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MVTR (Moisture Vapour Transmission Rate)

Breathability of disposable protective coveralls

Having worked in the manufacture and sale of disposable coveralls for over fifteen years I have seen many ideas, products and fabrics come and go in a market that has evolved considerably since the early days when, if the stories are true, a bright young technician in a lab somewhere spotted some high density polyethylene fibres bursting from a crack in a pipe and as a result went off and developed, eventually, the first disposable coveralls that became known popularly, despite the best efforts of the marketing men, as 'paper' overalls.

Covered in this article:

Nowadays of course viewers of programmes like “CSI” and “Bones” are used to seeing the white “bunny-suits” worn by Scene-of-Crime (SOC) officers at the grisly murder scene, though of course the purpose here is actually the opposite of the biggest part of the market, SOC officers (as workers in clean rooms) wear disposable coveralls to protect the crime scene from contamination by themselves, whereas most disposables are worn to protect the wearer from hazardous, possible life-threatening chemicals.

Compromise between comfort and protection

All protective clothing is of course a compromise between comfort and protection: in warmer environments the best comfort would be derived from wearing a vest, shorts and sandals or even less, it just wouldn’t help much when you’re splashed with hydrochloric acid; on the other hand the best protection might be derived from sealing yourself inside a lead-lined eight inch thick steel box, but it wouldn’t be too comfortable and you wouldn’t get much work done.

So as all of us in the industry know the panacea for disposable coverall manufacturers is a suit that provides adequate protection for the task along with a good level of comfort. And comfort is derived mostly from the “breathability”of the fabric. Softness, flexibility and garment fit can help of course, but if you’re wearing a very soft, flexible and Saville-Row fitted polythene bag it will still be quite uncomfortable!

And there is my gripe. The key to comfort is breathability, and the issue of breathability has now been made so confusing by the industry itself with claims and counter claims that it is now difficult to identify what is breathable and what isn’t. This particularly applies to the two leading types of fabric used in the market and especially to microporous films.

I will come onto an analysis of whether these fabrics are breathable or not, but meanwhile it is with some amusement and at the same time not a little frustration that I read some of the literature published by manufacturers about their respective microporous film products. One manufacturers’ web site claims that their product is...

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“...made from a new revolutionary fabric which is soft and breathable...”.

“Soft” I can readily accept... but “revolutionary”..? It wasn’t particularly revolutionary in the early nineties when it was first used for disposable coveralls (it was already widely used as a liner in the diaper industry) and even if it was this particular manufacturer was a long way from being the first to produce such a product... and it certainly is not revolutionary in 2008. And breathable..? We shall see.

Another manufacturer describes a microporous film coverall as a...

“...breathable PE laminates fabric...”

Again, I will address this so called breathability shortly. However this fabric certainly does not offer any level of breathability implied by such a statement as this as we shall see.

At a major European show recently I even saw another manufacturer describing on a large poster for his product that it was...

“...soft and breathable like cotton...”!

This is such a bold statement that I wonder if perhaps I am mistaken and that the poster really was not referring to a microporous film but some other new product, no doubt manufactured from some new and “revolutionary” new material. If so I apologise to this manufacturer unreservedly...

Manufacturers literature, and I stress that at least two of the above quotations come from major European manufacturers, abound with such claims; claims that in my opinion are misleading about the performance properties of such fabrics. For me, this market will finally grow up properly when manufacturers start telling the simple truth about whether a fabric is breathable or not - in terms of its real world, practical effect on the comfort of a wearer.

Breathability

But how can we define what breathability is? The first thing to clarify is that the way the word is commonly used in this industry is not black and white. In fact, at present the way the term is used is a little like describing a fabric as grey; it doesn’t really tell you anything because “grey” could mean anything from white with a hint of grey to something that is almost, but not quite, black. A “lighter shade of black” if you like - to paraphrase a 1960s popular beat combo. This is quite correct of course; it’s just not very useful. All the shades in-between can be rightly described as grey, but that gives you no help at all if you need to match it with your bathroom cabinets. What you need to know is its “greyness” in comparison to your bathroom cabinets. In other words and in terms of our disposable coveralls, (and the clue is in the wording at the end of the above paragraph) does the breathability of the fabric have any real, practical effect in maintaining wearer comfort over time... can it actually be described as comfortable?

