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Air Monitoring Equipment

Quick Tips #231

When workers are exposed to chemicals, it's important to make sure they are not overexposed – which is why air monitoring is an important part of any safety program. Review some of the air monitoring equipment available and choose what will work best for you. This document gives information on the types of air monitoring equipment available and defines basic terms.

Air can be monitored in several ways. To sample worker exposure, diffusion-detector tubes, vapor-monitoring badges, or personal air-sampling pumps can be used. To monitor specific areas, detector tubes and pumps, hand-held electronic monitors, or fixed wall-mounted electronic monitors are used. The following is a description of each type of monitor.

Several manufacturers make diffusion-detector tubes, which have limited applications. Diffusion tubes can be used to determine a time-weighted average (TWA) or a short-term exposure limit (STEL). They usually consist of a glass tube with a chemical reagent impregnated in a silica layer. One end is broken off and the tube is placed in a tube holder. If the tube has a clip, it is often hung on the worker's lapel, near the breathing zone, to get an accurate reading of the worker's exposure. To calculate an accurate level, the sampling start time must be recorded on the writing area of the tube. The indicating area has a calibrated scale. This is where the reaction with the sample gas or vapor takes place, causing a discoloration that can be read off the scale.

 

When the workday is complete, the tube is taken off the worker and the stop time recorded. The tube is read by looking at 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.

Diffusion-detector tubes meet OSHA guidelines for air sampling, but they are not the most accurate method of air sampling.

Advantages: They give an immediate reading, and you don't have to wait for lab results.
Disadvantages: There are often chemical-cross sensitivities.

Vapor-monitor badges are a good way to monitor a worker's breathing zone to determine the worker's exposure. They work through the simple principle of diffusion. The worker is given the badge, which clips onto the collar. The exposure time, temperature, relative humidity, date exposed, employee and monitor number must be recorded. This information is needed to correctly calculate the exposure level. The badges can be used to determine an eight-hour time-weighted average or a 15-minute short-term exposure limit.

Badges are available for organic vapors, formaldehyde, ethylene oxide, mercury, nitrous oxide and a few other compounds. Some of the badges are sold with a prepaid analysis. All the employer has to do is have the worker wear the badge and then return it in the addressed mailer to the laboratory to be analyzed. Some of the badges can be purchased without the analysis, requiring the analysis to be done by the employer's laboratory of choice. Analysis is usually done by desorbing the vapors trapped on the badge. The desorbed vapor is then run through a gas chromatograph to determine the level.

 

Advantages: The results are more accurate than those of diffusion tubes.
Disadvantages: Badges have to be sent away for analysis, and results cannot be given immediately. They are often more expensive than diffusion tubes. Badges are also only available for a limited number of chemicals.

Personal air-sampling pumps are a little more difficult to use. They consist of a small pump that pulls a constant amount of air (usually in liters per minute), a charcoal tube or filter cassette, and a clip for attaching the collection media near the worker's breathing zone.

These pumps require calibration before each use and must have the flow rate set to the correct level depending on what is being monitored. The worker usually wears the pump and the collection media for the entire day. Once the day is finished, the charcoal tube or filter cassette is sent to the laboratory of choice to be analyzed.

 

Advantages: The results are usually more accurate than other methods and allow for a broader sampling of chemicals. It is also one of the few ways to get a time-weighted average for dusts in the air.
Disadvantages: Requires a knowledge of National Institute for Occupational Safety and Health (NIOSH) sampling methods, as well as which type of filter or tube to use. It is also more bulky and cumbersome than other sampling methods, so workers might not warm up to it as easily. If a person is not experienced in this type of work, it would be beneficial to get a consultant to help.

Detector tube and pump systems are a very effective way to do on-the-spot air monitoring and meet the OSHA requirements of 25% error. They are accurate enough to get an idea of the hazards in the workplace. Detector tubes are typically used when surveying an area, and can also give an idea of worker exposure in ppm or percent volume.

Two main types of pumps are available: piston and bellows. The piston style requires the user to pull a piston to pull air through the tube. With the bellows style, the user squeezes the bellow, and upon release, air is pulled through the tube as the bellow opens.

 

Several types of tubes are used to get a measurement:

  • Scale tube–On the tube is an actual ppm scale that is read by measuring the length of discoloration.
  • Qualitative tube–This tube does not give a quantitative reading, but it lets the user know that the gas or vapor is present.
  • Pretube–For some chemicals, a test that produces a simple color change is not available, so a pretube is required. It is used to convert the test gas or vapor into another compound that can be detected in the indicating tube by a color change.
  • Color-matching tube–The sample is taken until the color of the sample matches the color intensity of the control tube. The number of pump strokes taken to match the color of the control tube is then looked up on a chart to calculate the concentration of the test gas or vapor.
  • Color comparison–This method compares the intensity of the discoloration on the sample tube with a control tube. The comparison tube has three color intensities to compare with three levels.

