|
<<< Back to
Dust Control Handbook for Minerals Processing |
Printing Instructions |
Chapter 8: Sampling Dust in the Work Environment
|
For problems with accessibility in using figures and illustrations in this document,
please contact the Directorate of Technical Support and Emergency Management at (202) 693-2300. |
Personal Dust Sampling
Personal dust sampling should be a central part of the occupational
health program of every mining or minerals processing operation.
Employers have an ethical and legal duty to provide a safe, healthy
working environment for their employees.
The Federal Mine Safety and Health Act was established in 1977 to
protect the health and safety of miners. Section 103 of the act
authorizes inspections by U.S. Government personnel; Section 101 addresses
health and safety standards. Compliance with the established
standards requires regular monitoring of employee exposure to harmful
substances. Title 30 of the Code of Federal Regulations requires
checking dust, gas, mist, and fume emissions as frequently as necessary to
determine the adequacy of control measures.
In addition to ensuring compliance with the regulations, regular
monitoring also provides information on-
- Existence of potential health hazards
- Possible sources and concentrations of airborne dust
- Extent of individual employee exposure to toxic substances
ABC's of Personal Dust Sampling
Before conducting any personal dust sampling, a preliminary evaluation
of the facility should be made. It should be conducted in two steps:
- Collect and evaluate information about the operation, such as
process flowsheets describing flow of material and types of equipment
used; number, type, and toxicity of raw materials, products, and
by-products and the manner in which they are handled; potential
sources of dust; number of workers; and types of control measures in
use.
- If possible, use an instant dust monitor such as a real-time aerosol
monitor (RAM) or GCA respirable dust monitor (RDM) to evaluate the
existing environmental conditions. This information will save
time by pinpointing dust sources, high-risk occupations/areas, etc.
Valid measurements are needed to determine if health hazards exist and
dust controls are needed. The same sampling plan may not be suitable
for every work and exposure environment; therefore, a sampling plan should
be developed for each specific situation. Following are some of the
important criteria for devising an appropriate sampling strategy:
- Type and nature of contaminant
- Location of workers and nature of work operations
- Availability of sampling equipment
- Availability of sample analytical facilities
- Availability of personnel for survey
It is not necessary to sample all workers in a facility.
Suspected and potential health hazards may be evaluated by sampling a
maximum risk worker-the person believed to have the greatest potential for
exposure.
A worker may experience high risk because of the work area (location)
or work procedures (tasks). The work area may have more than one
maximum risk worker if activities or operations are not uniform or if
several different exposure sources exist.
The following are important considerations when selecting the maximum
risk worker:
- Proximity to contaminant sources
- Frequency of proximity to contaminant source
- Number of contaminant sources
- Worker complaints and illness
Threshold Limit Value (TLV)
The following equation should be used to calculate TLV's for respirable
dust:
TLV
respirable dust |
= |
10mg/m3 |
|
| % respirable quartz + 2 |
To calculate TLV's for total dust, the following equation should be used:
TLV
total dust |
= |
30 mg/m3 |
|
| % quartz + 3 |
Time-Weighted Average (TWA)
Since TLV's are time-weighted based on a 7- or 8-hour work day and a
40-hour work week, MSHA uses a time-weighted average to determine
compliance with the TLV's. The TWA is calculated using the following
equation:
| Net dust weight (mg) |
= |
mg |
|
|
| Flow rate (L/min) x 0.001 (m3/L) x time (min) |
m3 |
Short-term time periods with dust levels above the TLV are averaged
with time periods below the TLV using the TWA method. When the time
factor in the TWA formula is less than an 8-hour exposure, MSHA normalizes
to an 8-hour exposure and calls it a shift-weighted average (SWA).
This is acceptable practice provided the time interval samples is
representative of the entire shift.
Personal Dust Sampling Equipment
Personal dust samplers are used to conduct both respirable and total
dust sampling. Components of a respirable dust sampler are a
cyclone, a filter-cassette assembly, and a sampling pump. A total
dust sampler does not have a cyclone; a filter-cassette assembly and
sampling pump are its only components.
 |
 |
| Respirable Dust Sampling Head |
Total Dust Sampling Head |
Cyclone
 |
| Cyclone |
A cyclone is a size-selective device used to separate respirable and
nonrespirable-sized particles from the air. The cyclone has the
following parts:
- A vortex finder that brings the dust-laden air in at an angle and spins it.
- A cyclone body where respirable and non-respirable dust particles are separated.
- A grit pot that collects the separated nonrespirable particles.
MSHA uses a 10-mm nulon cyclone for enforcement sampling of respirable
dust.
Filter- Cassette Assembly
 |
| Filter-Cassette Assembly |
The respirable fraction of dust that passes through the cyclone is
deposited onto a filter inside a cassette. The completely assembled
cassette consists of inlet and outlet plugs, top and bottom sections of
the cassette, backing plate for the filter, and the filter.
MSHA specifies the following:
1. Polyvinyl chloride (PVC) membrane filters must be 37 mm in diameter
and have a pore size of 5 m (for example, Millipore type PVC-5 and MSA
Corporation type F WS-B).
2. A stainless steel or plastic backing screen should support the
filter.
3. A two- or three-piece plastic cassette, 37 mm in diameter, should be
used for respirable dust sampling while an pen-face, three-piece
cassette should be used for total dust sampling.
Sampling Pump
 |
| Sampling Pump |
The sampling pump moves the dusty air through the sampling train. It
consists of a diaphragm or piston pump driven by a battery-powered
electric motor. The air volume can be controlled through a rotameter,
a stroke counter, or automatically through a micropressure sensor.
The sampling pump should operate continuously for at least 8 hours between
charges.
