Download Health Effects of Silica Overexposure

Module 9: Management of Silica
Exposure
#outcomes
Upon successful completion of this module you will be able to:
1. Describe in detail what silica is and how it relates to the related terms silicon
and silicate.
2. Explain the similarities and differences between crystalline and amorphous
silica.
3. List the workplaces and work activities that present the highest risk of
worker overexposure to crystalline silica dust.
4. Describe the health effects of overexposure to crystalline silica dust.
5. Access and interpret the occupational exposure limits for various silicates.
6. Identify key resources that you will use to develop safe work procedures and
exposure control plans for silica-related work.
/outcomes
What is Silica?
Silica, more properly called silicon dioxide, SiO2, is certainly one of the most
commonly occurring minerals on earth. You are very likely to have some very near
you right now. But because it occurs in many forms and is used in many workplaces
and because some of the words do sound alike (silicate, silicon, siloxane and
silicone) there is still confusion in workers and employers as to which presents a
risk to worker health and which does not.
Perhaps the easiest way to learn about silica is to first do a basic introduction to the
chemistry and geology of the mineral and then to have a first look at which forms of
silica are subject to OHS laws.
#reflection
Exercise 1
Open these links and use them to complete this exercise:
Silicon (Wikipedia)
Silicon Dioxide (Wikipedia)
Silica (Minerals Education Coalition)
Silicate (Wikipedia)
1. __________________ is a chemical element with symbol _______
#reveal
Silicon is a chemical element with symbol Si.
/reveal
2. ___________________________ is the eighth most common element in the universe by
mass, but ____________________________________________ in the Earth's crust.
#reveal
Silicon is the eighth most common element in the universe by mass, but very rarely
occurs as a pure element in the Earth's crust.
/reveal
3. Silica is made mostly from _________________ (______).
#reveal
Silica is made mostly from silicon dioxide (SiO2).
/reveal
4. Silicon dioxide, also known as__________, is a _____________________ that is an
_________________ with the chemical formula SiO2.
#reveal
Silicon dioxide, is a chemical compound that is an oxide of silicon with the chemical
formula SiO2. (For those of you that do not have a chemistry background, many
elements such as silicon and iron rapidly combine with oxygen to form an oxide.
When iron combines with oxygen it forms a chemical compound called iron oxide,
much more commonly known as rust.)
/reveal
5. Not to be confused with the chemical element silicon, _____________ are polymers
that include any inert, synthetic compound made up of repeating units of siloxane,
which is a chain of alternating ___________atoms and oxygen atoms.
#reveal
Not to be confused with the chemical element silicon, silicones are polymers that
include any inert, synthetic compound made up of repeating units of siloxane, which
is a chain of alternating silicon atoms and oxygen atoms.
/reveal
6. In geology and astronomy, the term _____________________ is used to denote types of
rock that consist predominantly of _______________________. Silicates constitute the
majority of Earth's crust, as well as the other terrestrial planets, rocky moons, and
asteroids. _________ , _________________ and thousands of minerals are examples of
silicates.
#reveal
In geology and astronomy, the term silicates is used to denote types of rock that
consist predominantly of silicon oxides. Silicates constitute the majority of Earth's
crust, as well as the other terrestrial planets, rocky moons, and asteroids. Sand
(silica sand), Portland Cement and thousands of minerals are examples of silicates.
(Just be clear: Concrete is not the same thing as cement. Concrete is a mixture of
Portland Cement and an aggregate, often silica sand or a silicate rock. As engineers
and geologists say, it is a concretion.)
/reveal
7. Have you already studied some silicates as hazardous materials in this course? If
so, which ones? (Hint: Mg3(Si2O5)(OH)4 = chrysotile)
#reveal
Yes. All forms of asbestos are silicate minerals in that they contain silicon atoms.
Perhaps it is not surprising that the health effects of overexposure to asbestos are
very similar to those associated with overexposure to dusts of other silicates.
/reveal
/reflection
Wow! No wonder some people get confused: silicon is one of the elements that make
up silica and siloxane. Silicone, which is a sticky polymer and which has absolutely
nothing to do with this module, actually sort of does because it contains siloxane
which itself contains silica. A mineral that contains mostly silica is called a silicate
mineral. All forms of asbestos are silicate minerals.
And we now have to add to the complicated language:
#reflection
Exercise 2
In the WorkSafeBC Guidelines scroll way down and just past G5.59 you will find the
Table of Exposure Limits for Chemical and Biological Substances. Find the listings
for silica in the Table.
1. Note that some of the listings are for silica, amorphous and one is for silica,
crystalline.



