Gloves as PPE: Standards for permeation and penetration

Latex, polychloroprene, nitrile and vinyl thin film single use gloves are used by the million in cleanrooms around the world every day. These gloves are worn for one of two reasons; either to protect products from contamination or to protect the wearer from harm caused by exposure to chemicals. This article looks at protecting the wearer and tries to unravel some of the
technicalities that should be considered when specifying gloves.

Protective gloves work by forming a physical barrier between the wearer and the chemicals that they are handling and will remain effective until the barrier is breached. Many users are lulled into a false sense of security because they are given protective gloves to wear. They believe that they can don them at the beginning of their shift and that they will remain protected throughout their work time. How wrong they are. There is a possibility that, during the course of their shift, the barrier will be breached via one of two mechanisms; permeation or penetration.

The definitions of permeation and penetration are provided by the European Standards covering gloves used for personal protection, but before we look at permeation and penetration in detail let us consider the background and purpose of standards.

Standards
Standards are technical documents intended to be used to define state of the art specifications, definitions, methodologies and guidelines.

Standards are created by bringing together interested parties such as manufacturers, consumers and regulators of a particular material, product, process or service. The resultant committees draw inputs from as many sources as possible to develop practical standards that can be adopted by manufacturers and service providers as minimum specification requirements. All parties benefit from standardisation through increased product safety and quality.

Unless required by regulation, the application of standards is voluntary and companies may use them to demonstrate that their products, services or systems have been assessed by an independent body, achieving at least the minimum performance levels required. Compliance with standards helps customers to specify their purchased products with the confidence that they are buying a quality product which will meet their requirements. Because published standards play such a large part in unifying product designs (particularly in specifying minimum safety requirements) they are often used as a means of meeting safety related regulations.

In this context the example that springs readily to mind are the various European CE Marking Directives. Each directive has an associated list of standards, compliance with which is deemed to satisfy specific parts of the regulation. All standards that are associated with European Directives have been harmonised for use throughout Europe and are recognisable by the prefix EN to the standard’s number.

EN standards (or Euro Norms) are created through the European committee for Standardization (CEN). CEN is a non-profit organization set up under Belgian law. Once approved and issued, an EN automatically becomes a national standard in the 31 member countries.

Gloves for protection against chemicals are classed as Personal Protective Equipment (PPE) and are regulated by the implementation of the European PPE directive, 89/686/EEC. The harmonised standard covering gloves used as PPE against chemicals is EN 374: 2003. The standard is broken down into 3 parts:
1.    EN 374-1: 2003 Protective gloves against chemicals and micro-organisms – Part 1: Terminology and performance requirements
2.    EN 374-2: 2003 Protective gloves against chemicals and micro-organisms – Part 2: Determination of resistance to penetration
3.    EN 374-3: 2003 Protective gloves against chemicals and micro-organisms – Part 3: Determination of resistance to permeation by chemicals

Permeation EN 374-3: 2003 Defines permeation as follows:
Permeation is the process by which a chemical moves through a protective glove material on a molecular level. Permeation involves the following:
•    Absorption of molecules of the chemical into the contacted (outside) surface of a material
•    Diffusion of the absorbed molecules in the material
•    Desorption of the molecules from the opposite (inside) surface of the material

Put more simply, chemical permeation can be considered as the passage of a chemical through a barrier layer at a molecular level. Molecules of the chemical are absorbed by the barrier material and as the number of molecules absorbed increases, the chemical gradually works its way through the material and out of the other side. By way of illustration, imagine a layer of dry sponge that has a trickle of water poured upon it. At first the water sits on the surface of the sponge. It then starts to soak in (absorption) and as it does so it spreads out throughout the sponge (diffusion). As more water is poured on, the sponge ‘fills up’ until the volume held reaches the point where water starts to trickle from the opposite side (desorption). The trickle of water emerging will increase in volume until a steady rate of flow is reached.

When assessing the suitability of a glove for its intended use, the performance of the material against the permeation of chemicals needs to be considered along with the proposed duration of use/exposure. The rate of permeation through protective gloves varies dependent upon the glove/chemical combination. The elapsed time between the initial application of a test chemical to the outside surface of a protective glove and its subsequent presence on the other side of the material, when it reaches a specific flow rate, is known as the breakthrough time and is the parameter which is used to determine the resistance of the glove material to the chemical.

