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Why is a bound seam effective for a garment protecting against asbestos dust?

The new regulation published in 2013 by the French Ministry of Labour relating to garments used for protection against asbestos dust introduced new requirements beyond those standard for Type 5 dust protective coveralls. It includes the requirement for:-

“…de vêtements de protection à usage unique avec capuche de type 5 aux coutures recouvertes ou soudées (classification issue de la norme NF EN ISO 13982-1 et son amendement de mars 2011), fermés au cou, aux chevilles et aux poignets”

or, translated:-

“…single use Type 5 protective clothing with hood, with covered or welded seams (classification following the standard NF EN ISO 13982-1 and its amendment in March 2011), elasticated at the neck, ankles and wrists…”

There has been some debate regarding meaning or intention of this requirement, specifically in respect of the term “coutures recouverte” or “covered seams”. Some claim the requirement is for a taped or welded seam – in other words a fully sealed seam, on the basis that no stitched seam is suitable because it does not seal the seam. However, not only does this argument ignore the actual regulation wording, it is simplistic and ignores the greater complexities and unique nature of dust protection (which is different to liquid protection).

This guidance document covers the issues relating to asbestos and dust protection, discusses the principles behind protection against hazardous particles and explains why in some types of suits for dust protection a sealed seam, either taped or welded, makes little difference to the protection afforded.


1. The three seam types used for disposable protective clothing are stitched (or “serged”), bound, and stitched & taped (or welded).

2. Particles move as a result of airflows in the

atmosphere in which they are suspended and are only likely to pass through seam holes if there is an air-flow through the seam hole to carry them.

3. In non-breathable fabrics (such as microporous films and flashspun polyethylene) the “Bellows Effect” results in air-flows through seam holes, so sealing of the holes will make a difference and improve protection.

4. However, where fabric is breathable the “bellows effect” is minimal and there is little air-flow through seam holes, so no tendency for particles to pass through them. Thus a sealed seam on a breathable fabric garment makes little appreciable difference.

5. A bound seam on a breathable fabric meets the letter of the regulation both factually AND more importantly in terms of the question of whether it provides protection and improves protection over a standard stitched seam. In fact this analysis suggests a bound seam on a breathable fabric may be more effective than a sealed seam on a non-breathable fabric.

Seam Types

There are three types of seam commonly in use for disposable protective clothing:-

Serged / stitched seam: The most common is the standard serged or stitched seam, used for most Type 5 and 6 garments. Clearly the French regulation, not unreasonably, was intended to EXCLUDE this basic type of seam as it features open stitch holes through which particles might penetrate.

Stitched and taped seam: This construction completely seals the seam against ingress and is normally reserved for chemical protective Type 3 & 4 garments. However, this is a more expensive seam type (primarily because it is slower and involves two operations – stitching and taping) and so has a considerable effect on the cost of the garment. Clearly the intention of the French regulation was to INCLUDE these seams as acceptable as they seal up seam holes.

Bound Seam. The debate primarily revolves around the third of these seam types – the “bound” seam. In this seam a strip of doubled-over fabric is wrapped around the seam during the stitching process. This results in both a stronger seam with an improved resistance to the holes opening under stress and a greater tendency to trap particles that might attempt penetration. Some have claimed that a bound seam is not suitable because it does not offer a completely sealed seam.

Thus the key the questions relating to bound seams are:-

1) Does a bound seam meet the requirements of the regulation?

2) Are there sound reasons why a bound seam offers suitable protection against dust including asbestos?

Asbestos particles

There are several types of asbestos particle. Generally they are long and thin in shape and it is the relatively large ones (>5 microns in length and with a length to width ratio of 5:1) that are considered the most dangerous. Asbestos is a respiratory hazard and small quantities inhaled in a short time are unlikely to represent a real health hazard, whereas inhalation of large quantities and / or repeated inhalation over a long period can result in asbestosis and eventually death. Thus, for workers involved in operations where asbestos dust is present, the purpose of wearing protective clothing is to prevent contamination of the wearer’s own clothing in order to avoid inhaling the dust after exiting the critical area and after any respiratory protection is removed.