"There are many applications where only flash-spun polyethylene or microporous film laminates are used because users conclude that SMS garments do not provide sufficient protection"

Protective clothing fabrics

The global market leading fabric in the disposable protective clothing industry is flash-spun polyethylene. In shorthand, the fabric is produced by mixing polyethylene with a solvent, pressurising it at high temperature and then extruding it through fine holes. During the extrusion process the solvent literally explodes or “flashes” off (hence the term “flash-spun”) leaving behind many fine polyethylene fibres which are then compressed into a flexible dense sheet form which combines the properties of strength, softness and liquid barrier (bearing in mind each of these terms is relative and subjective). It is also often described as “breathable”.

Its key competitor of the last ten years, microporous film, is a different two layer construction, the inner layer being standard spunbonded polypropylene, and the outer being a microporous polyethylene film. With my non-scientist, layman’s mind I tend to think of this film as having the structure of a sponge on a microscopic scale. There are no holes directly through it, but it consists of lots of tiny chambers some of which interlock and overlap creating winding tortuous pathways through the fabric. This creates a fabric that combines the properties of strength, softness and liquid barrier (bearing in mind each of these terms are relative and subjective). It is also often described, as we saw above, as breathable.

Now I of course acknowledge the fact that flashspun polyethylene definitely does have measurable air permeability. However, can this permeability be translated into a definition of breathability in user terms and in our terms of having a clear effect on comfort? My simple response is this: if these fabrics are breathable, try making a bag out of either of them, placing the bag over your head and then see how long you last. In fact a better idea may be to try out these bags over somebody else’s head - likely suggested candidates being your solicitor, your ex-wife or husband, any handy estate agents, insurance salesmen and Arsenal, Manchester United or Chelsea supporters depending on personal preference. This on the basis that you, or they, won’t actually last very long at all, because in the terms described above - that is, their actual effect on comfort, neither material is really breathable at all. (Please do not actually try this as a real test. It is a rhetorical suggestion and I can’t afford any law-suits!).

In fact I recommend to users that the best and quickest way to check breathability is to simply hold the fabric over your mouth and try to blow through it. This of course in purely empirical terms is not a fair test since it creates an artificial condition of a pressure differential between the two sides of the fabric which would not be replicated in a real scenario... but it will indicate whether air can get through the fabric easily or not.

Learn about when PPE (including coveralls) was first used and how it is used today:

https://www.hsimagazine.com/article/what-is-ppe-an-historical-overview

SMS material (and the many and growing band of latter day variations, SMMS / SMMMS, SMSMS etc.) of course by contrast really is breathable. The simple fabric-over-the-mouth test above will readily demonstrate this and you can wear an SMS bag over your head and carry on quite normally all day long - other than you may keep bumping into things as you won’t be able to see very well. On the other hand (surprise, surprise!), SMS doesn’t have quite the same level of protection although (and this is important), it does still meet the requirements of EN Types 5 and 6 and in many applications will provide perfectly adequate protection.

This is in fact a key point in this discussion. There are many applications where only flash-spun polyethylene or microporous film laminates are used because users conclude that SMS garments do not provide sufficient protection. In fact my experience suggests the opposite; that there are many applications where currently flash-spun polyethylene or microporous film products are used but are actually over-specified. Perfectly adequate protection could be provided by an SMS or similar coverall (especially when it is more likely to be worn properly as noted below), and of course it will be more comfortable and more economical to boot.

A further option currently gaining more ground in the industry is the idea of a composite or hybrid coverall. These coveralls tend to consist of a microporous film coverall that features a breathable SMS panel in the back. This provides better protection to the areas where it is needed the most - the legs, arms and front of the torso, whilst providing adequate protection and more importantly, true breathability to the back. This seams to me to be an excellent compromise and whilst not suitable in all applications, does provide a useful alternative to the comfort - protection compromise in the safety manager’s armoury.

