Balancing breathability with protection in turnouts

Fire service needs to tell NFPA committee about its needs, issues and what it considers to be a priority

Editor's note: In the following article, Jeffrey and Grace Stull outline how the stress impact of clothing on firefighters has been lessened in recent years. Now, as the NFPA begins a revision of the 1971 standard, consideration is being given to raising the breathability criteria to a higher level. Read on to learn the purpose of the total heat loss test, its consequences and the tradeoffs involved in design and share your views on proposed changes to the standard and own personal experiences in the member comments area below.

By Jeffrey O. and Grace G. Stull

It has only been in the past 10 years when all garments were required to have a breathable moisture barrier. Prior to that time, most of the fire service wore protective coats and pants with impermeable coated barrier materials. In the late 1990s, the International Association of Fire Fighters fought endlessly to have the total heat loss test implemented as part of the NFPA 1971 standard for turnout gear and sponsored a major study that set the basis of the current requirement.

Presently, firefighters benefit from garments that are breathable. Breathability is a property of materials and composites, which permits the escape of moisture and the heat associated with that moisture from the interior of the garment to the outside environment. This property is considered important because in wearing heavy clothing such as turnout gear, the clothing traps heat that the body produces through metabolism and activity.

If left unchecked, the body core temperature can rise to dangerous levels and lead to the onset of heat stress. Heat stress can manifest itself in many different ways that include heat exhaustion, heat stroke, and ultimately death. The effects of heat stress may also include impairment of judgment and limit endurance. Given the extreme conditions of firefighting, it is no surprise that more than half the fatalities and a significant proportion of injuries to firefighters can be linked to stress-related causes.

The total heat loss (THL) test evaluates "clothing composites" (the layers of outer shell, moisture barrier, and thermal barrier) that are used in the construction of protective garments. Samples of the clothing composite are placed on a special hot plate that measures the energy for the plate to stay at a constant set point temperature (94 F).

The plate and the composite samples are located in a controlled environmental chamber at a lower temperature (77 F). The THL test is then performed in two different ways to measure the energy required to keep the plate at the set point temperature. In the first procedure, the testing is conducted dry.

Heat transfer
This way of testing evaluates the heat transfer from the skin to the outside environment that occurs through conduction, which is one of the major ways that the body dissipates heat. The second way attempts to simulate evaporate heat transfer (sweating), which is the other primary mode for how the body expels heat. In this part of the THL test, water is pumped through small holes in the hot plate (representing skin pores) to simulate sweating.

Heat loss occurs from the plate as the moisture moves through and exits the fabric system. The energies associated with both conductive and evaporative heat transfers are combined to estimate the total heat loss. This test result is reported in Watts per square meter of fabric composite materials. Higher total heat loss values indicate clothing systems that permit more heat to escape the garment and consequently result in less stress to the wearer.

The total heat loss test first appeared as a recommended turnout clothing evaluation method in the 1991 edition of NFPA 1971. At the time, the method was relegated to an appendix where, if desired, manufacturers could apply the THL test (or departments could specify the test) to characterize material breathability as an aspect of garment stress relief.

It is important for the NFPA committee to hear from the fire service as to their needs, issues, and what they consider to be a priority.

An attempt was made to make the test mandatory as part of the next revision of the standard (in 1997) but was blocked on administrative grounds in an appeal to the NFPA Standards Council.

The IAFF raised significant concerns about this decision, which provided the incentive to conduct comprehensive evaluation to show how garments having more breathable moisture barriers (with higher total heat loss values) could lessen the stress-related physiological effects of clothing on firefighters. This testing became known as the Indianapolis Field Study as it was conducted with the support of the Indianapolis Fire Department.

Significant reduction
The study showed a significant reduction in body core temperature for garments with higher total heat loss values and recommended a minimum requirement of 205 W/m2, citing the additional finding that a difference of 45 W/m2 was necessary to perceive a physiological impact for garment breathability. In comparison, most non-breathable material systems had total heat loss values in the range of 100 to 120 W/m2. Yet, the committee chose to conservatively set the new total heat loss requirement at 135 W/m2 based on the thinking that it was best to have the TPP test as a mandatory part of the standard with a lower limit to allow the industry to adjust their product offerings.

It was not until several years later that the total heat loss criterion was set at the 205 W/m2 recommended level scientifically demonstrated nine years earlier. The incorporation of the total heat loss test as part of the criteria for protective garments resulted in the elimination of impermeable moisture barrier materials.

Since higher total loss values mean less stress on firefighters, it would seem that total heat loss values should be set as high as possible. Yet there would be multiple tradeoffs created with this approach.

First, there are limits for the levels of total heat loss that can be achieved in firefighter protective clothing. All turnout gear must include a three layer material composite or at least clothing layers that provide the functional performance of the three layer system.

The principal function of the moisture barrier is to keep liquids out of the clothing interior. There is a narrow range of films that can accomplish this feature and still be breathable. Materials with very, very high breathability often cannot meet the liquid and bloodborne pathogen requirements of the NFPA 1971 standard.

Second, increasing total heat loss usually comes at the expense of decreased insulation against heat. It follows that keeping thermal energy from entering the clothing from the exterior environment is directly opposed to allowing body heat energy to escape from the clothing interior.

Thus it becomes a balancing act to select a material composite with relatively high total heat loss value without sacrificing thermal insulation (as measured by thermal protective performance or TPP). The criteria in NFPA 1971 for both total heat loss and TPP create a window of acceptable material systems.

Fewer systems
If the minimum total heat loss test requirement is raised significantly, it would mean that fewer material systems would be available for selection, some thermal barriers would be eliminated altogether, and that TPP values for composites would generally be lower with consequentially lower thermal insulation for protecting firefighters.

There are also limitations in the application of total heat loss testing. Just as TPP testing does not evaluate the entire garment for thermal insulation, total heat loss testing is a material test and does not evaluate the entire capability of the garment to dissipate heat. Areas of garments where there is trim, pockets, and reinforcements have lower or less breathability.

In contrast, these same areas of the garment have greater thermal insulation. The specification of garment design must account for which areas of the garment outside the three-layer composite need reinforcement and other materials and how these additional materials or components may contribute to stress.

While it is a worthy objective to increase the total heat loss criteria applied to firefighter protective garments, careful consideration must be given to weighing the advantages and disadvantages. Certainly, a higher total heat loss test can promote improvements in lessening the clothing and equipment imposed stress on firefighters. Yet, the total heat loss test is probably one of the few requirements in the NFPA 1971 that has been scientifically validated (through the IAFF Indianapolis Field Study).

Industry has become accustomed to balancing insulation and breathability needs. End user organizations are also recognizing that how other parts of the garments are designed also affect stress and that the total heat loss test does not address garment design, only material selection. Still, it is important for the NFPA committee to hear from the fire service as to their needs, issues, and what they consider to be a priority. We look forward to your comments below.


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