Humectant, drawing moisture into the skin
Emollient, helping to soften skin
Solvent, dispersing and stabilizing formulas
Preservative booster, enhancing the antimicrobial activity of other preservatives

Alcohol Denat.

Antimicrobial, Astringent, Masking

Alcohol Denat. is an alcohol with a denaturant property. It is created by mixing ethanol with other additives.

The "denat" part just means "denatured"; common denaturants include Denatonium Benzoate, t-butyl alcohol, and Diethyl Phthalate. This step makes the alcohol undrinkable (and lets brand skip taxes related to beverage alcohol).

This ingredient gets a bad rep because it is irritating and drying due to its astringent property. Astringents draw out natural oils in tissue to constrict pores and dry out your skin.

However, alcohol denat. is not all that bad.

Due to its low molecular weight, alcohol denat. tends to evaporate quickly. One study on pig skin found half of applied alcohol evaporated in 10 seconds and less than 3% stayed on skin.

This also helps other ingredients become better absorbed upon application.

Studies are conflicted about whether this ingredient causes skin dehydration. One study from 2005 found adding emollients to propanol-based sanitizer decreased skin dryness and irritation. Another study found irritation only occurs if your skin is already damaged.

Small amounts of alcohol are generally tolerated by oily skin or people who live in humid environments.

The rule of thumb is this ingredient will probably not affect your skin much if it is near the end of an ingredients list.

One thing to note:
People with ALDH2 (aldehyde dehydrogenase 2) deficiency may experience skin irritation from continued alcohol use. About 8% of the world's population have this deficiency.

The Cosmetic Ingredient Review Expert Panel has determined denatured alcohols to be safe for use in concentrations between 0.05% and 12% (depending on which denaturant is used).

Also...

This ingredient has antimicrobial and solvent properties.

The antimicrobial property helps preserve products and increase their shelf life. As a solvent, it helps dissolve other ingredients.

Look for formulas that contain glycerin, hyaluronic acid, or panthenol if you want to offset any drying effect.

This ingredient will trip away your skin's natural oils/lipids that help it lock in moisture. This can worsen dryness, trigger eczema flare-ups, and aggravate rosacea.

Be sure to patch test any product with this ingredient if you have dry or sensitive skin, eczema, or rosacea.

Learn more about Alcohol Denat.

Butylene Glycol

Humectant, Masking, Skin Conditioning

Butylene Glycol (or BG) is used within cosmetic products for a few different reasons:

Overall, Butylene Glycol is a safe and well-rounded ingredient that works well with other ingredients.

Though this ingredient works well with most skin types, some people with sensitive skin may experience a reaction such as allergic rashes, closed comedones, or itchiness.

Learn more about Butylene Glycol

Citronellol

Perfuming

Citronellol is used to add fragrance/parfum to a product. It is often derived from plants such as roses. In fact, it can be found in many essential oils including geranium, lavender, neroli, and more. The scent of Citronellol is often described as "fresh, grassy, and citrus-like".

Since the Citronellol molecule is already unstable, Citronellol becomes irritating on the skin when exposed to air.

Citronellol is a modified terpene. Terpenes are unsaturated hydrocarbons found in plants. They make up the primary part of essential oils.

Citronellol is not able to be absorbed into deeper layers of the skin. It has low permeability,

Citronellol is also a natural insect repellent.

Learn more about Citronellol

Citrus Aurantium Bergamia Fruit Oil

Masking, Perfuming

Citrus Aurantium Bergamia Fruit Oil is the oil from the bergamot orange and is primarily used as a fragrance. It has a "fresh" and "bright orange" scent.

The main aroma compounds found in this ingredient are limonene (~27-52%), linalool (~2-22%), and linalyl acetate (~27-40%). These are known EU fragrance allergens.

The term 'fragrance' is not regulated in many countries. In many cases, it is up to the brand to define this term. For instance, many brands choose to label themselves as "fragrance-free" because they are not using synthetic fragrances. However, their products may still contain ingredients such as essential oils that are considered a fragrance.

When used topically, Citrus Aurantium Bergamia Fruit Oil is a photosensitizer due to the furanocoumarin content. Furanocoumarins absorb UV-A and cause phytophotodermatitis; this can look like redness, blistering, and lasting brown pigmentation on sun-exposed skin.