Primarily asbestos protection is a Type 5 (EN 13982) application in terms of EC standards. This standard deals with protective clothing against hazardous particles. The standard Type 5 garment is a low cost, often breathable garment with stitched seams and is considered perfectly acceptable for most applications involving hazardous dust. However, with or without the French regulation, in some asbestos (and other hazardous dust) related applications, for example in enclosed areas with poor ventilation and a high concentration of particles in the atmosphere, a much higher level of protection should be specified: such as a fully sealed Type 3 / 4 garment consisting of barrier fabric and sealed seams or in extreme cases a fully encapsulated suit that seals the worker from the surrounding atmosphere. For this reason the assumption that a standard Type 5 garment, whether with stitched, covered or sealed seams is suitable for all asbestos related applications is a fundamental error in any case.

Why would a dust particle penetrate through the seam holes of a stitched seam garment?

Does the addition of a sealed seam improve the protection of a Type 5 garment? Clearly a stitched seam has (stitch) holes in it and a taped or welded seam does not, so the simple answer - certainly in the case of liquid hazard protection - is an obvious “yes”. However, dust protection is a different matter and the situation is more complex.

The nature of dust particles is that they float freely in the atmosphere (they would present much less of a hazard if they did not). Unlike liquid sprays they have no or little intrinsic kinetic energy or trajectory and will move only as a result of an air flow (in still air dust particles will remain suspended until they float to the floor as a result of gravity). So why would a particle pass through the seam holes of a garment? The answer is only if there is airflow through the seam hole to carry it. For this reason whether a taped or sealed seam will add protection to a coverall is more complex; the “Bellows Effect” means the answer will depend on the garment fabric and whether the fabric has a measurable level of breathability – whether air can pass through it or not.

Type 5 coverall fabric types

Three basic fabric types are in use for construction of disposable (Type 5) coveralls:-

*some manufacturers claim forms or levels of breathability for these fabrics but any that exists is both very low and only occurs under pressure – so is irrelevant in these terms as such pressure will not occur in a garment scenario.

SMS Polypropylene is very breathable and allows air to pass through freely. Its central layer of meltblown fibre also creates an effective dust barrier - hence its common use and effectiveness for Type 5 protective coveralls. It is this type of fabric that that has been traditionally used for garments for asbestos and other hazardous dust protection.

Microporous Film Laminates and flashpun polyethylene however feature no or very low levels of air permeability. Air cannot pass through the fabric unless under pressure, and this has consequences for their use in dust protection.

The 'Bellows Effect'

The bellows effect is created by non-breathable fabrics as a result of the air trapped inside the suit. Movement of a wearer is in effect a “pumping” action, causing the air to be circulated inside the suit and creating micro-areas of pressure differential. In a breathable fabric such as SMS this does not occur. The air can pass freely through the fabric so no air is trapped and no pressure differential is created. However, in the case of the other two fabrics the air is trapped being unable to pass through the fabric, so greater areas of pressure differential result (this can be seen visually in the way the legs of a garment made from non-breathable fabric will “balloon” repeatedly as the wearer walks or moves). In this case the holes created by a standard stitched seam become a problem; any stress on stitched seams causes the holes to open – and any pressurised air will rush through – taking any dust particles in the atmosphere with it.

The net result in a garment with no breathability is a constant, repeated “sucking and blowing” (hence “The Bellows Effect”) of air through open seam holes. As a result particles suspended in the surrounding atmosphere can be actively sucked or drawn into the suit through the seam holes.

(This effect can also be seen visually in the use of such garments for protection against very fine particles of a dark colour such as carbon black. User wearing white clothing underneath will be aware of a line of dots on the sleeve where particles have been drawn through seam holes by the bellows effect).

