This study detected the presence of 1,4-dioxane in an industrial product containing the polyethoxylated surfactant SLES. 1,4-dioxane is formed during the production of SLES, by an ethoxylation process, as a toxic and carcinogenic by-product (1).

Sodium laureth sulfate belongs to a group of salts of sulfated ethoxylated alcohols whose safety has been evaluated by the Cosmetic Ingredient Review (CIR) panel for use in cosmetic products. Sodium laureth sulfate can produce skin and eye irritation, but is not a sensitizer. The Panel acknowledged that there are data gaps regarding the use and concentration of these ingredients (2).

This study convincingly found skin irritation caused by Sodium laureth sulfate while another detergent, alkyl polyglucoside, showed no significant reaction (3).

Sodium laureth sulfate is an anionic detergent, which has been used for decades for its foaming power.. However, SLES is somewhat invasive and stimulating to the skin, and many consumers with sensitive skin desire gentler cleansers for everyday skin cleansers. This research finds that the addition of sodium laureth carboxylate and lauryl glucoside to Sodium laureth sulfate, may provide gentle and less aggressive surfactant (4).

References________________________________________________________________________

(1) Saraji, M., & Shirvani, N. (2017). Determination of residual 1, 4‐dioxane in surfactants and cleaning agents using headspace single‐drop microextraction followed by gas chromatography–flame ionization detection. International Journal of Cosmetic Science, 39(1), 36-41.

(2) Robinson VC, Bergfeld WF, Belsito DV, Hill RA, Klaassen CD, Marks JG Jr, Shank RC, Slaga TJ, Snyder PW, Alan Andersen F. Final report of the amended safety assessment of sodium laureth sulfate and related salts of sulfated ethoxylated alcohols. Int J Toxicol. 2010 Jul;29(4 Suppl):151S-61S. doi: 10.1177/1091581810373151.

(3) Löffler H, Happle R. Profile of irritant patch testing with detergents: sodium lauryl sulfate, sodium laureth sulfate and alkyl polyglucoside. Contact Dermatitis. 2003 Jan;48(1):26-32. doi: 10.1034/j.1600-0536.2003.480105.x. 

(4) Takagi Y, Shimizu M, Morokuma Y, Miyaki M, Kiba A, Matsuo K, Isoda K, Mizutani H. A new formula for a mild body cleanser: sodium laureth sulphate supplemented with sodium laureth carboxylate and lauryl glucoside. Int J Cosmet Sci. 2014 Aug;36(4):305-11. doi: 10.1111/ics.12127.

                                                    ____________________________

Compendium of the most significant studies with reference to properties, intake, effects.

Begum, G., Mohamed, I., Zahoor, H., & Gomaa, R. (2016). A study of potential cytotoxic effect of 1, 4 Dioxane on human hepatic cell line (HEP10).

Abstract. 1, 4 Dioxane is a highly toxic inflammable substance present in contaminated air, tap water and a large variety of products ranging from shampoos, moisturizers to detergents, toothpaste and cosmetics. Canada and Qatar have even banned cosmetics and shampoos that contain 1, 4 Dioxane. This study aimed to investigate the potential cytotoxic effects of 1, 4 Dioxane on human hepatic cell line (HEP10) following exposure to different concentrations; which are 500 ppm, 2000 ppm, 5000 ppm and 10,000 ppm w3wwfor four different durations of time; 1, 7, 14 and 21 days . Cell viability and apoptosis level was assessed for all plates using comparison between MTT Assay and LDHAssay. Lower concentration of 1,4 dioxane has showed relatively high cell toxicity for LDH Assay which wasn’t witnessed in previous studies. MTT Assay’s reliability is questionable for checkingcell viability.

Xu L, Amin S. Microrheological study of ternary surfactant-biosurfactant mixtures. Int J Cosmet Sci. 2019 Aug;41(4):364-370. doi: 10.1111/ics.12541.

Abstract. Objective: The main objective of this paper is to understand the rheological impact of rhamnolipids biosurfactant (mono/dirhamnolipids mixture, CCB) on a common personal care mixed surfactants system: anionic sodium laureth sulfate (SLES) and zwitterionic cocamidopropyl betaine (CAPB). The ternary biosurfactant/surfactants mixtures were evaluated at three different formulation conditions. The experimental results can provide a formulation guideline when applying rhamnolipids in cosmetics and personal care products....© 2019 Society of Cosmetic Scientists and the Société Française de Cosmétologie.

Charbonnier V, Morrison BM Jr, Paye M, Maibach HI. Subclinical, non-erythematous irritation with an open assay model (washing): sodium lauryl sulfate (SLS) versus sodium laureth sulfate (SLES). Food Chem Toxicol. 2001 Mar;39(3):279-86. doi: 10.1016/s0278-6915(00)00132-0. 

