Introduction to Air Sampling
Thursday, August 30th, 2018
Air sampling is a part of the discipline of industrial hygiene. It has been described as those in occupational health who quantify what a safety professional qualifies. Some applications would be:
- If you operate a gasoline or propane powered forklift, the measurement of carbon monoxide (a gas) would be something that needs to be known.
- During the modification or addition to your place of work you are mixing dry concrete with water and a cloud of dust envelops you while mixing with water. Crystalline silica, a component of sand, is a particulate where you would need to know the concentration.
- As a part of your job, you are required to clean your work with a solvent to remove contaminants. Solvents typically release a vapor that can be harmful if inhaled in enough quantity.
Concentration of gases, particulates and vapors are expressed in parts per million (PPM) or milligrams per cubic meter of air mg/m3) of micrograms per cubic meter (µg/m3). There are other expressions used that are unique to the physical attributes such as asbestos, which uses fibers per cubic centimeter (f/cc).
Gasses, particulates and vapors are three types of contaminants that can be measured in the workplace, and there are many other applications that engineering controls, work practice and controls and the use of respirators can control what you breathe in the workplace.
The Occupational Safety and Health Administration (OSHA) has established standards, or Permissible Exposure Limits (PEL) for approximately 600 substances. They are published in 29 CFR 1910.1000. The 1968 PELs were adopted by OSHA with the passage of the Occupational Safety and Health Act in 1970. There are two other consensus best practice organizations that you need to consider when developing an air sampling plan, these are the American Conference of Governmental Industrial Hygienists (ACGIH), the consensus best practices organization that established Threshold Limit Values® (TLV®) and another federal government agency, the National Institute of Occupational Safety and Health (NIOSH) that establishes Recommended Exposure Limits (REL). Both the ACGIH and NIOSH are recommended standards and OSHA is the regulatory standard that business and industry need to comply with.
An air sampling strategy plan is a planning tool to determine how you’re going to sample and where the contaminants are used in the workplace. A good starting point is reviewing the safety data sheets (SDS) of the suspected materials, reviewing the technical information, reviewing the work practice controls and engineering controls, as well as seeing if any occupational health illnesses have been noted and determining how many employees are performing the same operation to determine that you have a representative number of samples to make a determination of exposure risk.
Personal Sampling Explained
The type of sampling required by OSHA's substance-specific standards is called personal breathing zone sampling. The sampling is "personal" because it evaluates an individual employee's exposure to a chemical as opposed to area sampling that measures the concentration of a substance in a given area (e.g., the amount of carbon monoxide in a warehouse where gasoline-powered forklifts are being used). Although certain inferences can be made about exposure by considering the length of time an employee is in the area, the best indicator of a person's actual exposure comes from personal sampling since the sample is collected by equipment that is worn by the employee during the work day. Because the samples are collected at the employee's nose and mouth, they are called "breathing zone" samples. The breathing zone can be visualized as a hemisphere about 6 to 9 inches around the employee's face. Breathing zone samples provide the best indication of the concentration of contaminants in the air the employee is breathing. Two types of instruments are commonly used to do personal breathing zone sampling: passive monitors and personal sampling pumps.
Passive monitors are small plastic enclosures about half the size of a pager. They are filled with a granular solid sorbent such as activated charcoal that has an affinity for organic gases and vapors. One section of the enclosure is open to the air. Organic gases and vapors in the air that pass through the opening by diffusion are adsorbed, or trapped, by the sorbent material.
At the beginning of the sampling period (for instance, at the beginning of the work shift), the monitor is placed in the employee's breathing zone by clipping it to his shirt collar. A protective cover that seals the opening in the monitor is removed, allowing air to diffuse into the monitor. The time the cover is removed is noted and the monitor remains attached to the employee throughout the work shift, breathing the same air the employee breathes.
At the end of the sampling period, the cover is replaced, the monitor is removed and resealed and the time it is removed is noted. The total time the open monitor was exposed to the air is calculated by subtracting the time it was put on from the time it was taken off. A laboratory submission sheet is completed, and the monitor is sent to the laboratory for analysis. The laboratory will extract the contaminant from the sorbent and use a diffusion coefficient to calculate the concentration of the gas or vapors the employee was exposed to and subsequently provide you with a written report containing that information.
Personal sampling pumps
One drawback to passive monitors is they are only able to measure some gases and vapors, but many contaminants exist as particulates such as dust, metal fumes or mists. A different type of personal monitor is required for particulate sampling, but that equipment can also be used to measure gases and vapors. The sampling system consists of two components a personal sampling pump, which is a small battery-powered vacuum pump, connected with Tygon® plastic tubing to a collection media, usually a filter or sorbent tube on which the substance of interest can be collected.
The pump is attached to the worker's belt either behind his back or above his hip using a belt clip on the pump and the collection media, supported by a holder, is clipped to the employee's shirt collar in his breathing zone. The media holder is attached to the air inlet fitting on the pump with a short length of plastic tubing that is run behind the employee's back. The combination of pump and attached collection media is called a sampling train. When the pump is turned on, it pulls air through the collection media and contaminants in the air are trapped for subsequent laboratory analysis. Two types of media are commonly used for industrial hygiene sampling sorbent tubes and filters.
Sampling with Sorbent Tubes
Sorbent tubes are small glass tubes about a quarter inch in diameter and 2 to 4 inches long that are filled with a granular solid substance such as activated charcoal or silica gel or a proprietary substance. One end of the tube is attached to the pump with a short piece of plastic tubing. The other end is open to the environment. Air is drawn through the tube by the sampling pump and vapors in the air are trapped by the sorbent. At the end of the sampling period, the tube is removed, the open ends are covered with tight-fitting plastic protective caps and it is sent to the laboratory for analysis.