Returning to the key point about the breathability of these fabrics, if flash-spun polyethylene and microporous films are breathable and make comfortable garments, why have I spent the last fifteen years going into factories and seeing users wearing them with the zip down, with the sleeves tied around the waist, with holes torn in them to "let some air in", even with the whole back of the garment ripped out? During a recent visit to a foundry in Turkey, I was discussing this issue in the office with the Safety Manager and suggested this type of “self-designed air-conditioning” of garments is very common. “Not in this factory” he stated categorically. “They would not be allowed to wear protective garments in this way. This is a safety issue”.

Of course the first thing we saw during the factory tour thirty minutes later, to his embarrassment, was two users with their zips down and a third with the sleeves tied around his waist. The serious point of course is that such habits as these do defeat the object of wearing the coveralls in first place! If users are wearing them like this, the argument that the protection level of flashspun polyethylene and microporous films are better than SMS becomes somewhat null and void! Users would quite possibly be better off wearing an SMS suit and wearing it properly then wearing one of the alternatives and not wearing it properly.

Definition of breathability

Let’s look in detail at the actual breathability of flash-spun polyethylene. The breathability or air permeability of the fabric is quoted as “20s”. After some fairly painful searching of some of the more obscure corners of the internet I discovered that this refers to 20 seconds according to the Gurley Hill test of air permeability. Further rooting about at the back of the bottom drawers of “T’Interweb” as my daughters call it, revealed that “Gurley Hill” is an institute in the USA that developed a method of measuring the air permeability of papers and boards for the packaging and printing industry (which begs the question of what relevance is it here)... and that this is the test being used here - now referenced in ISO 5636-5. Now this bit will demand a degree of concentration, and you may wish to go and make a cup of tea before tackling it... but try to stay with me...what it means is this...

100 cubic centimetres of air (so say about the volume of air in cube 10cm x 10cm x 10cm - let’s say roughly a mug full)

At a pressure equal to a depth of 12.5cm (567g) of water (so say the pressure at the bottom of a large mug of tea)

will pass through 1 square inch of fabric (6.45cm2) (why do they mix measurements? That’s an area of fabric 2.54 x 2.54cm - so say a piece of fabric about the size of a larger 2 pound or 2 Euro coin) in 20 seconds... (That’s about the time it will take a top class sprinter to run 100 Metres twice...)

So to summarise, in theory and very roughly it means that a mug full of air will pass through a 2 Euro coin sized piece of fabric in 20 seconds. Provided it is under the same pressure as at the bottom of a mug of water. In other words... if it is forced through under pressure.

"In the last ten years microporous films have become the industry standard as the possible alternative to flash-spun polyethylene"

In the light of our broad definition of breathability, that is, whether a measured breathability has any real practical effect on comfort, this seams a pitifully small and slow trickle of air passing through the fabric - even without taking into account the pressure differential requirement. Perhaps the more important question is under what circumstances would the pressure differential between the inside and outside of a coverall achieve a level equivalent to that at the bottom of a mug of water and be sustained for any length of time in order to result in any air passing through?

Pressure differentials do occur of course within a suit because of the “bellows” effect, but such differentials are unlikely to be maintained over time and in any case the air will always take the path of least resistance to equalise any differential which is to move around in the suit or pass through larger openings (zips and seams etc) rather than through the fabric. (And of course the very fact that measurable pressure differentials are occurring is evidence that the air permeability of the material is very low... if it wasn’t the pressure differentials would not occur).

It seems to me, taking an overall common-sense view, that when wearing such a coverall, with everything except your face, hands and feet enclosed and whilst you may be doing strenuous work, this level of air permeability is not going to help to keep you comfortable much at all. This of course might be an unscientific view, but I don’t think it is an unreasonable one... and supported by the common self-designed air-conditioning alterations added by users mentioned above. So I think it would be fair to say that whilst technically flash-spun polyethylene does have some measurable air-permeability, any claim that this breathability has a real and practical positive effect on making a coverall more comfortable to wear over time in comparison to similar alternative types of coverall is, to say the least, a little dubious.

In the last ten years microporous films have become the industry standard as the possible alternative to flash-spun polyethylene. Now it is undoubtedly true that the measured air-permeability of these fabrics is even less than that of flash-spun PE; in many cases and depending on the test and parameters used, it is measured as zero. But in the same breath (no pun intended) it is also fair to say that the difference between the two is small and that in terms of our desired practical effect on garment comfort, neither are really, genuinely, in our defined terms, breathable.

