Shedding is a term used to describe the release of loose fiber — usually unsecured staple or shear lint — from carpet. This condition is most evident when carpet is new, and it usually diminishes within a few weeks or months with frequent, effective vacuuming.

Consumers sometimes notice that dust settled on furniture consists largely of small fibers, and fibers can sometimes be seen floating in the air in beams of sunlight.

These observations lead some consumers, especially those with allergy problems, to question the appropriateness of carpet as a floor covering; they wonder whether such fibers may be inhaled and cause respiratory irritation. Even some physicians have, without really thinking about it, recommended that patients with allergies remove carpet.

What are those floating fibers?

A concerned homeowner in Northern California contacted me with this exact question. She had just moved into a new home with new carpet, and noticed fibers floating in the air and accumulating on table tops and other horizontal furniture surfaces.

I asked her to sent me dust samples collected from furniture, the contents of her vacuum cleaner’s bag, and a sample of the installed carpet. Microscopic examination of the dust samples confirmed that the dust was in fact comprised almost entirely of fibers; however, the fibers in the dust were found to be primarily cotton and polyester, evidently from clothing and other textiles in the home.

The carpet sample she sent confirmed what she had told me: Its pile was 100% continuous filament nylon. Shedding is normally observable only in carpet of staple yarns; continuous-filament yarns release very little fiber, if any, and usually they release virtually none.

I sifted through the dust samples and the contents of her vacuum cleaner bag and, as you might expect, not a single carpet fiber was found in any of the samples.

Can airborne fibers be inhaled?

An attorney once asked me, “How large a particle can a person inhale?” My first answer was, “It depends how determined they are to inhale it.” More or less accurate, but more levity than he was looking for.

Here’s the serious answer:

This chart shows the size ranges in microns (millionths of a meter) of various particles and fibers. The human respiratory system is remarkably effective at filtering out airborne particles larger than 10 microns, which is about the threshold of visibility. So as a general rule, if it’s visible to the unaided eye, it’s too big to be inhaled into the lungs. Particles larger than 10 microns are arrested in the nose and sinus, where they are trapped and either disposed of or expelled.

Even small apparel fiber fragments, which are around 3-10 microns across, are at least thousands of microns long. (One quarter inch is over 6,000 microns.) That’s small enough to remain suspended in the air for quite some time, but far too large to make it into the lungs.

Can carpet fibers remain airborne?

No, carpet fibers cannot stay aloft for more than a few seconds at most. As textile fibers go, carpet fibers are enormous. Their smallest dimension — their cross section — is around 20-50 microns across, hundreds of times the cross-sectional area of fibers used in applications like clothing, upholstery, towels and bedding materials. (See figures 1 and 2.)

If they do somehow become airborne, kicked up by foot traffic for example, their sheer size and density cause them to drop out of the airspace very quickly. (You may demonstrate this for yourself by holding a handful of loose carpet fiber and gently releasing individual filaments; they fall like stones.)

What about allergies?

The word allergy is frequently used incorrectly to refer to just about any irritation. For example, a person who experiences lactose intolerance may say that they are allergic to milk, when in fact an allergy to milk proteins is an altogether different ailment.

A true allergic reaction is a response of the body’s immune system. In the indoor environment, the most common allergens are animal dander and saliva, dust mite excreta, cockroaches and other insects, food products, and pollen.

As discussed above, there is no scenario where carpet fibers (or even apparel fibers) can present an inhalation problem, and I am unaware of any case of an actual allergic reaction being cause by any component of carpet itself.

Tests have demonstrated that well-maintained carpet makes a strong positive contribution to the quality of the indoor environment, including the cleanliness of the air. Available evidence indicates that carpet tends to collect and hold soil, dust, and other materials until they can be removed by cleaning, rather than allowing them to float freely in the air. This is true, of course, as long as this “soil trap” is kept clean.

In the introductory article to this series, I defined performance as “the ability to withstand use and maintenance while retaining physical integrity and minimizing changes in appearance.”

I’ll tackle the “minimizing changes in appearance” part first. However, before I do that, it’s important to do a brief refresher on fiber characteristics, so I’ll use this post to review the properties that are most relevant to the life of a carpet fiber.

