By Jeffrey O. and Grace G. Stull
One of the more common questions we routinely get is how thermal insulation and other properties provided by protective clothing are affected by moisture or being wet. It is a good question because inevitably moisture gets on or in firefighter clothing in a number of different ways during emergency operations.
The clothing industry and other sources have put out a lot of information on this topic, some of it good and some of it perhaps misleading. The very simple answer is that there is no simple answer. When clothing is wet, sometimes the extra moisture improves insulation; in other cases, it can be dangerous.
Wet clothing can have other performance effects, such as increased weight (adding to stress) and impact on hand function (grip). Probably the only conclusion that can be drawn in this area is that the insulation of dry clothing is more predictable than for wet clothing. In this article, we talk about clothing insulation and performance and how specific clothing attributes are affected by water.
Range of exposures
Firefighters get wet much of the time. Direct suppression of fire using hose lines with large amounts of water creates mist, splash back, and accumulation of water that many firefighters come in contact with. Water dripping off ceilings and water accumulating on the floor easily makes engine company personnel wet.
Even those firefighters that are not handling a hose line can become wet during fireground operations. From an exterior perspective, the outer shell is the layer that will be exposed first. Clothing outer shell materials, though treated with repellent finishes, will soak up varying amounts of water.
Clothing that is relatively new or properly maintained will tend to absorb less water, as some finishes must be replenished in order to effectively lower water absorption. Water that gets through the shell wvill come in contact with the fabric side of the moisture barrier, which can also absorb some moisture.
However, the moisture barrier film and seams, if intact, will prevent the water from traveling any further unless it entered through openings such as the collar, sleeves, front closure, or cuffs. Similarly, glove shell materials, whether leather or textile, can absorb water outside of the moisture barrier liner.
These materials are also treated to limit absorption, but under many conditions can still become wet. The exterior of rubber boots does not absorb water, as rubber is impermeable, but well maintained leather boots will also pick up very little moisture.
Pooled water
Exterior wetting can also occur from contact with surfaces. For example, in crawling or duck walking, firefighters can come into contact with pooled water or other wet areas. Depending on the material, the water can wick through the clothing to other areas and also find its way into the interior of the clothing. Some reinforcements on clothing may tend to limit liquid pick up but do not eliminate it.
For example, a coated material knee reinforcement may seem to prevent any liquid pick up on the knee, but in many circumstances, wetting around the knee reinforcement just provides another avenue of liquid penetration to other layers.
Water soaking around the legs, for example, can travel through the shell material to other areas simply by wicking. If water pooling is deep enough, liquid can enter the clothing interior by passing through the interface between the pants and the footwear, regardless of the overlap of the pants over the footwear.
At the very least, wearing clothing under emergency conditions produces sweat, which causes moisture in the clothing interior. Depending on how much sweating occurs and the environmental conditions, this sweat either evaporates or becomes liquid where it stays on the skin or is absorbed into various clothing layers.
Micro-environment
Sweating is occurring constantly whether liquid is visible or not. Just sitting in your chair and reading this article, your body is sweating. The ability for sweat to evaporate off the skin happens when the air layer inside the clothing is relatively dry. This condition does not usually last for long because the clothing tends to trap air and the air space between the clothing and skin becomes saturated. When the micro-environment inside the clothing is fully saturated, evaporation diminishes and liquid builds up.
Clothing layers that contact the skin will soak up some of this sweat. Materials, like 100 percent cotton used in many undergarment fabrics, readily wick and absorb sweat at the points of liquid contact, generally holding the moisture in place because the materials are so absorptive. On the other hand, materials such as Nomex, unless specially blended or treated, soak up much less liquid. Some body areas are directly in contact with the thermal barrier.
The skin side of the thermal barrier is typically a cloth-like material, commonly called the facecloth, which in turn is quilted to batting or nonwoven material that provides the principal insulation of the composite. Different thermal barriers use different face cloth materials. Some materials are very absorptive while others are not. These same attributes apply to glove, footwear, and hood materials.
Sweat that is produced on the skin only has a couple of places it can go, particularly if the firefighter continues to sweat. If it accumulates in absorptive underclothing, some of the sweat will transfer to adjacent turnout clothing, depending on how absorptive the thermal barrier facecloth is. If the facecloth is relatively non-wicking or non-absorptive like some new filament fiber based materials, then the sweat may pool and run into low areas just from gravity.
This is why some firefighters complain that they feel the sweat running off their skin into their boots. Materials that absorb some sweat and permit wicking spread the moisture over a larger area, which tends to allow the clothing not to saturate and permit quicker drying. This is the principle that is being applied in the sport apparel industry where moisture management is all about taking sweat off the skin and spreading it out to evaporate faster.
Types of material
It is not surprising that clothing gets wet in a variety of functions. However, the types of materials used in the clothing, their relative position in the clothing, and their general condition (newness or condition of soiling) will affect how wet the clothing does get.
This is important because moisture in a clothing system affects performance. Water, as a medium, has some remarkable properties, including a tremendous capability for storing heat. This property means that a lot of energy has to be absorbed by water in order for it to vaporize into steam. This heat capacity thus becomes a way that wet clothing can store more heat than dry clothing.
But an opposing property is conductivity. As a liquid, compared to a gas, water more efficiently conducts heat. The insulative properties of fabrics are partly dependent on the principle of trapped air. Trapped air exists in the fibers themselves and the way that fibers overlap or come together in a fabric structure as well as the voids between fabrics that are placed together.
Water fills these voids and thus provides a bridge for heat conduction. So, depending on the level and amount of heat exposure, the moisture in clothing serves to store more heat than would normally be stored by the fabric and conducts heat faster than normally would be conducted by the fabric.
There are many other factors that affect heat flow in clothing, not the least of which are the changing conditions of a fire, but moisture provides variability in clothing that cannot be avoided. Firefighters and their clothing will get wet, even if only from sweating, and how the system works to manage this moisture will have a large impact on how clothing performance is affected.
In our next article, we will explore exactly the ways that moisture affects heat transfer and some other selected properties of clothing performance. We value your input and questions on this topic and any other topics of interest to you.
