Wrinkle Collagen Cream
Ingredients overview
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The Skin House Wrinkle Collagen CreamIngredients explained
Good old water, aka H2O. The most common skincare ingredient of all. You can usually find it right in the very first spot of the ingredient list, meaning it’s the biggest thing out of all the stuff that makes up the product.
It’s mainly a solvent for ingredients that do not like to dissolve in oils but rather in water.
Once inside the skin, it hydrates, but not from the outside - putting pure water on the skin (hello long baths!) is drying.
One more thing: the water used in cosmetics is purified and deionized (it means that almost all of the mineral ions inside it is removed). Like this, the products can stay more stable over time.
Butylene glycol, or let’s just call it BG, is a multi-tasking colorless, syrupy liquid. It’s a great pick for creating a nice feeling product.
BG’s main job is usually to be a solvent for the other ingredients. Other tasks include helping the product to absorb faster and deeper into the skin (penetration enhancer), making the product spread nicely over the skin (slip agent), and attracting water (humectant) into the skin.
It’s an ingredient whose safety hasn’t been questioned so far by anyone (at least not that we know about). BG is approved by Ecocert and is also used enthusiastically in natural products. BTW, it’s also a food additive.
- A natural moisturizer that’s also in our skin
- A super common, safe, effective and cheap molecule used for more than 50 years
- Not only a simple moisturizer but knows much more: keeps the skin lipids between our skin cells in a healthy (liquid crystal) state, protects against irritation, helps to restore barrier
- Effective from as low as 3% with even more benefits for dry skin at higher concentrations up to 20-40%
- High-glycerin moisturizers are awesome for treating severely dry skin
It seems to us that squalane is in fashion and there is a reason for it. Chemically speaking, it is a saturated (no double bonds) hydrocarbon (a molecule consisting only of carbon and hydrogen), meaning that it's a nice and stable oily liquid with a long shelf life.
It occurs naturally in certain fish and plant oils (e.g. olive), and in the sebum (the oily stuff our skin produces) of the human skin. As f.c. puts it in his awesome blog post, squalane's main things are "emolliency, surface occlusion, and TEWL prevention all with extreme cosmetic elegance". In other words, it's a superb moisturizer that makes your skin nice and smooth, without being heavy or greasy.
Another advantage of squalane is that it is pretty much compatible with all skin types and skin conditions. It is excellent for acne-prone skin and safe to use even if you have fungi-related skin issues, like seborrhea or fungal acne.
The unsaturated (with double bonds) and hence less stable version of Squalane is Squalene, you can read about it here >>
Cyclomethicone is not one type of silicone, but a whole mixture of them: it's a mix of specific chain length (4 to 7) cyclic structured silicone molecules. (There seems to be a confusion on the internet whether Cyclomethicone and Cyclopentasiloxane are the same. They are not the same, but Cyclopentasiloxane is part of the mixture that makes up Cyclomethicone).
All the silicones in the Cyclomethicone mixture are volatile, meaning they evaporate from the skin or hair rather than stay on it. This means that Cyclomethicone has a light skin feel with none-to-minimal after-feel. It also makes the formulas easy to spread and has nice emollient properties.
The famous or maybe rather infamous mineral oil. The clear oily liquid that is the "cheap by-product" of refining crude oil and the one that gets a lot of heat for its poor provenance. It is a very controversial ingredient with pros and cons and plenty of myths around it. So let us see them:
The pros of mineral oil
Trust us, if something is used for more than 100 years in cosmetic products, it has advantages. Chemically speaking, cosmetic grade mineral oil is a complex mixture of highly refined saturated hydrocarbons with C15-50 chain length. It is not merely a "by-product" but rather a specifically isolated part of petroleum that is very pure and inert.
It is a great emollient and moisturizer working mainly by occlusivity. Occlusivity is one of the basic mechanisms of how moisturizers work and it means that mineral oil sits on top of the skin and hinders so-called trans-epidermal water loss, i.e water evaporating out of your skin. When compared to heavy-duty plant oil, extra virgin coconut oil, the two of them were equally efficient and safe as moisturizers in treating xerosis, a skin condition connected to very dry skin.
The other thing that mineral oil is really good at is being non-irritating to the skin. The chemical composition of plant oils is more complex with many more possible allergens or irritating components, while mineral oil is simple, pure and sensitivity to it is extremely rare. If you check out the classic French pharmacy brands and their moisturizers for the most sensitive, allergy prone skin, they usually contain mineral oil. This is no coincidence.
