BY RON BOVE
Turnout gear and Its components are becoming increasingly complex, as are the issues surrounding their selection. Since they are required and relied on to durably do far more than they have in the past, new materials and test methods are being developed to satisfy these needs. Among these new test methods is the Total Heat Loss Test, now a requirement of NFPA 1971, Standard on Protective Ensemble for Structural Fire Fighting-2001 edition.
Total heat loss (THL) is the amount of conductive (dry) and evaporative (wet) heat loss that occurs through the three layers of a turnout ensemble-outer shell, moisture barrier, and thermal liner. It is a performance evaluation that’s drawing more and more attention among a range of interested parties, and it promises to confront the industry with a complex array of competing facts and variables.
Certainly, no one ever doubted that heat trapped inside a turnout suit would have ill effects on a firefighter. But as the universe of knowledge surrounding firefighter physiology expands and as the impact of clothing materials becomes more precisely measurable, the level of THL performance in turnout ensembles is becoming a matter of increasing visibility and importance. For instance, it is a well-known fact that relatively small increases in body core temperature (e.g., 37.0°C to 38.5°C) can translate to immediate and serious threats, particularly to a firefighter’s ability to make sound judgments and rational decisions.
Acknowledging that the heat stress hazard must be addressed, however, is only the beginning in a series of facts, figures, results, and variables that make a common approach to THL problematic. Compounding the complexities of the THL issue is the enormous assortment of possible component materials that make up turnout ensemble combinations-which provide varying levels of THL while also having a significant impact on thermal protective performance (TPP).
LAYERS OF COMPLEXITY
As manufacturers and department decision makers sift through the data, the following are thumbnail descriptions of the intertwined and competing variables they will encounter. Start with the premise that, to date, the only real point of agreement begins and ends with the conclusion that there is such a thing as heat stress. From there, facts and viewpoints diverge.
- Assuming that moderate to hard physical activity performed in a turnout suit generates heat trapped next to the body, how is the rate of heat generation (or dissipation) measured? Should it be measured in the lab or in simulated field tests? Which methodology is more accurate and reliable?
- Assuming agreement that trapped heat should be allowed to dissipate or evaporate, at what level should an acceptable THL value be set? At a minimum level just barely above the low value that a nonmoisture vapor breathable-based ensemble (e.g., neoprene) could achieve? Or at a higher level that provides more potential for heat stress reduction to a firefighter?
- Of the three main turnout ensemble components, which has the greatest effect on THL? And how do the various combinations of thermal liner, moisture barrier, and outer shell measure up when evaluated on a THL vs. TPP basis?
Some of these questions are well on their way to being clearly answered. Others will remain the focus of debate and different perspectives for the foreseeable future. In the meantime, however, there are some signposts that are useful for navigating through the maze of total heat loss issues.
HEAT EXCHANGE
Among all the THL issues, the one that is universally agreed on is that physical activity performed while wearing turnout gear generates excess body heat, which, if not allowed to evaporate or dissipate, is then cumulatively stored by the body. This is especially true, obviously, in an unusually warm or high-heat environment.
Of course, one of the functions of turnout gear is to protect a firefighter from high levels of ambient heat. More specifically, gear is designed to protect against outside sources of conductive, convective, and radiant heat transfer. Therein lies the conundrum. As the component layers of a turnout ensemble perform their role of protecting against heat ingress from outside heat sources, these layers, depending on the various fabric technologies, negatively impact the amount of body heat that can escape by typical heat transfer mechanisms. Evaporative heat transfer, therefore, becomes the primary avenue through which excess or stored body heat can be dissipated. Prior to the addition of the minimum THL requirement in NFPA 1971, the heat buildup problem was worsened by the use of nonbreathable moisture barriers, which prohibited evaporative heat from transferring through the ensemble.
As physical exertion generates internal heat, the heat that cannot dissipate represents heat stored in the body. This results in an increase in the body’s core temperature. Enough of an increase in core temperature leads to heat stress in its various forms. For instance, just a 1.5°C to 2°C (2.7°F to 3.6°F) increase in core temperature leads to fatigue, exhaustion, nausea, and diminished decision-making capacity. A rise of only 5°C to 6°C (9°F to 10.8°F) above normal body core temperature is enough to cause a fatality. Furthermore, core temperatures may, unwittingly, continue to rise for as long as 30 minutes following the cessation of activity or exertion. This means that a firefighter’s core temperature may actually continue to rise-and heat stress conditions may progressively develop-during a rest phase after a firefighter has stopped working.
