Sodium lauryl sulfate (SLS), or one of its cousins like Ammonium lauryl sulfate (ALS) or Sodium laureth sulfate (SLES), is the main cleaning agent and second-largest ingredient after water in most regular brands of shampoo, shower creams, liquid soap, toothpaste, shaving gel, bubble baths, etc. It is used in more than 1000 different types of cosmetic products in a concentration ranging from 0.01-50%. SLS is also on every top 10 list there is over toxic chemicals to avoid in your personal care products. On the internet, the list of allegations over what horrors this molecule can do seems to grow longer and longer. If you believe them, using SLS containing shampoo can have serious health effects including cancer, permanent eye damage, skin corrosion, hair loss, in addition to being comedogenic, skin sensitizing and a general skin and eye-irritatant. So what is true and what is just internet rumors? How dangerous is this everyday cleansing agent really and why are there so many rumors about the dangers of SLS?
Over the years there have been several thorough and independent investigations of the scientific research on SLS [1-5]. These reports methodologically go through all experimental studies on possible toxicity, mutagenicity, sensitization, irritability, etc. They also explain why certain studies have been some misunderstood, causing unnecessary worries. Here we take a closer look at what is true and what is not what are really the facts behind these claims.
Can 16,000 Studies About SLS Be Wrong?
This a common statement referring to a link on EWG’s Skin Deep database leading to a general search on PubMed, the database for all biologically and medically oriented scientific articles. Today this number has in fact reached over 90,000 and does not in any way relate to a number of articles or studies showing that SLS is toxic. Just scientific articles that mention SLS. The fact that SLS, or its more common name in science, SDS (sodium dodecyl sulfate) is found in so many studies is the same reason why you find it in a lot of cosmetics, household and industrial cleaners – it is cheap! SLS is a cheap surfactant, commonly used in scientific labs to dissolve other molecules, for analytical studies, for toxicological studies and for any other application that does not require more natural conditions such as preserving the 3D-structure and activity of proteins. Because that is the downside of this very cheap surfactant, it damages protein structures or, correctly expressed, denatures the proteins, and thereby they lose both shape and function. In fact, one of the most used laboratory analytical methods for proteins is based on the ability of SLS to denature proteins (SDS-PAGE).
Is SLS Skin Corrosive or Skin Irritating?
Destroying the structure of skin proteins, that sounds very brutal. That must mean that SLS is skin corrosive? Well, obviously a protein with ruined 3D-structure is not going to function correctly. However, skin corrosion means permanent damage to the skin, think pouring acid on the skin, and although SLS may be harsh on skin proteins, the skin will make new proteins rather quickly, so the damage is not in any way permanent. However, SLS is a harsh surfactant capable of disrupting your natural protective epidermal water barrier, washing away both the lipids and the natural moisturizing factors (NMF) as well as destroying skin proteins in the deep epidermis. Although this damage is reversible and the skin is capable of healing, you need to think about how many times a day you actually wash your hands. In a study from Germany , a medium strong solution (7.5%) of SLS was applied daily for 20 minutes for 8 days. During this time there was a rapid increase in TEWL (trans-epidermal water loss) and erythema (skin redness) followed by increased dryness of the skin (measured as decreased capacitance), flaking and cracking. In fact, after the treatment had stopped, it took 16 days for the skin to heal and regain normal hydration levels.
There are no limits within EU how much SLS a product can contain. Cosmetic Ingredient Review (CIR) recommends a concentration of maximum 1% in leave-on products . However, after 24 hours' exposure, even 1% SLS results in a 4 times increase in TEWL and a change in gene expression of proteins involved in the maintenance of the epidermis water barrier and the natural exfoliation process .
It is not only cheap household brands that contain SLS. Many high-end brands include SLS, SLES or ALS in their shower gels and shampoos. It is generally claimed by cosmetic formulators that with the right addition of other ingredients, SLS is completely non-irritating. But why not just reformulate and change the surfactant to something less irritating instead of covering it up with expensive oils and plant extracts?
Is SLS Skin Sensitizing?
Unlike skin irritation, sensitization is an allergic reaction resulting in inflammation and itchiness. The reaction becomes stronger after repeated exposures. SLS has in no study elicited an allergic response and can therefore not be considered sensitizing .
However, some companies that produce SLS for the cosmetic market sell it as a liquid stock solution that contains formalin/formaldehyde as a preservative. It has been shown that the small amount formaldehyde that remains in the final consumer product can cause sensitization [9, 10]. This has later been shown to be the real cause of the allergy in the American woman who in the 1950s seemingly reacted to SLS in a hydrocortisone cream .
How Toxic Is SLS?
Can you get sick if you swallow a little toothpaste? Not really. The toxic dose, LD50, of SLS is about 1,3g/kg . LD50 is the dose per kg body weight that kills 50% of the experimental animals. This is not a very high toxicity value and in a 60kg person that equals eating 78g pure SLS or drinking 520ml of a shampoo that contains 15% SLS. For comparison, table salt (NaCl) has an LD50 value of 3g/kg. Everything is toxic if ingested in high enough amounts.
Is SLS Cancerogenic?
What about cancer then? Can SLS cause cancer? No, this a myth that stems from the fact that SLS has been used in numerous toxicology studies to dissolve the substances being tested. It is good laboratory practice to do controls, meaning that you repeat the test with just the solvent solution itself. This is considered as background noise. No studies have ever been able to show any signs of SLS mutagenicity or carcinogenicity . In one study, dogs were fed a diet containing 2% SLS for 2 years and yet it did not cause any tumors .
Because of all the internet rumors, the CIR (Cosmetic ingredient review) looked into the issue again in 2005 and concluded that “None of the available data suggested any possibility that SLS or ALS could be carcinogenic. Despite assertions to the contrary on the Internet, the carcinogenicity of these ingredients is only a rumor” .
