BY ROGER BARKER, SHAWN DEATON, JOHN MORTON-ASLANIS, AND MARC MATHEWS, NORTH CAROLINA STATE UNIVERSITY (NCSU) TEXTILE PROTECTION AND COMFORT CENTER (TPACC)
This supplement describes the state-of-the-art system-level testing methodologies for evaluating the thermal protective performance and heat strain contribution of firefighter protective ensembles. System-level evaluations can be described as using either the skin-outward approach (where material composites are testing in the sample configurations that they are worn in, including all layers) or the full-garment or ensemble approach (where the entire garment or ensemble is evaluated “as-worn” with all other ensemble elements).
This sidebar discusses the following test methods in Annex G of the newly revised and consolidated NFPA 1970:
- Full Ensemble and Element Thermal Testing
- —Full Ensemble Flash Fire
- —Cylinder Thermal Protective Performance Test (C-TPP)
- Full Ensemble and Element Physiological Testing
- —Measured Ensemble Heat Loss Test
- —Measured Ensemble Evaporative Resistance Test
- —Glove Evaporative Resistance Test
- —Footwear Evaporative Resistance Test
- —Hood Evaporative Resistance Test
Full Ensemble Flash Fire Testing
This testing is based on ASTM F1930-23, Standard Test Method for Evaluation of Flame-Resistant Clothing for Protection Against Fire Simulations Using an Instrumented Manikin. The test allows the measurement of the thermal protection provided by different materials, garments, clothing ensembles, and systems when exposed to a fire. In the test, a stationary upright instrumented manikin is exposed to a simulated fire environment having a controlled heat flux, flame distribution, and duration (Figure B1). The average exposure heat flux is 84 kW/m2, which can last as long as 20 seconds.
Energy transferred through and from the test specimen during and after the exposure is measured by sensors that are distributed as uniformly as possible within each area on the manikin, located at the surface of the manikin. These are used to predict second- and third-degree burn injuries resulting from the exposure. For full garment evaluations, F1930 reports if there were predicted second- and third-degree burn injuries and their respective locations on the manikin (Figure B2). Subjective observations can include after flame or ignition of the garment the amount of smoke generated; if the garment had shrinkage; char formation; and if it melted, broke open, or fell apart.
Figures courtesy of authors.
One of the drawbacks of F1930 is that the effects of movement on thermal protection are not addressed. A firefighter may also have a significant reduction in fire protection under dynamic conditions when compared to the standard static testing conditions performed in ASTM F1930.
Separate Element Thermal Testing
Heat transmission through clothing is largely determined by its thickness, including any air gaps. The air gaps can vary considerably in different areas of the human body. This method provides a means of grading materials when tested under standard test conditions where an air gap exists between the fabric and the sensor. During the exposure, fabric temperatures can exceed 400°C. At these temperatures, some fabrics are not dimensionally stable and can shrink or stretch. The cylindrical geometry used in this test method allows such motion to occur, which will affect the time to achieve the end point of the test.
This equipment for the test method is like ASTM F2700 (used for TPP testing) in that it uses the same energy heat source, water-cooled shutter, and data acquisition. It measures the heat transfer through protective clothing materials using a copper calorimeter (Figure B3). This test method differs from Test Method F2700 in the usage of an instrumented cylinder mounted horizontally, which allows for the thermal shrinkage of materials when tested. A test specimen is wrapped around an instrumented test cylinder that is horizontally positioned and exposed to a combined convective and radiant heat source with an exposure heat flux of 84 ± 2 kW/m2 (Figure B4). Using the X-inch spaced configuration allows samples to shrink, eliminating the air gap and increasing the rate of heat transfer as shown in Figure B5.
The data in Figure B6 shows testing a material that is not dimensionally stable at higher temperatures. The TPP test shows a 78% increase in protection when going from contact to spaced configuration while the C-TPP only shows a 45% increase in protection.
Systems Tests for Evaluating Contribution to Firefighter Heat Stress
Sweating manikins. Guarded sweating hot plates measure heat loss only through flat swatches of clothing materials. Sweating manikins are used to measure the heat loss and breathability of full-scale PPE clothing systems. The sweating manikin (Figure B7) measures insulation, evaporative resistance, and heat loss using the same principles as the sweating guarded hot plate that is used for both THL and Ret. The difference is that the sweating manikin has a human form that accounts for the air layer between garments and the human body, reinforcements, garment design, and fit. The manikin form consists of 22 separately heated zones. Moisture brought to the manikin’s surface causes it to “sweat,” enabling measurement of a garment’s ability to dissipate evaporative heat. The Newton type manikin at NCSU is 175 cm in height, with a skin surface area of 1.8 m2, to simulate an adult male. It has 134 sweating holes evenly distributed over the manikin surface.