Watch this short video that explains the difference between amorphous and
crystalline solids: (lots more information available on the web if you want a
deeper understanding).
Note the relative OEL values: the 8 hour TWA limit for crystalline silica is
much lower (0.025 mg of crystalline silica per m3 of air breathed) than any of
the limits for amorphous silica (lowest is 1. 5 mg/m3).
It is correct to say that all forms of crystalline silica are more toxic than all of
the forms of amorphous silica.
#reveal
 So………..we now understand crystalline silica sand is used to make some
kinds of glass by transforming the silica to an amorphous form and so it is the
mining and processing of the silica sand that presents the higher
occupational disease risk than does the making of the glass.
 Noted
 Yes; crystalline silica is more toxic than amorphous silica: it will produce
disease at a smaller dose (therefore the smaller OEL); it is NOT correct to say
that crystalline silica is more hazardous or riskier: toxic vs hazard vs risk are
3 completely different things in OHS: if you are not 100% clear on this please
discuss with your tutor.
/reveal
2. The next terms to note are related to diatomaceous earth, a form of amorphous
silica:
 Diatomaceous earth, also known as DE, is a naturally occurring, soft, siliceous
sedimentary rock that is easily crumbled into a fine white to off-white
powder. Diatomaceous earth consists of fossilized remains of diatoms, a type
of hard-shelled protozoa (much more on the web on DE if you are
interested).
 Depending on what DE is being used for in the workplace it may be need to
be heat treated (calcined). Calcined DE is more toxic than uncalcined DE
because heating converts amorphous silica to crystalline silica.
 Read this information on the difference between calcined vs non-calcined
diatomaceous earth.
#reveal
DE is used in many applications but often for its ability to be used as a fine filtering
medium. There is a lot of information on the web on the use of DE in the workplace.
/reveal
3. Finally, note the terms Total, Inhalable and Respirable:
These are standard occupational hygiene terms to describe the size of particles and
where in the respiratory tract that they can cause illness.
Total means any size of particle that can be taken into the respiratory tract (RT) and
which can cause illness throughout the RT. Total particulates range in size from
about 100 microns to much smaller.
Inhalable means the particles are small enough to not be removed by the defense
mechanisms of the upper respiratory tract and that can reach the middle parts of
the RT.
Respirable means the smallest particles that can escape all of the RT defenses and
move right to alveoli, the parts of the lungs where gas exchange occurs (much more
on this in OCHS 4320).
#reveal
 These terms are fundamental occupational hygiene/occupational health
terms related to diseases caused by inhalation of various chemical agents in
particle form.
 Just for reference, the diameter (not length) of an average human hair is
about 5–70 microns: a rule of thumb is that a particle must be no larger than
5–10 microns in diameter to be respirable, that is, have any significant
chance of avoiding the defense mechanisms of the RT and making it all the
way to the air sacs/alveoli.
/reveal
/reflection
Silica as a Workplace Material
Silica in the workplace and as a hazardous material has, not surprisingly, now that
you know about silicate minerals, many similarities to asbestos:





Generally used as part of a manufactured material (e.g., concrete)
Crystalline form causes pneumoconiosis and RT cancer
Was and still is extensively used in the workplace, and will be for many years
Used in many different types of products in many different kinds of
workplaces
Generally only a health concern when products containing the material are
disturbed and create small airborne particles that can be inhaled
Silica is used extensively in many products and workplaces. By weight, it is used as
simple silica sand or as an aggregate in concrete. Silica-containing dust is produced
practically any time rock is crushed or broken.
Review OSHA’s list of Probable Uses of Silica.
Were we correct in saying earlier that “you are very likely to have some very near
you right now”?
#reflection
Exercise 3
Significant numbers of workers are believed to be exposed to potentially harmful
levels of silica in Canada. Read this newsletter from CAREX on crystalline silica
exposure in Canada:
1.