The cons of mineral oil
The pros of mineral oil can be interpreted as cons if we look at them from another perspective. Not penetrating the skin but mostly just sitting on top of it and not containing biologically active components, like nice fatty acids and vitamins mean that mineral oil does not "nourish" the skin in the way plant oils do. Mineral oil does not give the skin any extra goodness, it is simply a non-irritating moisturizer working mainly by occlusivity.
The myths around mineral oil
Badmouthing mineral oil is a favorite sport of many, it is a cheap material and being connected to petrolatum makes it fairly easy to demonize.
While it is true that industrial grade mineral oil contains carcinogenic components (so-called polycyclic compounds), these are completely removed from cosmetic and food grade mineral oil and there is no scientific data showing that the pure, cosmetic grade version is carcinogenic.
What is more, in terms of the general health effects of mineral oils used in cosmetics, a 2017 study reviewed the data on their skin penetration and concluded that "the cosmetic use of mineral oils and waxes does not present a risk to consumers due to a lack of systemic exposure."
Another super common myth surrounding mineral oil is that it is comedogenic. A 2005 study titled "Is mineral oil comedogenic?" examined this very question and guess what happened? The study concluded that "based on the animal and human data reported, along with the AAD recommendation, it would appear reasonable to conclude that mineral oil is noncomedogenic in humans."
Overall, we feel that the scaremongering around mineral oil is not justified. For dry and super-sensitive skin types it is a great option. However, if you do not like its origin or its heavy feeling or anything else about it, avoiding it has never been easier. Mineral oil has such a bad reputation nowadays that cosmetic companies hardly dare to use it anymore.
It’s the - sodium form - cousin of the famous NMF, hyaluronic acid (HA). If HA does not tell you anything we have a super detailed, geeky explanation about it here. The TL; DR version of HA is that it's a huge polymer (big molecule from repeated subunits) found in the skin that acts as a sponge helping the skin to hold onto water, being plump and elastic. HA is famous for its crazy water holding capacity as it can bind up to 1000 times its own weight in water.
As far as skincare goes, sodium hyaluronate and hyaluronic acid are pretty much the same and the two names are used interchangeably. As cosmetic chemist kindofstephen writes on reddit "sodium hyaluronate disassociates into hyaluronic acid molecule and a sodium atom in solution".
In spite of this, if you search for "hyaluronic acid vs sodium hyaluronate" you will find on multiple places that sodium hyaluronate is smaller and can penetrate the skin better. Chemically, this is definitely not true, as the two forms are almost the same, both are polymers and the subunits can be repeated in both forms as much as you like. (We also checked Prospector for sodium hyaluronate versions actually used in cosmetic products and found that the most common molecular weight was 1.5-1.8 million Da that absolutely counts as high molecular weight).
What seems to be a true difference, though, is that the salt form is more stable, easier to formulate and cheaper so it pops up more often on the ingredient lists.
If you wanna become a real HA-and-the-skin expert you can read way more about the topic at hyaluronic acid (including penetration-questions, differences between high and low molecular weight versions and a bunch of references to scientific literature).
An odorless and colorless emollient ester (cetyl alcohol + ethylhexanoic acid) that gives a velvety and silky feel to the skin. It has great spreadability and a non-oily feel. It's a popular ingredient in makeup removers.
The golden yellow oil coming from the Macadamia nut, a native Australian nut. Similar to other plant oils, it's loaded with emollient and nourishing fatty acids. It's a high oleic acid oil (50-67% oleic acid and only 0-5% linoleic acid) that makes it very emollient and ideal for dry skin types.
Its unique property is that it contains high amounts of a rare fatty acid called palmitoleic acid (12-25%) that give Macadamia oil a "cushiony" feel. It's also easily absorbed and makes the skin soft and supple.
Probably the most common silicone of all. It is a polymer (created from repeating subunits) molecule and has different molecular weight and thus different viscosity versions from water-light to thick liquid.
As for skincare, it makes the skin silky smooth, creates a subtle gloss and forms a protective barrier (aka occlusive). Also, works well to fill in fine lines and wrinkles and give skin a plump look (of course that is only temporary, but still, it's nice). There are also scar treatment gels out there using dimethicone as their base ingredient. It helps to soften scars and increase their elasticity.
