Considerations for Selecting Protective Clothing

23 Dec.,2023


When the transmission route is defined as “direct contact transmission,” such as in the case of Ebola and HIV, employers should consider gowns and coveralls that demonstrate resistance to synthetic blood, as well as passage of virus. Standard test methods can be used to evaluate the resistance of fabrics or seams/closures to synthetic blood penetration and viral penetration, as described in Table 1.

The United States commonly uses American Society of Testing and Materials International (ASTM) methods, while Europe commonly uses International Organization for Standardization (ISO) methods.

Table 1. Standard test methods to evaluate the resistance of fabrics to synthetic blood and virus penetration

Table 1. Standard test methods to evaluate the resistance of fabrics to synthetic blood and virus penetration Barrier Property
(Type of Penetration) ASTM Test Methods ISO Test Methods Synthetic Blood Penetration ASTM F1670—
Standard test method for resistance of materials used in protective clothing to penetration by synthetic blood. ISO 16603—
Clothing for protection against contact with blood and body fluids—Determination of the resistance of protective clothing materials to penetration by blood and body fluids—Test method using synthetic blood. Viral Penetration ASTM F1671—
Standard test method for resistance of materials used in protective clothing to penetration by
bloodborne pathogens using
Phi-X174 bacteriophage penetration as a test system. ISO 16604—
Clothing for protection against contact with blood and body fluids. Determination of resistance of protective clothing materials to penetration by bloodborne pathogens— Test method using Phi-X174 bacteriophage. Note: These tests are typically conducted on fabrics, but they can be conducted on the garment seams as well. It is recommended that end users inquire from the garment manufacturers about seam barrier test results, in addition to the fabrics, in order to appropriately protect healthcare workers from blood and viral penetrations.

ASTM F1670 and ISO 16603 are “screening-tests” that evaluate the resistance of a material to synthetic blood penetration [ASTM 2003a; ISO 2004a]. The synthetic blood used for these tests is a mixture of cellulose, coloring, buffer solution, and stabilizing agents. Synthetic blood has a surface tension (0.042 ± 0.002 Newton per meter [N/m]) and viscosity representative of blood and some body fluids (see Table 2 for surface tension of the body fluids).

Within the context of gowns and coverall testing, the surface tension of the challenge liquid is critical. This is because liquids with higher surface tension, like water (0.070–0.072 N/m), are more likely to bead on a surface than liquids with lower surface tension, which are more likely to wet and penetrate through the garment. Consequently, some test methods that use water as a challenge agent may not be representative for evaluating the barrier effectiveness of the healthcare PPE and may overestimate the effectiveness of the PPE for blood-borne pathogens. Test methods evaluating the water resistance of garments will be discussed later in this document.
Table 2: Surface tension values for water, synthetic blood, and human blood and body fluids1

The viral penetration resistance tests, namely ASTM F1671 and ISO 16604, are similar to ASTM F1670 and ISO 16603, but they use a bacteriophage (Phi-X174) challenge suspension instead of synthetic blood [ASTM 2003a; ISO 2004b]. At the conclusion of the exposure period in the ASTM F1671 or ISO 16604 viral penetration tests, the opposing surface of the material is rinsed with an assay fluid, and this fluid is then cultured in the presence of the host bacterium, E. coli. Plaques form when a bacteriophage is present, with the number of plaques indicating the number of penetrating bacteriophages. Materials pass the viral penetration test when no liquid is observed to penetrate the specimen and the E. coli bacteriophage is not detected in the assay fluid.

The choice of virus challenge agent in the standard methods is a critical test condition. For these test methods, the bacteriophage serves as a surrogate to simulate viruses that are pathogenic to humans. Phi-X174 bacteriophage has nearly spherical morphology similar to HIV, Hepatitis B, and Hepatitis C. At 27 nm in diameter, it is similar in size and shape to Hepatitis C (30 nm in diameter), which is the smallest-known bloodborne viral pathogen.

