HEAT STRESS IN FIREFIGHTING
SAFETY & HEALTH
The Relationship Between Work, Clothing, and Environment
Firefighters are exposed to a variety of stressors at a fire. These include heavy work over relatively short time spans, dense smoke, noxious gases, flames, intensely heated atmospheres, and having to wear heavy, restrictive protective clothing. In comfortable or warm climates, heat illness is a serious problem. Even during winter, heat stress may seriously hamper personnel performance and limit individual work time.
FACTORS IN HEAT STRESS
Heat stress usually results from the interaction of three factors: the heat produced during work, clothing worn, and the operational environment.
Metabolic heat (M) is the heat produced and given off by human cells and organs. This heat is measured in watts, where 1.163 watts equal 1,000 calories per hour. A firefighter’s body produces about 100 watts while at rest and 150 watts while responding to the scene. TTie heat production rate at a fire scene is estimated at a time-weighted average of between 300 and 400 watts. Ibis estimate includes short periods of light activity (150 watts); longer periods of moderate work, such as laying hoselines or searching the building wearing full protective gear and SCBA (250 watts); and shorter periods of peak work, such as hauling hose up stairs, venting with an ax, or working on ladders (500 watts or more).
Protective clothing, because of its weight, stillness, and extra bulk at body joints (“hobbling”), increases heat production. Insulation against heat penetration is an important feature if air, contact, or radiant temperatures (measured by radiometer) are greater than skin temperature. On the other hand, if the air and radiant temperatures are less than skin temperature, this insulation may prevent the body from losing heat produced at rest or work.
The clothing also may interfere with evaporative cooling of the body in a heated atmosphere because of both its limited permeability (“Im” — the ability of a material to pass liquid and vapor) and its insulation (clothing insulation units, or “clo,” equivalent to 1.14 R units, where R is the standard unit for building insulation). The Permeability Index Ratio (Im/ clo) of the complete clothing ensemble can be used to indicate the percentage of the maximum possible evaporative cooling obtainable with that clothing in a given environment.
The environment at the fire scene consists of air temperatures that usually approach 500°F but that can reach 1,8()()°F. Wind will reduce the value of protective clothing insulation, a benefit if more heat loss from the body is needed but a risk if heat gain is a concern. The water vapor pressure of the air (related to humidity at a given temperature) should be as much below the vapor pressure of sweat at the skin as possible.
Ideally, heat production by the body should match the amount of heat able to pass through the clothing and be lost by radiation and convection (Hr + c) to the environment. There would be no evaporative requirement (Ereq), and whether the clothing was totally impermeable would not make any difference. However, as air and radiant termperatures approach skin termperature (about 95°F when comfortable), the requirement for sweat evaporative cooling increases. When these temperatures exceed skin temperature, Ereq must eliminate not only body heat production but also the heat gained through the clothing to the skin from Hr + c; thus the evaporative requirement equals the sum of metabolic heat plus the amount of heat lost to the environment by radiation and convection (Ereq = M + [Hr +c]).
The maximum evaporative cooling (Emax) possible is limited both by the vapor pressure difference between the skin and the environment and by the permeability index ratio of the clothing (Im/clo).
As the requirement for sweat evaporative cooling increases (due to harder work, higher clothing insulation, or a hotter environment) or the maximum evaporative cooling decreases (due to a more humid environment, higher insulation, or decreased permeability), more of the skin must become sweat-wetted as the body produces more sweat in an attempt to get the evaporative cooling it needs. When skin humidity (or wettedness, estimated as a percent given by Ereq/Emax) exceeds about 20 percent, heat discomfort begins. At such mild levels, increases in air motion or clothing permeability can be sensed as increasing comfort, although the actual improvement may be quite small. Performance of mental tasks (judgment, decision making, and so on) may be affected at Ercq/Emax ratios above 40 percent; performance of physical work tasks is affected above 60 percent. As skin-wettedness approaches 80 percent, the risk of heat exhaustion and collapse increases greatly and work time is severely limited—to as little as 20 minutes under the most severe conditions.
COMBATING HEAT STRESS
There are three solutions to heat stress problems:
Modify the worker by selecting fitter individuals, by instituting physical conditioning or heat acclimatization programs, by increasing drinking water intake (and minimizing alcohol intake) to help replace sweat losses, and so on. These solutions work best when sweat evaporation is not limited by clothing or environment, but they may have little effect on firefighters wearing full protective gear, with the possible exception of increasing water intake. Work practices that reduce heat stress by allowing personnel to remove garments or wear them open until just before getting involved on the scene are also advantageous.
Modify the clothing to reduce insulation and increase permeability. Reducing insulation is far more resultoriented than increasing the permeability of the moisture barrier. Fire clothing, normally value-weighted at 3 clo, will reduce by one-third any cooling improvement brought on by a permeability change. Reducing weight and hobbling also will be helpful.
New clothing designs allow easier ventilation of the spaces in and under the clothing. Introduction of radiant heat barriers and/or lamination of fabric layers to eliminate some of the insulating air trapped within the three-layer assemblies used in modern firefighter clothing also are more effective than permeability increases.