Sweating manikins are used to measure garment insulation and evaporative resistance according to ASTM FI291, Standard Test Method for Measuring the Thermal Insulation of Clothing Using a Heated Manikin, and ASTM F2370, Standard Test Method for Measuring the Evaporative Resistance of Clothing Using a Sweating Manikin. Both dynamic and static tests can be conducted of clothing breathability. Dynamic tests involve using an external mechanical drive to “walk the manikin” to create forced convection through body motion. Dynamic manikin tests typically produce larger heat loss through both evaporative heat transfer and dry heat transfer than static manikin tests.
Evaluating heat loss through firefighting elements (gloves, boots, and hoods). Sweating thermal body form instrumentations (hand, foot, and head) is designed to evaluate heat and moisture management properties of glove, boot, and hood systems (Figure B8). These instruments simulate heat and sweat production, making it possible to assess the influence of ensemble elements on the thermal comfort properties for a given environment. Simultaneous heat and moisture transport through the various element systems and variations in these properties over different areas of the body form can be quantified.
The hand, foot, and head manikins operate on the same principles as the sweating manikin system. They consist of several features designed to work together to evaluate comfort and/or heatstress. Housed in a climate-controlled chamber, each body form is divided into separate sections or zones, each of which has its own sweating, heating, and temperature measuring system. Using a pump, preheated water is supplied from a reservoir located outside of the environmental chamber. An internal sweat control system distributes moisture to “sweat glands” across the surface of the manikin. Water supplied to the simulated sweat glands is controlled by operator entry of the desired sweat rate. Each sweat zone is individually calibrated, and the calibration values are used by the control software to maintain the sweat rate of each section.
Continuous temperature control for the hand segments is accomplished by a process control unit that uses analog signal inputs from separate resistance temperature detectors (RTDs). These evenly distributed RTDs are used instead of point sensors because they provide temperature measurements in a manner such that all areas are equally weighted. Distributed over an entire section, each RTD is embedded just below the surface and provides an average temperature for each section. Software establishes any discrepancy between temperature set point and the input signal and adjusts power to section heaters as needed.
Insulation and breathability of each ensemble element are measured following procedures for sweating thermal manikin testing adapted for testing of gloves on a hand manikin (ASTM F3426, Standard Method for Measuring the Thermal Insulation of Clothing Items Using a Heated Manikin Body Forms and ASTM F 3672, Standard Method for Measuring the Evaporative Resistance of Clothing Items Using a Heated Manikin Body Forms, respectively).
Heat Loss Potential (W/m2) is calculated for a standard environment by combining both the dry and sweating components of heat loss measured in their respective states.
Note: Redder, light colors represent lower heat loss.
Human Evaluated Wear Testing of Firefighter Ensembles
Full ensemble wear testing is the ultimate assessment for indicating user opinion and acceptance of a garment ensemble. A specific and limited human subject pool, following a protocol based on ASTM F2668, Standard Practice for Determining the Physiological Responses of the Wearer to Protective Clothing Ensembles, is used to evaluate protective garments where heat stress might impact wearer performance of job tasks.
Participants are instrumented to collect physiological data on heart rate, skin temperature, and core temperature while following a prescribed protocol, often on a treadmill at a controlled rate ofexercise (Figure B10). Wireless sensor technologies are used for data transmission and collection of readings on bodily stress indicators.
This advanced physiological monitoring platform is cutting edge technology that combines human physiology measurements with human movement. Wearer vital signs are transmitted noise-free via a Bluetooth interface. Data can be transmitted continuously or in intervals and can be viewed in real time. These data can then be plotted to show ensemble differences (Figure B11).
Tradeoffs Between Protection and Wear Comfort in Firefighter Gear
Traditionally, protective ensembles for firefighters were designed to primarily address the delicate balance between thermal protection and thermal comfort, with protection from emergency fireground conditions such as flashover being of highest importance due to the nature of responding to structural fires. Over the past few decades, the number of line-of-duty deaths associated with thermal injury on the fire scene decreased, while the fatalities associated with heat strain and cardiac events remained. This trend began to indicate to the fire service community and manufacturers that the protection-to-comfort balance was favoring thermal protection at the expense of the firefighter’s physiological health and well-being. With this realization, the NFPA committees began to revise the product standards across the first responder applications to place higher importance on heat loss requirements for all ensembles. For several years, these two parameters—thermal protective performance (TPP) and total heat loss (THL)—continued to be the main two concerns for the development of firefighter garment and ensemble designs. However, just a few years ago, this balancing act became more complicated with the newfound awareness of firefighter exposure to toxic fireground contaminants and the epidemic of cancer in the fire service. Now research studies are investigating exposure to both smoke particles as well as fireground vapors, manufacturers are developing smoke-resistant turnout garments and protective hoods, and the NFPA committees are once again attempting to revise the standards to address this newly realized threat to firefighter safety.