Due to this, this ingredient is capped at 0.4% in leave-on products applied to sun exposed skin.

Many modern formulas used a "furanocoumarin-free" version that sidesteps the phototoxicity issue, but still contains the fragrance allergens.

Learn more about Citrus Aurantium Bergamia Fruit Oil

Geraniol

Perfuming, Tonic

Geraniol is used to add fragrance/parfum to a product. It is the main component of citronellol. It is a monoterpenoid and an alcohol.

Monoterpenes are naturally found in many parts of different plants.

Geraniol can be found in many essential oils including Rose Oil and Citronella Oil. The scent of Geraniol is often described as "rose-like". Many foods also contain Geraniol for fruit flavoring.

Geraniol can irritate the skin when exposed to air. However, irritation depends on the ability of geraniol to penetrate into the skin. In general, geraniol is not able to penetrate skin easily.

Geraniol is colorless and has low water-solubility. However, it is soluble in common organic solvents.

Like citronellol, it is a natural insect repellent.

2,6-Octadien-1-ol, 3,7-dimethyl-, (2E)-

Learn more about Geraniol

Glycerin

Humectant, Skin Conditioning, Skin Protecting

Glycerin (or glycerol) is a compound naturally found in your skin. It's a powerhouse humectant that pulls water into the stratum corneum.

Topically, glycerin does several things at once:

Your skin makes glycerin on its own (mostly from sebaceous oil breakdown) and shuttles it to your outermost layer of skin, or your epidermis, via aquaporin-3.

Aquaporin-3 is a transporter that is essential for normal skin hydration, elasticity, and repair. Interestingly, mice lacking in AQP3 have dry and less elastic skin that can be fully corrected with glycerin.

This ingredient is non-irritating, plays well with almost every ingredient, and works across all skin types. Typical use is anywhere between 3-10% but can go up to 79% in some leave-on products.

Just know very high concentrations (>40%) can feel tacky in low humidity.

Glycerin is the name for this ingredient in American English. British English uses Glycerol/Glycerine.

Learn more about Glycerin

Hydrogenated Lecithin

Emulsifying, Skin Conditioning

Hydrogenated Lecithin is a more stable version of lecithin.

It's made by taking lecithin (a phospholipid commonly found in soybeans and egg yolks) and hydrogenating it. This just means the unsaturated fatty acids are turned into saturated ones so they don't go bad as easily.

This ingredient is an emollient, emulsifier, and penetration enhancer. As an emollient, it helps soften and hydrate skin by trapping moisture within. As an emulsifier, it prevents oil and water ingredients from separating.

Hydrogenated Lecithin can form tiny spherical structures made of phospholipid bilayers called liposomes. These liposomes are able to capture compounds inside their structure and deliver them through the skin barrier.

Because phospholipids are a natural component of our cell membranes, this ingredient is inherently compatible with skin.

A 2021 study found lecithin-based surfactants were less harsh and more tolerable comared to Sodium Lauryl Sulfate (SLS).

Learn more about Hydrogenated Lecithin

Limonene

Perfuming, Solvent

Limonene is a fragrance that adds scent and taste to a formulation.

It's found in the peel oil of citrus fruits and other plants such as lavender and eucalyptus. The scent of limonene is generally described as "sweet citrus".

Limonene acts as an antioxidant, meaning it helps neutralize free radicals.

When exposed to air, oxidized limonene may sensitize the skin. Because of this, limonene is often avoided by people with sensitive skin.

Learn more about Limonene

Linalool

Perfuming

Linalool is a fragrance and helps add scent to products. It's derived from common plants such as cinnamon, mint, citrus, and lavender.

Like Limonene, this ingredient oxidizes when exposed to air. Oxidized linalool can cause allergies and skin sensitivity.

This ingredient has a scent that is floral, spicy tropical, and citrus-like.

Learn more about Linalool

Protease

Exfoliating, Skin Conditioning

Protease is an enzyme that works as a gentle exfoliant by mimicking something your skin already does naturally.

Your skin uses proteolytic enzymes to carry out desquamation; this is the process of shedding dead skin cells from the stratum corneum.