The requirements for evaporative cooling during moderate-to-harder work in the heated environment of a fire scene cannot be matched even by removal of all liquid barrier materials from the ensemble. However, improvements in permeability can be detected as improved comfort, but only at rest. Providing some form of auxiliary cooling (for example, ice vests, ventilation at the scene with cool or conditioned air, or using a wettable but nonflammable outer cover) is probably the most practical solution. Auxiliary cooling has become routine for hazardous waste site workers wearing protective clothing and seriously should be considered for firefighters.
Modify the task by substituting mechanical for human work (for example, water towers rather than hose hauled up ladders); providing added firefighters at the scene to allow shorter work and longer break periods; and providing air-conditioned “rest break vans” at the site or at least cool air jets to blow into the clothing during breaks are probably the most practical to adopt.
HEAT STRESS TASK FORCE
The NFPA 1590 subcommittee organized a task force on heat stress in 1986 to look at improved protective clothing materials. It first developed a Draft Standard for evaluating the thermal properties of protective clothing materials using a device called a “heated, sweating flat plate.” This apparatus also is referred to as a “skin model.” Comfort Technology, W.L. Gore Associates, Kansas State University, and North Carolina State University all participated in developing this Draft Standard. Then, working individually, the four groups demonstrated that they could produce comparable values for measured insulation and permeability on four types of fireprotective material assemblies consisting of the now common three layers: an inner heat insulating batting, a moisture barrier, and an outer shell of fire-resistant fabric.
The heated, sweating flat plate, in an environmentally controlled chamber, has electrical heating wires on its surface and a temperature sensor to measure and control the plate surface temperature (“skin temperature”). The skin temperature is usually controlled at 95°F, typical human skin temperature in hot weather. Air movement across the surface is set at 50 feet per minute—a typical value for air motion around a human body—which is about twice the movement rate called for by the ASTM standard test method (ASTM D1518).
The amount of electrical heat required by the skin surface to stay at 95°F once the apparatus settles into a “steady state” is exactly equal to the amount of heat being lost from the surface. The insulation of any material placed on the plate, or even of just the air layer at the plate surface, can be expressed in clothing insulation units. One clo allows a nonevaporative heat loss of 6.5 watts per degree of the difference between the skin (at 95°F) and the environment; two clo limit this nonevaporative heat loss to half that, three clo to one-third that, and so on. In essence, a firefighter’s typical clothing ensemble, which provides three clo of insulation, allows the wearer to lose only 2.2 watts through the clothing, without sweating, for each degree that the air temperature is below 95°F.
The skin model plate also can be run in a sweating mode. Water at the plate surface will evaporate to the conditioned air (temperature and humidity) in the chamber. The additional heat demanded by the plate to maintain its surface skin temperaure while this “sweat” is evaporating can be measured. This “evaporative cooling” can be expressed in proportion to the maximum evaporation possible from a 100-percent-wetted surface with a high air flow across it. If there is no resistance to evaporation, then the Im = 1; if there is an impermeable plastic sheet directly over the plate surface, there can be no evaporation and Im = 0.
However, if there is a layer of fabric (or even just trapped air) between the sweating surface and the impermeable layer, evaporation can remove heat from the plate and deposit it at the inner surface of the vapor barrier. Because this moves heat across an insulating space so that it is now more easily lost to the outside air by nonevaporative transfer and because probably there has been a loss of the insulation as it absorbs the water vapor collecting and condensing inside the vapor barrier, there is an “effective evaporative heat loss” even with a complete vapor barrier layer.
Typically, because of lack of such high wind over the body and the evaporative resistance of everyday clothing fabrics, a normal assembly will have an Im of about 0.45. The Im for a relatively thick insulation layer covered by an impermeable membrane might be 0.10. The actual evaporative cooling obtained by the garment wearer will be limited by both the Im value and the garment-insulating clo value; in essence, the actual evaporative cooling obtained by a garment wearer is set by the factor Im/clo. Thus with a typical 3-clo firefighter clothing ensemble, any change in the Im designed to increase evaporative cooling will be almost nullified by the insulative quality of the rest of the ensemble.
Of course, the potential of the environment to allow sweat evaporation is limited by the difference between the vapor pressure of water at the skin surface temperature (which at 95°F = 42 Torr) and the vapor pressure of the water in the air. As air temperature approaches skin temperature and the vapor pressure of the air approaches 42 Torr (the vapor pressure of sweat), there is no way for heat to be passed through the clothing by either dry or evaporative means. Whether a firefighter is nude or heavily insulated, in permeable or impermeable clothing, no heat can flow from him unless there is a temperature or a vapor pressure difference between the skin and the air outside the clothing.
(Photos courtesy of author.)
The four laboratories also measured the evaporative cooling potential through some typical three-layer firefighter clothing material arrays. The arrays included one with a Neoprene® (impermeable) barrier as the middle layer (over the insulating batting and under the Nomex® cover fabric) and one with a microporous (some vapor permeability) barrier as the middle layer. Even on the heated, sweating flat plate skin model, the difference between these two types of barrier membranes was small.
