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Industrial Hygienists responsible to evaluate health hazards in the work environment and make recommendations in an effort to keep employees safe from any potential health hazard. For the evaluation and control of inhalation hazards, some duties performed by industrial hygienists include preparing and calibrating equipment used in collecting samples, collecting samples of potentially toxic materials and evaluating samples in a laboratory setting. This topic will look into general of air sampling methodology, different type of air sampling, the collection device and method used to sample particulates and how to select a sampling method.
9.2 TYPE OF AIR SAMPLING
Air samples may be collected by placing sampling devices directly on the worker (personal sampling) or by placing a sampling device in the work area (area sampling). Sampling equipment with alarms can be set up in a work area to alert workers to unusual or unexpected concentrations of substances in the air. This is called sampling according to location. Classification of method of air sampling can be done according to physical state of contaminant of interest, according to how the air is moved through a sampling device (active or passive), according to length of sampling (grab, integrated or continuous) and according to nature of toxin (acute or chronic). This section will explain briefly part of air sampling method.
Personal Air Sampling
Personal sampling is used to both detect and measure exposures. It is done to determine the quality of the air the worker is breathing or would breathe if not protected. Samples are usually collected by placing a battery-operated air pump on the wearer's belt and clipping a collection tube or filter in the breathing zone, usually on the collar near the nose. Air from the environment is pulled into the collection device where the contaminants are trapped, and then the sample is sent to a laboratory for analysis.
Area Air Sampling
Area air samples can be used to evaluate background concentrations, location source of exposure or evaluate the effectiveness of control measure. The sampling device is strategically placed in a fixed location in the area that might be exposed. However, this type of sampling is not used to provide an estimate of worker exposure because condition at fix location may not be the same as those experienced by the worker.
Grab samples are taken to measure the airborne concentration of substance over a short period of time, usually less than 5 minutes. Personal or area grab samples are used to identify peak or ceiling concentration.
Integrated sampling is used to estimate a worker's 8 hour or 15 minutes exposure to a particular substance by collecting one or more personal air samples for duration of a particular task or work shift. The result integrates all of the various concentrations to which worker have been exposed during the sampling period.
SELF CHECK 9.1
List classification of method of air sampling
Differentiate grab sampling and integrating sampling
9.1 SAMPLING FOR PARTICULATES
Air sampling is the process of quantifying or determining the identity of a contaminant by the use of standard sampling instruments, techniques and strategy. In workplace sampling is conducted for these reasons:
To monitor the integrity and performance of hazard control systems. Concerned with measuring changes in ambient levels in order to follow trends of improvement or deterioration in air quality and located in fixed positions and normally in close proximity to contaminant sources.
To access health risk of persons at work place
To compliance with the requirement of the law
To do epidemiological study
In a workplace, air contaminants can be divided into two broad groups depending on their physical characteristics. These are particulate matter, gases and vapour. In this chapter, the discussion only focus on air sampling of particulate matter and for the gas and vapour the discussion will be made in the next chapter. As we know, particulate is a small discrete mass of solid or liquid matter that remains individually dispersed in gas or liquid emissions.
SAMPLE PUMP AND VOLUME DETERMINATION
There are varieties of sample pump available, depend on the sampling approach. Figure 9. shows the sample pump for personal air sampling and area sampling pump.
Figure 9. : Sample pump (a) Personal pump (b) Area sampling pump
Compare to area sampling pump, most personal sampling pumps are lightweight and quiet, use nickel/cadmium rechargeable batteries,and can be easily attached to the worker's belt. Each has a flow rate control valve and some are programmable. Air-sampling pumps are generally available in the following airflow rate ranges: low-flow (0.5-500 mL/min), highflow (0.5-5 L/min), and dual range (high- and low-flow). Low-flow pumps are used for solid sorbent tube sampling. High-flow pumps are used for filter, cyclone, and impinger sampling.