As for hair care, it is a non-volatile silicone meaning that it stays on the hair rather than evaporates from it and smoothes the hair like no other thing. Depending on your hair type, it can be a bit difficult to wash out and might cause some build-up (btw, this is not true to all silicones, only the non-volatile types).
A super common emollient that makes your skin feel nice and smooth. It comes from coconut oil and glycerin, it’s light-textured, clear, odorless and non-greasy. It’s a nice ingredient that just feels good on the skin, is super well tolerated by every skin type and easy to formulate with. No wonder it’s popular.
An extremely common multitasker ingredient that gives your skin a nice soft feel (emollient) and gives body to creams and lotions. It also helps to stabilize oil-water mixes (emulsions), though it does not function as an emulsifier in itself. Its typical use level in most cream type formulas is 2-3%.
It’s a so-called fatty alcohol, a mix of cetyl and stearyl alcohol, other two emollient fatty alcohols. Though chemically speaking, it is alcohol (as in, it has an -OH group in its molecule), its properties are totally different from the properties of low molecular weight or drying alcohols such as denat. alcohol. Fatty alcohols have a long oil-soluble (and thus emollient) tail part that makes them absolutely non-drying and non-irritating and are totally ok for the skin.
A common multi-tasker fatty acid. It makes your skin feel nice and smooth (emollient), gives body to cream type products and helps to stabilize water and oil mixes (aka emulsions).
A white, elastomeric silicone powder that gives a nice silky and powdery feel to the products. It also has some oil and sebum absorption capabilities.
A super common, waxy, white, solid stuff that helps water and oil to mix together, gives body to creams and leaves the skin feeling soft and smooth.
Chemically speaking, it is the attachment of a glycerin molecule to the fatty acid called stearic acid. It can be produced from most vegetable oils (in oils three fatty acid molecules are attached to glycerin instead of just one like here) in a pretty simple, "green" process that is similar to soap making. It's readily biodegradable.
It also occurs naturally in our body and is used as a food additive. As cosmetic chemist Colins writes it, "its safety really is beyond any doubt".
A very common water-loving surfactant and emulsifier that helps to keep water and oil mixed nicely together.
It's often paired with glyceryl stearate - the two together form a super effective emulsifier duo that's salt and acid tolerant and works over a wide pH range. It also gives a "pleasing product aesthetics", so no wonder it's popular.
The chemically chopped up version of the big protein molecule, collagen. It is often derived from fish or bovine sources and works as a nice moisturizer and humectant that helps the skin to hold onto water.
To understand a bit more what Hydrolyzed Collagen is, you have to know that proteins are large chains of amino acids connected with so-called peptide bonds. These bonds can be broken up when a water molecule is added and the resulting thing is a mix of shorter length amino acids, also called peptides. So Hydrolyzed Collagen is not really collagen, it is rather an undefined and varying mix of largish peptides. Based on a manufacturer's data, the whole, soluble collagen has an average molecular weight of 300 000 Da, while this chopped up mixture has an average MW of 12 000 Da (still pretty big).
The main thing of these largish peptides is to act as water-binding agents, and to make the skin nice and smooth (aka emollient). Hydrolyzed Collagen is also often used in cleansers as it can make harsh surfactants milder and in hair conditioners as it improves the flexibility and manageability of hair.
If you wanna know more about collagen in cosmetics, we have a shiny explanation about soluble collagen here >>
A sugar beet-derived amino acid derivative with nice skin protection and moisturization properties. Betaine's special thing is being an osmolyte, a molecule that helps to control cell-water balance. It is also a natural osmoprotectant, meaning that it attracts water away from the protein surface and thus protects them from denaturation and increases their thermodynamic stability.
It also gives sensorial benefits to the formula and when used in cleansers, it helps to make them milder and gentler.
A corn sugar derived, water-soluble, pale yellow syrup, that nicely moisturizes the skin. It has a light and smooth skin feel, it is non-tacky, and it can improve the after-feel of the formula. It is also mild and gentle, popular in sensitive skin formulas.
A silicone emulsifier that helps water and silicone oils to mix nicely together. It can also be used together with plant oil + silicone oil mixtures.
A common little helper ingredient that helps water and oil to mix together, aka emulsifier.
A handy helper ingredient that helps water and oil to mix nicely together, aka emulsifier. It is especially recommended for protective, baby care and general purpose emollient creams.
