When it comes to selecting our firefighting personal protective equipment (PPE), too often, we make decisions based on subjective factors, such as appearance or color. Fit and cost are important factors for consideration in the selection, but we must also consider more scientific factors, such as heat transfer characteristics and thermal protection. Because these scientific factors involve technical formulas and tests, it’s easy to simply assume that the manufacturer knows best—but as end users, we must at least understand the basics.
So, when it comes to NFPA 1971: Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting, do you know what the numbers really mean?
Thermal Performance Tests
NFPA 1971 establishes not only the minimum performance requirements for PPE; it also specifies the test methods by which the PPE will be measured. Many of these tests are performed on individual layers or composite swatches representative of the PPE item itself.
There are more than 70 tests specified in NFPA 1971, but we’ll look at only the most relevant ones.
The Vertical Flame Test evaluates the flame resistance. This is a very rigorous test to ensure materials will not continue to support combustion upon removal of a direct flame assault. The material is exposed to flame for 12 seconds and must extinguish in 2 seconds. The char length of the material after exposure must not exceed 4 inches and the material cannot melt, drip or ignite.
The Forced Air Oven Test is a convective heat exposure used as an evaluation technique for materials under catastrophic conditions and isn’t intended to replicate field conditions. Materials shall not melt, drip or ignite after exposure to 500 degrees F for 5 minutes and fabrics cannot shrink more than 10% in any direction.
Now, let’s look in more detail at the tests that produce one of the levels you’re probably most familiar with—TPP.
Thermal Protective Performance (TPP)
The TPP rating is probably the most familiar value associated with a garment’s thermal performance. It represents the emergency firefighting conditions where there are high thermal exposures, such as during flashover or backdraft. Turnouts on the market today range from TPP levels of the minimum of 35 to more than 50. This is not including thermal enhancements or reinforcements. The NFPA values are also from materials pre-wash. Laundering of garments increases the loft or the airspaces in fabrics, thus creating a higher TPP value.
Thicker and heavier materials will generally have higher TPP levels. However, the ability to trap more air inside the system can also boost the TPP level. Conversely, when moisture is introduced into the system it can have a negative effect on heat transfer, thus lowering the thermal protection.
But what does a specific TPP level really mean? NFPA 1971 specifies that firefighting turnouts must have a minimum 35 TPP rating. A composite sample is used, and the test simulates an extreme exposure similar to flashover and measures the time in seconds before a second-degree burn would occur. The TPP test is conducted under a combined convective and radiant heat energy of 2.0 calories/cm2, therefore the TPP rating obtained is divided by two to obtain a minimum time to burn criteria of 17.5 seconds. Something called the “Stoll curve” is used to determine burn time, and the curve is only accurate to 30 seconds, therefore values over 60 TPP are extrapolated. NFPA 1971 states any value over 60 should be reported as “> 60.”
One important TPP consideration when speccing gear: The base composite is used for determining the TPP value of your turnouts and does not take into account reinforced areas of the garment, such as the shoulders or knees. More than 50% of the surface area of the average set of turnout gear is reinforced by additional fabrics, such as overlap, pocketing, trim and components. These enhancements can increase the TPP value for those areas of the garment.
Example: The Houston Fire Department (HFD) has one of the most highly engineered coats in the fire service for thermal protection, but the base TPP value is only 40. The garment has reinforcement materials in the shoulders and arms where the highest TPP value is found.
Though some think the garment was tactically designed, it was not. HFD designed the coat to protect firefighters in case of entrapment or being caught in rapidly changing conditions. Each department must evaluate their needs during their risk assessment, and use the information to develop the specification for what level of TPP they desire.
Radiant Protective Performance (RPP)
The RPP test is traditionally used on elements of PPE other than turnouts as well as proximity firefighting ensembles. Similar in nature to the TPP test, the RPP test is performed solely with radiant heat and no flame. Quartz heat lamps are used to generate a radiant exposure and the sample is orientated vertically.
Just like with TPP, the test measures how long it takes for the composite sample to be heated sufficiently to cause a second-degree burn. Per NFPA 1971 specifications, PPE must have an RPP level of at least 20, which translates to 20 sec at 1.0 cal/ cm2 sec heat flux radiant exposure.