Before we get started, I should qualify the sweeping generalizations I’m going to make. Every time I cover this subject, someone gets fixated on some tangential exception and misses the point of the overview, starting with the fact that it’s an overview. So if you find yourself thinking that’s not always true, you’re probably right. Few things are always true. This article describes things that are generally true most of the time, and in fact these generalizations hold up in all but the most exceptional cases.

So, lest anyone split hairs, get their undies in a bunch, or start speaking in clichés, here are the qualifiers:

1. The comparisons below assume all other factors are equal, so when I compare the resilience of polyester to that of nylon, I’m comparing them in equivalent constructions. None of this “well, if you make polyester carpet extra dense…” stuff here. Strictly apples to apples.

2. This article is only about carpet fibers — nylon, wool, polyester, acrylic and polypropylene. (For now I’ll skip specialty fibers like sisal, cotton, coir and silk.)

When you assess a performance property of a carpet fiber, the only comparisons that matter are those against other carpet fibers, so those are the only ones we’re concerned with here. When I say wool has poor stain resistance (and I’m going to), don’t respond with something like, “well, not compared to linen.” That’s relevant for clothes, but not for carpet.

3. Finally, yes, there are variations in how each fiber type exhibits each characteristic. For example, there are noteworthy differences between nylon 6, nylon 6,6 and cationic-dyeable nylon; there are variations between different polyesters and huge differences between various wools. Even so, those variations usually aren’t enough to change the way one fiber type ranks relative to other types.

The Big Six

Of all the fiber properties we could talk about, six are of particular interest to carpet specifiers:

Resilience — Resilience is the ability of fibers to return to their original upright position after being bent over or compressed under dynamic (momentary) or static (prolonged) loads. It’s the characteristic that enables pile to spring back after being stepped on and prevents it from being prematurely compacted under traffic.

Nylon offers the best resilience. Wool comes in second, followed by acrylic. Both nylon and wool recover well with cleaning. Polyester has only moderate resilience and tends to flatten rather quickly in trafficked areas. Polypropylene has the poorest resilience of the bunch and becomes crushed even more quickly and severely than polyester.

Twist retention — Twist retention is the ability of tufts in cut-pile constructions to resist bursting, flaring and untwisting under traffic. (Twist retention is not a factor in loop-pile constructions.) In most cases, twist retention is roughly inversely correlated with resilience; the better the twist retention, the worse the resilience, and vice-versa.

Based on that, you might conclude correctly that polypropylene is the king of twist retention, followed by polyester and acrylic, which also do quite well. Relatively speaking, nylon and wool rank last; however, it’s worth noting that their twist retention is highly variable and can range from very bad to quite good according to the size of the yarn (smaller is better) and the quality of yarn processing.

Abrasion resistance — Abrasion resistance is the ability of a fiber to resist becoming scratched and dulled by abrasive soil and traffic. When fibers become abraded, they appear dull and cloudy in much the same way a piece of clear plastic would be scratched and dulled by sandpaper. Regardless of how effectively it’s cleaned, the surface would be permanently dulled. (Figures 1 and 2.)

Abrasion can be a visible problem when a high-luster fiber becomes dulled by traffic and soil. The original luster of the pile cannot be restored regardless of the effectiveness of cleaning. Cleaners call this condition “traffic lane gray.”

The visual effects of abrasion can be disguised by delustering the fiber with a pigment such as titanium dioxide. Nylon and polyester exhibit very good abrasion resistance, whereas polypropylene tends to become abraded rather easily.

Wool exhibits the poorest abrasion resistance, and it is not terribly unusual to see wool carpet in heavy traffic areas or on stairs that has worn completely through to the backing materials. On the other hand, aside from physically wearing away the pile, abrasion on wool doesn’t degrade its appearance in the same way it does on, for instance, polypropylene. Wool often tends to “age gracefully” with a kind of classic-looking patina, rather than just looking “old” like synthetics tend to.

Colorfastness — Colorfastness is the ability to resist becoming discolored by light (particularly ultraviolet light in sunlight), cleaning, chemicals, atmospheric agents (i.e., oxidizing gases such as ozone and oxides of sulfur and nitrogen), and the friction of traffic (crocking).