Sometimes concerns are raised that SLS could be contaminated by 1,4-dioxane. However, 1,4-dioxane is a byproduct of ethoxylation processes, that is, to introduce ether bonds in molecules. Although 1,4-dioxane is a suspected carcinogen, it cannot be contaminating SLS since SLS is not ethoxylated. Be aware though, that 1,4-dioxane contamination is possible for products containing ethoxylated surfactants and that some cousins of SLS, like SLES (sodium lauryl ether sulfate/sodium laureth sulfate) is ethoxylated. To avoid contamination by 1,4-dioxane in your shampoos and shower gels, be on guard for surfactants containing laureth, myreth, oleth or ceteareth in their names.
SLS has also been linked to carcinogenic nitrosamines through statements that SLS would become nitrosylated by reacting with triethanolamine forming N-nitrosodiethanolamine (NDELA). Fact is, if NDELA is formed in the shampoo bottle, it would be by nitrosylation of triethanolamine and not of SLS. SLS simply has nothing to do with this reaction.
Formaldehyde, on the other hand, could trigger such a reaction . Formaldehyde is a highly reactive molecule and a known cancerogen. As mentioned before, SLS stock solutions are sometimes sold with formaldehyde as a preservative, so if the product is made with this type of SLS, small amounts of formaldehyde will be present in the final consumer bottle. If the final concentration is below 0.05% it is not necessary to state in the ingredients list. Since it is impossible to find out from where and at what grade cosmetics companies buy their ingredients, we would like to see a clear “Formaldehyde-free” label in the future.
Is SLS Eye Irritating, Eye Damaging or Even Cause Blindness?
There is no doubt that SLS is irritating to eyes . Everyone who has ever got shampoo in their eyes can vouch for that. There are also numerous studies on rabbits showing that higher concentrations of SLS or SLS containing shampoo are eye-irritants, especially if you don’t rinse immediately . Strong solutions of SLS left in the eyes for more than 10 seconds can result in red and swollen eyes and even lead to temporary corneal opacity. A damage that may take weeks to heal from but, nonetheless, is reversible.
The main cause for the rumor about SLS “causing blindness and preventing children’s eyes from developing correctly” stems from an alleged statement of a Dr. Green, Medical College of Georgia, "SLS is a systemic that can penetrate and be retained in the eye, brain, heart, liver, etc., with potentially harmful long-term effects. It can retard healing and cause cataracts in adults, and can keep children's eyes from developing properly."
Dr. Green denies ever saying these things. Furthermore, Dr. Green states in an interview with Paula Begoun, the Cosmetics Cop that "There is no part of my study that indicated any eye development or cataract problems from SLS or SLES and the body does not retain those ingredients at all."  The study in question , actually shows that if the thin protective cell layer on the cornea is damaged, it is wise to avoid getting SLS in your eyes because it slows down the healing process. That’s not news, that’s just logical from what we know about SLS’s effects on the skin.
Two other studies are also sometimes cited in the context of SLS causing cataracts [16, 17]. In these studies, dissected lenses from cows or calves are immersed in high concentrations of SLS or other protein denaturing substances to induce the type of protein changes seen in cataracts. That’s right, dissected lenses, not whole eyes. Lenses are normally not naked and readily exposed to shampoo, ever. They are well protected, safely situated behind the cornea.
Can SLS Accumulate in the Body?
The absorption of SLS through the skin is very limited , in fact so limited that specifically labeled SLS is difficult to detect inside the body after contact with the skin. A big part of the SLS is stuck on the skin, even after rinsing, and it does not cross into the bloodstream. A very small portion (0.1%) has been traced as broken down and exhaled as CO2 or excreted in the urine. SLS readily dissolves in water and is not the type of molecule that can be stored in the body fat like some environmental toxins. The rumor about SLS accumulating in various organs is from the same alleged statement of Dr Green, which also caused the blindness rumor.
How about breakdown? Whatever small amounts of SLS that gets inside the body, do they stay there forever? No, SLS is readily biodegradable, typically within 96 hours (via hydrolytic cleavage of the sulfate ester bond catalyzed by alkyl sulfatases, which leaves inorganic sulfate and fatty alcohols that is then oxidized by hydrogenases to fatty acids and degraded through normal β-oxidation). So, no bioaccumulation of SLS whatsoever, not inside the body, nor in the environment.
Is SLS a Penetration Enhancer?
Yes, it perturbs the epidermis water barrier, which makes it easier for other molecules to cross into the deeper skin layers. For example, the presence of SLS caused nickel allergy in guinea pigs .
SLS is also used in the provocative patch test, which is an allergy test where SLS increases the uptake of the test substance to enhance a positive response from weakly allergenic substances .
Does SLS Cause Blackheads?
Yes, SLS is most likely comedogenic. It has scored positive in the classical rabbit's ear assay . In spite of that, it is regrettably often found in acne cosmetics.
Can SLS Cause Hair Loss?
SLS has been found accumulated in hair follicles [4, 5, 18] but so far there is no study that supports that SLS could cause increased hair loss.
Does SLS Cause Dry Hair and Split Ends?
Yes, SLS damages the outer surface of the hair strands, the cuticle, resulting in loss of shine, a rougher surface and split ends [19-21]. Even though all anionic surfactants to some extent damage the hair surface compared to only using water, SLS is harsher than all other surfactants used in skin care products.
To sum up, SLS is generally bad for the skin and hair, it seriously damage the epidermis water barrier, it is comedogenic and increases the risk of contact allergies. It may not kill you and there are definitely more toxic cosmetic ingredients out there, but it is definitely not something you want to include in your daily cleaning routine either. There are many better alternatives out there.
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