Although this test is not currently used for turnouts, it can provide valuable information on how they might perform under radiant heat conditions.
Compressive Conductive Heat Resistance (CCHR)
The Compressive Conductive Heat Resistance (CCHR) test evaluates the PPE’s “pinch points,” or areas of the garment that commonly become compressed—specifically, the knees and shoulders—to evaluate the garment’s performance when compressed. The sample composite represents the layers used in the actual construction of the garment shoulders or knees. The test measures how long it takes the garment to reach a rise in temperature of 24º C when exposed to a heat source of 280º C. Per NFPA 1971 specifications, PPE must have a CCHR rating of at least 25.
The test is conducted with the thermal liner in a dry and wet condition. The shoulder area is tested under 2 psi of pressure (to approximate the weight of an SCBA 2″ strap when carrying a full cylinder) and the knee area is tested under 8 psi of pressure (to approximate the force that the average 180-lb. firefighter exerts on the knee area when kneeling or crawling).
What to consider about CCHR when purchasing PPE? In earlier editions of the standard, the minimum CCHR rating was 13.5 seconds, which was arrived at by testing a composite with a TPP of 35 to the shoulder conditions of the standard. Reinforced areas of the garment (e.g., shoulders or knees) may differ by manufacturer to meet the CCHR requirements. Manufacturers also offer more than one CCHR option in composite areas to meet varying department needs. Because of the reinforcements, these areas will also not only have increased CCHR, but increased TPP too.
Stored Energy Test (SET)
The newest thermal performance test, added to the 2013 edition of the standard, is the Stored Energy Test (SET). In contrast to TPP, the test uses a lower energy source and longer exposure time to simulate routine and ordinary firefighting conditions. The SET is performed on the garment’s sleeves where there are any “enhancements” to the outer shell (visibility markings, elbow reinforcements, etc.), excluding the cuffs. The composite sample with outer shell enhancements is exposed to a heat source and then compressed. Per NFPA 1971, the composite must not produce a second- degree burn in less than 130 seconds.
This test was added to the standard because TPP was not telling us everything. It was reported that firefighters were receiving burn injuries, in less than flashover conditions, in many cases under these outer shell attachments, and in some cases where the garment itself showed no thermal damage. This is possible due to the fact that PPE is a great insulator; it can effectively trap hot air in the layers. When a firefighter then compresses the PPE (e.g., by bending an arm, leaning on a wall), the heat can be rapidly transferred to their skin, thus causing a burn even though the PPE itself isn’t damaged. This effect can be enhanced when the garment is wet, because water conducts heat faster than air. For this reason, unlike TPP, the SET specimens are tested wet.
Regardless of the amount of thermal protection designed into our gear, the fire service must begin training our members to move away from heat and discontinue the practice of moving from pain. If we wait for pain to be our indicator for conditions, we may carry the scars of the lesson.
Now that we’ve examined the tests that measure aspects of thermal performance, we must also discuss the test that measures heat loss. As a firefighter works hard, heat is produced, and the same properties that lead a garment to keep heat out also keep the body heat in. Fortunately, we have another mechanism for shedding this body heat, due to the evaporation of sweat. However, for the firefighter to take advantage of this mechanism, the turnout gear must be “breathable”—meaning that it enables moisture vapor to escape.
Total Heat Loss (THL) is the test in the NFPA standard used to determine the capability for a turnout composite to lose heat. This is a static test performed on a “skin model” that simulates heat transfer through the skin. It tests how much heat is lost from the combined effects of wet (evaporative) and dry (conductive) conditions. Per NFPA 1971, the base turnout composite must have a THL of at least 205 watts/m2 (NFPA uses a three-sample average to arrive at the number). As previously mentioned in the TPP section, enhancements and/or areas reinforced by additional fabrics, overlap, pocketing, trim and components will affect THL by driving the number down.
It was not until 1989 that the 1971 Technical Committee (TC) started discussing the importance of moisture barrier breathability. In part due to the technical challenges of measuring THL, the test was slow to be adopted. In the 1991 edition of 1971, it was included in the appendix (where it was not a mandatory requirement), but it was then removed from the 1997 edition.
