Apparel Selection for Chemical Barrier
Understanding when to apply permeation and when to apply penetration data is key to choosing the best garment, one that is ideal in all aspects.
- By Kimberly Dennis
- May 01, 2004
CHOOSING the best chemical protection garment can be complicated. An ideal garment for one situation may be too much or too little protection in different conditions. However, by assessing workplace risks and applying the right evaluation methods, the garment selection process becomes less complicated. In fact, some may find their current selection process for chemical protection garments is actually overprotective, creating undue worker stress and productivity loss, not to mention higher cost.
EPA Protection Levels
As part of the apparel selection process, it's helpful to understand industry-standard categories of protective apparel. The Environmental Protection Agency has designated four levels of protection for workers during chemical cleanup. These levels help define personal protection in terms of respiratory and skin protection:
- Level A--The highest respiratory and skin protection under the highest threat conditions. This includes totally encapsulating chemical-resistant suits, chemical-resistant gloves and footwear, and a self-contained breathing apparatus.
- Level B--For high respiratory exposure threats and moderate skin threats. Level B ensembles include one- or two-piece chemical splash suits, chemical-resistant gloves and boots, and the same respiratory protection as Level A.
- Level C--For moderate respiratory and moderate skin exposure threats. Level C ensembles include a one- or two-piece chemical splash suit, chemical-resistant gloves and boots and an air-purifying respirator.
- Level D--Indicated when no respiratory or skin protection is needed.
- It's important to note there is no difference between Level B and Level C garments, because both levels provide protection from moderate skin threats. The difference is the type of respiratory protection required.
Understanding Skin Hazards
Selecting the right garment for protection against chemical splashes depends on the skin hazards in the workplace. The hazard assessment process starts with identifying the chemicals present and their physical state (solids, liquids, gases, or combination-phase chemicals such as volatile liquids). Because garments are designed to protect against skin hazards only, it's important to determine which of those chemicals pose a risk of skin exposure (versus respiratory or ingestion routes of exposure).
The likelihood of exposure must also be determined. This depends on three factors: the chemical state, the work task, and the work environment.
Chemical state. The likelihood for skin exposure is always higher with gas or vapor-phase chemicals such as ammonia and ethylene oxide, because vapors can spread throughout the environment and can continuously contact every exposed square inch of a garment. Some liquid chemicals, such as dichloromethane and hexane, also create vapors. However, these volatile liquid chemicals evaporate quickly, leaving the clothing before they can pass through to the skin.
Even with non-volatile liquid chemicals such as sulfuric acid and sodium hydroxide, the liquid typically runs off the clothing after a spray or splash, before it can soak through the garment and reach the skin. In addition, standard operating procedures typically require the wearer to remove the clothing immediately following exposure.
Work task. If the job involves cleaning parts with a solvent, for example, exposure to that solvent is likely as it occurs as part of the job, whereas equipment failure during a repair operation would be accidental and less likely.Work environment
. There is a great deal of difference between an open vat of acetone in an enclosed space, and a quart of acetone in a well-ventilated room. High ambient temperature also plays a role because it can increase the evaporation of volatile chemicals, leading to higher concentrations of those chemicals in the surrounding air.
After identifying various hazards in the workplace and the likelihood for exposure, it's time to assess the short- and long-term consequences. This depends on the nature and concentration of the exposure. Some chemicals may cause skin irritation or burns, even at low concentrations. Others may not cause immediate effects but could cause long-term health problems; and some chemicals may do both.
A good example of a chemical with both short- and long-term skin effects is phenol, which can cause a serious burn or poisoning in the short term and cause damage to the kidneys or liver over the long-term. Check the ACGIH book on Threshold Limit Values and Part 1910.1000 of the OSHA Title 29 Code of Federal Regulations for more specific notation on skin hazards.
Assessing the Risk
Once the consequences and likelihood of exposure are understood, the level of threat or risk involved can be assessed. Consider three scenarios:
- Production of acrylonitrile. Under normal circumstances, acrylonitrile would represent serious consequences for exposure because of its high toxicity. But in a self-contained production environment, where engineering controls dramatically reduce the likelihood of exposure, the threat to skin is moderate.
- A worker filling a container with a dilute solution of sodium hydroxide. Although sodium hydroxide can cause caustic burns to the skin, a dilute solution presents moderate consequences. Because splashing is possible in this scenario, the likelihood of exposure is moderate. As a result, the skin threat is moderate.
