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January 22, 2008

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Fire resistant protective clothing

Fire service and emergency personnel have to wear protective clothing for their work but as well as the physical properties of such garments, physiological comfort is also a crucial consideration, argues Dr Wolfgang Nocker.

When a fire, accident, or other disaster occurs the first responders on the scene – be they fire-fighters, ambulance workers, or rescue personnel – depend on functional protective clothing. The garments they wear must provide reliable protection at all times and in every situation, against such possible hazards as heat, flames, moisture, chemicals, bacteria, and mechanical damage. At the same time, it should also be comfortable to wear, so as to ensure the well-being and ability to perform of the wearer on active service.

But who is to say what exactly is comfortable? What do we understand as comfort? What are the factors that influence it? And what are the performance characteristics of physiologically optimised clothing?

Asking people how comfortable they feel in protective clothing will produce a variety of opinions. Some will say they are comfortable if the clothing fits properly and offers enough freedom of movement. This is known as ergonomic comfort. Others will say it depends on how the clothing feels on the skin, i.e. smooth, soft or rough. This is sensory comfort. Still others will emphasise (thermo)physiological comfort, i.e. the ability of the clothing to support the wearer’s body temperature-regulating function in varying climate conditions, or at varying levels of physical activity.

Influencing factors – internal

Being warm-blooded, normal human body temperature must be between 36.7°C and 37.3°C in order to maintain vital functions and feel well. Humans constantly produce heat through organ and muscle activity during periods of rest and movement, and excess heat is continuously released so that the body’s thermal balance remains at a constant level.

Of this heat loss, 10 per cent occurs through breathing (via the lungs) and 90 per cent via the skin. The latter loses “dry heat” through thermal conduction, convection and radiation. The body’s other effective cooling mechanism is vaporisation, i.e. evaporation of sweat.

On cooling, the body reacts to the drop in core temperature by “taking” various measures. Cutaneous circulation is first reduced by the contraction of the blood vessels, so that there is less heat evacuation to the surface of the skin. The restriction of blood flow causes the temperature of the extremities (hands, feet, etc.) to fall, and, last but not least, the body attempts to produce additional heat by raising its metabolic rate by, for example, shivering.

Influencing factors – external

Factors outside of the body can also create a heat imbalance and thus limit the comfort of those wearing protective clothing. First, there is the climate in which the wearer is working. Air temperature, relative humidity, air movement (wind), and thermal radiation (sun) can all affect the body’s heat balance. The lower the atmospheric temperature and moisture content of the air (relative humidity) the greater the quantity of “dry” heat flowing from the body into the environment. Or, to put it another way, the colder and drier the ambient air the greater the tendency for humans to cool down and, in extreme cases, to freeze.

In most workplace situations the ambient temperature is above 15°C. Physiologically speaking, below 15° C is already where the cold zone (where humans begin to feel uncomfortable) starts, so cold stress is often underestimated. Feeling cold can affect people’s performance level because of restricted mobility, reduced concentration, and slower reflexes.

The wind, too, greatly influences the loss of heat from the body. Humans experience moving air as colder than still air. It takes heat from the body by destroying the 1mm-thick layer of air trapped next to the skin, which creates a micro-climate that functions as a thermal insulator. As a result, the body cools down more rapidly and so perceives the moving air as colder than it really is.

Other factors are damp and wet from outside (e.g. rain) and inside (sweat), which can cause an increase in clothing weight. In a cold environment, damp or wet clothing leads to greater heat loss. Wet clothing can also cut the clothing’s thermal insulation by up to two thirds. This is exacerbated when combined with the wind-chill factor, which can cause local chilling. Cold feet may not sound like much of a problem but they can have a knock-on effect, causing complaints in the neck, nose and throat area, for example.

When responding to an emergency, active service personnel must expect to have to work in unfavourable environmental conditions at all times. In the toughest deployment conditions – fire-fighting, for example – atmospheric temperatures of up to 900°C can quickly develop due to radiant and convective heat. Obviously, this causes extreme heat stress for the fire-fighter.

Flashover (darting flames) and superheated steam are also a problem: when the water vapour produced by the water used to put out the fire penetrates the protective clothing and soaks it, skin scalds can occur when there is sudden contact heat.

Work intensity

The second external factor that affects the body’s heat balance and, ultimately, the wearer’s comfort, is workload – type, intensity and duration. The greater the workload the more heat the human body gives off. In the everyday working lives of many fire and rescue personnel, the intensity of physical effort is comparable to that of competitive athletes. Personnel on active service can achieve physical peaks during their operations that, in extreme cases, can correspond to a metabolic rate of up to 1200 watts.

The body responds to this intensive heat production by increasing heat loss: the heart and breathing rates increase and the skin sweats more to cool the body down via vaporisation.

However, because protective clothing is essentially designed as a barrier to the outside, it often has the effect of allowing the excessive heat produced to be discharged only to a limited extent, or the sweat to vaporise insufficiently. As a result, the body’s core temperature can rise dangerously, leading to a drop in performance, loss of concentration, dizziness, and even heat stress (in the US, this has been found to account for 49 per cent of all fatalities among fire officers).

If sweat cannot be wicked away to the outside, and there is no “quick drying out” of the wet insulation layer, heat or cold bridges are created that, on exposure to heat, can lead to scalds or, on exposure to cold, to chilling.

Most fire and rescue workers have to wear and/or carry cumbersome equipment – breathing apparatus, for example – and this can also stress the entire body by increasing heat production. And if protective clothing is too thick, freedom of movement is further restricted, which, again, can lead to more heat production.

Conclusion

Because it is a direct link between the human wearer and the environment clothing takes on a quasi-physiological significance. Depending on its specific properties, it can support the body’s temperature-regulating function and in varying climates or levels of physical effort can help maintain the body’s heat balance.

Physiologically optimised protective clothing for fire and rescue personnel should therefore consist of materials that sufficiently help take heat and moisture away from the heated body. This “breathable” clothing must also be thermally insulating, i.e. protect against the chill caused by cold or wind, and against the wet. In short, thermal insulation and the passage of moisture are the two most important properties of protective clothing for active service personnel in terms of maintaining thermoregulation in the wearer.

However, fire and rescue personnel must also be provided with garments that offer effective protection from heat, flames, and wet. This usually requires protective clothing with a multi-layer fabric structure, comprising a flameproof outer fabric, moisture barrier, insulating layer and inner lining. For a long time the requisite protection against heat was achieved by an air pocket in a thick textile insulation layer made of several plies of textile fibre. The disadvantages of this solution, however, were that it allowed heat and moisture to build up, and the garments were heavy and limited freedom of movement.

These days, new technologies, such as the use of embedded air cushions, guarantee a constantly high level of heat protection. At the same time, the physiological comfort for the wearer has been massively improved thanks to significantly higher levels of breathability, sweat evaporation, sweat absorption and quick drying.

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