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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
Respirable Dust Sampling Head Total Dust Sampling Head

Cyclone
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
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
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 120 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.