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Mist methodologies

June 1st, 2014 / By: / Technical

NCSU lab rethinks chemical/biological
hazard protection testing and derives
important new conclusions.

When you ask first responders if they are more concerned about having enough protection against flash fire exposures, ballistic and physical hazards, heat stress, or chemical and biological threats, more times than not they are going to say they are most concerned about the chemical and biological threats. It is human nature to be apprehensive about things that are not well understood
or identified.

Overcoming misconceptions

Chemical and biological threats are completely different depending on the actual compound or agent that is present. All that the first responder may see, in most cases, is a puddle of liquid that may be as innocuous as water, as corrosive as concentrated acid, or as deadly as organophosphate nerve agents. Since the identification of the chemical and the hazards associated with it are not possible by sight alone, first responders need to have the most protection possible from the widest array of chemical and biological threats.

With all protective clothing, the ideal for the manufacturers and first responders would be one material and one suit that provide protection against all threats. In reality, this ideal suit does not exist and may not be required. Instead, the design, manufacture, testing, evaluation, and certification of chemical and biological protective clothing is focused on providing appropriate protection from possible threats balanced with minimal thermal burden for specific end uses.

In the field of chemical and biological protective clothing, a common misconception is that the most impermeable or impenetrable materials will automatically provide the highest level of protection against toxic industrial chemicals (TICs), chemical warfare agents (CWAs) or biological hazards. But even the most impermeable material can fail if the seams, seals, closures and other interface areas don’t provide the same level of protection as the material itself. This drives the necessity to perform full ensemble evaluations, in addition to the standard material-level chemical permeation resistance testing.

The MIST method

The American Society for Testing and Materials (ASTM) International provides test methodologies for evaluation of numerous material and garment characteristics and properties. The National Fire Protection Association (NFPA) provides the performance criteria that the protective ensembles must meet to be certified for the specific end uses. Included as a test method in the NFPA and ASTM standards is one of the only full ensemble evaluation methods that is currently used to evaluate and certify chemical protective ensembles.

The Man-In-Simulant-Test (MIST) provides an evaluation of how the ensemble performs as it would be used in the field. The test requires that human subjects don the chemical protective suit along with the gloves, boots, respirator/SCBA (self-contained breathing apparatus) and any other component that would normally be worn. Thirty passive adsorbent dosimeters (PADs, which are air samplers) are placed in areas with different skin sensitivities and at ensemble interfaces.

During the test, the human subjects perform exercises and job-related tasks that are designed to stress the interfaces of the ensemble so that the protection level can be evaluated in a dynamic scenario. This is done in a controlled environment chamber. The PADs adsorb any of the simulant that infiltrates the suit through poor closures or inappropriate zippers or seams.

Following the test, the amount of simulant on each PAD is measured and related to a dosage inside the ensemble at that particular body region, and finally a protection factor is calculated by creating a ratio of the dosage outside the suit to the dosage inside the suit.

Making MIST work

Even though the standard committees and organizations have been working to improve many of the test methods for years, the MIST methodologies were mainly ignored because the MIST was thought to be too difficult to conduct, too time consuming and too variable. The most important misconception about the test that was addressed in NCSU’s research was the source and meaning of the variability.

Most people associated the variability in the test results with the use of human subjects because each person wearing the garments is different. The results from the MIST evaluations showed that the level of the protective ensemble and therefore the design drastically affected the variability associated with the resulting protection factors across the body: the MIST results had nothing to do with the subjects inside the ensemble and everything to do with the design of the suit itself. Basically, the variability indicates where the weaknesses are in the suit design.

What was originally thought to be a downside of the test was shown through research to be an extremely beneficial aspect of a test that for once did exactly what it was designed to do. Only through conducting the test in the appropriate manner and understanding the data was this concept realized.

Bryan Ormond, Ph.D., is CBRN research assistant professor at the
Textile Protection and Comfort Center, North Carolina State University.

Advanced Textile Products, a division of IFAI, represents the interests of the textile industry in safety, protective, interactive, medical and other high-tech applications.

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