Polypropylene has outstanding colorfastness. Polyester and acrylic also are generally excellent. Nylon and wool exhibit comparatively poor colorfastness; however, nylon can be pigmented (solution dyed) to give it colorfastness equal to or better than polyester and acrylic.

I said that I wasn’t going to dig into the variances within fiber types, but as it relates to colorfastness it’s worth two short detours. The first is that nylon 6,6 provides substantially greater colorfastness than nylon 6, particularly to gases like ozone, nitrogen oxides and sulfur oxides. The vast majority of gas-fading cases are on type 6 nylon.

The second point is that nylon can be solution-dyed, like polypropylene, in which case its colorfastness is nearly as good as that of polypropylene.

Cleanability — Cleanability is the ability of a fiber to readily release soil — water-soluble, oily and insoluble particulates — during vacuuming and cleaning.

Generally, more absorbent fibers like nylon and wool have better cleanability; however, absorbency also makes a fiber more susceptible to staining. Conversely, more hydrophobic fibers like polypropylene and polyester, which also have an affinity for oily soils, are more difficult to clean, but have excellent stain resistance. Though hydrophobic fibers often require more aggressive cleaning, their chemical makeup allows them to withstand it.

Cleanability can be enhanced considerably by soil-repellent fluorochemical finishes, usually Scotchgard or Teflon. These finishes enhance fibers’ willingness to release particulate and oily soils from their surface and increase the effectiveness of vacuuming and cleaning.

Wool is the only carpet fiber with inherently good cleanability without the addition of a fluorochemical. The cleanability of flourochemical–treated nylon is also quite good. Polyester and acrylic don’t rank quite as well; they have only fair cleanability and can be relatively difficult to clean even if treated with a fluorochemical. It is important to note that fluorochemical finishes alone add little resistance to staining by dyes.

Polypropylene is quite difficult to clean, but its outstanding chemical resistance allows the fiber to withstand unusually aggressive cleaning.

Stain resistance — Stain Resistance is the ability of a fiber to resist becoming permanently stained (having color added) by foreign substances like soil and dyes. Stains differ from soil in that stains are not removed by cleaning alone.

Stain resistance is determined primarily by a fiber’s inherent chemical resistance, lack of absorbency, and affinities for foreign substances.

Polypropylene is the undisputed winner in the stain resistance category. Polyester and acrylic also offer impressive stain resistance, though prolonged exposure to oily soils may leave them stained.

Nylon has the poorest inherent stain resistance of any of the synthetics. On acid-dyeable nylon, stain resistance may be enhanced by modifying the fiber with a finish that inhibits its receptiveness to anionic acid dyes; e.g., some food colorings like FD&C red #40. (Note that this “stain blocking” finish imparts no real resistance to any type of stain other than acid dyes.) Even the best stain-resistant nylon ranks a distant last among the synthetics; however, solution-dyed nylon can withstand some aggressive stain-removal chemicals, including careful, professional use of dilute hypochlorite bleach. (Hypochlorites are extremely destructive oxidizers and cause physical damage to nylon, even when its color remains intact.)

Wool’s stain resistance ranks dead last among carpet fibers, and the fiber can neither be modified enough to change its ranking nor cleaned with aggressive chemistry.

Some Notes About Blends

There are at least two reasons a manufacturer would design a carpet using two or more pile fibers:

Cost — One reason fibers are blended is to reduce cost. A wool/nylon blend costs less than pure wool, at least in the North American market. A nylon/polyester blend costs less than 100% nylon.

Styling — There are some interesting things you can do with carpet containing different yarn types, particularly with coloration. For example, you can cross-dye it or print a pattern that only colors some of the yarns, or dye two different fibers two different colors in a single dying process.

Despite those reasons, the specifier should think carefully before choosing a product with mixed pile fiber types.

Blending different fiber types introduces issues that don’t come into play when the carpet has only one pile fiber. When you mix two fibers, you don’t get the best of both; you get the combined weaknesses and limitations of both.