The sample of volume of air must known in order to calculate the airborne contaminant. The volume can be calculated by using equation below
V=Q x T
Where V is volume, L
Q is flow rate, L/min
T is sample time, min
To know the actual flow rate Q, the sampling pump must calibrate first before it use. Additionally, some sampling device such as impingers and respirable dust cyclones are design at specified flow rate. Pump are generally calibrate before and after sampling with the entire sampling "train" assembled as it will be or was used in the field. The average flow rate, before and after sampling should be used to calculate the sample volume. Several devices are used to calibrate the pumps. The accuracy of each of the following device varies with manufacturer and model.
Soap Film Flowmeter.
All pumps can be calibrated using a soap film method, and sometimes called as bubble burette. This calibration procedure is based upon using a burette of known volume and measuring the time it takes for the pump to move a soap film in the burette over a specific volume. There are different size commercial device available in the market. Figure 9. show a laboratory film flowmeter kit consists of a glass tube with red calibration lines at specified volumes, a squeeze bulb to contain sufficient soap film solution for flow measurement, an adapter to accommodate either vacuum or pressure sources, a stopper and connecting tubing, a tripod stand and film solution.
To determine the flow rate of the soap film flowmeter, equation below can be used
C:\Users\Ennie\Desktop\OUM WRITING\RAW MATERIAL\RAW_CHAP9\bubble burette.jpg
Figure 9. : Laboratory film flowmeter kit
Automated Soap Film and Piston Flow-meters
This type of calibrator resembles an automated bubble burette or operates on a similar principle. In a typical unit, the user release a soap film which moves past infrared sensor in a tube that measures the time durations the film cross a known distance. Another model features a very low friction piston to replace the bubble film. This method brings better precision and faster compared to the bubble burette calibration because the flowrate determined electronically and sometimes (depend on the model), the calibration data can be stored for electronic record keeping. Figure 9. show a calibrating a sample pump with an electronic calibrator
auto bubble burette.jpg
Figure 9. : Calibrating a sample pump with an electronic calibrator
The above approach is a primary standard because it relies on a volume which can physically measured. Secondary calibration standard are calibration device that are checked against primary calibration standard. They do provide an acceptable level of precision and are often more convenient and practical to carry into the field for use during sampling. Flow meter and rotameter are examples of convenient of secondary standard.
SELF CHECK 9.2SelfCheck
The industrial hygienist timed the movement of soap bubble from 500 to 0 ml, with the following result:
Volume timed: 500ml
Time taken :13.47 second
Determine the flow rate.
SAMPLING TECHNIQUES FOR COLLECTION AIRBORNE PARTICULATES
A number of devices can be used to collect particulates, each using different mechanisms for particle capture. This topic explains about sampling techniques for collection airborne particulates and it is summarized in Table 1.0.
Filters are some of the most common sampling media used by industrial hygienist. Air sampling filters are made using different material and method. The type of filter to be used will depend upon the contaminant being samples as well as on the sampling method chosen. During sample collection, the filter is place into rigid holder, called cassettes. (See Figure 9. ). Then a support pad is usually placed under the filter in the cassette. In some cases, the support pad is analyzed along with the filter. The filter used for air sampling work like a screen to capture airborne contaminant. Most filters used for industrial hygiene application work much differently.
Figure 9. : Filter and cassette assembly
There are several types, including glass fiber (GF),mixed cellulose ester fiber (MCE), and polyvinyl chloride (PVC) filters. They are selected based on their ability to collect material and their suitability for laboratory analysis. For mineral and nuisance dusts, for example, the total weight of the collected particulate is of concern.
Mixed Cellulose Ester Filter (MCEF)
These filters are manufactured from a polymer that starts a liquid, which is spread out in a thin layer to solidify or dry, like gelatine setting up. MCEF filter are available with pore sizes ranging from 0.4 to 0.8µm. Metal fumes and asbestos fibres are collected on MCEF filters with pores sizes of 0.8µm. Analysis of the filter is dependent upon the material that was collected. Filters used for welding fumes, for example, are dissolved or digested in an acid solution, which is then analyzed for metal content. Fibres are quantified by removing a section of the filter and treating it with acetone vapour, so that it becomes clear. It then possible to count the fibres while viewing the cleared filter through a microscope.
