Sodium Lauryl Sulfate is one of the oldest and harshest surfactants used in skincare. A surfactant has properties that allow it to act like a detergent, making it suitable for a cleansing job. Products like Cetaphil Gentle Skin Cleanser claim to be very gentle on the skin, yet contain Sodium Lauryl Sulfate (SLS).  It is considered a safe ingredient when used in suitable skincare formulas, but its concentration is not regulated, nor do we have extensive and long term studies on the effects of SLS on the skin (when within skincare formulas). Here I will present how SLS works and how its action can lead to detrimental effects on the skin over time, especially for sensitive skin. Due to the extensive referring to scientific papers, they are not linked, but rather listed at the end of this post.

WHAT IS SODIUM LAURYL SULFATE

Sodium lauryl sulfate (SLS), also known as sodium laurilsulfate or sodium dodecyl sulfate, is an ester of sulphuric acid. It acts as anionic surfactant, meaning it lowers the surface tension and makes the solution foam.

It is a very effective cleansing agent, used widely not just in cosmetics and personal hygene products, but also industrial cleaning. It attracts both water and oil (with different parts on it), which makes it suitable for washing away oil and grease with water, as is the goal with washing the face.

However, this is not the only use of SLS. In scientific practices, SLS is put on the skin for a limited amount of time with the aim to cause skin irritation for various purposes (Vié et al. 2002; Yan-yu et al. 2011; Katsarou et al. 2000).

As such, it is a known skin irritant (Loffler & Happle 2003; di Nardo et al. 1996; Lee & Maibach 2000; Monteiro-Riviere 2001) .

This raises an immediate question: Is SLS safely used in skin products, considering that it is commonly accepted as a skin irritant?

There is currently no EU regulatory guideline or recommendations in place relating to the acceptable levels of SLS in medicinal or cosmetic products, nor it is to the best of my knowledge, regulated as such in the rest of the world.

In the report “Background review for sodium laurilsulfate used as an excipient ” by the European Medicines Agency in 2015 (Andersen 2011; Committee for Human Medicinal Products (CHMP) 2015) , SLS is still considered to be safe as long as it is used as directed in cosmetic formulations, although the actual percentages are not regulated.

This view will be discussed in this paper. I will try to answer the following question:

IS SLS DAMAGING TO THE SKIN WHEN USED IN SKIN CLEANSERS?

In part 1, I explain how SLS irritates the skin in experimental settings. In part 2 I discuss how this translates into regulations for personal use, and why I believe the general view that SLS is non-irritating for the skin when present at low concentrations in skin cleansers, is wrong.

Part 1. a) How does SLS cause skin irritation?

To understand how SLS might cause a skin irritation and possible long-term damaging effects, we have to understand the basic skin structure and how SLS interacts with the skin, especially in a short period of time, after which it is washed away.

The outermost layer of the skin is called stratum corneum (SC). It consists of dead skin cells ( corneocytes ) full of a protein called keratin and water-binding molecules that constitute the Natural Moisturizing Factor (NMF) (Marks 2004).

The corneocytes are surrounded by a matrix of lipids, holding them in place (like a glue), which makes the structure robust and nearly impermeable. Stratum corneum is very important for the skin health because it forms a barrier in two ways: a barrier against the environment, thus hindering entry of external irritants and harmful microorganisms, and a barrier that prevents the water from evaporating from the skin into the environment (Schliemann et al. 2014; Marks et al. 2002) .

This function is referred to as skin barrier function, and it keeps the skin healthy and moisturized.

In addition, there is another very thin layer on top of the stratum corneum that contributes to the skin barrier function. It is a mixture of sebum, produced in sebaceous glands underneath the skin surface, and sweat, together referred to as acid mantle (Fluhr & Elias 2002) .

The acid mantle plays an important role in the skin health as well, as it keeps the skin’s pH at 5.5, which is essential for healthy skin because a plethora of biochemical processes still ongoing in the corneocytes only work at the narrow range of pH (Ali & Yosipovitch 2013; Kim et al. 2009).

These include for example, natural exfoliation process of the skin, called desquamation , which is dependant on the correct pH (Schmid-Wendtner & Korting 2007).

SLS solution as present in the skin cleansers can effectively remove the the acid mantle during washing, substantially elevating the skin’s pH (Kim et al. 2009; Korting & Braun-Falco 1996) , after which it can take up to 28-30 hours for the skin to regenerate itself and establish the correct pH (Moore et al. 2004; Bárány et al. 1999) .

