“A chink in one’s armor” – an expression intended to denote an area of vulnerability. For firefighters, our turnout clothing ensemble is our “armor.” But what’s our weakest spot?
Structural firefighting PPE encompasses a full ensemble that includes the coat, pants, helmet, hood, gloves, boots and SCBA. Each of these elements is intended to provide a commensurate level of protection. And while firefighters and manufacturers have tried to make this so, there are, in fact, challenges in providing uniform protection for each part of the firefighter’s body – and one area that’s particularly difficult to evaluate.
Homogeneous testing methods prove problematic
Back in 1997, the concept of a PPE ensemble was formalized when NFPA combined all the separate garments standards into one standard – NFPA 1971: Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting. This consolidation was intended to ensure that the separate requirements for each of these elements would be brought together so protection could be more homogenous for each part of the firefighter’s body.
Initially, this proved more difficult than expected because, historically, there were significant differences in the ways individual products were tested. The unique methods used for garments were quite dissimilar than those applied to helmets, gloves and footwear. In many cases, these differences were simply because of the nature of the product. For example, most testing of garments is conducted on the individual materials. In contrast, many tests performed on helmets, gloves and footwear involve the evaluation of the full item. These disparate approaches point to the vastness of choices for garments as compared to the other elements of the ensemble. Most major manufacturers offer hundreds of choices in the ways that different materials can be combined to create a variety of performance characteristics for garments. This is not the case for the other elements where choices are much more limited.
The consolidation effort did strive to achieve balance among key performance parameters, such as flame resistance, heat resistance and thermal insulation. This goal was pursued to provide the same levels of skin protection, but this effort has proved challenging. Both garments and gloves are designed to provide a thermal protective performance (TPP) of 35 calories per square centimeter. There are varied insulation requirements for helmets and for footwear; completely different metrics are used to ensure safe levels of heat protection. This is because footwear products are generally much thicker than garments or gloves, and heat protection is less of an issue for helmets and footwear.
Elsewhere, hoods must have a TPP of 20 cal/cm2 because they are considered an interface device that is partly protected by helmet ear covers, which also have a TPP of 20 cal/cm2. The same is true for coat, wristlets and glove wristlets, which have TPP requirements of 20 cal/cm2. These lower TPP levels are based on a philosophy that interface areas (head/neck and wrist) have a combined TPP that is greater than 35 cm/cm2.
So, if TPP requirements are the same for garments and gloves, helmets and footwear do not need TPP requirements because these products exceed the minimums for garments and gloves, and other parts of the ensemble are addressed by having a combined TPP value that is more than twice that of garment or gloves, then where is the chink in the armor? The answer lies in understanding whether TPP truly is balanced among the different elements of the ensemble.
Bottom line: It is my belief that TPP can fall short on accurately representing glove heat insulation. To understand why, we must first understand the TPP test process.
Understanding the TPP test
TPP has been a fundamental performance property for protective clothing since the mid-1980s. Before that time, adequate garment thermal insulation was gauged by the overall thickness of the composite. TPP measures the amount of heat energy that passes through the clothing materials from an exposure that is equivalent to a flashover or backdraft and determines the time that would be required to cause a second-degree burn injury of a person’s skin. TPP is a valuable test because it relates burn injury to the protective qualities of the clothing item; however, like any test, one must have a true understanding of the measurement and be aware of its shortcomings.
For testing purposes, the exposure conditions are constant at 2.0 calories per square centimeter (cal/cm2) per second, which is within the range of heat produced by a flashover or backdraft. The exterior side of protective clothing materials are exposed to this heat level from a combined source of burners and a radiant heat panel below the sample. The exposure itself is controlled by a shutter that is opened to allow heat impingement on the clothing sample. On top of the sample is a device known as a copper calorimeter, which is a copper disk that has embedded thermocouples mounted in an insulating board. The copper disk is used instead of the thermocouples by themselves because the mass of copper has to heat up in order to register temperature increases, whereas thermocouples produce instantaneous temperature measurements. This characteristic is important because heat injury is caused by the skin itself attaining threshold temperatures where damage is caused. The TPP calorimeter has been designed to mimic skin exposure for relatively short-term exposures (generally less than 1 minute) and thus offers a scientifically valid way of predicting burn injury for human skin. Many decades ago, detailed experiments were conducted to correlate the heat energy measured in the copper calorimeter versus heat levels that cause burn injuries on actual people to come up with this relationship.