For example, a 50/50 nylon/polyester blend performs very much like 100% polyester, but costs more. And while you could use more aggressive chemicals to remove difficult stains on polyester, you would be limited by the presence of nylon. In other words, it’s more like an overpriced polyester than a bargain-priced nylon, and it lacks polyester’s colorfastness and chemical toughness.

A more extreme example I saw once was a berber loop style with 50/50 wool/polypropylene pile, which flattened and fuzzed just like you would expect. Cleaning-wise, the two fibers make a terrible mix: Polypropylene often requires aggressive cleaning, but wool can’t tolerate it.

Another problem with blends is that different fibers respond differently to things like light, traffic and cleaning, which can result in some pretty strange-looking performance characteristics. For example, a nylon/polypropylene level loop will develop a rough or patterned texture (depending on the construction) in traffic paths, where the polypropylene yarns become permanently flattened and the nylon remains resilient. This behavior can lead to the traffic paths appearing to change color, and sometimes this phenomenon appears suddenly when the carpet is cleaned because the nylon springs back to life and the polypropylene stubbornly remains flattened.

The Optimum Balance

If the overview above made fiber selection seem too complicated, rest assured that it’s not that bad. It boils down to selecting the fiber with the best balance of the “Big Six” properties. Which fiber is that?

The long, textbook answer: It depends. The fiber that is best for the job is determined by how its mix of properties matches the traffic, soiling and cleaning requirements, with special consideration for job-specific things like sunlight exposure, indoor swimming pools, and the like.

The short answer: Solution-dyed nylon.

More on this in the next article in this series.

Shading, pile reversal and pooling are characteristics exhibited by many cut-pile carpets, rugs and other textile floor coverings. Though normal, they are sometimes unexpected and result in complaints. This bulletin discusses these conditions and their causes in an attempt to assist carpet and fiber manufacturers, dealers and buyers to make informed decisions regarding carpet selection and complaint resolution.

What Are Shading and Pile Reversal?

To varying degrees, most cut–pile carpets exhibit a characteristic known as shading — apparent shade variations caused by relatively slight changes in pile lay from traffic, vacuuming and general use. Since the sides of fibers reflect more light than their tips, pile laying away from the observer appears lighter, while pile laying toward the observer appears darker. Areas that appear dark when viewed from one direction appear light when viewed from the opposite direction, and vice–versa. These changes in pile lay are temporary and usually can be removed easily by vacuuming or brushing the pile.

In time, the forces of traffic may strongly orient the pile in a particular direction. This condition is commonly referred to as pile reversal, which, in contrast with simple shading, occurs in fairly predictable patterns and cannot easily be removed by vacuuming or brushing the pile.

The simplest form of traffic–induced pile reversal occurs where changes in the direction of traffic flow create shear forces that strongly orient the pile; i.e., the force of traffic turning to the left causes the pile to lay to the right, and vice-versa. This form of pile reversal occurs in fairly predictable patterns cannot be corrected permanently.

Pooling (aka Watermarking)

The most controversial form of pile reversal — commonly called pooling or watermarking — is characterized by sharp changes in pile direction at apparently random, wavering lines called interfaces. The pile at the interface lies in roughly opposite directions, usually away from the interface, without regard for the pile’s inherent lay created during manufacturing. (Figures 1, 2, 3.)

The interfaces typically are located in or immediately adjacent to trafficked areas, but they often appear not to correspond to the flow of traffic. In many cases the patterns of pile reversal continue across seams, even onto different carpet, while in other cases it stops or even reverses at seams.

While the pile on either side of the interface usually exhibits very little distortion, the pile within the interface often is severely distorted. (Figure 4.) This distortion appears to be attributable to the combination of traffic and loss of density created by the pile laying away from the interface.

The terms pooling and watermarking are synonymous and describe the typical appearance of an affected carpet, in which areas of the pile may appear wet. Puddling is a term sometimes used to describe areas of pile reversal that are completely surrounded by interfaces. (Figures 6 and 7.) Despite the terms used to describe it, this form of pile reversal is not related to exposure to moisture.

The textbook pattern of pile reversal, illustrated in figure 5, occurs far more frequently than apparently is recognized. This condition is not rare, but in fact occurs in most installations with moderately to very dense cut–pile carpet and concentrated, directional traffic. Though the condition exists, factors such as poor lighting conditions often prevent it from being noticed.