Compromising the integrity of the skin barrier function and skin pH can result in visible skin irritation (Moore et al. 2004; Ananthapadmanabhan et al. 2003). Elevation of stratum corneum pH also alters the lipid synthesis in this layer, quite possibly as a result of the local pH changes (Fiume et al. 2010; Kubo et al. 2012) .

Skin irritation is further exacerbated by the direct binding of the SLS to the keratin structures in the stratum corneum and subsequent swelling of the corneocytes (Fluhr & Elias 2002; Downing et al. 1993).

Some SLS also remains in the skin after cleansing, while some lipids from the skin surface are being washed away (Downing et al. 1993) , which can lead to a number of skin diseases (Sahle et al. 2015).

Measurements of lipid solubilization by SLS indicate that, at concentrations between 0.1 and 2%, the surfactant initiates the removal free fatty acids, cholesterol, and esters, which are components of the skin barrier and essential for the skin health and integrity (Ananthapadmanabhan et al. 2013).

SLS can also accumulate in the hair follicles (Fiume et al. 2010) , likely in very low concentration, but we have no knowledge whether even very small concentrations are irritating to the skin or potentially aggravating the skin conditions like acne, rosacea, atopic dermatitis and eczema when used in cleansers which are being washed away .

Part 1. b) How is the skin irritation by SLS tested in scientific experiments?

Skin irritation is by definition a reaction to an irritant substance that results in the inflammation of the skin and itchiness.

SLS is a known skin irritant (Horita et al. 2017; Proksch & Nissen 2002; Loffler & Happle 2003) , and is often used in scientific experiments to test the healing effects of another substance after it has been irritated or damaged with a SLS solution, typically 0.5 – 10% (Atrux-Tallau et al. 2010; Szél et al. 2015; Geier et al. 2003).

Skin irritation is normally induced by an application of a patch containing the SLS solution onto the skin for a period of time, usually 30min – 24h, which is an established experimental practice to determine the skin susceptibility to irritation and the irritancy potential of the substance, measured subsequently by Transepidermal Water Loss ( TEWL) (Oberg et al. 1981; Nilsson 1977; Geier et al. 2003), capacitance and visible dryness, all of which are considered the most important in evaluating chronic irritation in the skin (Marks 2004) .

A recent report has shown for example that the cumulative exposure to 0.5% SLS, when applied twice daily for 30 minutes under a patch, may lead to skin barrier damage and significantly decrease the amount of NMFs, which could lead to chronic irritant dermatitis, even in healthy individuals (Angelova-Fischer et al. 2014).

Part 2. A critical evaluation of the safety reports and studies on SLS irritant action

Despite the common acceptance that SLS is a skin irritant, it is often argued that research indicating its dangers are flawed. Particularly, it is pointed out that the concentrations and the exposure times used in scientific experiments are not realistic; The common user would wash the product out almost immediately, not leave it on the skin for 30 minutes or more under occlusion.

According to this view, SLS is considered to be safe because it is normally washed from the skin within a minute of application.

Given that these are established scientific practices for testing irritant properties of a substance (Schwitulla et al. 2014; Anon 2007; Hoffman et al. 2014) , I find it surprising that the findings are so easily discarded and referred to as irrelevant, and do not prompt for more detailed research.

These also do not take into account of how SLS causes protein denaturation, cell membrane disruption, keratin matrix disruption, elevation of pH in the stratum corneum and increase level of inflammatory cytokines, as mentioned in Part 1. All these take place likely within seconds of applying SLS to the skin and are capable of inducing chronic skin irritation.

Even though in virtually all cases of patch application clear indications of the skin irritation occur, mild to moderate skin irritation induced by SLS is usually reversible, meaning that the skin can regenerate itself and reestablish the correct pH within several hours, or up to 30h in some cases. Healthy skin is generally more capable of this than skin prone to atopic dermatitis.

Another argument for the safety of SLS is that although it is a known skin irritant and penetration enhancer (Jakasa et al. 2006) , this largely depends on the overall formulation of the product and the percentage of SLS used (Andersen 2011) , which I think is a valid one, but it should call for more independent testing and research.

Although SLS-containing cleansers might be formulated to be non-irritating, there is rarely peer-reviewed, scientific testing for irritancy of specific formulations, as those used in skin cleansers.

