Chapter 4 Strategies for Exposure Monitoring and Instrumentation
4.5 Occupational Safety and Health Characterization
Exposure monitoring equipment is a critical and essential element in the process to characterize exposures. Monitoring equipment is used to help characterize and evaluate work areas where there is a potential for a hazardous environment to exist.
Environmental conditions can be monitored in a variety of ways. When collecting samples to quantify worker exposures, the following equipment are generally used.
• Diffusion detector tubes
• Vapor monitoring badges
• Personal air sampling pumps
If the desire is to sample specific areas, then consideration should be given to the use of the following equipment as an option to collect data:
• Detector tubes
• Sampling pumps
• Handheld electronic monitors
• Fixed wall-mounted electronic monitors
When implementing the sampling strategy plan, careful consideration must be given to the type of equipment needed to collect the data. A brief summary of the utility of each type of sampling equipment is discussed further below.
4.5.1 DIFFUSION DETECTOR TUBES
Gas diffusion detector tubes are easy to use when sampling for gases and vapors.
They also offer a low-cost method when collecting a large number of samples. These
tubes are accurate enough to provide an indication of the contaminant concentration in the workplace. A detector tube is a graduated glass tube filled with a chemical reagent that will produce a color change when exposed to the gas in question. It is used with a hand pump that will draw a sample into the tube.
There are two main types of pumps available to use in this system: a piston pump or a bellows pump. The piston-style pump requires the user to pull a piston to pull air through the tube. With the bellows style, the user squeezes the bellows, and upon release, air is pulled through the tube as the bellows opens. The tubes generally are sealed at both ends and ready to use when the tips are broken off. Gas detector tubes are available for hundreds of compounds and have been used for many years. A major advantage in using this system is that it allows for sampling of an area quickly, with minimal expense. Although the cost associated with the use of these tubes is low and these tubes are easy to use, there are some disadvantages of using them, such as
• The potential for chemical cross-sensitivity and interference with other chemicals
• Detector tubes are not the most accurate method for collecting air samples and quantifying exposures. The accuracy assigned to these tubes falls in the area of approximately ±20%.
• Detector tubes have a shelf life, and many of them have to be stored in refrigerated environments. Many of the tubes are temperature sensitive.
• Many of these tubes require that multiple pump strokes be taken to achieve the desired sensitivity.
• Errors can occur from failure to complete the necessary amount of pump strokes or from not allowing the time needed for each sample to work its way through the tubes prior to activating another pump stroke.
• Some chemicals can interfere with the tube and give false-positive readings.
To effectively use these tubes, the following steps are necessary:
• One end is broken off and the tube is placed in a tube holder. If the tube has a clip, it can be hung on the worker’s lapel, near the breathing zone, to get an accurate reading of the worker’s exposure. Be cautious, as the end of the glass tube has been broken and if not cut smoothly, the worker may sustain cuts by coming into contact with the tube end, which can cause injury.
• To ensure that the accurate exposure level is calculated, record the sampling start time on the writing area of the tube. The indicated area has a cali- brated scale. This is where the reaction with the sample gas or vapor takes place, causing a discoloration that can be read on the scale.
• At the end of the work shift, remove the tube from the worker and record the removal time.
Exposure can be determined by observing the discoloration and recording the parts per million (ppm) value from the scale. The time-weighted average is obtained by dividing the scale value by the total amount of time sampled (in hours).
4.5.2 VAPOR MONITOR BADGES
Vapor monitor badges work through the concept of diffusion and are an excellent option that can be used to monitor a worker’s breathing zone to determine expo- sures. These badges are typically used to determine an 8-hour time-weighted aver- age or a 15-minute short-term exposure limit. The exposure results obtained from vapor monitor badges are more accurate that those obtained from diffusion tubes.
However, these badges have several disadvantages, which include
• They must be sent to a laboratory for analysis.
• The results are not immediately available.
• These badges are only available for a limited number of chemicals.
• They are oftentimes more expensive than diffusion tubes.
Vapor monitor badges are available for compounds such as formaldehyde, organic vapors, ethylene oxide, mercury, and nitrous oxide. These badges are lightweight and clip onto the worker’s collar. In order to correctly calculate and report exposures at the end of the work shift, the following information must be collected:
• Exposure time
• Temperature
• Relative humidity
• Date exposed
• Employee identification data
• Vapor monitor badge number
Vapor monitor badges are frequently used as a monitoring option because they are easy to use; all that is needed is to have the worker wear the badge and submit it to the laboratory to be analyzed. Vapor monitor badges are usually analyzed by a laboratory by desorbing the vapors trapped on the badge. The desorbed vapor is then run through a gas chromatograph to determine the contaminant level.
4.5.3 PERSONAL AIR SAMPLING PUMPS
Personal air sampling pumps are a little more difficult to use than vapor monitor badges and detector tubes. However, the results are generally more accurate than those of other methods and allow for the sampling of many more different types of chemicals. On the other hand, there are some distinct disadvantages of using sam- pling pumps, which include
• They require familiarization and knowledge of laboratory collection meth- ods, such as NIOSH sampling methods.
• The type of collection media used is specific to the laboratory analysis method.
• They are bulkier and more cumbersome than other sampling methods.
• Because of the bulky construction, workers are not always willing to wear the pumps for an entire work shift.
The air sampling pump assembly and sampling train consists of the following:
• A pump that pulls a constant amount of air through a charcoal tube or a filter cassette
• A tube to connect the pump to the charcoal tube or filter cassette
• A clip or method for attaching the collection media near the worker breath- ing zone
These pumps require calibration before each use and must have the flow rate set to the appropriate level based on the chemical constituent being monitored. The worker is expected to wear the pump with the collection media attached for the entire work shift or until the task or exposure period ends for the workday. Once the work shift is finished, the charcoal tube or filter cassette is then sent to the laboratory for analysis.
4.5.4 HANDHELD ELECTRONIC MONITORS
Handheld monitors can range from simple single-gas monitors to somewhat complex four-gas monitors with data logging capability. These monitors have distinct advan- tages and disadvantages, which are listed below.
Disadvantages include
• Handheld electronic monitors can experience interference that can alter the reading from gases that are similar in components.
• The operator must have adequate knowledge to be able to interpret the data obtained and must also be familiar with the calibration and limitations of the device.
• In comparison with other types of monitors, they are often more expensive.
Advantages include
• The readout is instantaneous and in real time.
• There is no waiting for a lab to analyze the results.
• The result displayed is the concentration at that moment in time.
• Many handheld meters have a visible and/or audible alarm to alert the user if the concentration is above a safe level.
4.5.5 FIXED AIR MONITORS
Fixed air monitors are similar to handheld monitors, except that they are mounted in a specified area and do not require an operator on location. Fixed monitors use sensors similar to those of handheld monitors and often do not have a wide range of chemicals that they can detect. These monitors are convenient because they operate at all times and often have audible and/or visual alarms to alert workers of dangerous environments. Fixed monitors have advantages and disadvantages similar to those of handheld monitors.