Abstract. Compared to exaggerated hand washing procedures, an open non-exaggerated assay better approximates consumer surfactant use. Our goal was to observe skin surface modifications induced by an open test with regard to discriminating between surfactant solutions. This human in vivo assay provided information about the effect of only three washes at the laboratory and a week of at-home use. Dorsal hand and volar forearm were compared. The results demonstrated that this clinical model permits exploration of subclinical surfactant-induced irritation. Both the volar forearm and the dorsal hand are capable of discriminating between the effects of sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES). Squamometry proved to be a sensitive assessment technique for detecting surfactant-induced subclinical skin surface alterations and for differentiating surfactant effects in this open application assay, in as few as three washes.

Robinson VC, Bergfeld WF, Belsito DV, Hill RA, Klaassen CD, Marks JG Jr, Shank RC, Slaga TJ, Snyder PW, Alan Andersen F. Final report of the amended safety assessment of sodium laureth sulfate and related salts of sulfated ethoxylated alcohols. Int J Toxicol. 2010 Jul;29(4 Suppl):151S-61S. doi: 10.1177/1091581810373151.

Abstract. Sodium laureth sulfate is a member of a group of salts of sulfated ethoxylated alcohols, the safety of which was evaluated by the Cosmetic Ingredient Review (CIR) Expert Panel for use in cosmetics. Sodium and ammonium laureth sulfate have not evoked adverse responses in any toxicological testing. Sodium laureth sulfate was demonstrated to be a dermal and ocular irritant but not a sensitizer. The Expert Panel recognized that there are data gaps regarding use and concentration of these ingredients. However, the overall information available on the types of products in which these ingredients are used and at what concentrations indicates a pattern of use. The potential to produce irritation exists with these salts of sulfated ethoxylated alcohols, but in practice they are not regularly seen to be irritating because of the formulations in which they are used. These ingredients should be used only when they can be formulated to be nonirritating.

Sodium Laureth sulfate or SLES, is a chemical compound and belongs to a group of salts of sulfated ethoxylated alcohols. It occurs in liquid form or clear transparent slightly yellow gel or white fine powder.

The name describes the structure of the molecule:

• Sodium is the sodium ion (Na+), an alkaline metal.
• Laureth is a term used in the cosmetics industry for a mixture of polyethylene glycol (PEG) and lauryl alcohol. The term 'laureth' is a contraction of 'lauryl ether', where 'lauryl' refers to the 12-carbon lauryl group derived from lauric acid, a medium-chain fatty acid found in coconut oil and palm kernel or palm oil.
• sulphate is a sulphate group (SO4-2-), a sulphur atom bonded to four oxygen atoms and having a negative charge.

The synthesis process takes place in different steps:

• Extraction.The raw materials are lauryl alcohol and ethylene oxide. Lauryl alcohol is typically derived from coconut or palm oil, while ethylene oxide is a petrochemical product.
• Ethoxylation. Lauryl alcohol is reacted with ethylene oxide in a process called ethoxylation. This reaction is catalysed by a base, usually a strong alkali such as potassium hydroxide. The term 'eth' in 'laureth' indicates that ethylene oxide has been added.
• Sulphonation. Ethoxylated alcohol is reacted with sulphur trioxide (SO3) to produce a sulphate.
• Neutralisation. The sulphate is neutralised with sodium hydroxide (NaOH) to produce sodium sulphate.
• Purification. The resulting product is purified through distillation and filtration to remove any unreacted materials and by-products.
• Quality control. The final product is tested to ensure that it meets quality standards.

SLES (Sodium laureth sulfate) must not be confused with SLS  because, although both are similar and have sulphuric acid and lauryl alcohol as their formula, differ in chemical properties. In SLES, which is less aggressive than SLS but is ethoxylated (obtained from ethylene oxide), it is not uncommon to find in SLES ethylene oxide and 1,4-dioxane residues, chemical compounds that are considered carcinogenic.

The term 'eth' refers to the ethoxylation reaction with ethylene oxide after which residues of ethylene oxide and 1,4-dioxane, chemical compounds considered carcinogenic, may remain. The degree of safety therefore depends on the degree of purity of the compound obtained. No manufacturer appears to provide this information on the label, at least as of the date of this review.

A preliminary remark must be made about synthetic surfactants, which can be divided into four groups:

• Anionic (SLS, ALS, SLES), which solubilises well with lipid monolayers but not with lipid bilayers model of liposomes or cytotoxicity tests towards cultured skin cells
• Nonionic amphoteric (CAPB), which solubilises well with both mono- and bilayers
• Biotensioactive (SAP), which penetrates monolayers at 1% dry mass without solubilisation, and probably penetrates bilayers, increasing their size (without solubilisation).