It also helps to disperse insoluble particles (think color pigments or zinc/titanium dioxide sunscreen) nice and even in cosmetic formulas.
It's the yellow, solid stuff that you probably know from beeswax candles. It's a natural material produced by honey bees to build their honeycomb.
As for skincare, it's used as an emollient and thickening agent. It's super common in lip balms and lipsticks.
A big molecule created from repeated subunits (a polymer of acrylic acid) that magically converts a liquid into a nice gel formula. It usually has to be neutralized with a base (such as sodium hydroxide) for the thickening to occur and it creates viscous, clear gels that also feel nice and non-tacky on the skin. No wonder, it is a very popular and common ingredient. Typically used at 1% or less in most formulations.
A popular, vegetable-derived oil-loving emulsifier that helps water to mix with oil. In itself, it is suitable for water-in-oil emulsions (where water droplets are dispersed in oil), but it is more often used as a co-emulsifier next to other, water-loving emulsifiers.
Chemically speaking, it comes from the attachment of sorbitan (a dehydrated sorbitol (sugar) molecule) with the fatty acid Stearic Acid, that creates a partly water (the sorbitan part) and partly oil soluble (stearic part) molecule.
A superabsorbent polymer (big molecule from repeated subunits) that has crazy water binding abilities. Sometimes its referred to as "waterlock" and can absorb 100 to 1000 times its mass in water.
As for its use in cosmetic products, it is a handy multi-tasker that thickens up water-based formulas and also has some emulsifying and emulsion stabilizing properties.
It's a petroleum derived emollient and thickener. It often comes to the formula as part of an emulsifier, thickener trio (with Polyacrylamide and Laureth-7). This trio is an easy-to-use liquid that helps to create nice, non-tacky gel formulas.
A not-very-interesting helper ingredient that is used as an emulsifier and/or surfactant. Comes from a coconut oil derived fatty alcohol, lauryl alcohol.
It's the chemically chopped up version of normal lecithin. Most often it's used to create liposomes and to coat and stabilize other ingredients.
The famous omega-6 fatty acid, the mother of all ω-6 fatty acids in our body. It is a so-called polyunsaturated fatty acid meaning it has more than one (in this case two) double bonds and a somewhat kinky structure that makes LA and LA-rich oils a thin liquid.
It is also an essential fatty acid meaning our body cannot synthesize it and has to take it from food. This is not hard at all as plenty of nuts (such as flax, poppy or sesame seeds) and vegetable oils (such as sunflower or safflower) are rich in LA. The hard thing seems to be eating enough omega-3-s, more specifically eating a healthy ratio of omega-6 to omega-3, but that is a topic for a what-is-good-to-eat-site and not for us.
As for linoleic acid and the skin, LA is a really important little guy found naturally in our skin. It is the most abundant fatty acid in the epidermis and it serves as a structural precursor for important skin lipids called ceramides. Knowing this, it will not come as a surprise that Linoleic acid has a central role in the structure and function of stratum corneum permeability, aka healthy skin barrier. LA deficiency leads to an impaired more permeable skin barrier and the topical application of LA-rich sunflower oil can fix this issue rapidly (while oleic-rich olive oil did not have the same barrier repairing effect).
LA is not only important for dry, barrier damaged skin types but also for acne-prone skin. Research shows that problem skin has lower levels of linoleic acid (and higher levels of oleic acid) than normal skin. So LA-deficiency in the skin seems to be connected not only to an impaired skin barrier but also to acne and smearing LA all over your face might help with your problem skin. A double-blind study using a 2.5% LA gel for 4 weeks found a 25% reduction in the size of microcomedones, the tiny blocked pores that can later lead to acne.
If that was not enough, we have one more thing to report about LA. It lightens hyperpigmentation (aka UVB caused sun spots) both by blocking the melanin production of melanocytes (the skin cells that make the pigment melanin) and by enhancing the desquamation of melanin pigment from the upper layers of the skin.
Overall, linoleic acid is a multi-functional skin goodie with barrier repairing, acne-reducing, and skin-lightening magic abilities. It's a nice one to spot on the ingredient list pretty much for any skin type.
One of the many types of ceramides that can be found naturally in the upper layer of the skin. Ceramides make up about 50% of the goopy stuff that's between our skin cells and play a super important role in having a healthy skin barrier and keeping the skin hydrated. It works even better when combined with its pal, Ceramide 1.
We wrote way more about ceramides at ceramide 1, so click here to know more.