Polyvinyl Chloride (PVC) Filters
PVC filters are commonly used to collect dusts, such as silica containing dusts. PVC filters have good resistance to acidic and basic substances and do not absorb much water vapour from the air.
Teflon filter is another polymer type filter that used in industrial hygiene sampling. It is sometimes referred as the PTFE (polytetraflouroethlyn) filter. These filter like PVC filters are chemical resistance and hydrophobic.
Glass Fibre Filters
Glass fibre filters are sometimes referred to as AE filters, are used for collecting particulates and some droplets of contaminants such as mercury and acid gases.
Inertial impactors collect particles by impacting them onto a surface. If an obstacle causes a moving airstream to deviate from a straight course, the particles in the airstream tend to leave the airstream and impact on the obstacle. Obstacles include filter paper, glass, stainless steel, and in the case of bioaerosol sampling, nutrient agar. The collection efficiency of this method is affected by the mass of the particles, the size and shape of the obstacle, and the velocity of the air. Inertial impactors can be used to determine particle size distribution. Figure 9. , shows the submicron-sized particle inertial impactor.
Figure 9. : A picture of the submicron-sized particle inertial impactor
Source: CMU. J. Nat. Sci. (2008) Vol. 7
The impinger is one of the oldest methods of particulate sampling, but it is little used today. It is used in situations where the number of particles must be expressed in millions of particles per cubic foot of air (mppcf). Impingers use the same particle collection method as the inertial impactor, except that the particles are collected in a liquid (usually water). Air is drawn at high velocity into a liquid- filled flask through a glass nozzle or jet. The particles impinge on a flat plate or the bottom of the flask, lose their velocity, and are trapped in the liquid. A small sample of the liquid is collected and then placed in a special cell that allows the particles to be counted and sized as they are viewed under a light microscope. Impingers do not collect very small particles (less than 0.7 Î¼m) well. For maximum collection efficiency, the air must be drawn at such a high velocity that larger particles are often shattered, thus producing erroneous results.
Figure 9. : Midget impingers are sometimes used to collect
personal air samples. They are placed in holsters so they can
be worn in the worker's breathing zone
Source: SKC Gulf Coast Inc
Elutriators are used in front of a sampling train to remove coarse particles. The coarse particles are removed by gravity; the smaller particles remain suspended and are collected for subsequent analysis.There are two types of elutriators: horizontal and vertical. The vertical elutriator is commonly used for cotton dust sampling. It consists of a large vertical tube through which the direction of airflow is opposite to the direction of gravity. Due to the airflow requirements and elutriator size, it must be operated in a stationary position. The flow rate is very important because if it is too high, the larger particles will not settle out but will be collected on the filter. If the flow rate is too low, some of the smaller particles may settle out and not be collected.
Electrostatic precipitators use an electric charge to remove particles from the sampled air. As the particles pass through a high-voltage electric field, they acquire a charge and are attracted to an oppositely charged electrode. Collection efficiency increases with the length of passage through the collector, so precipitators are often used in series. Electrostatic precipitators are used when the required sample air volume is large, high-collection efficiency is required for very small particles (such as fumes), there is a possibility of filter clogging, or high-temperature airstreams must be sampled.