What it is for and where

Chemical intermediate, anionic surfactant, densifying and foaming agent, with good solvency, wide compatibility, strong resistance to hard water, high biodegradation and relatively low skin and eye irritation.

Sodium Laureth sulfate is used in cosmetics, in liquid detergents, such as hair and bath shampoos, dishwashing detergents, toothpastes, bubble bath and hand washing, soap etc.. In chromatography as a reagent, it has excellent properties as a solvent. In printing and dyeing industry, oil and leather, textile,,, it can be used as lubricant, dyeing agent, cleaner, foaming agent and degreasing agent.

Cosmetics

Cleansing agent. Ingredient that cleanses skin without exploiting the surface-active properties that produce a lowering of the surface tension of the stratum corneum. 

Foaming.  Its function is to introduce gas bubbles into the water for a purely aesthetic factor, which does not affect the cleaning process, but only satisfies the commercial aspect of the detergent by helping to spread the detergent. This helps in the commercial success of a cleansing formulation. Since sebum has an inhibiting action on the bubble, more foam is produced in the second shampoo. In practice, it creates many small bubbles of air or other gases within a small volume of liquid, changing the surface tension of the liquid.

Surfactant - Cleansing agent. Cosmetic products used to cleanse the skin utilise the surface-active action that produces a lowering of the surface tension of the stratum corneum, facilitating the removal of dirt and impurities. 

Surfactant - Emulsifying agent. Emulsions are thermodynamically unstable and are used to soothe or soften the skin and emulsify, so they need a specific, stabilising ingredient. This ingredient forms a film, lowers the surface tension and makes two immiscible liquids miscible. A very important factor affecting the stability of the emulsion is the amount of the emulsifying agent. Emulsifiers have the property of reducing the oil/water or water/oil interfacial tension, improving the stability of the emulsion and also directly influencing the stability, sensory properties and surface tension of sunscreens by modulating the filmometric performance.

Safety

It is used in mild soaps and shampoos. However, it can cause eye irritation if used in large quantities. Since it is not mandatory to indicate the percentage or quantity of the chemical on labels, it is still difficult to know how much Sodium Laureth sulfate is in the product. The scientific literature that has dealt with this chemical compound for decades has concluded in favour of a recognition of the irritant properties of Sodium lauryl ether sulphate. Since it is not uncommon for ethylene oxide (1) and 1,4-Dioxane, a synthetic cyclic ether traditionally used as a stabiliser (2), to be found in this chemical compound during the production process, the IARC (International Agency for Research on Cancer ) warns that ethylene oxide is carcinogenic to humans (3) and 1,4-Dioxane is potentially carcinogenic to humans (4). The real problem is that no manufacturer declares SLES as free of these two compounds on the label. So we cannot know if and how much ethylene oxide and 1,4-dioxane are present in the product we have purchased. In addition, 1,4-dioxane does not degrade easily and is therefore considered to be a water pollutant that must be removed using special techniques (5).

From the above, a rather negative picture emerges for SLES, and my opinion is to be cautious and not to buy products containing SLES unless the absence of ethylene oxide and 1,4-dioxane is clearly indicated.

 It is also important not to mistake the acronyms SLES (Sodium lauryl polyoxyethylene ether sulfate Sodium Laureth Sulfate) with SLS (Sodium lauryl sulfate), also a surfactant but much less aggressive.

The most relevant studies on this chemical compound have been selected with a summary of their contents:

Sodium Laureth sulfate studies

Typical characteristics of the commercial product Sodium lauryl polyoxyethylene ether sulfate Sodium Laureth Sulfate (SLES)

PSA	84.04000
LogP	4.48150
Safety

• Molecular Formula: C₁₄H₂₉NaO₅S
• Molecular Weight: 332.431 g/mol
• CAS: 15826-16-1
• EC Number: 239-925-1 
• UNII 410Q7WN1BX
• DSSTox Substance ID: DTXSID2029298    DTXSID70274019
• MDL number  
• PubChem Substance ID 
• InChI=1S/C14H30O5S.Na/c1-2-3-4-5-6-7-8-9-10-11-12-18-13-14-19-20(15,16)17;/h2-14H2,1H3,(H,15,16,17);/q;+1/p-1  
• InChl Key      ASEFUFIKYOCPIJ-UHFFFAOYSA-M
• SMILES    CCCCCCCCCCCCOCCOS(=O)(=O)[O-].[Na+]
• IUPAC   sodium;2-dodecoxyethyl sulfate
• RTECS    KK7890000