Historically, this delicate balance between the protective capacity of a specific type of gear and the physiological burden that it adds to the first responder has been assumed to be achieved mostly by material-level performance evaluations and requirements specified by the NFPA product standards. The issue with this assumption is that material-level testing alone cannot predict the complex interactions that occur when these material composites are made into garments and donned with multiple other pieces of equipment.
For instance, the thermal protective performance of the turnout composite (outer shell, moisture barrier, and thermal liner) must exceed a value of 35 (which relates to 17.5 seconds before a second-degree burn is predicted). This evaluation does not consider any areas of overlap in the ensemble such as with pockets, the jacket overlapping the pants, or the fact that all firefighters wear some sort of base layer beneath the turnout garment, which further increases the protective performance of the system.
On the other hand, the base composite also must meet or exceed a total heat loss value of 205 W/m2, but additional insulating layers can then be added during garment manufacturing to improve thermal protection without ever evaluating the effect on the heat loss of the final product.
Figure B12 illustrates the variability in TPP and THL values across a typical turnout ensemble as measured at NCSU TPACC in a recent AFG-funded research project.
These combined effects are not measured using current material-level tests. While the base turnout composite that is shown exceeds the minimum TPP and THL requirements, the inverse relationship between these two values can clearly be seen; when TPP increases, THL in that area decreases significantly.
Because the modern structural firefighting (also used in WUI environments) ensemble is comprised of a turnout jacket and pants with multiple fabric layers, reflective trim, pockets, and padding, in addition to a base ensemble (station uniform), protective hood, gloves, boots, a helmet, and SCBA, the only approach to assess the true performance as a firefighter would experience it is with a system-level evaluation. These types of evaluations that consider the firefighter as a whole system instead of individual, unconnected components provide the capability to investigate the interoperability of different ensemble components, the effects of garment fit and designs, and the integrity of interfaces between ensemble elements. An evaluation of the same turnout composite and ensemble using PyroMan™ and a thermal sweating manikin (Figure B13) provides an example of the utility of evaluating the ensemble at the system level. The detailed data that can be collected across the manikin body provides many more insights into the actual performance of the whole ensemble than a simple material-level TPP or THL evaluation can provide.
The realism of fire manikin evaluation of responders’ ensembles by adding dynamic movement to the manikin during the fire exposure has enabled evaluation of the effects that create mechanical stress on the gear, revealing weakness in thermal protection not always apparent in static testing. Heat from flames produces significant thermal degradation to the materials directly exposed to flame contact. Some types of char-forming outer shell materials are badly degraded in flame exposures, yet the turnout suit continues to provide protective insulation in static conditions. In actual fire exposures, mechanical stresses produced by firefighters’ natural body movements in escaping the flames may cause degraded clothing layers to break open, leading to catastrophic breakdown of the ensemble’s thermal protective envelop.
Therefore, data on ensemble TPP provide opportunity to evaluate the value of material-level flame and thermal shrinkage tests. These advances will present opportunities to harmonize TPP and heat-resistant requirements for materials used in ensemble elements.
Authors’ note: Portions of this supplement were adopted from the project proposal and report for a research project supported by the United States Department of Homeland Security, Federal Emergency Management Agency, and Assistance to Firefighters Grants Program [FEMA Grant No.:EMW-2016-FP-00744] and from “Relationship between heat loss indexes and physiological indicators of turnout-related heat strain in mild and hot environments,” by Huipu Gao, A. Shawn Deaton, Roger Barker, Xiaomeng Fang, and Kyle Watson in the International Journal of Occupational Safety and Ergonomics, 29:2, 562-572, DOI:10.1080/10803548.2022.2058746.
Link to this article: https://doi.org/10.1080/10803548.2022.2058746
Reference to the effect of a walking sweating manikin on heat loss from turnout suits is supported by research described in McQuerry, M.; Barker, R.; DenHartog, E., “Functional Design and Evaluation of Structural Firefighter Turnout Suits for Improved Thermal Comfort: Thermal Manikin and Physiological Modeling.” Clothing and Textiles Research Journal 2018, 36 (3), 165-179.
