Firefighting protective clothing is designed to protect you against thermal exposures in structural fires. Yet like all safety products, protective clothing has its limitations. Therefore, the expectations for being protected are dependent on the levels of heat you encounter and how long you are exposed, as well as several other factors.
Several groups have found ways of characterizing the fireground exposures. In some of these approaches, thermal exposures are defined in terms of the air temperature and the radiant exposure level. For example, in a system of the classification of fireground exposure, Utech described three categories — routine, ordinary, and emergency.
In the routine category, air temperatures range from 68 to 140 degrees Fahrenheit, with thermal radiation levels of 0.025 to 0.05 calories per square centimeter per second. The thermal radiation level is the amount of heat transmitted to a specific area of clothing with a calorie being the heat required to raise a very small volume of water (about a teaspoon) by one degree.
Radiant heat is the heat you feel when you are standing close or at a distance from a burning fire. Under these conditions, the clothing is expected to provide more than 10 minutes of protection, but burns can still occur if the exposures are very long or at the upper limit of the category or with other circumstances of condition that lead to increased heat transmission through portions of the clothing.
Ordinary fireground conditions are the more commonly faced exposures during a working fire. In this category, the air temperatures may be from 140 F to a maximum of 572 F. The thermal radiation levels are much higher, spanning from 0.05 to 0.6 cal/cm2s. These conditions occur when exposed in the fire room with the contents on fire, but at a distance.
The expectation for the protection provided for your gear is on the order of minutes, sometimes considerably less depending on the actual levels of the exposure. For these circumstances, it is very possible for the firefighter to obtain a burn; however, the gear does not always show the effects of the exposures. A firefighter may sustain a serious, lost-time burn injury with no indications of damage to their clothing, or the changes to the clothing may be so subtle as to go unnoticed until the gear is carefully examined.
The worst fireground conditions are represented by the emergency category. Here, air temperatures go from 571 F to those approaching 2000 F. Thermal radiation levels start at 0.3 cal/cm2s to the thermal protective performance testing level of 2.0 cal/cm2s or more.
These conditions are found in the severe exposures of a flashover or backdraft. Protection times are in the order of seconds, with the intention that you can escape the situation with minimum harm. The intensity of these exposures generally causes damage to your gear in a variety of ways.
Different signs of damage
Exposure to these ordinary and emergency categories of fireground exposures will affect your gear. For the most part, your clothing will show no damage in the routine levels of exposure. At ordinary fireground exposure levels, some effects will be seen in many cases, while for emergency fireground conditions, you pretty much expect that damage will take place.
Some of the fabric suppliers have developed charts that show the effects of heat on different outer shell fabrics when they reach certain temperatures. The fabric supplier charts show the condition of the shell fabric after they are exposed to a given temperature, usually after five minutes of exposure in an oven under controlled conditions.
Because the length of the exposure is controlled at the set temperature, this means that the fabric itself reached the temperature and thus the fabric becomes an indicator of what temperature the clothing exterior reached in the given situation.
This does not necessary mean that the indication is of the exposure temperature — exposure temperatures are generally higher since the actual clothing exposure times may be significantly shorter than the test time of five minutes.
Damage to the materials can indicate the maximum temperature reached by the clothing or the total amounts of energy absorbed by portions of the clothing, but it cannot tell the length of exposure as the temperature can be reached at a variety of rates. Furthermore, fireground exposures are rarely static, but constantly changing, making the forensic work in examining clothing damage all the more difficult.
All outer shell fabrics in use today are made of inherently flame-resistant fibers — Nomex, Kevlar, PBI, Basofil, etc. When fabrics constructed from these fibers are exposed to high heat, the first observable effect is loss of dye. The dye sublimes; that is, it goes from a solid to a vapor. Thus the fabric goes to its natural color, if it is dyed (not all shell fabrics are dyed).
This is why black fabrics turn brown or beige when exposed to high heat. When only small areas of the fabric are affected, dye sublimation by itself is not harmful to the protective qualities of the material. However, any signs of dye sublimation always warrant that the clothing be examined very closely for other forms of damage, particularly to trim and underlying material layers in the area of exposure.
Trim and some other materials used on the clothing exterior do not have the same heat tolerance as shell fabrics. The moisture barrier material under the dye loss area may show signs of discoloration, charring or other damage that can compromise the liquid protection capabilities of the liner system.
Why fibers break down
As the exposure temperature increases, the fibers start to break down — they remain flame resistant, but they respond to heat by charring. The fibers now rid of dye become darker as the chemical bonds break down in their polymer molecular structures. When this happens the fibers weaken.
Some fibers char at lower temperatures compared to other fibers. For example, PBI is more char resistant than Nomex. However, this phenomenon is best demonstrated by comparing how much of the original strength the fabric retains after charring. Charring sometimes allows the shell material to be torn by hand. Any evidence of charring is a basis to have clothing removed from service and thoroughly inspected.
It is possible in the judgment of the manufacturer or other experts that clothing may be repaired if the charring is limited to a very small area; however, charring of the outer shell invariably affects the underlying moisture barrier and sometimes heat damage penetrates to the thermal barrier as well.
Inspections for clothing with charring must be carefully performed to identify all areas of extended damage. Charring of the shell will usually destroy adjacent trim materials, forcing their replacement as well, if the clothing remains usable.
The next stage of damage to an outer shell material is embrittlement. Embrittlement is generally associated with emergency fireground exposures. In these cases, the degradation is so severe that the material will break open with simple handling or movement. Nomex-based fabrics will more easily break open than PBI-fabrics based on the same level of heat exposure.
When clothing embrittles, it must be removed from service. Embrittlement of an outer shell will usually be accompanied by severe charring of the moisture barrier and often some degradation of the thermal barrier. Damage to multiple layers of the clothing unfortunately in many cases results in burn injury and is always cause for retirement of the gear.
There are some heat effects on gear that can occur from localized effects such as contact with hot surfaces or exposure to molten metal or other liquids. Prolonged contact of clothing surfaces with hot embers or other materials can affect shell materials in the same way that exposure to radiant heat, but heat transfer by conduction occurs more quickly in raising fabric temperatures.
Conductive heat damage is often shown with very distinct lines of damage to the outer shell if high enough temperatures are reached, but can be further explained by their position on the gear — shoulders, elbows, and knees are more likely to show this damage compared to other clothing areas, unless of course, materials fall on the firefighter or the firefighter falls.
Molten metals cause damage to the shell fabrics in the area of contact. Some metals, particularly aluminum, will adhere to outer shell fabric instead of running off, increasing the likelihood of heat damage to the clothing and burn injury to the firefighter. In these cases, there are telltale signs of the damage, usually by residual metal left at the point of contact.
Understanding the levels of heat exposures that firefighters face, combined with the ways that this heat can affect the clothing, can provide clues to the types of actual exposures that occur in given responses. It is essential to remember that there are many, many circumstances where firefighters get burned without any evidence of damage to their protective clothing.
On the other hand, if gear damage does occur, experienced clothing inspectors can often learn about some of the conditions faced by the respective firefighters and judge when the clothing has been subjected to heat levels beyond its capabilities. Furthermore, these assessments become very useful in deciding the serviceability of the clothing.
