The Sodium Lauryl Sulfate Debate:
What Science Says About This Controversial Ingredient
Have you ever wondered what's behind the foamy lather in your shampoo, body wash, or toothpaste? If you have checked the ingredient lists, chances are you have come across Sodium Lauryl Sulfate (SLS).
SLS is one of the most common cleansing agents in personal care products, but it is also one of the most controversial.
In recent years, the personal care industry has seen a surge in products labeled "SLS-Free." This trend has prompted consumers to question the safety of Sodium Lauryl Sulfate (SLS) and its effects on our skin, bodies, and the environment.
But is this reputation deserved? What does the science say?
In this blog, we will explore the world of SLS—what it is, what it does, why it is used in so many personal care products, and what the evidence says about its effects on our skin and the environment.
What is Sodium Lauryl Sulfate?
To create products that foam, lather, and clean, whether for hair, body, or dishes, a surfactant (surface-active agent) is needed. SLS is a sulfate surfactant.
What is a surfactant?
Simply put, a surfactant reduces surface tension and bridges the gap between oil and water, allowing dirt and grease to be easily rinsed away. Think of it as a "middle-man" that facilitates the mixing of oil and water. SLS excels at this—in fact, it is one of the most effective surfactants out there.
Soap, a natural surfactant, has been used for centuries. It was the very first “cleaning surfactant” commonly used. Synthetic surfactants, like SLS, are key ingredients in modern-day detergents.
Learn More Blog: How Soap Works
Historical Background
Imagine a time when soap was a precious commodity and the world was in turmoil due to World War I. In 1916, the shortage of soap ingredients led to the invention of synthetic detergents, or "syndets," which revolutionized the way we clean.
Fast forward to World War II, and Sodium Lauryl Sulfate (SLS) emerged as a powerful engine degreaser, showcasing its incredible ability to break down oil and grease.
By the 1950s, syndets, like SLS, had overtaken traditional soap products in America, transforming everyday items like toothpaste and shampoo and changing the landscape of personal care products forever.
Today SLS can be found in many personal care products such as liquid shampoo, body wash, syndet soap and shampoo bars, facial cleansers, shaving foam, toothpaste, bubble bath, baby wipes, and many more. SLS is widely used because it is a good surfactant, is inexpensive, and creates that satisfying foamy lather that many people associate with cleanliness even in cold water.
How is Sodium Lauryl Sulfate Made?
The process begins with the isolation of one particular fatty acid called Lauric Acid. Lauric acid is used as the starting material due to its unique properties, which make it an effective surfactant and lathering ingredient.
The lauric acid can be extracted from coconut oil, palm kernel oil, or petroleum.
Regardless of the source, the chemical process for making SLS is generally the same, involving a series of reactions to transform the fatty acid into a surfactant.
Here is a simple step-by-step overview:
- Lauric acid is converted into lauryl alcohol by a process called hydrogenation.
- Lauryl alcohol is then converted into a sulfate ester called lauryl sulfate by reacting it with sulfur trioxide or sulfuric acid.
- In the final step, the lauryl sulfate is neutralized with a base like sodium carbonate or sodium hydroxide.
- The result is sodium lauryl sulfate.
SLS By Any Other Name Would Be The Same
If you are shopping for SLS-free products, be aware that sodium lauryl sulfate can masquerade under various names on product labels, such as lauryl sodium sulfate, lauryl sulfate sodium salt, sodium dodecyl sulfate, sodium n-dodecyl sulfate, etc. as well as many trade or brand names.
Additionally, some products proudly display "SLS-free" labels simply because the chemical Sodium Lauryl Sulfate is not used as an ingredient. However, they may still contain other synthetic surfactants, such as Sodium Coco Sulfate, which naturally contains a significant amount of SLS due to its chemical composition.
This highlights the importance of researching ingredients.
Note: The words "Sulfate" and "Sulphate" are both correct. The preferred spelling varies depending on the region. Sulfate is more commonly used in American English, while sulphate is preferred in British English.
Sodium Lauryl Sulfate: Safety for Body and Skin
The sodium lauryl sulfate (SLS) controversy stems from information circulating online, which often misinterprets or distorts scientific research. Further confusion arises from the mistaken identity of SLS with other similarly named chemicals.
Companies and bloggers promoting "natural" often use bits and pieces of scientific information, to support claims that SLS is highly toxic, irritating, corrosive to the skin, and even linked to hair loss, blindness, infertility, cancer, and death. Unfortunately, hype and misinformation often take on a life of their own and if repeated often enough, become an accepted as indisputable truth that clouds reality. Be skeptical and consider the author's motivations and potential biases when evaluating claims.
Scientific Literature Review
To answer questions about the effects of SLS on the skin and body, I reviewed a lot of primary source scientific literature. Unfortunately, this review revealed discrepancies, conflicting results, and few controlled trials or human studies. While some researchers believe SLS is unsafe, others point to its decades-long use without proven serious health risks. SLS has been an ingredient in shampoos since the 1930s.