Then, in 1998, a landmark study demonstrated the importance of breathable moisture barriers, and in the 2000 edition the THL test returned, with a required minimum of 130 watts/m2. This requirement caused some moisture barrier technologies used at the time to be eliminated. In the 2007 edition, the THL requirement was raised to 205 watts/m2; once again, the increase eliminated some moisture barriers and thermal liners on the market. In garments on the market today, you will see THL ratings from the minimum 205 to those in excess of 300. Some studies show that the typical human cannot distinguish less than 50 THL point increments between garments.
Why THL matters: USFA firefighter fatality statistics continue to show nearly 50% of firefighter deaths annually are due to cardiac and stroke events. As firefighters continue to die from and be injured by these cardiac events on the fireground, the effects of heat stress have come under examination. Heat stress can lead to an increase in core temperature and heart rate. As this happens, a firefighter’s performance diminishes and they become more at risk for heat stress-related medical complications and heart attack. Recently, the National Institute of Standards and Technology (NIST) released preliminary data about heat stress in firefighters, which could possibly affect future editions of NFPA 1971.
A Final Word
Designing an ensemble to provide appropriate thermal protection while also ensuring heat stress relief is a complicated process. These two functions actually work against one another: thermal protection tries to shield us against a wide range of thermal exposures, while heat release seeks to prevent our bodies from becoming overheated while wearing the PPE.
As new research and test methods become available, NFPA 1971 will continue to evolve to provide the optimal balance of performance against heat and flame while maximizing mobility and breathability. But as firefighters, we shouldn’t rely solely on the assurance that all PPE on the market today meets NFPA 1971 standards. We must educate ourselves as to what those standards mean and what choices we have when speccing PPE. Every design choice has a tradeoff. Choose wisely—your life may depend on it one day!
Sidebar – Important Terms
Before we can understand the numbers, we first must revisit some of the information from back in our training academy days.
Thermal radiation is defined as calories per square centimeter per second, or cal/cm2 sec. A calorie is the amount of energy required to raise 1 gram of water 1° C. Another unit of heat is a British Thermal Unit (BTU), the amount of energy needed to raise the temperature of 1 pound of water 1° F, which is equivalent to 252 calories. Heat flux is the rate at which heat is applied to a surface.
Burns occur when heat is applied to the skin at a rate greater than the body’s ability to dissipate that heat. When human skin reaches the following temperatures, the average person will experience pain at 111° F, and receive a first-degree burn around 119° F, a second-degree burn around 131º F and a third-degree burn around 161º F. It typically takes about 1–2 cal/sq cm to cause a burn. Put more practically: The energy output of a typical butane lighter is about 1 cal/sq cm-sec. If you hold your thumb in the flame (~1 sq cm exposed), a burn injury will occur in 1–2 sec. 1 cal/sq cm-sec radiant heat is identified as the benchmark when defining thermal protection.
Sidebar – PPE & Fireground Conditions
The 1993 USFA Minimum Standards on Structural Fire Fighting Protective Clothing and Equipment: A Guide for Fire Service Education and Procurement references a 1973 Utech document defining “The Range of Thermal Conditions Faced by Fire Fighters.” This document is widely used throughout the fire service for defining three levels of fireground conditions: These conditions are:
- Routine conditions occur when one or two contents are burning in a room. The thermal radiation range is .025–.05 cal/cm2 and the air temperature ranges from 64–131° F. The convective and radiant heat loads are similar to a hot summer day and the garment is more than capable of meeting the heat load.
- Ordinary conditions are encountered during more serious fires or proximity to a room that is flashing over. The thermal radiation range is .05–32 cal/cm2 and the air temperature ranges from 140–572° F. Under these conditions, the garment may offer 10–20 minutes of protection, allowing the firefighter to extinguish the fire or exhaust their operational time on their air supply. Note: NFPA 1971 determines this based on a 30-minute cylinder. Currently, most department are using 45–60-minute cylinders, which may allow firefighters extended time in an environment, but also leads to more energy being allowed to enter the garment.
- Extreme conditions are severe and/or unusual exposures, such as being caught in a flashover or next to the flame front. The thermal load exceeds .32 cal/cm2 and the temperature is greater than 572° F. Under these conditions, the garment design is to provide 15–30 seconds of protection—just enough to allow the firefighter to escape.