Advantages: Allow for quick sampling of an area. They are relatively inexpensive for quick monitoring.
Disadvantages: The tubes are limited in the types of chemicals they can pick up. Certain chemicals can also interfere with the tubes and give false readings, so it is important to check for interferences.

Continuous hand-held monitors come in a variety of shapes and sizes. They can vary from a relatively simple single-gas monitor to a complex datalogging four-gas monitor. They all make a quantitative analysis that is displayed on a digital or analog readout. Some also have the capabilities to store information, which can be downloaded to a computer.

Advantages: The readout is immediate. There is no waiting for a color change or waiting for a lab to analyze the results of a badge. The monitors also give real-time readout. What is being read on the display is what the gas concentration is at that time. Most hand-held meters also have a visible or audible alarm or both that will alert the user if a gas is above a safe level.
Disadvantages: This type of meter is usually calibrated with one type of gas, and interference from similar gases could alter the readings. The operator must have adequate training to interpret the information that the monitor is providing, and also be familiar with calibration and limitations of the device. In comparison with other types of monitors, they are often more expensive.

 

This type of monitor is very similar to a hand-held electronic monitor, except it is mounted and an operator is not required. Fixed monitors use sensors similar to those of hand-held monitors. They often do not have a wide range of chemicals that they can monitor. This type of monitor ranges from a simple carbon monoxide monitor to a very expensive multisensory system.

Fixed monitors operate at all times. They often have alarms to alert workers to a dangerous situation. Some of the more complex systems can start a ventilation blower when concentrations are too high or even sound an alarm in another part of the plant.

Whatever type of system you choose, make sure you understand what the monitor will and will not do. Most of these monitors are for surveying and not for accurate readings. OSHA only requires a 25% error, so do not plan to use any of these monitors for laboratory analytical work. Air monitoring is a very important part of any safety program, so make sure you get all of the details before sending your workers into a hazardous area.

 

Definitions

  • CeilingConcentration that must not be exceeded during any part of the workday.
  • Generalarea sampling—Sampling used to determine whether an area is hazardous or becomes hazardous while the workers are present.
  • Immediately dangerous to life or health (IDLH)Conditions that pose an immediate threat to life or health, or conditions that pose an immediate threat of severe exposure to contaminants.
  • Lower-explosive level (LEL)The lower limits of flammability of a gas or vapor at ordinary ambient temperature expressed in percentage of the gas or vapor in air volume. The limit is assumed constant for temperatures up to 120°C (250°F). Above this, you should decrease it by a factor of 0.7 because explosibility increases with higher temperatures.
  • Milligrams per cubic meter (mg/m3)Unit used to measure air concentration of dust, gases, mists and fumes.
  • Parts per million (PPM)Parts of air by volume of vapor, gas or other contaminant.
  • Permissible exposure limits (PELs)OSHA's time-weighted average concentration that must not be exceeded during any eight-hour work shift of a 40-hour workweek. PELs are found in 29 CFR 1910.1000 in the Z-1A or Z-2 tables.
  • Personal samplingSampling done to evaluate a worker's exposure during the workday. Workers usually wear the sampling device in their breathing zone.
  • Short-term exposure limit (STEL)Concentrations measured over a 15-minute period unless otherwise noted.
  • Time-weighted average (TWA)Concentrations of airborne toxic materials that have been weighted for a certain time, usually eight hours.
  • Threshold-limit value (TLV)Eight-hour time-weighted average concentration set up by the American Conference of Governmental Industrial Hygienists (ACGIH). They are used as a recommended level.
  • Upper-explosive level (UEL)The highest concentration, expressed in percentage of vapor or gas in the air by volume, of a substance that will burn or explode when an ignition source is present.
Commonly Asked Questions
Q.   How do I know what air monitoring equipment to use?
A.   If you want to measure your worker's exposure to hazardous chemicals, use diffusion-detector tubes, personal badges or personal air-sampling pumps.
 
Q.   How often is calibration required?
A.   The monitor should be bump tested before every use to ensure that your workers are safe. Consult your owner’s manual for recommended calibration frequency.
 
Q.   What type of monitor should I have for entering a confined space?
A.   The first thing to know is what types of hazards you expect to encounter. Most importantly, you need to monitor for oxygen, then combustible gas, and then any toxic gases that might be present, such as carbon monoxide. This can usually be done with a three- or four-gas monitor with a simple draw pump to test the space before entry.
 
Q.   How do you convert mg/m3?
A.   PPM = (mg/m3 x 24.45) / molecular weight
 
Q.   What does it mean when a tube reads out in percent volume?
A.   1% = 10,000 ppm

 

Sources

NIOSH Pocket Guide
U.S. Department of Health and Human Services, 2013.

Fundamentals of Industrial Hygiene, 6th edition, National Safety Council, 2012.

National Safety Council Air Sampling Instruments, 6th edition, ACGIH

Methods for Air Sampling and Analysis, 3rd edition, APHA International Committee

(Rev. 12/2013)


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