If you’ve ever Googled the causes of a skin complaint or damaged hair, chances are someone on the internet has pointed the finger at SLS, or sodium lauryl (or laureth) sulfate, a common ingredient in beauty products, washes, toothpastes and even cleaning products.
So what does this ingredient do, why is it in everything, and what does the evidence say about how safe it is?
When we use a wash or beauty product on our skin, it’s probably a liquid made of a water phase and an oily phase. As we know, oil and water don’t mix, so something is required to keep the ingredients together.
That something is called a surfactant. A surfactant allows the oil and water molecules to bind together – it’s what’s found in soaps and detergents so we can wash our oily faces or dishes with water and get the grime to disappear.
Sodium lauryl sulfate is a surfactant, and its efficacy, low cost, abundance and simplicity mean it’s used in a variety of cosmetic, dermatological and consumer products.
Our skin’s outermost layer is specially designed to keep harmful stuff out, and this is where a surfactant can cause problems. Using a chemical that weakens this defence mechanism can potentially cause our skin harm.
And some surfactants are more irritating to our skin than others. For something to be harmful, irritant or allergenic, it has to fulfil two criteria.
It has to have been found in studies to irritate human skin, and it has to have the ability to penetrate the skin. SLS ticks both of these boxes.
Researchers from Germany tested 1,600 patients for SLS irritancy and found 42% of the patients tested had an irritant reaction.
Another study, on seven volunteers over a three and a half month period, found regular contact caused irritation, and the irritation subsided once the skin was no longer exposed to SLS.
Another study found the warmer the water used with SLS, the more irritating it will be.
In fact, SLS is so known to cause irritation, it’s used as a positive control in dermatological testing. That is, new products being tested to see how irritating they might be to human skin are compared to SLS – something we know definitely to be irritating.
If a person is sensitive to SLS, they might find the area that has been in contact is red, dry, scaly, itchy or sore.
It’s also important to note there’s no scientific evidence SLS causes cancer, despite what you may read on the internet.
So why is it allowed?
So if it’s known to be irritating to human skin, why don’t the regulatory authorities ban its use?
For SLS to be considered dangerous, it would have to be in contact with the skin for a long period of time. Generally, with consumer products such as washes that contain SLS, it’s assumed they won’t be on the skin for very long, meaning the chance of your skin being affected is pretty low. So authorities don’t ban its use, but instead cap the maximum percentage at which it can be used in products.
This cap varies based on how long the product is likely to be in contact with the skin. So products that will be on the skin for a prolonged time can contain no more than 0.05-2.5% SLS in most countries.
All consumer and cosmetic product manufacturers are required to conduct thorough testing and include any adverse findings in the form of warnings on their labels. So on products containing SLS, you should see something like “if this product causes any skin redness or irritation, discontinue use and consult a medical practitioner”.
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 solubilisation 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 .
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.