Thus for garments of non-breathable materials such as microporous films and flashpun polyethylene a sealed seam will certainly improve protection at the seams because it will prevent the consequences of the Bellows Effect – at least at the seams. And therein lies the further problem.

Sealing the seams in a garment does NOT seal the garment, just the seams. The garment still has potential (and larger) openings at ankles, neckline, hood opening and wrists, and sealing seams can only logically lead to an even greater propensity for the bellows effect to occur at these other openings. The conclusion is that the sealing of seams on such garments is ONLY of benefit if the user also takes extensive measures to seal up joins between hood and face-mask, sleeves and gloves etc. – measures such as taping up of these joins with a suitable adhesive tape. (It is notable that in Type 5 testing of non-breathable fabric garments such taping up is vital: garments would fail the test without it. Yet this is much less of a need in testing of garments made with breathable fabrics).

However, with a breathable fabric such as SMS, because the air can pass through the fabric freely there is no bellows effect created at the seams or any other openings. There is no creation of air flow and so no active drawing of particles into the garment. Thus, providing that the fabric itself has sufficient filtration properties to prevent ingress of the particles through it (and bearing in mind that asbestos is a relatively large particle compared to many dusts these fabrics are used to protect against), then the suit will have effective protection with or without a sealed seam. Thus in a breathable fabric suit sealing by taping of the seams is not only expensive, it serves little purpose.

Thus although the non-breathable fabric itself might have a superior barrier against particles, the bellows effect means that a garment of these fabrics can result in a lower protection level than an effective but breathable particle filter fabric because the lack of air permeability in the former results in active drawing of particles through seam holes and any other possible opening, something that simply does not happen with a breathable fabric.

In summary:

Considering this knowledge, we can re-address the two original questions regarding bound seams and the French regulation on asbestos;-

1. Does a bound seam meet the requirements of the regulation?

Yes it does. The requirement is clearly for a “couture couvert” or “covered seam”. It does not state a requirement for a sealed seam (which would be “couture etanche”). The bound seam is by definition a “covered” seam as a strip of fabric is wrapped around the seam in the stitching process. There is no ambiguity about this. The reference in the regulation to a “covered” seam and not a “sealed” seam is clear. A bound seam meets this requirement.

Of course it might be argued that the intention of the regulation was to require a sealed seam. However, that is not what it states and until there is clarity on this the bound seam does meet the requirement. Further, the bellows effect (or lack of it in a breathable fabric) suggests that whilst a sealed seam on a non-breathable fabric can improve protection (though arguably only if joins and connections with other PPE are also taped up) a sealed seam on a breathable fabric, such as SMS – normally used for asbestos protection – has at best a very limited effect that is far outweighed by the additional cost involved.

2. Are there sound reasons why a bound seam offers suitable protection against asbestos?

Yes there are. The Bellows Effect means that a breathable fabric creates little or no airflow through seam holes so in such a garment there is no reason that particles will penetrate through the holes – there is nothing to draw them through. And in terms of a bound seam, whilst it does have stitch holes the covering fabric does two things:-

a) It reduces the tendency for the holes to open under stress by adding strength.

b) It provides a more tortuous pathway (for any penetrating particle to gain access, (because it must pass through several consecutive holes instead of one) severely reducing the likelihood of penetration, airflow or not.

In other words the bound seam adds a level of assurance and strength to the seam integrity not present in a standard stitched seam, whilst meeting the requirements of the regulation AND minimising the additional cost.

Only in the case of non-breathable fabrics (such as microporous films and flashspun polyethylene) for dust protection is a sealed seam likely to make an appreciable difference to protection (but will require sealing of other openings such as hood, wrists, ankles etc.). In fact, if garments seams are not sealed, the logic is that for particles larger than the filtration level of the SMS fabric it is likely to offer BETTER protection than the non-breathable versions because it will have a lower tendency to draw particles through the seams and any other openings.

Published: 08-05-15

Click here for more information: http://www.lakeland.com/europe/our-products/disposable-clothing-type-5-and-6/safegard-76-diamant.html


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