ENGINEERING TURNOUT SYSTEMS FOR HEAT STRESS REDUCTION: DuPONT`S NOMEX OMEGAT BUNKER GEAR
BY ROBERT F. DAHL
FIRE SERVICE PROTECTIVE APPAREL
“The leading cause of fatal injury for on-duty firefighters in 1996 was, in fact, stress, as it has been in almost every year of this 20-year study, and this stress usually resulted in heart attacks.”–“1996 Fire Fighter Fatalities,” NFPA Journal, National Fire Protection Association, July/August 1997.
The most threatening stress firefighters face is heat stress. A potential killer, heat stress is a combination of body heat trapped by turnout gear; heat from the fire; and the strain created by wearing heavy clothing in hot, humid environments–not to mention the hard work of fighting a fire. In the NFPA study cited above, 44 of the 45 firefighter on-duty heart attacks in 1996 were attributed to stress or overexertion. In the same year, almost 26 percent of all firefighter injuries were caused by overexertion and strain. This may be only part of the story, since stress can cause both physical and mental fatigue, which can lead to other kinds of fireground injuries.
Adding to the heat stress problem is the need for higher levels of thermal protection. Today`s fires, fueled by a proliferation of synthetic materials and accelerants, burn hotter and faster than before. In addition, firefighters are entering structure fire environments with greater frequency than days past, penetrating deeper and staying longer. And early fire detection systems are bringing firefighters to the fire in many cases prior to flashover. To increase the levels of thermal protection needed under such conditions, turnouts became heavier, stiffer, and more uncomfortable, adding to the heat stress equation.
THE THERMAL PROTECTION PARADIGM
For years, it was understood that to gain increased turnout thermal protection, it was necessary to sacrifice the firefighter`s comfort, flexibility, and mobility. When fully outfitted, firefighters had to work harder to execute any task, which consequently contributed to heat stress. On the other hand, to provide greater comfort for firefighters, the thermal protection had to be reduced by using materials of lighter weight and lower bulk. If we were going to make any kind of progress against heat stress reduction, we had to break out of that paradigm.
Since turnout gear contributes to the heat stress problem, it somehow had to become part of the solution. The answer required a fresh approach to thermal protection.
Testing by DuPont, both in the field and in the lab, showed that no single layer alone could deliver the maximum flexibility, mobility, and light weight necessary to provide the thermal protection needed and still reduce heat stress. The dilemma meant digging deeper for the solution. We had to consider all turnout components–outer shell, moisture barrier, and thermal liner–as integral parts of an entire “system” and engineer a “total materials” solution.
SETTING PERFORMANCE REQUIREMENTS
We began by identifying the key performance areas of a turnout system–thermal performance and heat stress performance–as well as the test criteria and methodology (see Figure 1) needed to measure and evaluate these areas. The NFPA and the Canadian General Standards Board (CGSB) already established some of the tests and testing parameters. The American Society of Testing and Materials, independent universities, and DuPont had developed others.
If you think about the tests and requirements as climbing a ladder to optimum performance, the first rung of the ladder would be the current edition of NFPA 1971, Standard on Protective Ensemble for Structural Fire Fighting. Any material would have to meet all aspects of this standard as a minimum. We also sought to add the performance requirements of the CGSB, since we wanted a materials system for all of North America. We then added the additional performance requirements and tests that, when taken together as a whole, would provide a solution to the thermal protection vs. heat stress protection problem.
THERMAL PERFORMANCE REQUIREMENTS
Based on our interactions with firefighters from around the country, we realized there was a desire to maintain the current level of thermal protection obtained in the material combinations available today. We evaluated these current protection levels and selected tests to help us evaluate thermal protection for various combinations of materials: the Thermal Protective Performance (TPP) test, Radiant Protective Performance (RPP) test, and whole-garment thermal protection testing using DuPont`s Thermo-ManT test manikin.
The TPP level prescribed in NFPA 1971 is 35, but most fire departments look to have a margin of safety beyond the minimum. So we selected a performance level of 40 TPP to offer the same level of protection provided by existing material combinations. (DuPont also has addressed firefighting situations requiring 48 TPP.)