Approximately how many workers are exposed to silica in Canada?
Which industry are the majority of these workers employed in?
#reveal
 About 380,000……..so chances are good you will eventually practice OHS in
an industry in which at least some workers are exposed.
 Construction, where the building construction industry (versus, say, road
construction) and trade contractors account for about 54% of exposed
workers.
/reveal
2. What are the 5 largest exposure groups by industry and what percentage of
workers in those groups is believed to be exposed to potentially harmful levels of
silica?
#reveal
/reveal
/reflection
Health Effects of Silica Overexposure
We have already noted that the health effects associated with overexposure to silica
will be the result of:



The form of the silica (amorphous or crystalline)
The size of the particle
The dose of the exposure
And we have noted that because asbestos is a silicate mineral, the health effects
associated with overexposure to silica from other materials will be similar.
Silicosis is recognized as an occupational disease in all jurisdictions in Canada. The
following is typical:
3 Pneumoconiosis:
(a)
Silicosis
Where there is exposure to airborne silica dust including metalliferous
mining and coal mining.
There is an absolutely huge amount of information on the web about the health
effects associated with inhalation of silica and research in this area continues to be
very active. Here we will do a review of the basics—you will be introduced to silicarelated diseases in much more detail in OCHS 4360.
#reflection
Exercise 4
Read this Work Safe Alberta bulletin on crystalline silica at the work site and
complete this exercise:
1. ____________________________ silica dust particles that are _____________________ to be
inhaled into the lungs can cause a number of health problems, including
_______________, __________________cancer, chronic obstructive pulmonary disease and
emphysema, as well as pulmonary tuberculosis.
#reveal
Crystalline silica dust particles that are small enough to be inhaled into the lungs can
cause a number of health problems, including silicosis, lung cancer, chronic
obstructive pulmonary disease and emphysema, as well as pulmonary tuberculosis.
/reveal
2. Silicosis is the most common disease associated with overexposure to respirable
crystalline silica dust. Watch this silica exposure video to learn more about what
silicosis is.
#reveal
Silicosis is a kind of pneumoconiosis, as is asbestosis. Pneumoconiosis means “dusty
lungs.” On X-ray, the kinds of scars that form in response to crystalline silica
entering the air sacs/alveolar region appear as small dark areas—the lungs appear
dusty.
/reveal
3. Silicosis is caused when crystalline silica particles ______________________________ in
diameter are inhaled and deposited in the lungs. This is known as
_________________________ silica. Lung tissue reacts by developing _________________________
around the trapped silica particles. If the lumps and scar tissue grow _________________,
breathing becomes difficult and death may result.
#reveal
Silicosis is caused when crystalline silica particles less than 10 microns in diameter
are inhaled and deposited in the lungs. This is known as “respirable” silica. Lung
tissue reacts by developing lumps and scarring around the trapped silica particles. If
the lumps and scar tissue grow too large, breathing becomes difficult and death may
occur. (Note that like asbestosis, simple silicosis can then range from mild, in which
the number and size of the scars is too small to significantly affect breathing to lifethreatening silicosis.)
/reveal
4. Factors that influence the development of silicosis include:





particle ________
particle ________
_____________ a person is exposed to silica dust
the __________________ of silica dust in the air
_______________ susceptibility
#reveal
 Particle type
 Particle size
 How long a person is exposed to silica dust
 The concentration of silica dust in the air
 Individual susceptibility (this one is the most difficult to manage—we have
no idea what makes some people more susceptible than others but in a group
of workers with every other aspect of silica exposure the same, most will not
get silicosis and a few will)
/reveal
5. Read about the 3 forms of silicosis that can develop.
 Is silicosis a disease that occurs quickly after exposure or does it have a long
latency period?
 What kinds of problems will silicosis cause a worker that has the disease?
#reveal
 Normally and in the vast majority of workers a long latency period (which
can create all kinds of issues for our disease prevention programs and for
workers compensation benefits—much more on this in OCHS 4360).
 As the silicosis progresses, coughing develops and breathing becomes
difficult. Persons with silicosis have an increased risk of contracting
respiratory infections such as pneumonia and tuberculosis. (Be very careful
here: silicosis is not lung cancer. Many workers continue to work until
retirement with milder cases of silicosis. This is not the case for silica-related
lung cancer.)
/reveal
6. Review this Carex Canada profile on silica.
 Summarize the current information regarding silica as a carcinogen.
#reveal
Crystalline silica is a confirmed human carcinogen. In the technical language in OHS
we say that it is an IARC Group 1 carcinogen. This is listed as a Notation in the OEL
listings you reviewed for crystalline silica. (For further information on the IARC
system of classifying carcinogens see this IARC monograph.)
/reveal
/reflection
Silica-Related Law
Silica-related OHS law is somewhat complex as it is a combination of:


More general “hazardous materials” OHS laws that we have noted in previous
modules
Specific OHS laws directed at sources and activities where silica exposure is a
well-known issue
The BC model is typical in Canada:

Laws that relate to any chemical agent that has a legal OEL (usually the
ACGIH TLV):
5.48 Exposure limits
Except as otherwise determined by the Board, the employer must ensure that no worker is
exposed to a substance that exceeds the ceiling limit, short-term exposure limit, or 8-hour
TWA limit prescribed by ACGIH.