We don't have description for this ingredient yet.
The extract coming from the juice containing leaves of the Aloe vera plant. It's usually a hydroglycolic extract (though oil extract for the lipid parts also exists) that has similar moisturizing, emollient and anti-inflammatory properties as the juice itself. We have written some more about aloe here.
A semi-essential (infants cannot synthesize it, but adults can) amino acid that is one of the primary building blocks of hair keratin and skin collagen. It's a natural moisturizing factor, a skin hydrator and might also help to speed up wound healing.
Arginine usually has a positive charge (cationic) that makes it substantive to skin and hair (those are more negatively charged surfaces) and an excellent film former. Thanks to the positive charge, it also creates a complex with AHAs (AHAs like to lose a hydrogen ion and be negatively charged, so the positive and the negative ions attract each other) that causes a "time-release AHA effect" and reduces the irritation associated with AHAs.
An easy-to-formulate, commonly used, nice to have ingredient that’s also called pro-vitamin B5. As you might guess from the “pro” part, it’s a precursor to vitamin B5 (whose fancy name is pantothenic acid).
Its main job in skincare products is to moisturise the skin. It’s a humectant meaning that it can help the skin to attract water and then hold onto it. There is also research showing that panthenol can help our skin to produce more lovely lipids that are important for a strong and healthy skin barrier.
Another great thing about panthenol is that it has anti-inflammatory and skin protecting abilities. A study shows that it can reduce the irritation caused by less-nice other ingredients (e.g. fragrance, preservatives or chemical sunscreens) in the product.
Research also shows that it might be useful for wound healing as it promotes fibroblast (nice type of cells in our skin that produce skin-firming collagen) proliferation.
If that wasn’t enough panthenol is also useful in nail and hair care products. A study shows that a nail treatment liquide with 2% panthenol could effectively get into the nail and significantly increase the hydration of it.
As for the hair the hydration effect is also true there. Panthenol might make your hair softer, more elastic and helps to comb your hair more easily.
We don't have description for this ingredient yet.
We don't have description for this ingredient yet.
Super common soothing ingredient. It can be found naturally in the roots & leaves of the comfrey plant, but more often than not what's in the cosmetic products is produced synthetically.
It's not only soothing but it' also skin-softening and protecting and can promote wound healing.
Adenosine is an important little compound in our body that has a vital cell-signalling role. Research on smearing it on our face is also promising and shows so far a couple of things:
- It can help with wound healing
- It’s a good anti-inflammatory agent
- It might even help with skin’s own collagen production and improve skin firmness and elasticity
- It helps with barrier repair and protection
- It might be even useful for the hair helping with hair thickness and hair growth
Super common little helper ingredient that helps products to remain nice and stable for a longer time. It does so by neutralizing the metal ions in the formula (that usually get into there from water) that would otherwise cause some not so nice changes.
It is typically used in tiny amounts, around 0.1% or less.
We don't have description for this ingredient yet.
We don't have description for this ingredient yet.
We don't have description for this ingredient yet.
We don't have description for this ingredient yet.
We don't have description for this ingredient yet.
We don't have description for this ingredient yet.
If you are into the K-Beauty trend, you must have bumped into snail slime like a thousand times. Korean brands love the ingredient and tout it for its miraculous repair and hydration properties. It's claimed to be able to repair everything from dry patches, acne breakouts to signs of aging and we are happy to say that it might be just true.
So snail slime is the yucky stuff that snails (in cosmetics the secretion of Cornu Aspersum, the garden snail is used) produce when they are in stress (it's not the same as the one they secret to be able to move nicely and smoothly). As the cosmetic chemists at the Beauty Brains blog write, "chemically speaking, snail slime is a complex mixture of proteoglycans, glycosaminoglycans, glycoprotein enzymes, hyaluronic acid, copper peptides, antimicrobial peptides and trace elements including copper, zinc, and iron." English translation equals it's loaded with a bunch of good-for-the-skin stuff.
As for scientific proof that snail slime does something for the skin, we did find a couple of studies to go by. A Spanish radiation oncologist, Rafael Abad Iglesias MD discovered for the first time that snail mucin can be used to treat radiation dermatitis (skin irritation caused by radiotherapy, a form of cancer treatment). He did a clinical study with 100 patients and reported a "statistically significant clinical improvement in erythema, itching and burning pain" in the group treated with snail slime.