Sampling with Filters
Most industrial hygiene sampling for particulates is done using a filter that is 37 millimeters (mm) in diameter. One exception is asbestos, which uses a 25 mm diameter filter. The two most commonly used filters are made of polyvinyl chloride with a 5-micron pore size and mixed cellulose ester with a 0.8-micron pore size. The filters look like very thin plastic discs with the consistency of stiff tissue paper because the filters are so delicate. They are placed in a small plastic cassette to facilitate handling. The filter is placed on a porous support pad in the middle of the cassette and spans its cross-sectional area. The cylinder has openings on both ends, like the sorbent tubes. One end is connected to the pump with a length of plastic tubing. The other is open to the air. When the pump pulls air through the filter, particulates are collected on the surface of the filter in same way that the air filter in a car collects road dust. Like the sorbent tube, the filter cassette is clipped to the employee's collar in the breathing zone.
At the end of the sampling period, the openings on the ends of the cassette are sealed with a plug and the cassette is sent to the laboratory for analysis. When sampling with sorbent tubes and filters, the laboratory must be provided with the volume of air sampled which is determined from the low rate of the pump known from calibration and the duration of the sampling period. Calculations for this determination will be discussed in the next installment of this series.
As important as the collection of the suspected contaminants on the correct media is, another consideration, and just as important, is the accuracy of the volume of air that passes through the media. Volume is measured by the speed of the air sampling pump multiplied by the total time the pump is running. Exact calibration of the pump is extremely important. Historically, an inverted open-ended one-liter burette with a soap film has been used to measure the flow rate of the sampling equipment or train. Statistical reliability dictates that three values are collected before and after sampling each sampling event and averaged to determine flow rate. There are secondary calibration sources like dry-calibration electronic flow meters and rotameters that can be used in the field.
Understanding OSHA's Substance-specific Health Standards
Because of minor differences that exist between each of OSHA's substance-specific standards, particularly with respect to the frequency of periodic monitoring and the point when monitoring may be discontinued, anyone interested in doing air monitoring to fulfill the regulatory requirements should carefully review the relevant standard and become intimately familiar with the exact requirements for the substance of interest.
Note that the discussion below is limited solely to requirements related to air sampling. Substance-specific standards also include detailed requirements for a plethora of other concerns such as employee training, medical surveillance, protective clothing and equipment. Some of the provisions pertaining to air sampling that are common to most of the substance-specific standards are described below but remember the exact requirements vary from substance to substance.
Acceptable exposure limits
Substance-specific standards establish a permissible exposure limit (PEL) that must not be exceeded and, in some cases, an action level, typically half of the PEL. Both are expressed as 8-hour time-weighted averages. The action level is a concentration that is used to establish the frequency of routine sampling, as well as other ‘actions’ the employer must take and is sometimes used as a factor in determining when air monitoring may be discontinued. For example, sampling for methylene chloride may be discontinued when two consecutive measurements taken seven days apart are below the action level. Some substances such as acrylonitrile establish ceiling or short-term exposure limits (STEL) that cannot be exceeded over a 15-minute period during the day.
All exposures are considered without regard to respiratory protection. In other words, if the employees being sampled are wearing respirators, the protection afforded by the respirator is not considered when considering the level of exposure.
Accuracy of the measurements
Substance-specific standards typically stipulate the level of accuracy that the sampling and analytical method must meet.
Initial or baseline sampling must be conducted to determine the existing level of exposure. The results of this monitoring are used to establish the frequency of periodic monitoring and may invoke other requirements of the standard, such as medical surveillance, protective equipment and written compliance plans.
The frequency of periodic monitoring varies from substance to substance and is based on where the measured exposure is relative to the action level or PEL. The vinyl chloride standard, for example, requires monthly sampling for any employee exposed above the PEL. The benzene standard, on the other hand, requires annual sampling for employees exposed above the action level but below the PEL, and sampling every six months for any employee exposed above the PEL.
Termination of monitoring
Results of the periodic monitoring are used to establish when monitoring may be discontinued. For example, the lead and cadmium standards permit monitoring to be discontinued when two consecutive samples taken at least seven days apart are below the action level.
The standards include a provision for conducting additional monitoring whenever there has been a production process, control or personnel change, or when there is reason to suspect other change which may result in a new or additional exposure.
Employee observation of monitoring
Some standards such as butadiene specifically require that employees or their representatives be provided with an opportunity to observe the monitoring. This provision is to ensure that union representatives be afforded the opportunity to observe the sampling process.
Informing employees of monitoring results
Employees must be informed in writing of the sampling results within a prescribed time after receipt of the results from the laboratory, but the notification period varies among substances. For example, notification of the results for acrylonitrile, lead and DBCP must be provided within five days but 15 days is allowed for cadmium and benzene.
Some standards dictate specific information that must be part of the employee's exposure record. For example, the coke oven emission standard stipulates that the record must contain the name, social security number and job classification of the employee and the type of respiratory protection worn, if any. Sampling records must be maintained in accordance with 29 CFR 1910.1020, "Access to Employee Exposure and Medical Records," which requires exposure records to be retained for the duration of the employee's employment plus 30 years. The standard also requires that records be made available to OSHA representatives upon request and that employees or their representative be provided a copy of their exposure record or the means to make a copy upon request.
OSHA Substance-specific Health Standards
Substance 29 CFR 1910
Vinyl chloride: .1017
Inorganic arsenic: .1018
Coke oven emissions: .1029
Cotton dust: .1043
1,2 -Dibromo-3-chloropropane: .1044
Ethylene oxide: .1047
Methylene chloride: .1051
Crystalline Silica: .1053