Silica-specific laws
#reflection
Exercise 5
Go to Part 6 of the BC OHSR and find the law starting at Section 6.110:
1. The laws here apply to rock crushing, drilling, mucking, excavation, loading,
transportation, road grading, road construction or conveying of rock or similar
operations.
 What does any of this have to do with exposure to silica?
 Why are operations such as drilling and conveying specifically listed?
#reveal
 Because of the valuable engineering properties of many silicate rocks and
gravels these materials are used in many applications and have to be
modified in many different ways, most of which are capable of producing a
silica-bearing rock dust.
 These operations produce large volumes of respirable size crystalline silica
that easily becomes airborne.
/reveal
2. An untrained worker or uninformed employer might ask: If what is being worried
about is rock dust why doesn’t the law in 6.111 and 6.113 just say “give the worker a
dust mask?” What would be your proper response?
Review Part 20 on Construction, Excavation and Demolition.
#reveal
Something along the lines of:
 It is true that we have many kinds of respirators available that would provide
high level protection against respirable size crystalline silica particles.
 But the control hierarchy and common sense say that we should first look at
engineering and administrative controls to more effectively reduce the risk.
 There are engineering controls that can effectively reduce the risk and most
of them are relatively low cost and easily to put in place.
/reveal
3. Read Section 20.112 and read the law. What does any of this have to do with
silica?—but before you answer the questions watch this video on cutting concrete
with a dry diamond blade.
#reveal
Silicate minerals and the crystalline silica dust that can be created when they are
handled are clearly hazardous materials (note the word “including” does not limit
the hazardous materials to those listed) and so reasonably the regulator requires a
high level program to control worker exposure to them.
/reveal
Ontario has one of the most comprehensive system of OHS laws to regulate silica
exposure, especially in construction. Use this silica on construction projects
document from the Ontario ministry of labour to complete the rest of this exercise.
4.


Read the information on Types 1–3 Operations.
What is the basis of classifying a construction operation in regards to silica
exposure?
#reveal
 Level of risk of the work creating a situation in which workers could
reasonably be overexposed to crystalline silica dust.
 This is the exact same concept as we have seen for asbestos and lead
(interesting isn’t it that silica, asbestos and lead all occur in the particulate
form and all are widely used in the workplace).
/reveal
5.

Read the information in Appendix 1 on medical surveillance.
The objective of a medical surveillance program is to protect the health of
workers by:




ensuring their ______________________ to silica
evaluating their ___________________ of silica
enabling _____________________ to be taken when necessary
providing health education.

Summarize the medical tests that are done when a doctor assesses a
worker’s fitness to work with silica and assesses their absorption of silica.
#reveal
The objective of a medical surveillance program is to protect the health of
workers by:




Ensuring their fitness for exposure to silica
Evaluating their absorption of silica
Enabling remedial action to be taken when necessary
Providing health education
Guess who is assigned responsibility in the workplace to manage the medical
surveillance program: ______________ (only 1 guess is allowed and needed!)

Work/personal history re: factors that would increase risk of silica-related
diseases, lung function tests and RT x-ray
The US is much further ahead on the use of such medical testing as a way of catching
silica-related disease early. If you are interested then this silica screen protocol
document provides a nice overview
/reveal
/reflection
As with all OHS laws, the laws surrounding silica are dynamic and subject to
change as occupational health knowledge improves and OHS and work practices
change. It is the job of every OHS professional to be aware of proposed changes in
the law and to be able to advise employers and others on the impact these
changes will have. For example, review this BCCSA information on some
proposed changes.
Safe Work Practices for Silica
Because of the large number of places where workers can be exposed to potentially
harmful levels of silica and, unfortunately, because even today many workers are
being overexposed to silica on the job, OHS regulators and other reliable sources
have produced a large amount of guidance information for OHS professionals and
others. There are literally hundreds of resources available to you.
Simply stated, you should not have to re-invent the wheel when it comes to
developing SWP and ECP for silica activities. Instead, your job should be:

Locate resources that are already provided for you by your local regulator(s)
and you need only to make use of them (it is always duly diligent practice to
make use of local regulator resources before using those from other
jurisdictions).
 Ensure you understand the technical information in the resources that you
make use of.
#reflection
Exercise 6
1. Here you will be introduced to just some of the hugely wide range of resources
that are available to you from a variety of OHS regulators (you have seen others
already earlier in the module).
Your job here will be to:
 Review what is available to you.
 Consider which kinds of workers the information would best be suited for.
In no particular order, review each of the following sets of resources:

Silica (NIOSH)

Silica (WorkSafeBC)

Silica, Crystalline OSHA

Silica in the Workplace (IAPA)

Abrasive Blasting Code of Practice (NSW)

Occupational Health: Silicosis (ILO)
#reveal
Take this advice to heart. There is so much reliable information out there that you
would be foolish to start writing something of your own. Instead, your job should be
to:

Ensure that the source of the information is reliable (we should always look
to our local OHS regulator first and then expand outward as necessary).
 Ensure that we know our audience and their needs (are we talking about
senior workers who have long experience or junior workers who don’t……)
 Find resource material that will meet the test of due diligence: is it
information that OHS professionals in similar industries would use? Is the
information legal (does it meet local regulatory requirements) and is it
practical for the workplace and its culture.
/reveal
2. Let’s get some practice ensuring that you understand the technical information in
the resources that you can make use of. Go to Appendix A of Developing a Silica
Exposure Control Plan (WorkSafeBC) and explain these terms and concepts used in
the appendix, giving as technical of an answer as you can:
a. Half-mask air purifying respirator equipped with 100 series HEPA filters (page
15)
b. Full-face powered air-purifying respirator (PAPR) with P100 series HEPA filters
(page 15)
c. Local exhaust ventilation (LEV)—use concrete grinders with HEPA vacuum
attachments (page 15)
d. When LEV cannot be used, construct an enclosure including a negative air unit for
dilution ventilation (page 16)
e. Disposable coveralls should be worn when using full-face respirators (page 17)
f. Blasting units that capture the dust (e.g., shot recycle systems) should be used
when practical (page 18)
g. Eye protection should be worn when using a half-face respirator (page 18)
h. A water flow rate of 2.3 litres per minute (0.5 gallons/minute) is the
recommended minimum for saws equipped with wetting controls (page 18)
#reveal
So, how did this go? It certainly is a good example of why you need to be able to
integrate what you learn in your junior OCHS courses into your more senior OCHS
courses. You certainly can’t say to yourself, “well, I passed that course so I will now
promptly forget everything I learned in it.”
/reveal
/reflection
Managing Silica Safety Programs
As this is the last chemical hazardous material that we will be covering in the course
you should already be quite familiar with what it is going to take to manage a silica
safety program. And as always, the ECP is the key document that puts it altogether.
#reflection
Exercise 7
Here we will have you review and critique four silica ECPs. As necessary you should
review your earlier module information on what exactly makes a duly diligent ECP.
1. This ECP for cutting concrete was developed by an OHS regulator:
Reasonably, it will be compliant and meet any test of due diligence. Your job here
will be to:
 Review the ECP.
 Determine whether you would use this ECP format or perhaps make some
changes to better meet the needs of a workplace.
#reveal
And your opinion is: ______________________________
/reveal
2. This silica ECP template is being provided by a construction industry safety
association for its member companies (open Final Template Silica Exposure Control
Plan from the list of documents):
 Review the ECP template.
 Critique it: what do you like and what don’t you like?
#reveal
 Pros and cons/goods and bads?
 This is a great document for you to really focus on. It is not a document
invented as an academic exercise. It is not a generic model put out by a
regulator. It is the real thing. It is being used by real companies as their real
response to real laws requiring an ECP. The template is the result of
extensive consultation between BCCSA, its members and its regulatory
sponsors.
/reveal
3. This silica ECP template is also being provided by an industry safety association
(ENFORM) but being provided by an industry association (Canadian Association of
Oil Drilling Contractors/CAODC) for its member companies:
 Review it.
 Do you think it is designed to specifically meet the needs of oil drilling
companies or is it generic?
#reveal
 The author personally thinks it is a bit generic but it is a good start.
 As above, it is a good thing to focus on because it is real.
/reveal
4. This silica ECP was developed by the OHS staff of a major excavation and
demolition company:
 Review the ECP template.
 Critique it: what do you like and what don’t you like?
#reveal
And your opinion is: ______________________________
/reveal
/reflection
Watch these video clips about how these organizations handle management of their
silica safety programs: insert video clips