The current NFPA 1971 standard does not include a performance requirement for radiant heat exposure; however, we recognized that firefighters regularly are exposed to radiant heat situations. Therefore, we selected the RPP test method used in NFPA 1977, Standard on Protective Clothing and Equipment for Wildland Fire Fighting, using an exposure heat flux of 0.5 cal/cm2/ sec to replicate similar radiant heat environments seen by structural firefighters.
Finally, we selected a test protocol that could evaluate the entire garment for thermal protection (not just a swatch of outer shell) in a simulated flash fire to test the integrity of the materials and garment in the worst of firefighting situations. The DuPont Thermo-ManT test employs a life-size, instrumented manikin equipped with 122 sensors that model skin and measure second- and third-degree burns and their locations. We dress the test manikin in complete turnout gear to test the level of thermal protection each garment material system provides. A computer-controlled flashover engulfs the figure in flames for 10 seconds at 2 cal/cm2 while the computer performs millions of calculations per second to determine potential burn injuries. We selected a performance requirement of 10 percent total body burn injury as the maximum allowed for this severe test because information from the American Burn Foundation indicates the survivability of people with burn injuries over less than 10 percent of their body is extremely high. The Nomex OmegaT system consistently achieved results of less than four percent body burn injury in this test.
HEAT STRESS PERFORMANCE
Achieving lighter weight and lower bulk allows for increased mobility with less exertion. Our testing and analysis show both factors can contribute to heat stress reduction, as does increased flexibility. We selected performance requirements and tests that could improve the critical areas affecting heat stress, such as total composite weight, composite bending stiffness, inner liner friction, and total composite heat loss.
Our field interactions indicated a continual firefighter request for a reduction in the total turnout garment weight. We therefore chose a performance requirement limiting the total composite weight to 17.0 ounces per square yard.
Next we looked for a way to assess flexibility and mobility of materials that make up the turnout ensemble–a test that could measure the ease of bending materials, similar to bending your knee or elbow. If we could make the materials easier to bend, it would reduce the effort required by firefighters to move. There was no such test, so DuPont invented a Composite Bending Stiffness test, which replicates the bending movement of a knee or elbow and measures the force required to bend the material composite to a 90-degree angle.
The inner liner also plays a big part in making the wearer`s movements more fluid and natural. Reducing friction of the inner liner reduces the hobbling effect of wearing a bulky garment like a turnout and can reduce heat stress. Here again, the idea is to reduce exertion in every way possible. DuPont selected the ASTM sled test, which measures the coefficient of friction of various fabrics, and set a maximum friction force.
Ridding the body of heat buildup is critical to reducing heat stress. Testing shows it is significantly contingent on interior components of the turnout. A complex interaction of moisture and heat transport is involved in the process. With so many moisture barriers and thermal liners on the market, there were hundreds of possible combinations to test. Using the guarded seating hot plate (the proposed NFPA test method), we set a performance requirement to maximize total heat loss of the material composite.
DuPont developed the various performance requirements and tests to optimize thermal protection and heat stress reduction. We then sought the combination of materials that could meet the demanding performance required by the tests. From this research we developed the Nomex OmegaT turnout system.
The key to this system is a “smart” fiber developed by DuPont to make a breathable, flexible outer shell fabric that expands slightly when exposed to extreme heat and flame (approximately 5007F), increasing the air layer between the outer shell and moisture barrier and creating an extra insulating barrier not unlike the insulating air in a thermal pane window. This extra insulation means the outer shell is capable of providing greater TPP.
With a better outer shell TPP, we could remove weight and bulk from the interior components and reduce friction in the inner liner fabric. The Nomex OmegaT turnout system is DuPont`s solution in its search to break the thermal protection paradigm and help reduce stress on the fireground. n
The data above outline tests and test standards for NFPA and CGSB performance testing. They also indicate the additional testing of the NOMEX OMEGA? System using ASTM, independent universities and the exclusive DuPont Thermo-Man? manikin.