Ever since I started reading about skin care, I always see people address serums and moisturizers as if they were distinct and separate types of products; same goes for moisturizers with SPF and sunscreens. Therefore, I’m writing this post to explain why they’re both inaccurate, and how a better understanding of these concepts can be of assistance as you attempt to comprehend and construct the ideal routine.
Moisturizers versus Serums
- Most people define a moisturizer as any heavier non-silicone-based leave-on product that prevents the skin from becoming dry.
- On the other hand, a serum is defined as a lightweight silicone-based leave-on product whose purpose is to deliver high concentrations of beneficial ingredients to the skin.
Because dry skin or xerosis is characterized by decreased water content in the stratum corneum (SC), the main purpose of a leave-on product is to prevent further loss, which is known as transepidermal water loss (TEWL). Therefore, the main component of a leave-on product is some type of occlusive agent, which will form a superficial barrier that is to an extent, impermeable to water. To further increase the water content of the SC, humectants are often included to bind to excess water, and deliver it into the skin. And since most leave-on products contain a lot of water, there’s plenty to go around. Finally, leave-on products may also contain emollients, whose purpose is to proliferate through and fill in the cracks of the SC, in order to soften and smooth the appearance and texture of the skin.
Okay, we’ve addressed the characteristics necessary to prevent the skin from becoming dry. But do the additions of silicones and other beneficial ingredients differentiate moisturizers from serums? Fortunately, that is easily answered. Silicones are just other types of occlusive agents; think of them as lighter forms of petrolatum. And high concentrations of beneficial ingredients like antioxidants can and should be included in all leave-on products, not just “serums.”
So is there any characteristic that is exclusive to either moisturizers or serums? Nope.
A moisturizer is a leave-on product that contains occlusive agents, and may also contain humectants, emollients, and other beneficial ingredients. A serum is a type of moisturizer that tends to have a lighter texture. But they’re essentially the same thing: a serum can function as a moisturizer for oilier skin types.
Moisturizers with SPF versus Sunscreens
I don’t actually know how people (in their minds) differentiate moisturizers with SPF from sunscreens. But I always see people give this distinction, without giving an explanation. For example, a reader recently commented that:
“… HOWEVER, the LRP is actually a MOISTURIZER with SPF rather than a straight sunscreen. So I was wondering if the PCA Sunscreen you recommend is moisturizing as well or would it require an additional moisturizer?…”
Now that we’ve defined what a “moisturizer” is, a moisturizer with SPF is therefore just a leave-on product that contains occlusive agents AND UV filters, and may also contain humectants, emollients, and other beneficial ingredients. What about sunscreens? I honestly don’t know what to say, except that sunscreens are the exact same thing. They can certainly be “moisturizing.” In fact, a common complaint is that “sunscreens,” especially those that contain inorganic UV filters like titanium dioxide and/or zinc oxide, are TOO moisturizing, greasy, emollient, and/or heavy. So I don’t understand how this distinction was imagined in the first place. I mean, anything with an SPF rating is measured the same way; a “moisturizer” with an SPF of 20 and a “sunscreen” with an SPF of 20, will both provide the same initial level of UVB protection.
Moisturizers with SPF and sunscreens are the exact same thing: leave- on products that contain occlusive agents AND UV filters, and may also contain humectants, emollients, and other beneficial ingredients.
So how can this newfound knowledge help you? Here’s an example. Another reader recently wrote this comment:
“…I’m a little concerned the Glycolix Facial Cream would prevent the BHA from penetrating, but I’m told applying moisturizer within several minutes of cleansing is key…”
Basically, the reader is hesitant on me asking her to apply a BHA product and then to wait around 30 minutes before applying the Glycolix Facial Cream, because she’s been told that you HAVE to apply moisturizer within several minutes after cleansing.
Now, dermatologists (and others) recommend applying a moisturizer a few minutes after cleansing in order to reduce TEWL. The longer you wait, the more water will be lost to the atmosphere.
But here’s the thing. Because salicylic acid is volatile, it requires some type of carrier that isn’t water (though water is necessary; think pH), in order to inhibit the rate of evaporation. But in doing so, the carrier will also act as an occlusive agent for the skin; thereby reducing TEWL. And if we go back to our revised definition of a moisturizer: “a leave-on product that contains occlusive agents, and may also contain humectants, emollients, and other beneficial ingredients,” a salicylic acid product would satisfy the requirements necessary for it to be a “moisturizer.” Why? It’s because it contains occlusive agents, which will reduce TEWL. Yes, it may not be a very good moisturizer, but it will do perfectly fine for the 30 minutes or so wait time that I recommended before applying another product.
See how handy these re-imagined definitions can be?
Anyways, feel free to share your thoughts and any of YOUR newfound insights and/or realizations in the comment section below.
Thanks for reading.
P.S. Thank you everyone for your support in the past week! The last post on the interaction between niacinamide and L-ascorbic acid is now in 1st place for being the most-commented non-giveaway post in the history of FutureDerm! That’s right; it’s first out of 1,272 posts! And we achieved this honor in less than a week! Yes, almost half of those comments were my responses to other people… but whatever. I’ve also never had two of my posts be the 1st and 2nd most popular ones of the week. Nice job everyone!
John Su describes himself as eccentric—you might find him having a conversation with himself. He’s a stickler for accuracy, so you might find him correcting one thing or another! His goal is to answer questions and provide unbiased, meaningful, and insightful information when it comes to skin care. His underlying motivations stem from a need to inform people who have doubts, questions, or even prayers for solutions to their problems. He has his own skin care blog, The Triple Helixian.View all John Su posts.
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