In thermal precipitation, the air stream is passed through a narrow space which has a significant temperature gradient perpendicular to the direction of flow. The movement of a particle in the direction of decreasing temperature called thermophoretic velocity causes the particle to be deposited on relatively cool collecting surface. In sampling instrument, asFigure 9. , the air is drown past a heated wire or plate and the dust collects on a cold glass or metal surface opposite the hot element. A high thermal gradient is needed so that the channel between the wire or plate and collecting surface is kept small. Because the migration velocity induced by the thermal gradient is small, the system is limited to low volumetric flowrates and thus it is used only for collecting sufficient particulate for microscopic examination.
thermal precipitation 1
Figure 9. : Sampling head of thermal precipitator
A cyclone is used to collect particles of respirable size. Respirable particles are those that are retained in the lung and are generally considered to be of an aerodynamic size below 10 Î¼m. Figure 9. shows the example of aluminium cyclones. Cyclones have traditionally been used to sample for mineral dusts containing crystalline silica because of the strong association between the respirable dust fraction and lung disease silicosis. Air is drawn into a cyclone tangentially through a small orifice. It is important that the cyclone is operated at the airflow rate for which it was designed. The centrifugal motion of the air inside the cyclone forces the larger particles to the periphery of the airstream, where they fall to the bottom of the cyclone. The respirable particles, in the center airstream, are drawn upward onto a preweighed filter. After sampling is completed, the filter is analyzed or weighed to determine how much material has been collected.
Figure 9. : Aluminium Cyclone is a lightweight respirable dust sampler that is used with a filter loaded into a three-piece filter cassette.
Table 9.0: Summary of Sampling Techniques for Collection of Airborne Particulates
Force or Mechanism
Combination of inertial impaction, interception,
diffusion, electrostatic attraction, and gravitational forces
Inertial-Impaction on a solid surface
Inertial-Impingement and capture in liquid media
Electrical charging with collection on an electrode of opposite polarity
Thermophoresis-Particle movement under the influence of a temperature gradient in the direction of decreasing temperature
Inertial-Centrifugal separation with collection on a secondary stage
SELF CHECK 9.3
Explain the following mechanisms for capture of contaminant by filters:
SIZE SELECTIVE SAMPLER
Occupational hygienists have been considering size-selective criteria for particulates to better assess exposures and to provide consistency in exposure measurements. It can be divided into three size fractions: inhalable, thoracic, and respirable.
Inhalable fractions include particles that are hazardous when deposited anywhere in the respiratory system, including the nose and mouth.
Thoracic fractions include particles that are hazardous when deposited within the lung airways and the gas-exchange region
Respirable fractions include particles that are hazardous when deposited within the gas-exchange (alveolar) region of the lung.
Figure 9. shows the size fraction for each size category.
Figure 9. : Size Fractions for Particles
Source: TSI Inc
The inhalable particulate mass fraction of an aerosol is the fraction of the ambient airborne particles that can enter the uppermost respiratory system compartment, the nose and mouth. Airborne material that deposits here may be absorbed or swallowed. Inhalability depends on wind speed and direction, breathing rate and whether breathing is by nose or mouth. There are at least three general classes of chemical compounds for which sampling should be done for all inhalable particles:
Highly soluble material that can quickly enter the blood becoming available to pass through membranes in many region and exhibit their toxicity,
Material that can be toxic after oral ingestion
Compound that can exert effects at their deposition site
In many work environment airborne particle size are within the thoracic particulate mass fraction rather than inhalable particulate mass, because larger particles are often removed via filtration or deposition on surface; however in situation where there is a source of aerosol, such as sprayers, cutting or abrasive machinery, there may be a significant inhalable particulate mass concentration.
As noted, the thoracic particulate mass fraction represents those airborne particles that are capable of entering the upper respiratory area and trachea during mouth breathing. It has been described as representing the worst-case potential exposure of the whole lung to particles. The thoracic particulate mass size selective sampling criterion has been established as a tolerance band consisting of those particles that can penetrate a separator whose size collection has 50% of its particles (50% cutoff) 10µm in size and geometric standard deviation of 1.5±0.01. A substance for which it would be useful to measure the thoracic particulate mass fraction is asbestos. Fiber level are related to their potential to cause bronchogenic cancer since the type of fibers causing this effect deposit in tracheobronchial and gas exchange region. Another compound has been recommended for thoracic particulate mass sampling is sulphuric acid aerosol.
