Using synthetic underwear to remain comfortable while working in extreme ambient conditions is enticing. Newer materials are said to provide increased moisture removal resulting in better comfort and higher productivity. But how will these materials stand up to the heat and fire a firefighter is exposed to?

This underwear question was put to the test at the University of Alberta's Flash Fire Facility. The U.S. Forest Service funded the experiment, held in the university's protective clothing and equipment research laboratory. The lab is used to evaluate flame-resistant clothing from single-layer coveralls to structural turnout gear; it also was used to develop a new fire shelter.

Fighting fires produces high rates of metabolic activity. When coupled with working in close proximity to fires, the body producing tremendous amount of perspiration in an attempt to keep from overheating. The recommended fluid intake while working is approximately one quart per hour. And as the ambient temperature rises, the work-to-rest balance should shift from 40/20 at temperatures below 82°F to 10/50 at temperatures above 90°F. Also, approximately 50% of firefighters' moisture is lost through the face and scalp. Protective materials such as shrouds and helmets will affect moisture removal rates and the body's ability to stay cool.

Polypropylene garments wick moisture and are commonly used by athletes in an attempt to cool the body. Yet using these undergarments is generally discouraged because synthetic materials can melt and aggravate burns.

In firefighting, underwear's principal role is to provide an additional layer of material between the hazard (radiant or direct flame contact) and the person's skin. One might expect that heavier, or thicker, underwear would provide better protection, but this depends on the construction of the fabric. Fabrics of equal mass could have different thicknesses, and in turn different thermal resistance, due to construction.

This study examined six underwear materials. In each case the materials were weighed to determine nominal conditioned fabric mass (65% relative humidity), and then saturated to determine moisture regain. This was done to determine the amount of water that could be present in the fabric. Moisture can play a part in either increasing or reducing the rates of heat transfer though clothing systems. The effects depend on where the moisture is located within the clothing system as well as the type and duration of the exposure.

The investigation was undertaken in three parts: small-scale radiant tests, full-scale flame exposure and full-scale radiant exposure. The effects of moisture were examined in only the small-scale testing. All of the materials were evaluated in conjunction with an outer layer of aramid-fiber clothing typically used by wildland firefighters.

For the tests, a mannequin was fitted with sensors then exposed to a propane-diffusion flame while the temperature of the sensors was monitored. The duration of the test was usually between 3 and 5 seconds while the data recording period was 60 seconds or longer. The longer recording period was necessary to capture what happens after the event when the surface of the mannequin is at a high enough temperature for thermal injury. The duration depends on what happens to the garments during the exposure: whether the garment continued to burn, whether there was any after flaming, or whether the garment melted and resolidified.

The test used T-shirt and brief combinations. Although the T-shirt material was changed, the brief was cotton in all cases. Thus the underwear covered the torso, upper arms and pelvis area, with the lower arms and legs covered only by an aramid layer. The shirt and pant used in testing were of different fabric masses so that the level of protection afforded by each was different.

The mannequin was equipped with 110 sensors distributed over the body in approximately equal areas. Although each sensor was assigned a surrounding area, the actual sensor had a 19-mm diameter. Data was taken from each sensor about 1,000 times per second and that information used to filter noise and average readings down to a rate of 10 readings per second.

Twelve propane burners surrounded and fully engulfed the mannequin in flames. The burners were carefully adjusted to ensure a very uniform exposure over the surface. The average heat flux to the nude mannequin was 84 kw per square meter. Exposure times were typically 3 to 5 seconds, although the system is capable of generating sustainable exposures to 20 seconds.

Preliminary tests were run using cotton underwear to determine the exposure duration that would produce a measurable result and at the same time not simply destroy all garment systems. It was thought that an exposure of between 3 to 5 seconds could simulate a trapped firefighter escaping through a flame front. Although this may not be a realistic scenario, the purpose was to differentiate clothing systems and look for potential problems with underwear materials. In all cases the shirt was tucked into the pant and the collar button was left undone so that in some cases the neck ring of the underwear showed.

A 4-second exposure to flames was sufficient to remove most of the dye from the aramid material and to shrink the material on the arms and lower legs. There was enough shrinking that the garment could not be removed by undoing closures and had to be cut off the mannequin. In places, the aramid became brittle to the touch.