As mentioned earlier, the size and shape of a virus are believed to affect viral penetration, and thus selecting a small virus (27 nm in diameter) would serve as a “worst-case” scenario for the barrier material. Smaller particles are expected to more easily pass through pores in the fabrics used in barrier materials. Some of the other viruses, such as Ebola virus, are larger in diameter compared to Phi-X174. Currently, there is no scientific evidence to suggest the Ebola and other larger viruses would be more likely to penetrate through protective clothing than a smaller virus.

The amount of pressure applied in the standard methods is another critical test condition. The biggest difference between the ASTM and ISO test methods is the pressure levels used when conducting test procedures. In ASTM F1670 and ASTM F1671, tests are conducted using 13.8 kilopascal (kPa) (2 pounds per square inch [psi]), and the criterion is that no penetration should occur. Whereas, in ISO 16603 and ISO 16604, the maximum pressure level before any penetration occurs is found by applying increasing pressure levels (0 kPa to 20 kPa)—14 kPa is the most equivalent pressure to that of the ASTM tests. Note that ISO 16603 and ISO 16604 are used to classify and rank materials, and they do not relate the classification of material barrier performance to any specific circumstances of use.

Penetration (often called strikethrough) can be initiated by an external force acting against clothing. The force generated by an external pressure, such as from a pressing or leaning motion, is likely one of the major routes of blood penetration, especially in the chest and sleeves of protective clothing. These pressures arise when individuals wearing protective clothing lean or press on a surface that may be wet with blood or body fluids, such as in the case of a healthcare worker leaning against a patient’s bed or an emergency medical responder kneeling on a contaminated roadway. Studies have documented a range of pressures to which protective clothing is subjected during clinical use. [Altman et al. 1991] reported that the pressures exerted on surgical gowns during pressing and leaning in surgery can range from 1 psi to 60 psi. Blood penetration has been shown to increase with increasing pressure [Granzow et al. 1998].

Although high pressures have been reported, other studies have found that many common surgical movements (including leaning, reaching, and arm resting) result in less than 2 psi pressure. For example, [Smith et al. 1995] evaluated the pressures generated during a variety of surgical procedures and found that most pressures applied to the front of surgical gowns are 2.9 psi or less for 15 seconds or less. Another study showed that leaning against the operating table caused a pressure of 0.52 psi (3.6 kPa), while reaching for an instrument showed the greatest (0.70 psi, which equals 4.8 kPa) [Smith and Nichols 1991]. The greatest pressure seen during any maneuver was 1.84 psi (12.7 kPa) while reaching. Smith and Nichols estimated representative abdominal pressures during surgical procedures to be between 0.25 and 2.0 psi.

Others have looked at the areas where blood/body fluid penetration occurs through the garment. One study found that blood penetration was most common on the chest, forearm, and abdomen, and was correlated with the areas of highest exposure and pressure [Quebbeman et al. 1992]. Others have noted that the cuff, forearm, thigh, chest, and abdomen are most vulnerable to blood strikethrough [Pissiotis et al. 1997]. Studies suggest that if a liquid is in prolonged contact with a fabric, prewetting can occur, and this can result in the fabric’s decreased resistance to penetration [Flaherty et al. 1993; Olderman 1984].

The viral penetration of surgical gowns by HIV has been compared with the soak-through point (the point at which fluid visibly soaks through the fabric) by multiple investigators [Tyler et al. 1989; Shadduck et al. 1990]. It was reported that HIV could penetrate some surgical gown materials in common use at the time of the studies, and HIV penetration was sometimes noted in the absence of visible soak-through. This is important to remember, because endusers can often have a false sense of security when they see no visible penetration in their garments.

The conditions of the ASTM F1671 test require subjecting barrier material specimens in a special test cell to the viral challenge for one hour, with the sixth minute of the exposure at 13.8 kPa (2 psi) for one minute. These conditions were selected because they are used in a related method, “ASTM F903 Standard Test Method for Resistance of Materials Used in Protective Clothing to Penetration by Liquids,” which assesses liquid chemical penetration through protective clothing materials. Research at Kansas State University [McCullough and Schoenberger 1992] was performed to show how these test conditions best correlated with a human factors evaluation where visible blood strikethrough occurred. This is referred to as the elbow lean test.

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