Synonyms:

• SLES
• Sodium dodecylpoly(oxyethylene) sulfate
• Sodium Alkyl Ether Sulfate
• Sodium laureth-5 sulfate
• Sodium laureth-8 sulfate
• Sodium dodeceth-1 sulfate
• Dodecyl sodium ethoxysulfate
• Sodium lauryl oxyethyl sulfate
• Sodium lauryl polyoxyethylene ether sulfate
• Sodium 2-(lauryloxy)ethyl sulfate
• Sodium 2-(dodecyloxy)ethyl sulfate
• Ethanol, 2-(dodecyloxy)-, hydrogen sulfate, sodium salt
• 2-(Dodecyloxy)ethyl sodium sulfate
• Etoxon EPA
• Sodium lauryl ethoxysulphate
• Sodium lauryl sulfate ethoxylate
• Sodium laureth-12 sulfate
• PEG-5 Lauryl ether sulfate, sodium salt
• PEG-7 Lauryl ether sulfate, sodium salt
• PEG-8 Lauryl ether sulfate, sodium salt
• PEG-12 Lauryl ether sulfate, sodium salt
• Sodium polyethylene glycol (7) lauryl ether sulfate
• Sodium polyethylene glycol 600 lauryl ether sulfate
• Polyoxyethylene (5) lauryl ether sulfate, sodium salt
• Polyoxyethylene (7) lauryl ether sulfate, sodium salt
• Polyoxyethylene (8) lauryl ether sulfate, sodium salt
• Polyoxyethylene (12) lauryl ether sulfate, sodium salt
• Polyethylene glycol (5) lauryl ether sulfate, sodium salt
• Polyethylene glycol 400 lauryl ether sulfate, sodium salt
• Polyethylene glycol 600 lauryl ether sulfate, sodium salt
• Dodecanol, ethoxylated, monoether with sulfuric acid, sodium salt
• Sodium laureth-3 sulfate
• Sodium laurylpoly(oxyethylene) sulfate
• Sodium polyoxyethylene (8) sulfate
• alpha-Sulfo-omega-(dodecyloxy)poly(oxy-1,2-ethanediyl) sodium salt
• Poly(oxy-1,2-ethanediyl), alpha-sulfo-omega-(dodecyloxy)-, sodium salt
• Glycols, polyethylene, mono(hydrogen sulfate), dodecyl ether, sodium salt
• Sodium polyoxyethylene (5) lauryl ether sulfate
• Sodium polyoxyethylene (7) lauryl ether sulfate
• Sodium polyoxyethylene (12) lauryl ether sulfate
• Sodium polyethylene glycol (5) lauryl ether sulfate
• 2-(Dodecyloxy)ethanol hydrogen sulfate sodium salt
• Laureth-8 carboxylic acid, sodium salt
• Polyethylene glycol sulfate monododecyl ether sodium salt
• Sodium polyethylene glycol 400 sulfate
• Polyethylene glycol (7) lauryl ether sulfate, sodium salt
• Sodium (lauryloxypolyethoxy)ethyl sulfate
• Sodium poly(oxyethylene) lauryl ether sulfate
• 15826-16-1
• Standapol ES 2
• Sodium 2-(dodecyloxy)ethyl sulphate
• Sodium laureth-7 sulfate

 References______________________________________________________________________

(1) Vleugels LF, Pollet J, Tuinier R. Polycation-sodium lauryl ether sulfate-type surfactant complexes: influence of ethylene oxide length. J Phys Chem B. 2015 May 21;119(20):6338-47. doi: 10.1021/acs.jpcb.5b02043.

(2) Black RE, Hurley FJ, Havery DC. Occurrence of 1,4-dioxane in cosmetic raw materials and finished cosmetic products. J AOAC Int. 2001 May-Jun;84(3):666-70.

(3) Ethylene oxide. IARC Monogr Eval Carcinog Risks Hum. 1994;60:73-159. 

(4) 1,4-Dioxane. IARC Monogr Eval Carcinog Risks Hum. 1999;71 Pt 2(PT 2):589-602.

Wilbur S, Jones D, Risher JF, Crawford J, Tencza B, Llados F, Diamond GL, Citra M, Osier MR, Lockwood LO. Toxicological Profile for 1,4-Dioxane. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US); 2012 Apr.

(5) Scaratti G, De Noni Júnior A, José HJ, de Fatima Peralta Muniz Moreira R. 1,4-Dioxane removal from water and membrane fouling elimination using CuO-coated ceramic membrane coupled with ozone. Environ Sci Pollut Res Int. 2020 Jun;27(18):22144-22154. doi: 10.1007/s11356-019-07497-6.