When the respirable dust sampling head is used, the pump must be
calibrated and operated at 1.7L/min. For total dust, the pump must
be recalibrated at 1.7 L/min to account for the different pressure drop of
the total dust sampling head in line with the pump.
Sampling Procedure
In Office
Be sure the battery has a fresh 16-hour charge prior to each use.
Calibrate the pump using prescribed calibration procedures.
In Field
Instruct Employee
Inform the employee that the sampling train is being used to determine
the amount of dust in the air and that the pump should not interfere with
normal work practices.
Tell the employee when and where the sampler will be removed.
Explain that if it is necessary to have the sampling device removed (for
example, to use the washroom), the employee should inform a supervisor who
will arrange for temporary removal. Be sure to arrange for removal
of the sampling train during the lunch period.
Attach Sampling Train
Attach the pump to the employee's belt, positioned so that it does not
interfere with the work operation- usually in the back on the opposite
side of cassette placement. Be sure the exhaust port (charging
inlet) is not obstructed. Supply a belt if the employee is not
wearing one.
Position the cyclone-filter-cassette assembly on the employee's lapel
or front shoulder area to approximate the breathing zone. As a
guideline, attach to the left lapel of a right-handed person and the right
lapel of a left-handed person. Be sure the inlet orifice is facing
forward and the assembly has minimum freedom of movement. A clean,
lightweight vest may be provided to secure both the pump and the
filter-cassette assembly.
Initiate sampling and Record Information
Turn on the pump. Record the starting time, pump serial number,
and sample number on the sample documentation form.
Observe the pump operation for a short time to ensure that the flow
rate is 1.7 L/min.
Complete the sample documentation form. Observe and record
weather data or call the local weather service around midday.
Observe the pump operation after approximately 20 minutes and about
every 2 hours thereafter to ensure that the sampling train is still
assembled and working properly. Record the time of the check and the
time of any pump flow rate adjustment in the field notes.
Collect the sample for 7 to 8 hours. If large concentrations of
dust are suspected, collect two 4-hour samples and combine weights.
Conclude Sampling and Record Information
Check the flow rate and turn off the pump. Be sure to note the
time that the sampling period ended.
Remove the filter cassette from the cyclone assembly. Blow loose
dust from the filter cassette and keep the assembly upright to
prevent dumping the nonrespirable-sized particles from the grit pot onto
the filter.
Dust will remain in the grit pot provided the cyclone is not tilted
more than 120º from vertical provided the pump is still running.
Put the inlet/outlet plugs in the filter cassette and set the sample
aside. Be sure to maintain custody of all samples at all times.
Secure the collected sample. Place adhesive tape or a shrinkable
cellulose band over the two-piece cassette assembly, covering the inlet
and outlet plugs.
Ask the employee about activities and their duration during the
sampling period to see if they match earlier predictions and if the
employee classifies the day as typical.
Quality Control
Prepare a "blank," a filter-cassette assembly that is never exposed to field
conditions. Remove the plugs from a prepared filter assembly in the workplace
(sampling site) and immediately replace them. Seal the blank filter assembly in
the same manner as a sampling filter assembly. Prepare one blank for each day of
sampling. Be sure the blank filters and the filters on which the
samples are collected come from the same batch since the background dust count
varies from batch to batch.
Post Sampling Procedure
If a respirable crystalline silica sample was collected, each filter
must be analyzed for quartz, and permissible limits must be calculated for
each personal exposure. A minimum of 0.1 mg of respirable dust per
filter is necessary for accurate analysis. If the presence of
cristobalite or tridymite is suspected in the sample, request analysis for
crystalline quartz, cristobalite, and tridymite.
Ship samples to an analytical laboratory for analysis. Be sure
collected samples are shipped in a container designed to prevent damage
during transit.
Errors in Respirable Dust Sampling
In minerals processing operations, compliance with the silica dust
standard is based on a 1-day, full-shift respirable dust sample using a
personal dust sampler. The errors associated with measuring
respirable silica dust concentrations should be quantified, because the
law requires a degree of certainty that the standard has been violated.
Described below are the errors associated with sampling equipment and
analytical methods. Human errors are not considered here because
they can be minimized or eliminated by proper care, calibration, and
standard procedures.
| Summary of Expected Errors |
Source of error
|
Percent expected error
|
| Quartz analysis |
11 |
| Instrument variability |
5 |
| Weighing |
5 |
| Airflow rate |
6 |
There are four primary errors in respirable silica dust sampling:
- Errors in Quartz Analysis - To establish TLV's, th
erespirable dust sample is analyzed for quartz content. MSHA
Standard Method #4 requires a precision of 11%.
- Errors Due to Instrument Variability - The variation between
personal samplers can contribute up to 5% of the overall error factor.
- Errors in Weighing - a maximum of 5% error is allowable
- Errors Due to Fluctuations in the Pump Flow Rate - The
generally accepted value for errors in measuring airflow rate is 6%.
It is important for operators to recognize that dust sampling is a
careful analytical procedure. Care should be taken to reduce errors.
The following formula for propagation of errors can be applied to
calculate the overall error factor:
Text Version |
| |
____________________ |
|
Eoverall = √ (E1)2 + (E2)2
.... + (En) 2
|
where:
|
|
|
| Eoverall |
= |
error propagated by n individual errors |
| En |
= |
the nth error |
Substituting the error values from the table above:
Text Version |
|
|
_____________________ |
|
|
Eoverall = √ (11)2 + (5)2 + (5)2
+ (6) 2 |
|
= 14% |
|
This 14% error can be applied to the TLV's to determine the
concentration above which it can be concluded that the respirable dust
standard has been violated.
MSHA metal and nonmetal policy requires that the TLV be exceeded by
1.28 times for respirable and 1.10 times for total dust, before a citation
is issued.
|
| |
|
|
Back to Top