PEG-100

PEG-100 is a synthetic polymer belonging to the polyethylene glycol family, specifically engineered with an average of 100 ethylene oxide repeat units per molecule. This water-soluble compound is created through the polymerization of ethylene glycol, a process that yields a clear, viscous liquid with a relatively low molecular weight compared to other PEG variants. Naturally, PEG compounds are not found in the environment—they are entirely laboratory-derived, which allows for precise control over their molecular size and functional properties. The "100" in its name refers to the average molar ratio of ethylene oxide units, giving it a specific balance of hydrophilicity (water-attracting) and lipophilicity (oil-attracting) that makes it versatile in formulations.

In cosmetics, PEG-100 primarily functions as a humectant, solvent, and secondary moisturizer. As a humectant, it works by drawing moisture from the air into the skin's outer layer, helping to temporarily boost hydration levels. Its solvent properties allow it to dissolve other ingredients that are not easily soluble in water or oil, improving the stability and texture of creams, lotions, and serums. Formulators typically use PEG-100 at concentrations ranging from 1% to 10%, depending on the product type—higher levels may be found in rinse-off cleansers where it aids in cleansing, while lower amounts are common in leave-on moisturizers to enhance spreadability. Unlike some thicker PEGs, this variant remains fluid, making it ideal for lightweight formulations without a greasy afterfeel.

The main benefits of PEG-100 center on its ability to improve product texture and skin feel while providing mild hydration. It helps create a smooth, non-tacky finish that allows other active ingredients to spread evenly across the skin, potentially enhancing their efficacy. Evidence for its humectant properties is well-established through decades of use in pharmaceuticals and cosmetics, though it is less potent than classic humectants like glycerin or hyaluronic acid. Studies show that PEGs of this molecular weight (around 4,000–6,000 Daltons) are too large to penetrate deeply into the skin, so they work primarily on the surface to soften and temporarily plump the stratum corneum. This makes them particularly useful in products designed for immediate, short-term hydration rather than long-term barrier repair.

PEG-100 is generally well-tolerated by most skin types, including sensitive and acne-prone skin, due to its non-comedogenic and non-irritating profile. However, individuals with severely compromised skin barriers or known sensitivities to polyethylene glycols may experience mild stinging or dryness, especially in high-concentration formulations. It is compatible with a wide range of ingredients, including vitamin C, retinoids, and AHAs/BHAs, without causing instability or degradation. One limitation is that PEG-100 can sometimes feel slightly tacky if used alone at high levels, but this is rarely an issue in well-formulated products where it is blended with other emollients. There are no known negative interactions with other common skincare components, making it a safe choice for multi-ingredient cocktails.

When reading labels, PEG-100 may appear simply as "PEG-100" or as part of a larger compound like "PEG-100 Stearate" (a common derivative used as an emulsifier). You'll find it most frequently in cleansers, toners, and water-based serums, where its solvent and humectant roles are most valuable. An interesting fact: PEGs were first developed in the 1940s and have since become ubiquitous not only in cosmetics but also in pharmaceuticals as laxatives and in industrial applications as antifreeze—though cosmetic-grade PEG-100 is rigorously purified for safety. Despite some online concerns about "contaminants" like ethylene oxide or 1,4-dioxane, reputable manufacturers ensure these impurities are removed during production, and regulatory bodies like the FDA and EU CosIng consider PEG-100 safe for use in rinse-off and leave-on products at typical concentrations.