Under EN 374, breakthrough is deemed to have occurred when the flow rate of 1 µg/cm2/min is reached.

Penetration
Penetration is defined in EN 374 as the movement of a chemical and/or micro-organism through porous materials, seams, pinholes, or other imperfections in a protective glove material on a non-molecular level.

Another way of considering this is where the chemical passes through holes in the barrier. The illustration for this mechanism is that of a polythene bag full of water. If the polythene is intact the water will stay in place. If the polythene has any holes (e.g. pinholes) the water will penetrate and escape.

Glove testing – penetration
If a glove does not protect against liquids and gas, it could be hazardous to wearers. EN 374-2 outlines the testing standard for penetration which involves a water leak test and an air leak test. Tests are performed with model liquids (e.g. water) and gases (e.g. Nitrogen).

Air Leak Test – A glove is immersed in water and its interior is pressurized with air. A leak will be detected by a stream of air bubbles from the surface of the glove.

Water Leak Test – A glove is suspended from a test frame and is filled with a minimum of 1 litre of water. A leak is detected by the appearance of water droplets on the outside of the glove.

A glove which resists penetration and passes both air leak and water leak tests to Performance Level 2 is deemed to be an effective barrier to microbiological hazards (however the standard stresses that this test does not guarantee protection against viruses). The air leak test may not be appropriate for all types of glove. For example, parts of some gloves might over inflate, while other parts of the same glove are only partially inflated or not entirely filled with air. If the air leak test proves to be inappropriate, only the water leak test need be performed to determine penetration.

The performance levels are ascertained from the Acceptance Quality Level (AQL) set in manufacturing and are defined in EN 374 .

Gloves from each single lot or batch are sampled according to ISO 2859-1 and inspected. ISO 2859 is an International Standard covering sampling procedures for inspection by attributes. Part 1 of the standard sets down the sampling schemes indexed by AQL and the inspection level for assuring quality during production. The performance level is determined by the required Inspection level and the AQL. The inspection level determines the sample size based on the manufacturing batch size and is given in a lookup table in ISO 2859-1. The AQL specifies the number of defective gloves that are allowed to be in the sample before the rejection of the entire batch and is also read from a lookup table in ISO 2859-1. By way of example, a manufacturer produces a batch of 500,000 and wants to claim Level 3 performance. Inspection level G1 predetermines that the manufacturer must draw a random sample of 500 gloves for each test (air and water leak).AQL 0.65 means that for each test six gloves from the 500 sample are allowed to have pinholes, but if a seventh is found the entire manufacturing batch must be rejected.

Glove testing –permeation
Permeation testing is carried out by placing a sample taken from the glove under test in a special test cell so that the glove material separates the test cell into two chambers. One chamber is filled with the challenge chemical and the other with a collection medium, which is a liquid in which the challenge chemical will dissolve. The collection medium is tested at regular intervals by mass spectrography to measure the amount of challenge chemical held in solution. This gives a measurement of the volume of challenge chemical that has passed through the sample material, enabling the flow rate to be calculated and monitored.

EN 374-1: 2003 also provides guidance on how the Measured Breakthrough Time (MBT) should be expressed in performance levels.

A performance level of 2 is obtained if the MBT is a minimum of 30 minutes and performance levels increase further the higher the MBT. When assessing the suitability of a glove, the performance level(s) should be considered. The higher the performance level, the better the level of protection.

Selecting the right glove for the job
When selecting protective gloves it is important to know what chemical is to be protected against, then select a glove that will offer suitable resistance. This may take a little detective work and a call to the glove manufacturer. With a pass at level 2 for only three from the list of twelve EN 374 chemicals required to classify a glove as offering chemical protection, it is highly unlikely that the chemical in use will be listed on the glove’s packaging.

Because EN 374 is very specific about the chemicals to be used in testing and qualification of protective gloves the resultant labelling of the packaged product can be confusing. The standard specifies three different symbols that must be used on packaging .

A shortcoming of EN 374 and its labelling requirements is that some very good protective gloves are required to be labelled with the open beaker symbol (indicating low level protection) just because the chemicals that they are designed to protect against are not on the prescribed list. For example, gloves that have been designed for use with chemotherapy drugs can have a performance in excess of Level 6 against a half dozen or so very aggressive and dangerous chemicals, but are still classed as low level protection by the standard. It is easy to fall into the trap of thinking that just because the flask symbol is missing from a glove’s labelling it doesn’t possess good protective properties.