Selecting protective apparel
Permeation testing typically applies to Level A garments that protect against unknown hazards or gaseous/vapor phase chemicals that represent the highest level of respiratory and skin threats. The penetration test method applies to Level B and Level C garments that protect against moderate-skin-threat liquid chemicals. (Note that there is no difference in EPA garment recommendations between Level B and Level C protection, because both levels provide protection from moderate skin threats. The difference is the type of respiratory protection provided.)
For industrial applications, it is important not to overemphasize permeation performance over penetration performance. Doing so can result in an overprotective garment, which can create undue worker stress and productivity loss, as well as higher cost.
Permeation resistancePermeation testing measures a chemical's movement through a material on a molecular level. The American Society for Testing and Materials (ASTM) has created ASTM F739 as a standard test method for measuring permeation resistance. Apparel manufacturers typically test the permeation of their garments' materials against 21 representative liquid and gaseous chemicals as recommended in ASTM F1001.
The permeation test is conducted for up to eight hours (480 minutes), during which time the chemical remains in continuous contact with the garment material specimen at full concentration. This testing scenario is much more severe than situations typically found in industrial settings, where only splash or intermittent contact occurs.
In reality, most high-level (Level A) garments are worn for an hour or less due to time limitations in SCBA air supply, or to prevent heat stress and exhaustion in the wearer.
The permeation test provides two types of results: permeation breakthrough time and permeation rate. Normalized breakthrough time is the elapsed time between the start of chemical exposure and the point when the amount of chemical permeating the material reaches a rate of 0.1 micrograms per square centimeter per minute.
To put this into perspective, at the time of breakthrough, the rate of permeation is so small, it is equivalent to one grain of sand falling through a one square inch net each minute.
The reported permeation rate is the maximum speed at which the chemical comes through the material during the test.
Penetration resistancePenetration testing is a more appropriate method for testing chemical barrier when splash exposure is anticipated, which is far more likely in industrial settings than full immersion in a liquid. Reference for its use in determining chemical splash protection for emergency response applications can be found in the National Fire Protection Association (NFPA) Standard 1992.
Penetration is defined in ASTM Test Method F903 as the bulk flow of liquids through a material or seam in the garment. When testing penetration, a garment material is exposed to a concentrated liquid chemical for one hour. The material is then observed for any sign of liquid coming through the fabric.
Any detectable sign of chemical passage through the fabric is reported as a failure. This is a key advantage of penetration testing over permeation testing: the test results are not open to interpretation. Instead, results are presented as either PASS or FAIL for each chemical tested.
How to chooseIt should be clear that permeation is more appropriate for evaluating Level A garments. These garments are designed to protect against truly unknown hazards and chemicals presenting the highest level of skin threat.
On the other hand, penetration resistance testing is more appropriate for evaluating Level B and C garments, where the garments are worn to protect against chemicals presenting moderate skin threats, primarily in situations involving splashes or limited exposure.
Penetration resistance testing is an appropriate garment evaluation method for protection against liquid chemicals whose vapors are not hazardous through skin absorption. It is also appropriate when the liquid does not produce vapors, but users are exposed to the liquid, primarily through splashes, and those splashes are harmful to the skin, such as with nitrobenzene or sulfuric acid.