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How moisture impacts thermal insulation in PPE
Several studies have been performed in the past to measure various forms of heat transfer through materials under different wetting conditions
Last month, we wrote about the different ways that firefighter clothing gets wet. While expected, this is significant because water or moisture in clothing can have a variety of effects on its performance.
The most obvious concern is the impact of moisture on clothing thermal insulation, but water can be a factor for other kinds of performance. For example, clothing that retains too much moisture from exterior sources can become heavier and result in greater stress to the wearer.
Moreover, wetness in gloves can affect dexterity and grip. Materials that easily pick up water may also more easily retain various hazardous contaminants. Understanding how moisture affects your clothing allows you to better judge your circumstances and realize conditions when your protection may or may not be compromised.
There are also a number of overly broad generalizations that have come about in the industry, some of which are simply not true and can result in poorer understanding of clothing protection. Several years ago, we worked with a large metropolitan fire department that had just started wearing hoods over the previous few years and the firefighters in that department had little experience with the ways that hood materials attenuated the levels of heat. One of the early concerns that arose was the wearing of wet hoods. The opinion spread throughout the department was that it was dangerous and therefore many firefighters opted to go without a hood rather than wear a wet hood.
In response to these concerns, we conducted a study for the department along with some other industry experts and measured insulation levels for hoods with the varying levels of moisture that could be expected (dry, damp, and saturated).
The hoods or hood materials were subjected to an array of different heat exposure tests ranging from the extreme (simulated flashover) to the more ordinary (low levels of radiant heat). For small scale tests, the predicted time to second degree burn injury was measured along with the time to pain.
The difference in these two measurements is known as the alarm time, which is the amount of time that the average individual would first feel pain to the time for the onset of burn injury. Full scale tests were also performed with a manikin instrumented with sensors in different locations that would predict what parts of the body would sustain burn injury during a simulated fireground flashover.
The results of this study showed some fairly remarkable findings. At the lowest level of radiant exposure, dry hoods outperformed damp hoods, but saturated hoods provided more protection than damp hoods. At a moderate level of radiant heat exposure, dry, damp, and saturated hoods had equivalent protection. At higher levels of radiant heat, the saturated hoods showed slight advantages over damp and then dry hoods.
These differences became more distinct as the radiant level increased; however, the overall relative protection diminished as compared to lower levels of radiant exposure. Lastly, the instrumented manikin results provided similar results for a flashover as the high radiant heat level tests — all areas of the face, neck, and ears were burned with no hood.
There was a substantial reduction in burn areas for dry hoods, but additional decreases were observed for increasing levels of moisture. Yet, the most definitive result was that in no cases was the wearing of no hood more protective with wearing a hood under any moisture condition.
Of course, the hoods that were tested in the study described above were of relatively simply construction — two plies of a homogeneous knit material. Moisture effects on thermal insulation grow increasingly complicated with multi-layer garments having different types of materials used in their construction.
In fact, several studies have been performed in the past to measure various forms of heat transfer through materials under different wetting conditions. Many of the early studies focused on differences between impermeable and permeable moisture barriers.
For example, one study done nearly 30 years ago demonstrated that the thermal protective performance of firefighter clothing composites (outer shell, moisture barrier, and thermal barrier) changed radically depending whether the moisture was in the shell, the liner (combination of moisture barrier and thermal barrier) or both.
This testing further showed in some cases, such as the lining being wet while the shell was dry, that alarm time was lowest and that permeable moisture barriers provided substantially more warning to the firefighter before the onset of a burn.
Related testing showed varying the amount of the water generally favored impermeable barriers over permeable barriers. Testing performed in separate studies 12 to 15 years later showed many of the same trends, but refined the results. To say the least, this test information simply demonstrated how complicated moisture effects could be on insulation.
More recent tests have examined the effects of stored energy in clothing over a relatively long exposure to low levels of radiant heat that is then followed by rapid compression of the clothing layers against the skin. In this testing, the worst condition has been found when impermeable materials are placed over breathable materials for certain levels of moisture in the liner only.
That is not to say any firm conclusion can be drawn by these findings, rather, a specific case has been identified where firefighters might be at greater risk for unexpected burn injury. Testing has also shown that this potential can be mitigated by other factors. All tests are nothing but snapshots of some very specific conditions, some of them realistic, others not and it is generally difficult to draw a broad conclusion from just one set of testing for relating moisture effects in clothing.
What you must remember
To understand moisture effects on clothing, it is important to remember that the moisture in clothing both absorbs and conducts heat. In absorbing heat, the water provides additional mass in the material. However, as the moisture is heated, it can reach scalding temperatures if the heat is not dissipated through the clothing or by other means.
This is why the source of moisture and where it is located becomes very important in determining possible effects of moisture on thermal insulation. Water temperatures on the exterior of the clothing will rise very rapidly because the moisture is closer to the source of heat. Yet at the same time, this moisture has a freer path for evaporation off the surface or wicking into other parts of the clothing.
Additional contact directly with liquids or with wet surfaces can serve to replenish the moisture. In contrast, moisture generated primarily from sweating within the clothing interior can also travel but remains within a closed environment. Furthermore, the interior clothing layers may be next to the skin.
But unlike the laboratory experiments, moisture in the material is replenished through additional sweating. This accumulation of moisture will or will not become a problem in terms of thermal insulation depending on the level and length of heat exposure combined with the moisture management qualities of the clothing.
Generally, the largest problems occur when there are radical changes — changes in the exposure conditions or changes in way the clothing fits on the individual. If the fire scene worsens with a rapid build up of intense heat, clothing insulation can be overwhelmed and the position of the moisture in the clothing can play a dramatic role in affecting the propensity for increased heat transfer and storage of thermal energy.
Likewise, if clothing tightens against the skin or underclothing from compression or movement, new pathways for heat transfer are created. This is one of the reasons why many manufacturers caution firefighters to regularly change their position to minimize heat transfer to one area of their body. Still, some material systems will retain heat. If in combination, these materials do not facilitate moisture spreading throughout the garment, then under some conditions burn injuries can occur with little or no warning.
It would be nice if there was a simple answer for recognizing what levels of moisture, together with where that moisture is located, the condition of the garment, and heat exposure levels could allow prediction of burn injury. That approach is extremely unlikely as exposure conditions vary dramatically as do the factors which affect moisture in clothing and its subsequent impact on thermal insulation.
For the time being, the best advice is to realize that moisture can impact your garment's thermal insulation and be alert to potential signs that long exposures followed by rapid changes in your environment can lead to increased variability in your protection. While this principle applies to garments, it equally applies to other parts of the ensemble, especially gloves and hoods.