While we are washing hands frequently in order to stop the spreading of viruses, please remember to try your best to wash away the soap or detergent from your skin completely. Do not think that leaving some soap or detergent residue on the skin could be helpful in protecting your skin. Most hand wash contains sodium lauryl sulfate (SLS, also known as sodium dodecyl sulfate, SDS), which can damage the skin if left for a prolonged period of time, making the skin more vulnerable to the invasion of bacteria and viruses.
The harm SLS causes to humans
You may be familiar with SLS as it is a well-known ingredient in shampoo that can cause hair loss. However, SLS can also cause irritant skin. In fact, the SLS is such a well-known irritant that it is used as a standard irritant in the positive control in dermatological tests.1 According to research studies, SLS causes irritation to the skin if it is left for a prolonged period of time.1,2,3
Researchers from Germany found 42% of 1600 tested patients had an irritation due to SLS.1 Skin irritation is usually assessed by the changing level of redness of the skin, stratum corneum thickness, and the level of transepidermal water loss (TEWL) before and after SLS treatment. Research studies on Caucasian and Japanese populations found significant erythema, stratum corneum dehydration, and elevated TEWL in a dose-dependent manner, when 0.025% to 0.75% of SLS was applied and retained in the forearm for up to 24 hours.1,2,3 When SLS was applied repeatedly, the levels of erythema and TEWL were augmented and the reactions developed more quickly.3
There are two main ways the SLS triggers skin irritation. One way is by physiologically damaging the skin. Our skin is protected by layers of cells that are composed of oil and protein. Prolonged exposure to SLS can disrupt the natural oil in lipid membrane that protects skin and thus damages the skin. This results in cracked, dry and tender skin which makes it irritant.4 More importantly, this also reduces the ability of the skin to keep out bacterial and viral invasions.
Additionally, SLS triggers skin irritation on the biological level. A study using confocal Raman microscopy reported that SLS can penetrate into human skin.5 Research studies showed that SLS triggers the expression of two inflammatory mediators, IL-1alpha and PGE2, upon topical application of SLS on keratinocytes.6,7 Given the recent findings that SARS-CoV-2 can also trigger a hyperinflammatory response in severe cases,8 we can imagine what would happen if the SARS-CoV-2 virus invaded cells which had been penetrated by SLS.
Why SLS is commonly used
SLS, with the formula CH3(CH2)11SO4Na, is a *surfactant (surface active agent), a substance made from molecules that have hydrophobic ("grease loving, water hating") groups as tails and hydrophillic ("water loving") groups as heads. Soap, an alkaline salt of fatty acids, is the oldest known surfactant.
Surfactant is the main ingredient in hand soaps, and is added into face washes, shaving creams and toothpastes because it lathers up to generate cleansing foam. The lather-creating feature also enables the core ingredients of the products to be dispersed effectively across the entire cleaning surface. Because of its amphiphillic property, surfactant is added in cosmetic products, dermatological products, and cleaning products, to help mixing oily ingredients with aqueous ingredients. However, when it is left in contact with the skin, the hydrophobic tail of the molecule can disrupt the lipid structure of the skin cell and causes skin damage.
SLS is a commonly used surfactant because of its easiness to produce. It is made by combining lauric acid (from coconut oil) with sulphuric acid (from petroleum) and sodium carbonate. Moreover, it has higher efficacy in generating lather, which is important for removing dirt. As an anionic surfactant, this means SLS has higher ability to solubilize fats and oil. However, unfortunately, this also means that SLS is more harmful than the other surfactants in causing more skin irritancy than non-ionic, or amphoteric surfactants.
What can we do to protect ourselves from the effects of SLS
SLS is not only commonly found in hand wash, it is also found in shampoo, toothpastes, and cleaning detergents. It is very hard to avoid using products with SLS. As there is no scientific evidence that it can cause cancer, and the skin irritation no longer exists once exposure to SLS has ceased,3 we do not need to worry overmuch about the use of SLS-containing products. However, as prolonged exposure to SLS damages the skin and make it vulnerable for virus invasion, it is important to avoid leaving SLS on the skin each time we finish using the relevant products, especially during the pandemic period.
There are a few practical tips, that you may have missed in your daily routine, to minimize the chance of leaving SLS on ourselves, apart from using SLS-free products.
1. While we should use warm water with soap for cleaning in order to increase the lather and thus increase the cleaning power to remove dirt, it is best to avoid using excessively hot water for cleaning or showering, as a high water temperature damages the skin.9
2. Put on gloves while we wash dishes to avoid direct contact with the detergent. Or simply use a dishwasher to do the washing.
