Hazmat protective clothing: Testing changes plannedThe measurement at the heart of this debate is the chemical permeation breakthrough timeBy Jeffrey O. and Grace G. Stull
Many people in the firefighter protective clothing industry probably do not remember that, up until 1986, the principal measure for thermal insulation in bunker clothing was just thickness of all the layers used in its construction.
That requirement specified that all three layers had to have a thickness that was greater than 0.175 inches. Of course, materials used in firefighter protective clothing had to meet other criteria in being strong enough as well as being able to resist the effects of heat and flame, but it was only thickness that established the basis of adequate thermal protection.
Fortunately, the test for thermal protective performance was developed, which established a realistic measurement of the amount of energy that went through the material composite that could be related to the prediction of a second degree burn.
After its introduction, the TPP test forever changed the way that clothing manufacturers selected and promoted material systems.
The hazardous materials protective clothing industry is facing a similar pivotal change, but unlike structural firefighting of years ago, some in the industry are questioning the need for revamping its methodology to a more meaningful measurement.
The measurement at the heart of this debate is the chemical permeation breakthrough time and its application to the totally encapsulating suits addressed by NFPA 1991 Standard for Vapor Protective Ensembles for Hazardous Materials Emergencies.
Breakthrough time has been the principal way of communicating the acceptability of a material used in chemical protective clothing when there are significant and serious chemical hazards.
In its general form, it is defined as the elapsed time where a chemical placed on one side of the material is able to diffuse through the material and be detected on the other side.
This would seem to be a simple concept but in fact the measurement is not what it appears to be. The name “breakthrough time” implies that no chemical gets through the material, but in reality, the technical basis for measuring breakthrough time is to define breakthrough time as the time the level of chemical permeation reaches a certain rate.
That rate is set at a low level but it was established only for the convenience of the laboratory and has no bearing on the toxicity or other hazards associated with the chemical.
Chemical permeation testing is performed in NFPA 1991 at completely unrealistic conditions. Chemical exposure is at 100 percent over the entire 1-hour test period. This is akin to being fully immersed in a liquid chemical or in an atmosphere that is at a 100 percent concentration for gases for the entirety of the 1-hour exposure.
These exposures are not only unlikely, they are impossible. The combination of an overly severe exposure (challenge) condition and overly sensitive measurement for chemical permeation results in criteria inhibit the potential for material technologies that are cheaper, more flexible, and more comfortable to end users.
Nevertheless, at this time, the committee is only seeking to modify the information that is reported to qualify the material. A recommendation is not being made to reduce the current test challenge exposure conditions at this time other than to use more representative (higher) test temperatures and humidity that end users face.
Cumulative permeation is being proposed as a replacement measurement for breakthrough time. Cumulative permeation is the amount of chemical that comes through a material in a given period of time.
Unlike breakthrough, this measurement provides an assessment of the overall dose over the exposure time, which in turn can be related to permissible exposure levels.
It is the same principle that is used for defining acceptable respiratory protection, where even current fire service self-contained breathing apparatus are evaluated for how much chemical agent can pass into the breathing stream as part of their CBRN rating.
The measurement of cumulative permeation also parallels the measurement of heat transfer via TPP testing, which as an analog measures the amount of heat energy that passes through a material over a given time.
Many people get confused with trying to relate breakthrough time to cumulative permeation because it seems that in the case of breakthrough time, they have no exposure, but with cumulative permeation they are now accepting exposure.
This is not true because chemical is actually already passing through the material before the breakthrough time is defined. The two values cannot be directly compared because they involve completely different measurements – one measures a time to a certain rate of permeation, while the other measures how much gets through the material in a given time.
The proposed limit of cumulative permeation for NFPA 1991 is 6.0 micrograms per square centimeter of material (μg/cm2). This is the equivalent of a speck of dust almost too small to see laying on a square yard of material.
In contrast, NFPA 1991 currently states the breakthrough time can be no shorter than one hour, but the breakthrough time is when the amount of chemical going through the material reaches a rate of 0.1 microgram per square centimeter of material per minute (μg/cm2min).
The units of measurement might seem similar but they are very different. Yet, it is possible to relate the permeation rate to a cumulative permeation or dose.
The testing exposure period specified by NFPA 1991 is for one hour. If the amount of chemical permeating a material were to immediately jump to a rate just below 0.1 microgram per square centimeter of material per minute, then the total amount of chemical would be nearly 6.0 micrograms per square centimeter of material.
This is no coincidence but rather the committee’s attempt to make a first step toward implementing a meaningful measurement by making a preliminary change in the standard that does not result in a reduction in the current expected material performance.
Two questions arise when the above analysis is provided: Is the proposed value of 6.0 μg/cm2 safe and if cumulative permeation can be related to breakthrough time, why is cumulative permeation a better measurement?
We'll go into detail on these items in the second part of this column.