In skincare, proteases act as biological catalysts that mimic this natural desquamation process. You can think of it as giving your skin's own renewal system a nudge.

By breaking down the bonds that hold dead skin cells together, proteases help accelerate cell turnover to:

One reason they're becoming a popular AHA alternative is because proteases are considered effective while also being well-tolerated on skin. Because they work at a protein level rather than by lowering the skin pH, they can be a good option for those sensitive to AHAs.

Available in vitro and in vivo studies show positive exfoliant results but clinical (human) trials specifically on enzymatic exfoliation are still limited at this time.

Just one thing worth noting: temperature, pH, and stabilization are important factors that affect enzyme activity. The formulation quality definitely matters with this ingredient.

This ingredient can be either microbial sourced or plant-derived (papain from papaya, bromelain from pineapple).

Learn more about Protease

Rosa Damascena Flower Oil

Masking, Perfuming, Skin Conditioning

Rosa Damascena Flower Oil is an essential oil made from the Damask Rose. It is often used as a fragrance in cosmetics.

Rose Oil has antibacterial and antioxidant properties due to its terpene, glycoside, flavonoid, anthocyanin, and Vitamin C content.

Other major parts of Rose Oil include citronellol and geraniol. Both of these are known EU allergens and cause contact-allergies.

The downsides of this ingredient outweight the positives.

Learn more about Rosa Damascena Flower Oil

Serine

Masking, Skin Conditioning

Serine is a non-essential amino acid (your body makes it on its own!). It is a major player in your skin's natural moisturizing factor (NMF), or the water-loving molecules in your outermost layer of skin (stratum corneum) that keeps everything hydrated and happy.

Serine is one of your NMF's most abundant components that works as a skin-identical humectant. Its hydroxyl group grabs onto water molecules to boost hydration without any heaviness or occlusion.

Research on a hydrogel with serine confirmed this serine got delivered to your stratum corneum and demonstrated enhanced skin moisturization.

Interestingly serine also helps your skin produce filaggrin, a protein that keeps your skin barrier strong and used to create collagen.

Learn more about Serine

Tocopherol

Antioxidant, Masking, Skin Conditioning

Tocopherol is a fat-soluble antioxidant known as Vitamin E.

You'll find this ingredient in the vast majority of skincare (for good reason). It works to neutralize free radicals, or unstable molecules generated by UV exposure, pollution, and other environmental stressors, before they can cause oxidative damage to your skin cells.

Topically applied tocopherol has been shown to protect against UV damage by ramping up the skin's own natural defense enzymes.

It also acts as a skin conditioning agent; some studies show that regular topical use can improve the skin's water-binding capacity over 2-4 weeks.

This ingredient is especially loved for being a team player. When combined with Vitamin C, the photoprotective effect of both ingredients roughly doubles and the combo also helps reduce UV-induced DNA damage.

This ingredient has some brightening potential but it's more of a prevention ingredient than spot-fader. Cell studies show it can slow down melanin production but it's worth noting that it's not the most powerful brightener out there.

In formulations, it also serves as a stabilizer that helps protect other oxidation-prone ingredients from degrading.

Concentrations usually range from 0.1-1% in most leave-on products.

Learn more about Tocopherol

Vinegar

In cosmetics, vinegar is often used to help adjust the pH of a product. It is also a light exfoliant. The pH of your skin is important to maintain a healthy skin barrier.

Vinegar is created by the process of double-fermentation. It is an aqueous solution consisting mostly of water and acetic acid. Typically, vinegar contains anywhere from 5-8% acetic acid.

Other components include small amounts of contains small amounts of tartaric acid and citric acid. Depending on what is used to ferment the vinegar, it can have traces of flavoring as well.

Learn more about Vinegar

Water

Skin Conditioning, Solvent

Water. It's the most common cosmetic ingredient of all. You'll usually see it at the top of ingredient lists, meaning that it makes up the largest part of the product.

So why is it so popular? Water most often acts as a solvent - this means that it helps dissolve other ingredients into the formulation.

You'll also recognize water as that liquid we all need to stay alive. If you see this, drink a glass of water. Remember to stay hydrated!

Learn more about Water