While I acknowledge that there might be no visible and immediate skin irritation after usage of SLS-containing cleansers, this does not imply that there is no accumulated irritation and skin damaging effects over time, due to the effects SLS clearly has on the stratum corneum and the skin barrier function, as explained in Part 1.

This lack of long term studies on cumulative damaging effects is also recognized in the report “Background review for sodium laurilsulfate used as an excipient ” by the European Medicines Agency, 2015., where it is stated that:

“…experimental studies using a single challenge of the skin to SLS is a transient reflection of skin susceptibility, which does not take into account the cumulative effect of long-term damage or repair mechanisms of the skin. Therefore, studies using methods that mimic the clinical situation, such as the repeated open application test and soak/wash test, may be more relevant.”

They acknowledge that “Repeated, prolonged exposure to dilute solutions of SLS can cause drying and cracking of the skin ensuing to contact dermatitis, although regulatory compliant randomised controlled trials are lacking…” and conclude the report by proposing a threshold of 0% SLS in topical products (Marks 2004; Committee for Human Medicinal Products (CHMP) 2015) .

On the other hand, a recent review, which is sponsored by the Seventh Generation, manufacturer of household cleaning items, suggests that SLS is safe and that all of the ‘misinformation’ circling the internet for decades is simply inaccurate (Bondi et al. 2015)

The review gives a very positive tone towards SLS and states that most internet rumors are simply false, and misinterpretation of the scientific practice.

They state that human patch tests “confirm that SLS concentrations >2% are considered irritating to normal skin, but that in reality, dermal exposure to SLS in cleaning products is more likely to last a matter of minutes rather than hours. ”

They then simply discard the possibility of skin irritation by SLS, ignoring the newer publications (De Jongh et al. 2006; de Jongh, Verberk, et al. 2007; de Jongh, Lutter, et al. 2007) which have revealed a clear indication of the dermal irritation by 0.5% SLS when applied under dermal patch for 30 minutes, and the volunteers could shower or do other things as they normally would.

They admit that the cleansing products have the potential to be dermal irritants “if not formulated correctly, but products that contain SLS are not necessarily irritating to the skin.”

Overall, there seem to be an abundance of cherry-picking the appropriate research papers to support the desired claims, while there is clearly a lot more accumulated knowledge acquired in the recent years, connecting for example the change in concentration of different cytokines (de Jongh, Verberk, et al. 2007; De Jongh et al. 2006), which are indicative of skin irritation and not mentioned at all, simply discarding the scientific uncertainty regarding the possible skin irritation by SLS, even when applied onto the skin and then washed away after minutes.

CONCLUSION

One of the main, but perhaps unreachable goals, are the long-term evaluations of how SLS within many different formulations impacts the skin health under real-life scenario situations are largely NOT present.

We have in the recent years just started to gain more insight into this area. One such study has shown that an alkaline skin care product impaired the skin barrier after repeated application over a 5-week period, and the skin barrier was disrupted severely by exposure to 1% SLS because it was already impaired by alkaline pH and sensitive to external stress (Kim et al. 2009).

This suggests that the pH of daily skin care products, including pH-disrupting skin cleansers with SLS, is very important for skin barrier homeostasis and further use of SLS-containing products will quickly aggravate the skin irritation over time.

Still, it is a prevalent opinion within dermatological society that SLS is not irritating to the skin as present in current formulations.

We simply know very little about the gradual, cumulative effects of the long-term, repeated exposures, which are in my opinion, the real concern.

It's not just repeated exposure to SLS —it's the combined effect of thousands of brief exposures to various substances, day after day, for years and decades, to which SLS can greatly contribute as skin penetration enhancer and due to its damaging effects on stratum corneum, skin barrier function and the skin pH level.

It is my belief that SLS mostly does not cause adverse skin reaction or irritation upon a single usage, but it very likely damages and irritates the skin when used for long periods of time via brief daily washing twice a day, as is the real life scenario, and aggravates some of the common inflammatory skin conditions, including dryness, acne, rosacea, atopic dermatitis and eczema.

The scientific uncertainties surrounding SLS appear overall to be very high. As our knowledge changes and evolves though, so will our perspective and scientific experiments, and the critical mass of scientific evidence supporting the skin damaging effects of SLS might gain more credibility and even lead to its regulation.

REFERENCES