Thermal Protective Properties of Firefighter's Personal Protective Equipment

This USFA/NIST research partnership has developed effective measurement equipment and techniques for the evaluation of the thermal environments experienced by firefighters and to examine the thermal performance of their protective clothing.

A heat transfer model has been developed that can be used in the design and development of future firefighter protective clothing and as a training tool for firefighters, aiding them in understanding the capabilities and limitations of their protective clothing under a variety of thermal conditions.

Another result of this project was the development of a thermal conductivity database for a representative cross-section of materials that are used in the construction of structural firefighting protective clothing. The measurements were made in both dry and wet conditions, including examining the effect of compression that would simulate real world operational use.

This project has produced a series of reports that; document firefighter protective clothing related research needs, characterize the thermal environments experienced by firefighters, document the development of potential standardized test measures of heat transfer through protective clothing, and the development of a heat transfer model and data base for predicting heat and moisture transfer through firefighter protective clothing. The reports and links are listed below in chronological order. A note about the current research within this project is given at the end of the page.

An accurate and flexible model of heat transfer through firefighter protective clothing has many uses. The degree of protection, in terms of burn injury and heat stress, of a particular fabric assembly could be investigated. The expected performance of new or candidate fabric designs or fabric combinations could be analyzed cheaply and quickly. This paper presents the first stage in the development of a heat transfer model for firefighters' protective clothing. The protective fabrics are assumed to be dry (e.g., no moisture from perspiration) and the fabric temperatures considered are below the point of thermal degradation (e.g., melting or charring). Many burn injuries to firefighters occur even when there is no visible degradation of their protective gear. A planar geometry of the fabric layers is assumed with one-dimensional heat transfer. The forward-reverse model is used for radiative heat transfer. The accuracy of the model is tested by comparing time dependent temperatures from both within and on the surface of a typical fabric assembly to those obtained experimentally. Overall the model performed well, especially in the interior of the garment where the temperature difference between the experiment and simulation was within 5 deg C. The predicted temperature on the outer shell of the garment differed most from experimental values (by as much as 24 deg C). This was probably due to the absence of fabric-specific optical properties (transmissivity and reflectivity) used for model input.

Firefighters' protective clothing has steadily improved over the years as new materials and improved designs have reached the market. A significant catalyst that has brought these improvements to the fire service is the National Fire Protection Association (NFPA) 1971 standard on structural firefighters' protective clothing. The fabric flammability test in this standard has resulted in the development of protective garments that resist flaming ignition. The Thermal Protective Performance (TPP) test has assisted in the development of garments that protect firefighters from short duration, high intensity, flash fire exposures. These two thermal tests methods have clearly lead to improvements in firefighter safety. However, thousands of firefighters are continuing to be seriously burned each year. Discussions with fire service personnel indicate that many of these serious burn injuries are occurring when firefighters are exposed to thermal environments that are significantly less intense than those addressed in the NFPA standard. Therefore, the National Institute of Standards and Technology (NIST) has begun the development of a method for measuring the thermal performance of firefighters' protective clothing under thermal conditions less severe than those currently specified in NFPA 1971. This report describes a test apparatus and investigates a method for measuring the thermal performance of firefighters' protective clothing. The test method measures temperature through the various layers that make up a firefighter's thermal protective garment. Temperature measurements are made at the surface of the outer shell, at locations between fabric or moisture barrier layers inside the protective clothing system, and at the, thermal liner surface where the firefighter's clothing or body would be in contact with the garment. When plotted, these temperature measurements show a detailed picture of how a protective clothing system performs when exposed to a given thermal environment. The apparatus may be used to expose protective clothing specimens to a wide range of heat flux conditions. These thermal conditions may be varied from 1.5 kW/m2 to more than 50 kW/m2. The test apparatus may be used for investigating the effects of moisture in protective clothing systems. In addition, this test apparatus and the measurement methods allow for specimens to be studied for a time period ranging from several seconds to more than 30 minutes.

Test apparatus for measuring heat transfer through materials used in firefighters' PPE.