A 2007 study examined the molecular basis for the great regeneration properties of the ingredient. It found that snail slime (SS) indeed does a bunch of positive things that could be the reason for its great repair abilities. First, it has serious antioxidant properties thanks to two great antioxidant enzymes, superoxide dismutase and glutathione s-transferase. Second, SS induces fibroblast proliferation, that's particularly important during wound healing. Third SS was also found to downregulate MMP, an evil enzyme that's out there to destroy skin-firming collagen. These properties add up to give SS not only wound healing and regenerative properties but also serious anti-aging potential.
Regarding anti-aging, a 2013 study in the Journal of Drugs in Dermatology examined the effect of SS on photoaged skin. The 14-week, double-blind, 25 participant study found that "there was a significant degree of improvement in fines lines", though the participants did not report a significant difference in the quality of their skin.
All in all, we think that snail slime definitely deserves the skin goodie status it already enjoys in the K-beauty space. If you have no problem with somewhat strange, yucky things or animal derived ingredients in your products, it's worth a try.
Ci 19140 or Tartrazine is a super common colorant in skincare, makeup, medicine & food. It’s a synthetic lemon yellow that's used alone or mixed with other colors for special shades.
FDA says it's possible, but rare, to have an allergic-type reaction to a color additive. As an example, it mentions that Ci 19140 may cause itching and hives in some people but the colorant is always labeled so that you can avoid it if you are sensitive.
We don't have description for this ingredient yet.
It’s pretty much the current IT-preservative. It’s safe and gentle, but even more importantly, it’s not a feared-by-everyone-mostly-without-scientific-reason paraben.
It’s not something new: it was introduced around 1950 and today it can be used up to 1% worldwide. It can be found in nature - in green tea - but the version used in cosmetics is synthetic.
Other than having a good safety profile and being quite gentle to the skin it has some other advantages too. It can be used in many types of formulations as it has great thermal stability (can be heated up to 85°C) and works on a wide range of pH levels (ph 3-10).
It’s often used together with ethylhexylglycerin as it nicely improves the preservative activity of phenoxyethanol.
If you have spotted ethylhexylglycerin on the ingredient list, most probably you will see there also the current IT-preservative, phenoxyethanol. They are good friends because ethylhexylglycerin can boost the effectiveness of phenoxyethanol (and other preservatives) and as an added bonus it feels nice on the skin too.
Also, it's an effective deodorant and a medium spreading emollient.
Exactly what it sounds: nice smelling stuff put into cosmetic products so that the end product also smells nice. Fragrance in the US and parfum in the EU is a generic term on the ingredient list that is made up of 30 to 50 chemicals on average (but it can have as much as 200 components!).
If you are someone who likes to know what you put on your face then fragrance is not your best friend - there's no way to know what’s really in it.
Also, if your skin is sensitive, fragrance is again not your best friend. It’s the number one cause of contact allergy to cosmetics. It’s definitely a smart thing to avoid with sensitive skin (and fragrance of any type - natural is just as allergic as synthetic, if not worse!).
Linalool is a super common fragrance ingredient. It’s kind of everywhere - both in plants and in cosmetic products. It’s part of 200 natural oils including lavender, ylang-ylang, bergamot, jasmine, geranium and it can be found in 90-95% of prestige perfumes on the market.
The problem with linalool is, that just like limonene it oxidises on air exposure and becomes allergenic. That’s why a product containing linalool that has been opened for several months is more likely to be allergenic than a fresh one.
A study made in the UK with 483 people tested the allergic reaction to 3% oxidised linalool and 2.3% had positive test results.
A common fragrance ingredient that smells like jasmine. It is one of the “EU 26 fragrances” that has to be labelled separately because of allergen potential. Best to avoid if your skin is sensitive.
A super common and cheap fragrance ingredient. It's in many plants, e.g. rosemary, eucalyptus, lavender, lemongrass, peppermint and it's the main component (about 50-90%) of the peel oil of citrus fruits.
It does smell nice but the problem is that it oxidizes on air exposure and the resulting stuff is not good for the skin. Oxidized limonene can cause allergic contact dermatitis and counts as a frequent skin sensitizer.
Limonene's nr1 function is definitely being a fragrance component, but there are several studies showing that it's also a penetration enhancer, mainly for oil-loving components.
All in all, limonene has some pros and cons, but - especially if your skin is sensitive - the cons probably outweigh the pros.
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