Cancer Claims and Toxicity
In the late 1990s, an email began circulating falsely claiming that SLS causes cancer. Some reports suggested that nitrosamines, a group of chemical compounds, 90% of which are believed to be carcinogenic, could form during the manufacturing process. However, it is chemically impossible for nitrosamines to be present in SLS. Currently, there is no scientific evidence to support the idea that SLS is a carcinogen or that it causes infertility, blindness, or other severe health issues.
Yes, SLS has a toxic dose. If you consume enough SLS it will probably kill you, but so will vodka, salt, and even water. Every chemical has a toxic dose.
Skin Penetration and Absorption
We know that SLS can penetrate the skin and enter the bloodstream because it is sometimes used as a penetration enhancer in topical drug formulations. It works by disrupting the skin’s lipid barrier, increasing permeability, and aiding the absorption of other chemicals. Some studies suggest that traces of SLS can be found in the bloodstream and tissues after topical application.
Another study indicates that while SLS can be absorbed through the skin "the majority of the material remains in or on the skin surface. SLS that is absorbed into the bloodstream is quickly metabolized by the liver."
SLS is sometimes used as an emulsifier in creams and lotions that remain on the skin, raising the question: Could prolonged exposure lead to buildup? Unfortunately, I could not find any research exploring the cumulative effects of SLS on the human body over extended periods.
Irritation and Concentration
Two primary factors affect the irritating properties of SLS, concentration and exposure time (how long the product remains on the skin). Skin irritation increases with SLS concentration and duration of exposure.
The SLS concentration of a product can range from 0.01%-50%. It makes sense that the higher concentrations are reserved for industrial and household products that require tough cleaners.
As you peruse the internet you will discover words like “when used in high concentration the corrosive nature of SLS strips natural oils from the skin disrupting the natural skin barrier and damaging the hair cuticles causing brittleness, breakage, and hair loss.”
Originally developed as a powerful degreaser, SLS was sold commercially as industrial cleaners like "Engine Brite" or "Gunk." While using this concentrated industrial cleaner on skin and hair would likely be corrosive, SLS is not used at these concentrations in personal care products.
We know that SLS is a skin irritant. In dermatological studies, it is used to induce skin irritation, helping researchers differentiate between allergic reactions and irritant reactions, test the irritation potential of new products, and evaluate soothing topical products.
Research has investigated the potential of Sodium Lauryl Sulfate (SLS) to cause skin irritation. A 2003 study published in Contact Dermatitis found that 41.8% of 1,600 participants experienced skin irritation after a 0.5% SLS patch was applied for 2 continuous days.
Another study using a 1% SLS concentration caused irritant contact dermatitis when left on the skin for 16 hours a day for over 3 days.
While research shows that SLS can disrupt the skin barrier and cause irritation, it is important to consider how we typically use personal care products containing SLS. We do not stand in the shower with body wash slathered all over our bodies for days. As we lather up and rinse off, these products remain on the skin for a very short time. So how relevant are these findings to everyday use?
Proper Formulation and Concentration Levels
SLS is one of the more irritating surfactants, but irritation usually occurs when it is used or formulated improperly.
When a personal care company includes SLS in a product, we expect them to use a safe formulation and concentration that minimizes the risk of irritation. The FDA does not regulate SLS concentrations in skincare products in the United States.
By contrast, the EU has set maximum concentration limits for leave-on products but not for rinse-off products like body washes or shampoos. Ultimately, improper formulation can cause skin irritation, regardless of the ingredient.
Sensitive Skin and Irritation
Since sodium lauryl sulfate is a powerhouse cleanser, it may exacerbate dry skin or scalp issues. Research suggests that individuals with sensitive skin, eczema, rosacea, psoriasis, or other skin conditions are more likely to experience irritation or worsening symptoms when using products containing SLS. If you are sensitive to SLS, you might experience redness, dryness, scaliness, itching, or even a burning sensation on the skin.
Everyone’s skin is unique—so pay attention to how yours reacts!
Is Sodium Lauryl Sulfate A Natural Ingredient?
You may have seen SLS or its relatives in so-called "all-natural" products. Since SLS can be derived from coconut or palm oil, some argue that it qualifies as a natural ingredient.
But here is where my bias kicks in—I disagree!
Terms like "naturally sourced," "naturally derived," "plant-derived," and "derived from nature" have no legal definition and can be misleading. While these labels suggest a connection to nature, they do not account for the extensive processing, chemical modifications, or additives that transform the original plant into the final ingredient.
Synthetic surfactants like SLS have no place in "natural" products, regardless of their safety or effectiveness.
Is Sodium Lauryl Sulfate Safe For The Environment?
As synthetic ingredients, like SLS, are washed down the drain, concerns arise about their potential toxicity to local ecosystems and aquatic life. Unfortunately, researching this topic revealed discrepancies and conflicting results, making it challenging to draw any real conclusions.
The biodegradability of SLS is a topic of debate. Some findings indicate that SLS does not biodegrade easily and can have a lasting impact on aquatic ecosystems. In contrast, other studies claim that SLS is readily biodegradable and does not persist in the environment.
A 2023 review of the literature regarding sodium lauryl sulfate and its implications on environmental safety highlights the limitations of current research, "Overall, the studies regarding SLS are patchy; the majority of the studies have been published in the EU, with only 28% of studies being published in the last 10 years."