- An emergency response scenario in which the chemical exposure is unknown. With unknown chemicals, one must assume the consequences and likelihood of exposure would be severe. Therefore, this situation is designated as a high skin threat.
Understanding Chemical Resistance Testing
To select protective clothing for the skin threats identified, one needs to understand how materials are tested for chemical resistance. There are two test methods for doing this: permeation and penetration.
The permeation test method 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 C garments that protect against moderate-skin-threat liquid chemicals.
Permeation. Permeation measures a chemical's movement through a material on a molecular level. The test method for permeation is American Society for Testing and Materials (ASTM) F739. Apparel manufacturers typically test the permeation of their 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 the chemical remains in continuous contact with the material 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 fact, the test is equivalent to being fully immersed in a liquid chemical or being in a gaseous environment at 100 percent concentration for an entire work shift. In reality, most high-level garments are worn for an hour or less because of time limitations in SCBA air supply, or to prevent heat stress and exhaustion in the wearer.
The permeation test provides two types of results: 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. (This is equivalent to one grain of sand falling through a 1-square-inch net each minute.)
The permeation rate is the maximum speed at which the chemical comes through the material during the test.
Penetration. Penetration testing is more appropriate for testing chemical barrier when splash exposure is anticipated, which is far more realistic in industrial settings than full immersion in a liquid. Reference for the use of penetration testing 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, making it an important test in evaluating the material's barrier to liquid chemicals.
When testing penetration, a 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 over permeation testing because the results are not open to interpretation; they are reported as either PASS or FAIL for each chemical tested.
To aid in the assessment, some manufacturers offer additional information in their chemical resistance guides to indicate which of the chemicals tested for penetration are carcinogens or have "skin notations."
Applying Chemical Resistance Data
Follow the flow chart in Figure A to determine what type of chemical protection garment to choose.
After conducting the hazard and risk assessment, ask whether the physical state of the chemical is a gas or liquid. If it's a gas that requires skin protection, as with chlorine or ammonia, then permeation data must be applied,and the clothing must be gas-tight and fully encapsulating. Clothing meeting EPA Level A criteria should be used in these situations.
If the chemical is a gas that does not require skin protection, such as helium or propane, no skin protection may be needed.
If the chemical is a liquid that produces vapors and those vapors pose a health threat to the skin (such as benzene and acrylonotrile, which have skin notations and are classified as carcinogens), then permeation data should be used. In addition to permeation resistance, this clothing must also be gas-tight and fully encapsulating. Users should look for clothing that meets Level A criteria against these vapor-producing chemicals, depending on the respiratory criteria.
If the vapors produced by the liquid are not hazardous through skin absorption, as determined by the consequences or likelihood of exposure, clothing may be selected based on penetration resistance testing. Garments selected for this type of scenario should meet Level B or C criteria. (Keep in mind that selection between Levels B and C depends on the respiratory protection needed, as the garment criteria is the same for both levels.)
If the liquid does not produce vapors but users are exposed to the liquid through splashes that are harmful to the skin (such as with nitrobenzene or sulfuric acid), then penetration resistance data should be used. This clothing should be liquid-resistant and should meet Level B or C criteria.
If users are exposed to liquid splashes or other incidental contact and the liquid is not harmful to the skin, penetration resistance data is not necessary; however, other PPE may be considered.
Finally, if chemical contact is not expected, then chemical protection is not needed. Level D protective clothing is recommended here for protection against nuisance materials.
The key to appropriate garment selection lies in a thorough hazard and risk assessment to determine the specific threat to the skin. Permeation resistance testing is more appropriate for evaluating Level A garments that are worn to protect against truly unknown hazards and chemicals presenting the highest level of skin threat. Penetration resistance testing is most appropriate for evaluating Level B and C garments, which are worn to protect against chemicals presenting moderate skin threats, primarily in situations involving splashes or limited exposure.
One advantage of applying penetration test data is the ability to evaluate more comfortable, lighter-weight clothing with acceptable splash protection, versus heavier-weight, high-permeation-resistant clothing. In fact, some in the chemical protective clothing industry overemphasize permeation resistance data in the selection of protective clothing. The result is insufficient focus on garments that provide appropriate protection and may actually improve worker comfort and productivity.
In short, understanding when to apply permeation and when to apply penetration data is key to choosing the best garment that is ideal in all aspects . . . protection, productivity, and cost savings.
This article originally appeared in the May 2004 issue of Occupational Health & Safety.