3. All of our cells, not only the skin cells, are protected by cell membranes composed of fat which can be disrupted by the SLS. SLS left on the kitchenware or the kitchen utensils which will be used later for food will come into our body. Therefore it is really important to rinse these things in running water to get rid of the soapy water from the cleaning.
There are many milder alternatives available (eg. sodium lauryl phosphate, **sodium laureth sulfate, alkyl phenol ethoxylate, fatty alcohol ethoxylate, or fatty acid alkoxylate) to replace SLS in cosmetic and cleaning products. You can seek advice from your pharmacist or GP on the usage of these products if you think SLS might be the cause of your dermatitis or worsening of your eczema.10,11
*Surfactant comes from the name surface active agent which means a substance that can lower the surface tension of a liquid. When surfactant is dissolved in water, the surfactant molecules orientate at the surface so that the hydrophobic regions are away from the aqueous environment. The surfactant molecules thereby adsorb at the water surface and weaken the forces between water molecules. The contraction force in the water thereby is reduced and thus the spreading and wetting properties of an aqueous solution are increased.
In water, micelles of surfactants are formed by aggregates of surfactant molecules in a way that the hydrophobic tails are directed inwards and the hydrophillic heads are directed outwards. In this way, the aggregates will form balls, cylinders or laminar layers depending on the concentration of the surfactant. When added into an aqueous solution containing oil, surfactant molecules aggregate around the oil so that the oil or fat molecules will be totally incorporated inside micelles. This way, the fat is dispersed into very small particles.
When added into non-aqueous solvent, the surfactant molecules aggregate the other way around, where hydrophilic heads form the core of the aggregate and hydrophobic tail are in contact with the surrounding fat/oil. The surfactant will act in such a way that they will disperse the water-soluble material, in the solvent, into very small parts by creating aggregation around the particle and forms a micelle. This makes it possible to remove the water-soluble material from substrates in solvent using surfactant.
** Sodium laureth sulfate, SLES, is a compound derived from SLS by introducing ethylene oxide through a process called ethoxylation. SLES is safe to use in bath and body care products and is gentler to skin than SLS. The compound won't aggravate your skin or strip any excess moisture off. On the other hand, SLES will be just as cleansing, foaming and emulsifying as SLS.
References
1. J. Geier, W. Uter, C. and Pirker, et al. Patch testing with the irritant sodium lauryl sulfate (SLS) is useful in interpreting weak reactions to contact allergens as allergic or irritant. Contact Dermatitis, 2003 Feb;48(2):99-107.
2. J. Aramaki, S. Kawana, and I. Effendy, et al. Differences of skin irritation between Japanese and European Women. Br J Dermatol., 2002 Jun;146(6):1052-1056.
3. Nara Branco, Ivy Lee, and Hongbo Zhain, et al. Long-term repetitive sodium lauryl sulfate-induced irritation of the skin: an in vivo study. Contact Dermatitis, 2005 Nov;53(5):278-284.
4. A. di Nardo, K, Sugino, and P. Wertz, et al. Sodium lauryl sulfate (SLS) induced irritant contact dermatitis: A correlation study between Ceramides and in vivo parameters of irritation. Contact Dermatitis, 1996 Aug;35(2):86-91.
5. G. Mao, C.R. Flach, and R. Mendelsohn, et al. Imaging the distribution of sodium dodecyl sulfate in skin by confocal Raman and infrared microspectroscopy. Pharm. Res. 2012, 29, 2189-2201.
6. C. Cohen, G. Dossou, and A. Rougier, et al. Measurement of inflammatory mediators produced by human keratinocytes in vitro: a predictive assessment of cutaneous irritation. Toxicol. Vitr., 1991, 5, 407-410.
7. S. Gibbs, H. Vietsch, and U. Meier, et al. Effect of skin barrier competence on SLS and water-induced IL-1? expression. Exp. Dermatol., 2002, 11, 217-223.
8. P. Mehta, D. F. McAuley, and M. Brown, et al. COVID-19: Consider Cytokine Storm Syndromes and Immunosuppression. Lancet, 2020 Mar 28;395(10229):1033-1034.
9. E. Berardesca, G.P. Vignoli, and F. Distante, et al. Effects of water temperature on surfactant-induced skin irritation. Contact Dermatitis, 1995 Feb;32(2):83-87.
10. M. Tsang, and R.H. Guy. Effect of aqueous cream BP on human stratum corneum in vivo. Br. J. Dermatol., 2010 Nov; 163(5): 954-958.
11. N. Kuzmina, L. Hagstromer, and M. Nyren, et al. Basal electrical impedance in relation to sodium lauryl sulphate-induced skin reactions--A comparison of patients with eczema and healthy controls. Skin Res Technol. 2003 Nov; 9(4): 357-362.
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