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The selection of appropriate protective gear is based on the hazards anticipated or recognized. Protective clothing protects primarily because of the material from which it is made.
In selecting the protective material, the following should be considered:
Chemical resistance, which is the most important. When clothing contacts a hazardous material, it must maintain its structural integrity and protective qualities.
Strength, which is based on resistance to tears, punctures, and abrasions, as well as tensile strength.
Flexibility, clothing easy to move in and work in. Flexibility is especially important in glove materials.
Thermal limits affect the ability of clothing to maintain its protective capacity in temperature extremes. Thermal limits also affect mobility in cold weather and transfer of heat to the wearer in hot weather.
Cleanability, difficult and expensive if protective clothing is not cleanable. Some materials are nearly impossible to clean adequately under any circumstances. Disposable clothing is sometimes used.
Lifespan, which is the ability to resist aging, especially in severe conditions over time. This should be balanced against the initial cost of the garment.
Protective material must be able to resist degradation, penetration, and permeation by the contaminant. Any of these actions may result upon contact, depending on factors such as concentration and contact time.
Degradation is the result of a chemical reaction between the contaminant and the protective material. Damage to the material may be slight or as severe as complete deterioration. The reaction may cause the material to shrink or swell, become brittle or very soft, or completely change its chemical and physical structure. Changes such as these may enhance or restrict permeation or allow penetration by the contaminant.
A chemical penetrates a protective garment because of its design and construction imperfections, not because of the inherent material from which it is made. Stitched seams, button holes, porous fabric, and zippers can provide an avenue for the contaminant to penetrate the garment. A well designed and constructed protective suit with self‑sealing zippers and lapped seams made of a nonporous degradation‑resistant material prevents penetration, but as soon as the suit is ripped or punctured it loses its ability to prevent penetration. A material may also be easily penetrated once degraded.
The ability of a protective material to resist permeation is an inherent property. A contaminant in contact with the protective material establishes a concentration gradient. The concentration is high on the contact surface and low inside. Because the tendency is to establish equilibrium, diffusion and other molecular forces "drive" the contaminant into the material.
When the contaminant passes through the material to the inside surface, it condenses there. The process of permeation continues as long as the concentration remains greater at the contact surface. The permeation rate is based on several factors. Rate is inversely proportional to the thickness of the material and directly proportional to the concentration of the contaminant.
The amount or degree of permeation is related to the exposure conditions, especially contact time, which ultimately dictates how much of the contaminant permeates the protective material. Thus a conscious effort should be made to avoid prolonged exposure or contact with any hazardous contaminant, even when wearing protective clothing. No material resists permeation by all agents.
Once a contaminant contacts a protective material, the garment must be decontaminated. With many materials, it is impossible to completely remove all contamination. Materials such as butyl rubber and Viton, which can be effectively decontaminated and cleaned, are also expensive. In some situations disposable clothing may be advantageous.
Tables are available indicating relative effectiveness of various protective materials against generic classes of chemicals (for specific chemicals). Most tables only reflect ability to resist degradation. A protective material may resist degradation by a contaminant, but still be very permeable to it. Such charts are useful when used with discretion and when the seriousness of the hazard is properly evaluated. If a chemical is extremely toxic, then any activity involving it should be re‑evaluated.
Permeability data are available from manufacturers and independent testing laboratories. If there is a question about permeability of a material in contact with a specific contaminant, a sample swatch of the material should be tested by a recognized laboratory for permeability to that chemical.