Like the simple traffic–induced pile reversal described above, pooling is effectively permanent.

Theories About Pooling

If there is anything humorous about pooling, it has to be its history of fueling imaginative and far–fetched theories to explain its occurrence. Following are some of the more popular ones:

Static Electricity: This theory actually is based partly on a real observation: sharp differences in static charge that sometimes can be measured on either side of interfaces. Problem: No evidence yet indicates that the static is the cause of the problem rather than a secondary effect of some other cause, such as traffic.

Electromagnetic Fields: A cousin of the static electricity theory, this hypothesis suggests that the changes in pile direction are caused by electromagnetic fields. Problem: No supporting evidence, and pooling occurs in the apparent absence of sources of electromagnetic fields.

Air Flow: The winding, swirling lines of pooling’s interfaces, combined with reports of its appearance near HVAC intakes and/or floor registers, apparently led to the perpetuation of this idea. Problem: No supporting evidence, and pooling has been documented in plenty of cases in the absence of such airflow.

The “Domino Effect”: This is the theory that the initial orientation of the pile is “set” the first few times the carpet is trafficked and that subsequent traffic exaggerates this orientation. As tufts in one area begin to lay over, they in turn begin pushing surrounding tufts over, and the pile reversal is thus spread throughout trafficked areas. Problem: None really. This is one theory that is fairly consistent with the observed characteristics of pooling, though it does not explain all of its behavior.

Subfloor Irregularities: This theory suggests that slight irregularities in the subfloor telegraph through the carpet and result in pile reversal when the carpet is trafficked. This idea is not totally without merit, as a number of documented cases show a degree of correlation between the pattern of the pooling and features of the subfloor. It appears that while subfloor irregularities do not cause pooling, they may influence the pattern in which it occurs. (Figures 8 and 9.) Problem: Pooling often occurs over surfaces without such irregularities.

What Really Causes Pooling?

Unfortunately for those of us who like complex answers to mysterious problems, the causes of pooling are unsatisfyingly straightforward.

There are two common factors present in virtually every documented case I am aware of:

1. Traffic: The forms of pile reversal discussed in this bulletin, including pooling, occur almost exclusively in and immediately adjacent to trafficked areas, though the volume of traffic required is minimal. Though stories of pooling occurring on untrafficked carpet still circulate, additional investigation almost invariably reveals that the affected areas received at least some traffic.

Stories of pooling being discovered on new, uninstalled rolls of carpet also circulate. Though some relatively rare forms of roll crush can be somewhat similar in appearance to pooling, the condition described in this bulletin occurs only with traffic.

2. Carpet Style: Pooling occurs primarily in relatively dense, untextured (or lightly-textured), cut–pile styles. Thickness does not appear to be a significant variable, as it occurs with comparable severity in short commercial styles as well as thicker residential products.

Low–density constructions (e.g., shags and lower-density friezes) and loop–pile styles appear to be virtually immune to visible pile reversal. (See figure 11.) I emphasize visible pile reversal here because, although the pile reversal itself often does occur, the surface texture of those styles disguises the condition so that it does not exhibit the apparent shade variances.

Because it occurs in carpet and rugs of all fiber types (wool, nylon, polyester, acrylic, polypropylene, coir, silk, etc.), all construction types (woven, tufted, fusion bonded, modular, etc.), in all methods of installation, under virtually every imaginable combination of circumstances, the conclusion that pile reversal and pooling are not related to any of these factors is well–supported.

Documented cases support the conclusion that normal traffic is the primary cause; i.e., pile reversal and pooling are normal reactions to traffic of certain styles of carpet. Therefore, pooling, like random shading, is accurately considered a performance characteristic rather than an abnormality. While some may find this explanation too simple to account for the wild, swirling patterns in which pooling appears, it is the most consistent with the hundreds of cases on which reliable documentation exists.

Can Pooling be Corrected?

The short answer is no. Pooling has been reduced or eliminated in nylon and wool carpet by aggressive steaming (actual steam, not “steam” cleaning) and pile lifting, but as long as there’s traffic, it inevitably recurs within a few days or weeks of correction.