A dynamic compression test apparatus has been developed that bases its design on fire ground conditions that produce burn injuries to the knees of firefighters. This apparatus may be used to measure the thermal performance of firefighters' protective clothing in either wet or dry thermal environments. Studies conducted by the New York City Fire Department (FDNY) show that the contact surface area of the human knee is approximately 3710 mm2 (5.75 in. 2) for a male firefighter with a body mass of about 79 kg (174 lbs). In addition, the FDNY shows that a fully equipped 79 kg (174 lb) firefighter operating a charged 44.5 mm (1 3/4 in.) hoseline had a mean average knee compression, force per unit area, of 133 kPa (19.3 lbf/in. 2). The test apparatus and operating procedures discussed in this report take these data into consideration. The test apparatus uses a timer controlled pneumatic piston to compress the thermal sensor against the test specimen. Test results show that the compression apparatus can discriminate between various levels of thermal performance for firefighters' protective clothing knee pad systems. In addition, results from the apparatus show that knee pad systems can have significantly different thermal performance when exposed to wet and dry thermal conditions.

Apparatus that simulates a firefighter's knee contacting a hot, wet surface.

Firefighters' protective clothing provides a limited amount of thermal protection from environmental exposures produced by fires. This level of thermal protection varies with the design, materials, construction, and fit of the protective garments. Limits of thermal protection may be analyzed using the thermophysical properties of garment materials. However, little information is currently available for analyzing and predicting protective garment thermal performance. To address this need, a research effort was begun to measure the critical thermal properties of firefighters' protective clothing materials. These thermal properties are: thermal conductivity, specific heat, and the thermal spectral properties of emissivity, transmissivity and reflectivity. This report presents thermal conductivity data for nine materials used in fabricating firefighters' protective clothing. These materials included outer shell fabrics, moisture barrier, thermal liner batting, and reflective trim. As a comparison, measurements were also made on a cotton duck fabric. The thermal conductivity of individual protective clothing materials was measured using the test procedure specified in ASTM C-518 Standard Test Method for Steady-State Thermal Transmission Properties by Means of Heat Flow Meter Apparatus. Measurements producing estimates of thermal conductivity for single layers of materials were carried out at mean test temperatures of 20 deg C (68 deg F), 48 deg C (118 deg F), 55 deg C (131 deg F), and 72 deg C (162 deg F). No visible physical changes were observed with any of the materials tested at these temperatures. Thermal conductivity estimates for materials used in the construction of firefighters' protective clothing ranged from 0.034 W/mK to 0.136 W/mK over the range of temperatures addressed in the study. Generally, thermal conductivity values increased for all materials as mean test temperatures were increased.

A detailed mathematical model is constructed to study transient heat and moisture transfer through multi-layered fabric assemblies with or without air gaps. The model accounts for changes in thermodynamic and transport properties of the fabric due to the presence of moisture. Numerical simulations are performed to study heat and mass transport through wet thermal liners (used in firefighter protective clothing), when subjected to a radiative heat flux from a gas fired radiant panel. Results were found to compare favorably with experimental measurements. The numerical solution is further analyzed to provide a detailed physical understanding of the governing processes. Moisture in the cloth tends to vaporize upon heating and part of it recondenses in the interior of the cloth. It was observed that the temperature of the fabric layers and total heat flux to the skin is significantly influenced by the amount of moisture and the distribution of moisture in the protective clothing. Finally, simulations are performed for a wet turnout coat assembly, to demonstrate the flexibility of the model for designing firefighter protective clothing.

The Thermal Protective Performance (TPP) test was developed to quantify the performance of firefighter protective clothing ensembles under an intense thermal exposure. This test method has certainly helped to improve the thermal protection of firefighter protective clothing. However, many fire service burn injuries can be traced to significantly lower thermal exposures than are simulated by the TPP test. A bench scale test method has been developed to evaluate the performance of firefighter protective clothing at low heat fluxes. In addition, a full scale test apparatus capable of exposing both complete firefighter ensembles and samples similar to those used in the bench scale test to various heat flux levels is under development. Both of these tests provide temperature measurements on the surface of the outer shell, at locations between the fabric or moisture barrier layers inside the protective clothing system, and at the thermal liner surface. When plotted, these temperature measurements show a detailed picture of how a protective clothing system performs when exposed to a given thermal environment. This report, Full Ensemble and Bench Scale Testing of Firefighter Protective Clothing, describes comparisons of results obtained using the bench scale test with data from the full-scale test apparatus. The data are also compared to results from a fire fighter protective clothing heat transfer model.