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These facts, then, leave little room for debate about the dangers posed by trapped heat and resultant heat stress. The questions and complications come, first, when the conversation turns to methods of replicating real-world exertion levels and measuring in a meaningful way the effect that clothing has on heat stress levels; and, second, they come when the industry debates how and where to set THL standards that are equitable and reasonable but, most importantly, beneficial.
MEASURING HEAT STRESS AND ITS EFFECTS
Part of the difficulty in finding common ground on heat stress and THL is the variety of approaches to testing and measuring. One school of thought focuses on testing human subjects in field trials. Given all the possible variables among individual human physiology, the materials being tested, and the test designs themselves, some observers believe that these methods have inherent shortcomings in obtaining consistent or reliable results.
Testing and measuring in the lab is another approach. Yet these methods have drawbacks as well. It is impossible, for instance, to replicate fireground conditions in a lab. This is particularly true for the nonuniform (i.e., unsteady-state) work rates that a firefighter would expend in the field. In addition, the simpler lab-based methodologies primarily test only moisture barrier performance and cannot be used to accurately evaluate multilayer ensembles.
Over time, the testing and measuring issues have sometimes raised as many questions as they have answered. If, for example, it is difficult to objectively measure heat stress levels for a layered ensemble, is it even possible to predict which component of the ensemble figures most prominently in alleviating the dangers?
In a series of human subject-based field tests over a period of years from 1982 through 1998, it was established that the moisture barrier is the most important factor in the heat buildup incurred by wearing turnout gear and that breathable moisture barriers are vastly preferable to nonbreathable barriers. This ultimately led to the phasing out of neoprene and other nonbreathable moisture barriers and to the ongoing evolution of alternative moisture barrier technologies. Each of the different technologies that exist today (e.g., microporous, monolithic, bicomponent) strives to balance the performance of THL with the TPP, durability, and overall cost of the product.
As ensembles with moisture vapor breathable barriers began to be compared against other ensembles with moisture vapor breathable barriers, it became apparent that the outer shell and the thermal liner also play significant roles in affecting heat stress. Yet the issue of, specifically, how to test and measure all three layers of an ensemble composite vs. just the moisture barrier itself remained a difficult discussion throughout NFPA 1971 Technical Committee meetings. The sweating hot plate had already been specified-with an associated minimum performance requirement-within NFPA 1977 (1998 edition) and was being considered in revision drafts of other NFPA standards. Eventually, test method ASTM F 1868 (Part C)-Standard Test Method for Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate-was accepted by the 1971 Technical Committee for measurement of THL. But, as with so many aspects of the firefighter physiology/turnout gear technology controversy, this was far from the end of the story.
TEST PATTERN
Explained in simple terms, the sweating (guarded) hot plate is a device that can test relatively large (typically 20- 2 20-inch) segments of turnout ensemble layups consisting of a thermal liner, a moisture barrier, and an outer shell. By measuring the dry (thermal) and evaporative (wet) heat transfer through the sample on this device, the THL value can be determined through a mathematical combination of the results. The THL value of a sample is expressed in Watts per square meter (W/m2) or the amount of heat (Watts) “lost” through a square meter of material, in a standard environment.
To the casual observer, it would seem that the whole matter would have logically concluded at this point in time. It was known that heat stress is a hazard, and now-with the ASTM test method-there was an accepted way to measure THL values for any given turnout suit ensemble. What more was left to discuss?
A great deal, as it turns out. While the NFPA 1971 Technical Committee acknowledged heat stress was a safety issue that could be reduced through turnout material performance requirements, there continued to be many debates over what the particular test methodology or test parameters should be. In particular, there was much discussion over whether a sample should be tested for THL as received or after some form of washing/preconditioning. Not only that, but once it was agreed that THL should be evaluated on as received samples, there were still further questions about the level at which the NFPA 1971 minimum performance requirement should be set.
