.)
 To be considered a fiber, the particle must:
 be at least 5 micrometers long (1 micrometer
equals 1/1,000,000 of a meter and has the
symbol μm)
 have an aspect ratio of at least 3 to 1 or
sometimes 5 to 1 (the aspect ratio is the ratio
of a fiber’s length to its diameter).
40
Amorphous Silica:
Synthetic Vitreous Fibers(cont.)
 The diameter of a fiber is an important property
because very thin fibers are more easily
suspended in air than thick fibers, and they can
be breathed in and deposited deep in the lungs.
 Only very thin fibers with diameters < 3 μm are
able to be breathed into the lower respiratory
tract of humans.
 In general, glass wool, rock wool, slag wool, and
refractory ceramic fibers have the smallest
diameters, while continuous filament glass fibers
have the largest diameters.
41
Amorphous Silica:
Synthetic Vitreous Fibers
(cont.)
 The primary uses of synthetic vitreous fibers
are:
 heat and sound insulating purposes
 reinforce other materials
 filtration materials
42
Amorphous Silica:
Synthetic Vitreous Fibers,
Health Hazards
 Results from animal experiments have
led to conservative classifications of
certain synthetic vitreous fibers as
possible human carcinogens
43
Amorphous Silica:
Synthetic Vitreous Fibers,
Health Hazards (cont.)
Source: http://www.osha.gov/SLTC/syntheticmineralfibers/table2.html
44
Synthetic Vitreous Fibers:
Exposure Limits, Regulated,
PELs
 General Industry : 29 CFR 1910.1000 Table Z-3
"Inert or Nuisance Dust”
and
 Shipyard: 29 CFR 1915.1000 Table Z
Mineral Wool and Fibrous Glass
 Respirable Fraction: 5 mg/m3
 Total Dust: 15 mg/m3
45
Synthetic Vitreous Fibers:
Other Guidelines: ACGIH
 Glass wool, rock wool and slag wool fibers
and special purpose glass fibers (confirmed
animal carcinogen with unknown relevance to
humans):
1 fiber/cc (cubic centimeter)
 Refractory ceramic fibers (suspected human
carcinogen)
0.2 fibers/cc
46
Synthetic Vitreous Fibers:
Other Guidelines: ACGIH (cont.)
 Continuous filament glass fibers (not
classifiable as a human carcinogen):
 Respirable
1 fiber/cc
 Inhalable (deposited anywhere in the respiratory
tract)
5 mg/m3
47
Synthetic Vitreous Fibers:
Other Guidelines: NIOSH
 TWA (8 hour average)
 Fibers with diameter  3.5 µm and length 10 µm
3 fibers/cc
 Total dust
5 mg/m3
48
Engineering Controls for
Synthetic Vitreous Fibers
 Local exhaust ventilation
 Wet methods for activities such as installation,
removal, cutting, grinding, sawing
 Use of plastic sheeting to enclose or encapsulate
 Clean up with a high-efficiency particulate air
(HEPA) vacuum cleaner followed by wet wiping
 And if other controls are not sufficient—
Use Proper Respiratory Protection
49
OSHA Internet Information:
Synthetic Vitreous Fibers
www.osha.gov/SLTC/syntheticmineralfibers/index.html
www.atsdr.cdc.gov/substances/toxsubstance.asp?toxid=185
www.cdc.gov/niosh/docs/2006-123/
50