The article further states, "There is empirical evidence of toxicity from exposure to SLS at varying concentrations with aquatic organisms being more susceptible than terrestrial organisms."
In contrast, an article published in 2015 suggests that the use of SLS in cleaning formulations does not pose an unnecessary environmental risk if properly formulated.
The fate of SLS in water treatment plants is another area of interest. While municipal water filtration systems can break down SLS, the effectiveness of this process varies. There is limited research on how well, how fast, or what factors affect this degradation process. As a result, we do not have a clear picture of how SLS breaks down over time, what it turns into, or how much is released into our waterways.
After wastewater treatment, a semi-solid residue called sewage sludge remains. Composed of organic matter and contaminants, it can contain synthetic detergents like Sodium Lauryl Sulfate (SLS), depending on how well they break down during treatment.
Sewage sludge can be sent to landfills or used in agriculture as compost, a soil amendment, and fertilizer. The USDA's National Organic Program (NOP) prohibits using sewer sludge in organic farming.
Despite my research, I am left with more questions and concerns than answers. A few of them include:
- Not all wastewater goes to treatment plants. What about those of us who have septic tanks and leach fields where the wastewater remains our property, and eventually filters into groundwater?
- Is it safe to use gray water, the non-sewage water that comes from showers, sinks, and washing machines, to irrigate vegetable gardens if it contains SLS
- "Aquatic life" is a very generic term. In studies examining SLS's effects on aquatic life, do researchers account for varying sensitivities among different organisms?
- Do we even have the means to accurately measure the concentration of SLS entering our waterways?
Beyond the environmental impact of SLS, its manufacturing process raises additional concerns, such as:
- What additives and byproducts are created during SLS synthesis, and how do they affect the environment?
- Do these byproducts and additives vary depending on the source—palm oil, coconut oil, or petroleum?
- When petroleum is the source, extraction and refining deplete non-renewable resources. Drilling and pipeline construction can also lead to habitat loss.
- If sourced from palm oil, production contributes to deforestation unless certified as sustainable palm oil is used.
The environmental impact of SLS is a complex and murky issue. Although SLS can be derived from natural sources its overall ecological footprint remains unclear. A lot more research is needed to fully assess its effects on the environment.
Embracing Uncertainty: What We Know, What We Don't
If you are a regular reader of Ida's Soapbox, there is one comment that continues to pop up, and it is worth repeating:
It's what we DON'T know that worries me!
Science is dynamic and ever-evolving, with new discoveries constantly challenging what we thought we knew. Our understanding is limited to the knowledge available today.
We only know what we know now!
The Earth was flat—until it wasn’t. The Sun orbited the Earth—until it didn’t. Pluto was a planet—until it wasn’t. Atoms were the smallest building blocks of matter—until they weren’t. Margarine was good—until it was bad.
As we learn more and our tools improve, scientific theories shift to match new evidence. That’s the nature of science.
I know that natural ingredients are not perfect—they will not cure all that ails our skin or our planet. But I firmly believe we do not need so many synthetic chemicals in skincare.
Ultimately, I am left wondering: What don’t we know? What are the long-term effects of decades of daily exposure to synthetic chemicals like SLS on our bodies and the environment? The sheer number of these ingredients makes it nearly impossible to study them all in depth. But maybe that is the point.
We may never truly know.
Final Thoughts
As someone who favors natural skincare, I recognize my own biases. It would have been easy to write a completely negative blog about sodium lauryl sulfate (SLS), cherry-picking studies, and anecdotal evidence to support my views. This is known as confirmation bias, seeking out only information that aligns with what we already believe. However, my goal was to try and move beyond the echo chamber and explore the topic in a more balanced way.
The truth is, there is still much to learn about the effects of sodium lauryl sulfate on both our skin and the environment. As conscious consumers, it is crucial to stay informed, critically evaluate information, and demand transparency from manufacturers.
I have come across articles defending the use of SLS, while others blame it for nearly every skin ailment. But are any of these claims supported by reputable scientific research, or have the findings been misinterpreted, misrepresented, or exaggerated?
The internet and social media make it easier than ever to find information that reinforces our existing beliefs.
At the same time, they have created an environment where misinformation spreads rapidly. It is much like a game of "telephone," where small misconceptions grow into widely accepted truths as they are passed along. Controversy fuels media frenzies, with people taking sides rather than engaging in meaningful discussion.
Information has the power to shape our thoughts, often in ways we don’t even realize. To make informed decisions, we must rethink our assumptions about what makes a source trustworthy.
For example, many assume that ".org" websites are more credible than ".com" sites, but anyone can register a .org domain.
Similarly, social media posts with more likes, shares, and comments are not necessarily more reliable. Popularity does not equal accuracy or truth.
In a world flooded with information, we must learn to navigate the noise. Question everything, even if it seems to be widely accepted. Seek out multiple sources. Demand evidence before accepting claims as fact.
By becoming more discerning consumers, we can make better choices for our skin and our planet while also contributing to a more informed and critical community.
In today’s digital age, a healthy dose of skepticism is not just helpful—it is essential.
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