The respirable particulate mass fraction is considered the portion of an aerosol available to the gas exchange region during nose breathing. It is based on ACGIH's previous recommendations with a tolerance band added. The 50% cutoff is the same as it was previously, 3.5µm and the geometric standard deviation is the same as for other fraction, 1.5±0.01.
Size Selective Sampling Device
The key criterion for these sampling devices is their accuracy in collecting particles according to the selection curve for three categories. Sampling devices that meet precise sizing requirements must be developed to collect the particles in each of these size fractions. To choose an appropriate sampler, occupational health and safety professionals should first review the occupational exposure limit (OEL) or guideline for the contaminant of interest to determine which particulate size fraction they need to collect. For example, ACGIH TLVs for individual particulate contaminants are issued for inhalable, thoracic, or respirable particulate fraction mass. After determining the appropriate size fraction, professionals should then select a sampling device that is designed to collect that size fraction.
Air + contaminant
Figure 9. : Size-selective air-sampling methodology
SELF CHECK 9.4
Name the three size fraction for particles and explain each of them.
DIRECT READING INSTRUMENT
The direct reading method for particulate matter is the way where samples can be taken and analyzed right at the fields thereby eliminating the time delay effort of sending fields samples to a laboratory for analysis. There are many type of direct reading instrument in the market that is design for specific monitoring purpose. Some comes in electronics device and some indicated by colour change called colorimetric device.
Display and alarm
Figure 9. : Diagram showing component of typical electronic direct reading instrument
Figure 9. shows the typical diagram of direct reading for electronic instrument for gas, vapour and particulate matter. It is consist of a detector, pump, signal processing unit, display and alarm indicator, a power supply unit and data storage section. All these included in one small package and sometimes it is enough to fit into a shirt pocket. The ideal of this type of direct reading instrument is rugged, lightweight, easy to calibrate, simple to operate.
While colorimetric device device is use colour change resulting from a chemical reaction to measure air contaminants is a colorimetric tape sampler. In modern devices, a chemically treated paper tape is drawn at a constant rate over the sampling orifice; the air contaminant drawn through the tape reacts with the chemical to produce a stain. The intensity of color is
directly related to the concentration of the contaminant and is read optically and then displayed on a digital readout.
Calibration of direct reading instrument is critical and must be perform prior to use and again at intervals specified by the manufacturer. The process of calibrating a direct reading instrument is different from calibrating a sampling pump, instead of verifying a flow rate; it is actually the process that the instrument is responding to the contaminant and providing an accurate reading or indication of detected concentration. Many direct reading instruments available today have hygiene function and include the ability to perform function such as computing TWA concentration and producing record that can be transferred to computer.
In this topic you have been explained about primary standard approach calibration for a sample pump. Explain two method of secondary standard approach and discuss the important of both calibration techniques.
Answer true or false:
Particles having diameter of approximately 75µm or less are generally thought to be inhaled in lower respiratory system.
Impaction device collect and retain particles of an air stream on surface.
Electrostatic precipitators use an air to remove particles from the sampled air.
Soap film flowmeter is a primary standard approach.
SELF CHECK 3.2
Air sampling is used to evaluate employee exposure, assist in the design or evaluation of control measures, and document compliance with government regulations.
Intergrated Sampling which Samples taken by drawing air through the sampling medium, which is then analyzed by a laboratory to determine the amount of contaminant present.
Grab samples are taken to measure the airborne concentration of substance over a short period of time
Filters are selected based on their ability to collect material and their suitability for laboratory analysis.
Inertial impactors collect particles by impacting them onto a surface.
Electrostatic precipitators use an electric charge to remove particles from the sampled air.
A cyclone is used to collect particles of respirable size
ACGIH TLVs for individual particulate contaminants are issued for inhalable, thoracic, or respirable particulate fraction mass.
The direct reading method for particulate matter is the way where samples can be taken and analyzed right at the fields thereby eliminating the time delay effort of sending fields samples to a laboratory for analysis
Area sampling pump
Soap film flowmeter
Direct reading instrument