Today, NFPA 1971 (2000 edition) includes a minimum THL requirement for each ensemble combination. The sweating guarded hot plate apparatus is established officially as the device of choice for compliance purposes, with the procedure in accordance to ASTM F 1868, Part C. Multiple test facilities now have operating sweating hot plate apparatus, and ASTM has recently conducted another round-robin evaluation of lab-to-lab precision for sweating hot plate test results. However, as to the proper or acceptable level at which the THL performance requirement should be set, the debate lingers on.
Most basically, the controversy centers on the minimum THL performance standard established in NFPA 1971 (2000 edition), which is 130 Watts per square meter (130 W/m2). To many, this THL requirement represents only a marginal improvement at best over the THL performance of a turnout suit with an impermeable moisture barrier (e.g., ensembles with neoprene moisture barriers typically generated THL results of ~100 W/m2). The 1998 International Association of Fire Fighters Physiological Study (i.e., the human subject-based “Indy Study”) concluded that the optimal THL recommendation for “heavy” work (i.e., from the ladder simulation) would be 205 W/m2 and that the optimal recommendation for “moderate” work (i.e., from the extrication simulation) would be 170 W/m2. Both are clearly above the NFPA 1971 requirement and at the time were thought to be at the high end of THL results possible for turnout gear ensembles. However, today-basically two years into the requirement-THL results above 200 W/m2 have been established for an extensive list of ensemble combinations (using competing moisture barrier alternatives), with some as high as 300 W/m2.
While there are widely disparate points of view on what constitutes an “acceptable” THL standard vs. a “prudent” one, there is still one more set of complicating factors for department decision makers to sort through. Given the possible combinations of ensembles among the shell, moisture barrier, and thermal liner options, the range of possible THL values across all these combinations is also enormous. Test results for one sample ensemble layup would be different, for instance, if just its outer shell component were changed, and so on with moisture barriers and liners. As well, different ensemble combinations provide different TPP results, which can also affect the THL values. (Typically, the basic trend in the THL and TPP data is that there is a trade-off; the higher the THL result is, the lower the TPP result will be; and the higher the TPP, the lower the THL.) And, too, not insignificant to a department’s component material selections are the other issues of care and maintenance of the gear once it is put into service, initial price, durability/life cycle costs, and so on.
THL and TPP numbers are available from manufacturers and suppliers for many turnout ensemble combinations; however, not every possible combination has been certified to NFPA 1971 or is offered for sale in the marketplace. The test results that have now been generated can be evaluated both visually and statistically. When THL results are plotted on a chart vs. TPP results, the data form clear patterns and provide graphically intriguing indicators as to the disparities among turnout suit component combinations. Moisture barrier selection remains the predominant variable for the THL results but not for the TPP results. And when comparing ensembles that incorporate the same moisture barrier selection, the plot immediately depicts that the outer shell and thermal liner also have significant impact in the overall balance of the ensemble THL-TPP performance. The choice of thermal liner is the second most significant factor in determining ensemble THL results.
Additionally, with a large and still increasing database, it is also possible to use statistical analysis software to calculate predicted TPP and THL values for any outer shell, moisture barrier, and thermal liner combination within certain standard deviations. These predictions can be-and already have been-used in computer demonstration programs that benefit departments or specifying individuals by allowing them to compare the THL-TPP trade-offs that would occur as a result of different material selections or ensemble combinations. Eventually, such predictive models could become CD- or Web-based, with the reliability of the predictions continuing to improve as more and more data are collected, for both new and repetitive ensemble combinations.
For the present, then, some of the ambiguities surrounding THL have been clarified, and appreciation for this performance measurement is gaining ground. Yet, there still are questions that remain in the shadows and uncertainties that need to be addressed. As a whole, the fire service community needs to, and will, face these challenges as it strives to provide better and safer personal protective equipment (PPE) for the individuals who require it.
Ron Bove is a business leader/product specialist with the Fire & Safety Services Group of W.L. Gore & Associates, Inc., manufacturer of the CROSSTECHT fabrics used in fire service garments, gloves, and footwear. He has a BChE degree in chemical engineering and has been involved in the protective clothing industry for more than 20 years.