Wearing underwear enhances the protection provided by the outer layers. This is illustrated in two studies from 2004. The first was the Novato Fire Protection District's investigation analysis following a wildland fire. One captain sustained burns through his Nomex wildland pants and Nomex work pants, but none where his cotton underwear provided a third layer. Likewise, a study of Norwegian firefighters also showed that burns were limited to areas not covered by underwear. That study did not list the underwear material; researchers assumed it was cotton.

To compare underwear materials, the test outputs were sorted and only the sensors covered by the undershirt were included The maximum possible thermal injury on the considered sensors would be 27%. Each result is an average of the three trials.

Even with no underwear, the level of thermal injury was not the total area covered by the underwear. This was expected because of how the shirt and pant combination is constructed. Sensors in the middle of the chest, around the waistband and under chest pockets were covered with multiple layers of material even thought the fabric was indicated as a single layer. In some cases there may be four or more layers of material on top of a sensor. Even a 4-second exposure was not enough to transmit sufficient energy to cause a thermal injury.

The addition of underwear in all cases reduced the predicted thermal injury below an aramid layer alone. The predicted thermal injury varied between 2.5% (wool) and 8.5% (silk). Cotton, the most commonly used underwear, produced a predicted thermal injury of 6%, while both the synthetic materials produced slightly higher results.

Yet, examining the predicted thermal injury alone does not present a complete picture. There was noticeable melting observed with both the polypropylene and Under Armour shirts. And, while energy transfer through the clothing systems can be measured on the mannequin, it is really a hard fiberglass shell. The mannequin system may, in fact, show that the energy transfer is reduced or slowed because of the energy absorbed in a phase change of the material. This may translate into a misleading, lower predicted thermal injury solely because the mannequin shell does not react the same way and cannot be used to predict the effects of materials melting and becoming attached to the skin. The materials seemed to preferentially adhere to the aramid outer layer, but there is no guarantee that this would be the case on a real person. Melting of the polypropylene shirt was more extensive than the Under Armour shirt, though the melting for both was significant. Both synthetics showed slightly higher thermal injuries than cotton.

In a wildland firefighting situation, it is more likely that firefighters will be exposed to a high radiant heat flux, away from the fire, than to be engulfed in flames. Further tests were performed with the mannequin outside of the fire to evaluate the effects of an intense, primarily radiant, exposure. With the mannequin's back to the flames, a heat flux of 40 kw per cubic meter was measured across the back of the mannequin's torso. Ten seconds was chosen as the exposure duration to effectively evaluate the clothing system.

The clothing systems in these tests consisted of the same inner and outer layers on the top of the mannequin but the pants and cotton briefs were removed. This left the area of the mannequin being considered, comprising 13 sensors (10.8% of the mannequin surface), entirely covered by a single layer of each the under and outer layers of clothing. The lack of pockets, seams, buttons and other design features reduces the number of factors outside of the fabric systems.

The polypropylene showed the lowest burn injury of the group; Under Armour and silk fared the worst. As in the flame engulfment tests, the Under Armour and polypropylene garments showed significant melting. While the predicted burn injury to the mannequin was minimized with the polypropylene undershirt, and the melted synthetic shirts seemed to primarily adhere to the aramid outer shirt. There are the same concerns regarding the difference between material melting on the mannequin shell and on human skin.

The researchers found that the predicted thermal injury, based on sensors covered by both an underwear layer and an outer FR layer showed significant differences. In both exposure cases, the synthetic underwear (polypropylene) showed lower thermal injury than the natural fibers. No accounting of either fabric thickness or weave was attempted in this study, but it is surmised that the low melt point of the material (energy absorbed in the phase change) led to a significant decrease in energy transfer.

They also observed significant melting and sticking with all of the synthetic materials tested. Based on observations made during the experiments, the risk of severe burn injury is much greater with synthetics if the energy transfer rates are sufficient to melt the materials. Since the materials showed melting during both flame engulfment and radiant exposures, there is no reason to believe that this would not occur if it were on a person rather than a mannequin.

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Mark Ackerman is a professional engineer at the University of Alberta and one of this study's researchers. He teaches courses in instrumentation and measurements as well as engineering design.

Anthony Petrilli, an equipment specialist for the U.S. Forest Service's Missoula Technology and Development Center and wildland firefighter, also contributed to this article.