When a TPP test is run, the shutter is opened, and the copper calorimeter begins measuring how much heat comes to the material until the exposure is terminated. For today’s automated test apparatuses, exposure to the heat sources is stopped after the amount of heat passing through material absorbed by the calorimeter projects is sufficient to predict that a second-degree burn would have occurred. This time to a second-degree burn is then multiplied by the heat exposure energy to come up with the TPP rating that many firefighters may be familiar with for garments at 35 cal/cm2. While this translates into a time of 17.5 seconds from start of the exposure to predicted second-degree burn, the attempted use of this information for determining a “safe” time for firefighter within a structural fire is fraught with all sorts of problems.
TPP test shortcomings
It is best to think of TPP as a “benchmark” test that allows the industry to compare clothing for determining which materials have greater insulation than others. However, the fire service must also understand that TPP values are affected by the relative precision of the test and that small differences in TPP values may be meaningless and simply due to testing variability. Recalling how TPP testing is performed also leads to recognizing that the test does not account for the air layer between the interior side of the clothing and the individual’s skin, the condition of the clothing (such as being wet), any prior levels of heat exposure when heat absorbed into the materials, and many other factors. Thus, TPP represents only one set of conditions among the myriad other types of heat exposures that firefighters can face. This is why it can be dangerous to assume that the TPP value relates to any given time for actual protection on the fireground. The committee responsible for setting the NFPA 1971 standard, which specifies the minimum level of TPP, has set the current level based on its review that this minimum requirement provides a reasonable level of protection from burn injury for the majority of firefighter exposures. However, this standard also cautions that firefighters can still become burned by analogous heat conditions where clothing protection capabilities are overwhelmed or, at the very least, are different from the exposure model addressed by the TPP test.
Relative to PPE vulnerabilities, TPP has another disadvantage. Since materials are set horizontally into the test apparatus, different types of materials can react in various ways that affect how much insulation is provided. Most intrinsically flame-resistant textile or laminate materials, such as those used in garments, are relatively stable in retaining their form, though they still deteriorate in place under the high-heat conditions simulated in a TPP exposure. On the other hand, there are materials that are less stable to high-heat exposures – for example, leather, which while staying intact can dramatically shrink. By shrinking during the TPP test, some of these materials will buckle and create folds that in turn comprise air pockets that lead to artificially high levels of predicted insulation. It is for this reason that TPP testing may inflate the performance of glove composites for thermal insulation.
The weak link: Glove testing
Protection of the hands is relatively unique because the hands have a large amount of surface area for very little volume. This means that heat absorbed into the skin of the hand has less mass for being dissipated. Moreover, this challenge is made more manifest by the fact that hands have a greater need of motion and function compared to other parts of the body. To simply build turnout gear for the hands would not be an acceptable solution because of the significant impact such a bulky product would create for severe hindrance of hand dexterity, tactility and grip. The use of leather and similar materials is an attempt to get around this dilemma, but shortcomings of the TPP test may result in less protection than actually provided by other elements of the ensemble.
The NFPA 1971 standard does include other thermal insulation tests for gloves, both for the back of the hand by using stored heat energy testing (a form of radiant testing under ordinary fireground conditions) and for the conductive heat resistance testing for the palm side of the gloves. Still, providing optimal thermal insulation (and physical protection) for the hands is always at odds with maintaining needed hand functionality. This perennial problem is being examined by manufacturers and many industry groups, including some groups that are leading research designed to examine zoned protection levels for gloves. However, for the time being, it is important for the fire service to be aware that hand protection must be carefully investigated with the recognition that the protection of hands is the chink in the armor for fire service PPE.
Note: The views of the author do not necessarily reflect those of the sponsor.