MVTR (Moisture Vapour Transmission Rate)

The “breathability” of microporous films is often claimed to relate more to their MVTR (Moisture Vapour Transmission Rate) rather than their actual air-permeability. There are manufacturers who appear to use these terms interchangeably, in particular claiming breathability when they actually mean MVTR - which is incorrect and misleading, though I suspect this is born more of ignorance than a deliberate attempt to mislead.

MVTR is not breathability. Breathability is air-permeability. MVTR is the tendency of a fabric to allow moisture vapour (which of course is generated inside a suit by condensation of sweat on the skin and surface of the fabric - hence the claim that it improves comfort) to pass through the fabric - in this case from the inside to the outside.

Whilst a high MVTR may in some limited circumstances have a beneficial effect of comfort, like the Gurley Hill breathability, the levels measured in microporous films are so small and the actual rate at any one time dependant on so many varying circumstances (air temperature, body temperature, skin temperature, ambient temperature, ambient air humidity etc) that it is unreliable if not downright dubious to say the least to claim it makes the garment more comfortable or even worse, breathable. When you are working in a warm area in a microporous film coverall, MVTR of any level will make very little difference to how comfortable you are over time.

"it is only breathability as defined as air-permeability that will have a practical effect on wearer comfort"

Moreover, there are several methods of measuring moisture vapour transmission, each using differing techniques and ambient conditions, and each likely to give widely differing results on the same fabric. Thus trying to compare the MVTR of two different fabrics from quoted information is almost impossible unless both manufacturers have used the same method and confirm the same ambient conditions used for the testing.

As with air permeability, higher levels of MVTR can be meaningful to comfort, but these levels are far above those available in disposable garment fabrics providing protection from hazardous chemicals. Additionally, even at moderate rates of work, the human body’s tendency to generate heat and moisture to cool itself far exceeds the capability of these materials to transport it away. The net effect is contrary to worker comfort.

The fact is, it is only breathability as defined as air-permeability that will have a practical effect on wearer comfort, and only a genuine level of air-permeability rather than a claimed one that is really just a cleverly disguised way of saying “it doesn’t breathe much at all really” will have a real practical effect.

For my part, I would like to see manufacturers in this industry begin to provide true useful information in this area and accept the (actually obvious) fact that...

“hey! If you want protection against hydrochloric acid then you ain’t gonna get a suit that is breathable and therefore comfortable... not at €4.00 per suit anyway!”...

I have certainly taken this honest view in the development of my business in the last few years and generally found that this refreshing honesty often pays dividends. It is better and more useful in the long term to educate the user rather than mislead or confuse him. I am tired however of seeing claims related to breathability in literature and advertisements that whilst perhaps not actually constituting outright lies, most certainly indicate at best a limited understanding of the issue and at worst a degree of convenient “bending” of the facts.

Furthermore, the European Commission could help by providing a standard for breathability of disposable fabrics using the “class” system currently used for other fabric properties such as tear strength and puncture resistance. I suspect this will not happen however.

I will leave aside for now the fact that it seems to me the way information is required to be presented to users by the CE marking requirements - the whole business of quoting a class rather than an actual result - is actually about preventing true comparison of different garments rather than enabling it (two fabrics can have differing results - one at the top of a class and one at the bottom - but still be quoted as being in the same class so to the user appear the same) that’s a whole other story...

Perhaps I’m being overly cynical, but if such a standard of classification was to be of any use to anyone in comparing fabrics and garments it would clearly have to define two of the main, market-leading fabrics currently in use as NOT breathable (or at the very least as having very low breathability), and I suspect this would lead to a shocked, sharp intake of breath from the powers that be and be considered a step too far. Actually tell people what’s really going on??? ...I feel the very foundations of society shuddering with nervousness at the thought...

For more information on Protective Clothing please visit ose.directory

Published: 10th Apr 2008 in Health and Safety International

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Martin Lill is General Manager of Lakeland Europe, the European & Middle East division of one of the world’s largest manufacturers of disposable protective clothing. He has over fifteen years experience in the industry and would be happy to answer any question...
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