It has been reported that diligent maintenance based on frequent pile lifting has minimized the degree to which pooling develops on nylon carpet in some commercial installations.

In one case of moderately affected nylon carpet, pooling was removed and did not recur. However, the furniture in the room was rearranged in such a way that the affected areas remained untrafficked.

Because of their poorer resilience and minimal response to steam, polyester and polypropylene usually do not yield even temporary correction.

Industry References

Shading and pooling are recognized throughout the carpet industry as inherent characteristics of most cut–pile products and are not considered a valid basis for claims.

In the Carpet and Rug Institute’s Carpet Claims Manual, shading is defined as an “apparent color difference between areas of the same carpet caused by … random difference[s] in pile lay direction. It is a characteristic of all cut-pile carpet and … is not a manufacturing defect. The sides of fibers reflect more light and appear brighter and lighter than the ends, which absorb more light and appear to be duller and darker in color.”

The Carpet Claims Manual also states, “Pile crushing, pile shading, watermarking, and soiling are not manufacturing defects and will not be considered as a basis for claims.”

The Carpet Manufacturers Association of the West’s Statement of Obligations and Responsibilities of Carpet Manufacturers and Purchasers of Carpet and Carpet Performance and Claim Guidelines states, “Highlighting and shading are differences in light reflection between surface areas and are not defects.” This document also states, “Watermarking or pooling is a color change effect which arises from reversal or bending of the carpet pile fibers so that light is either absorbed or reflected from the pile. This is a common condition and is not related to carpet construction or fiber type and is not the basis for a claim.”

The Carpet and Rug Institute’s bulletin Pile Reversal (“Shading”; “Water Marking”) states that “a thorough search of available literature has not revealed any reason as to why carpet, after it has been installed, may or may not develop pile reversal. The many theories have been studied, tested, and evaluated by one or more sources. It has been concluded that the theories were not valid. … At the present time, the only conclusion which can be drawn is that pile reversal may develop on the surface of some carpet after it is installed, and that pile reversal is not due to the materials which are used to produce the carpet, the manufacturing process, or any combination of these factors.”

That bulletin also makes the following observations:
• “it [pooling] occurs on the very expensive handmade Oriental rugs as well as machine–made carpet.”
• “The area may start to appear with several days after installation or may not become apparent for several months.”
• “A higher, cut pile, denser carpet which will not develop these areas is more than the consumer should expect.”
• “Those who object to this type of change should consider purchasing another style of carpet, as it is not possible to assure the purchaser that pile reversal will not develop [in dense, cut–pile carpet].”

The Carpet Institute of Australia’s Technical Information Bulletin Number 3, entitled Permanent Pile Reversal Shading, states, “Studies conducted by both independent researchers and major carpet manufacturers in Australia and overseas into the causes of Permanent Pile Reversal Shading have been largely inconclusive. A number of theories have been advanced over the years but most have been discarded or at best remain unproven. However, the consensus of expert opinion about Permanent Pile Reversal Shading is that:
• “it can occur in any cut pile carpet (or rug) including hand knotted, tufted, woven, bonded, knitted or hand-made carpets and rugs;”
• “its occurrence has not been linked with the various different carpet manufacturing processes or the component products used to make carpets;”
• “it can occur in carpets made from all common carpet fibres and blends of different fibres (e.g. nylon, wool, acrylic, polyester, polypropylene and their blends);”
• “its occurrence will not lead to premature wear of the carpet and it will have no effect on the durability of the carpet;”
• “it has not been linked to methods of installation.”

Preventing Pooling

The following guidelines have proven helpful in minimizing the visibility of the various types of shading, including pile reversal and pooling:

Pile Type: Because it occurs almost invariably in cut-pile styles, loop-pile styles are a virtual guarantee against pooling.

Pattern: The more busily and boldly patterned a carpet is, the better it will disguise shading and pile reversal.

Shade: Lighter carpets sometimes tend to show less contrast between darker and lighter shaded areas, whereas darker colors often tend to exaggerate these differences.

Luster: Fibers with duller lusters soften some of the “sheen” that is characteristic of bright fibers, thereby reducing the contrast between dark and light areas.