As stated in the current draft 1991 proposal for NFPA 1971 Standard on Protective Clothing for Structural Fire Fighting:
“The actual differences in garments made up from these fabric ensembles will be much smaller, because the insulation of the garments will be two to three times greater due to the air layers in garments which are not accounted for on the flat test plate. Therefore, the actual watts/m2 of heat transfer will be between ½ and ⅛ the values obtained by the test method. The actual heat transfer will be decreased even further by environmental temperatures and humidities that are higher than those used in the test procedure. These considerations lead the task force to the following conclusions, which should be remembered (when using the results from the following test method):
1.It is only in mild environments or at low work levels that differences in uniform materials are likely to have any appreciable effect on heat stress.
2. In the most stressful situations of high temperature and/or high work rates, material changes are unlikely to make any significant improvements in tolerance time.
3. NFPA should address heat stress through other means in addition to, or instead of, material specifications. For example, use of ice cooling vests is a practical and economical way to deal with heat stress problems. Such an approach allows the uniform to be designed for minimum heat stress without compromising protection. By comparison, material performance standards may have the effect of trading off protection for heat stress reduction.”
It may be useful to examine the current state of the art in one of the latest complete firefighter’s protective ensembles. The details of one such ensemble are presented in Figure 1, which gives measurements made by Comfort Technology during the summer of 1989 during a study of firefighting ensembles using a heated copper manikin designed to the size and weight of an adult human in 15 sections. The weight of the complete clothing ensemble, about 24 pounds, plus a 23-pound SCBA and the hobbling of arm and leg movement in such thick, heavy clothing increase the working heat production by about 30 percent over that of the same task done wearing only a station uniform. The insulation of such an ensemble, about 3 clo, is roughly twice that of a station uniform alone (see Figure 2). This limits nonsweating heat transfer from the body to about 2 watts per degree difference between the 95°F average skin temperature of a firefighter at work and the air temperature. In a 65°F environment, nonsweating heat loss would be about 60 watts through the ensemble, so 80 percent of the average heat produced fighting a fire (about 300 watts for an average hour spent at the scene) would have to be lost by evaporative cooling. With this much insulation, such a level is impossible even in desert humidity.
Figure 1.
ITEMS IN A MODERN FIREFIGHTER’S PROTECTIVE CLOTHING ENSEMBLE AND THEIR WEIGHTS
CLOTHING TOTAL: 24.27 SCBA (With Carrier and Regulator) 23.15
TOTAL SYSTEM: 47.42
- One pound of headwear equals 1.3 pounds “back weight”
- One pound of handwear equals 2 pounds “back weight”
- One pound of footwear equals 5 pounds “back weight”
The weight of the firefighter’s protective ensemble is measured as the relative weight felt on the back. The real weight of clothing parts can have a built-in multiplier when measuring “Back Weight”. According to the chart, a change of weight for effectiveness gained should be, in order: foot, hand and then head protection.
The high insulation is linked, although not linearly, to a requirement to use materials with sufficient thickness (and other properties) so that the Total Thermal Protection index (TTP) will be about 35 —the fabrics used will provide about 17.5 seconds of protection before a second-degree burn would occur, at least as measured in the test apparatus.
Largely because of such high insulation but in part because of the reduced permeability of even the most permeable of such waterproof layers, the maximum sweat evaporative cooling of a complete 1989 ensemble (with helmet, mask, hood, boots, and SCBA) as measured on the heated, sweating manikin is only about 12 percent of that allowed by the environment. Indeed, an ensemble using a totally impermeable liquid barrier will not be very different; the apparent Emax is still about 10 percent because of sweat moving through the clothing, evaporating from the skin, and condensing at the liquid barrier that is beyond the thick, insulating inner layer.
Looking at the latest materials proposed for fabrication of newer firefighter clothing, the maximum sweat evaporative cooling through the fabrics used in a turnout coat with even the most permeable liquid barrier available would be limited to 20 percent or less, compared with more than 30 percent when just the station uniform fabric was measured. Note that a three-layer turnout coat fabric assembly without any vapor or liquid barrier material also severely limited Emax—to only about 22 percent of the maximum cooling allowed by the environment because of the high insulation.
Figure 2.
INSULATION AND PERMEABILITY OF A FIREFIGHTER’S COMPLETE ENSEMBLE WORN WITH AN SCBA
Average weighted by surface area of each section of copper manikin.
Therefore, while not giving up on possible future improvements in protective clothing (focused on reduced insulation and weight; reduced hobbling by better design and fitting; introduction of radiant barriers and of designs, which allow easier or greater ventilation during wear; and development of liquid barriers with further improvements in permeability), in December 1989 the NFPA task force on firefighter heat stress recommended that the heat stress problem be referred from the protective clothing committee to the NFPA 1500 (Standard on Fire Department Occupational Safety and Health Program) committee.
The 1500 committee is the appropriate group to review the third class of solutions discussed above—modified work practices, greater use of mechanical rather than human effort, increased staffing levels, and use of auxiliary cooling. At present, auxiliary cooling appears to be the simplest of these solutions.
