Why and How to Mix Sunscreens

Question: Why should I mix sunscreens?

Answer: There are several plausible scenarios where mixing a sunscreen with another skin care product would yield a desirable outcome. The reasons can usually be placed under two broad categories: a cosmetic or formulary concern. A cosmetic concern would be if for example, you’d like the sunscreen to mattify more quickly or to be more compatible under foundation. The reasons that fall under this category tend to stem from personal preference. A formulary concern would be if for example, you’d like more antioxidant protection or if the existing formula is not emollient enough. The reasons that fall under this category tend to stem from necessity, which is dictated by science or personal experience.

Whatever the reason is, the combination of the two or more hypothetical products need to satisfy the following prerequisites in order of importance: provide adequate UVA- and UVB-protection, adequate antioxidant protection, and is aesthetically pleasing and compatible with foundation, assuming you wear some form of “cover-up.” The ideal scenario would be if a single sunscreen could satisfy all of these requirements. I have yet to find one, so if anyone knows of any, let me know!

Okay, so it’s obvious why we need a sunscreen with UVA and UVB protection: to prevent wrinkles, various types of skin cancers, hyperpigmentation, etc…  What most people don’t realize is that antioxidant protection is also vital because it neutralizes and prevents the UV-induced damage that the sunscreen misses. No amount or type of sunscreen can provide complete (100%) protection from UV rays. Furthermore, antioxidants like vitamins C and E have been shown to enhance the efficacy of chemical sunscreens like avobenzone. However, the mechanisms by which antioxidants neutralize UV rays and enhance chemical sunscreens are beyond the scope of this post. I have a dedicated page titled “Sun Protection,” for that discussion along with other material because “sun protection” is too massive and broad a topic to cover in a single article. I don’t have a timetable on when exactly that will be completed, but hopefully it will be soon.

Question: How do I mix sunscreens?

Answer: Like I said above, the three requirements necessary for a good sunscreen are:

• Adequate UVA and UVB protection;
• Adequate antioxidant protection; and is
• Aesthetically pleasing and compatible with foundation.

Before getting to the logistics, let me just recommend to not even think about mixing sunscreens that are chemically based, meaning the ones that contain ingredients like avobenzone, oxybenzone, or homosalate. This is because these ingredients are very sensitive and can give vastly fluctuating degrees of sun protection based on several factors such as: type of vehicular based used, interactions with other chemical sunscreens, interactions with metal oxides, and inherently variable concentration-to-efficacy ratios. It would just be too difficult for the average consumer that doesn’t have access to a chemical lab, to reliably create a new and dependable formulation. Therefore, this post will only discuss the mixing of physical sunscreens like zinc oxide and titanium dioxide. There are several other reasons why I prefer physical sunscreens of chemical ones, but that’s for the ”Sun Protection” page.

Step 1: Make sure the two products you’re trying to mix are primarily mineral-based sunscreens.

Why do we do this? Like I said above, chemical sunscreens are out of the question. But why not use another product that isn’t a sunscreen, such as an antioxidant-rich serum or a makeup primer? Here’s why. By mixing two sunscreens, the one with the lower sunscreen rating will guarantee a base level of sun protection at that rating. And since we’re using physical sunscreens, we know that it’s easier to reliably calculate the level of sun protection. Physical sunscreens are  inert minerals that aren’t affected too much by vehicular bases and have inherently stable concentration-to-efficacy ratios.

Step 2: Make sure one of the products contains large amounts of antioxidants proven to provide photoprotection such as: vitamins C and E, green tea, silymarin (derived from the milk thistle plant), and pomegranate.

Like I said above, antioxidants are essential to a good sunscreen because they provide an additional layer of protection.

Step 3: Mix ”X” parts of sunscreen 1 (SUN1) with ”Y” parts of sunscreen 2 (SUN2) (X:Y)

For example, mix (1:1), or (1:3). Whatever ratio you decide, keep in mind the THREE requirements that I enumerated above and the order of importance. Should you encounter a dilemma where you’re balancing the amount of sun protection against the how well it wears under foundation, I’d suggest finding that happy medium (not necessarily 1:1), that still satisfies the THREE requirements.

So how do you reliably calculate how much sun protection you’re getting from your personal concoction? Use the Sunscreen Simulator, one of the links displayed on the homepage, to calculate approximately how much sun protection you will be provided. I’d personally recommend an SPF rating of at least 20 (blocks 98% of UVB rays) and a UVA-PF rating of at least 10 (blocks 90% of UVA rays). However, an SPF rating of 15 (blocks 93% UVB rays) and a UVA-PF rating of 7.5 (blocks 87% UVA rays), while appearing signifcantly less potent, is actually acceptable as well. It’s not that big of a difference, really. Besides, the most important form of sun protection is unavoidance.

For those of you who are interested, I’ve created a very elementary algebraic system to use:

• A1 = Concentration of Sunscreen Type 1 in SUN1
• B1 = Concentration of Sunscreen Type 2 in SUN1
• A2 = Concentration of Sunscreen Type 1 in SUN2
• B2 = Concentration of Sunscreen Type 2 in SUN2
• X = Parts Present in A3
• Y = Parts Present in B3
• A3, B3 = Concentrations of Sunscreen Types 1, 2 in SUN3; these numbers are what you enter into the respective fields on the above-mentioned link

A3 = A1 x (X/(X + Y)) + A2 x (Y/(X +Y))

B3 = B1 x (X/(X + Y)) + B2 x (Y/(X +Y))

Let’s do an example just so everything is glass clear. (Get it? Haha!)

Here’s what we know:

• SUN1 contains 9% Titanium Dioxide and 5% Zinc Oxide.
• SUN2 contains 7% Titanium Dioxide and 2% Zinc Oxide.
• We’re going to mix 1 part of SUN1 with 2 parts of SUN2.

A3 = 9 x (1/(1 + 2)) + 7 x (2/(1 +2)) = 7 2/3 or ~7.667%

B3 = 5 x (1/(1 + 2)) + 2 x (2/(1 +2)) = 3%

So, we’d enter 7.667 into the titanium dioxide field, and 3 into the zinc oxide field.

Now, if you’re actually doing this with me, you’ll see that there’s separate fields for the oil and water phases of each ingredient. Here’s where it gets complicated. Both ingredients are mostly insoluble in water and therefore, in most creams and lotions (which exist as water-in-oil or oil-in-water emulsions) tend to naturally be in the oil phase. However, manufacturers have been able to have them them present in the water phase by creating water- and alcohol-based formulations. Why this matters is because mineral sunscreens tend to provide more protection in their oil phases. And how can you know what phase your particular product is in? Unfortunately, there’s no simple guideline to being sure what ingredients are in what phase. I mean, even with the water- and alcohol-based formulations, couldn’t the water and oil evaporate and all you’re left with are the physical sunscreen ingredients, which have typically been coated with a silicone to prevent free radical damage? Doesn’t that make them part of the oil phase, now that they are present in the absence of water? What about when you go swimming? It’s all quite complicated, and I don’t have a simple answer. I’ll be doing a more detailed and conclusive post about emulsions when I obtain more information and evidence.

For our scenario today, you can use the two phases to calculate the range of sun protection you would be getting; the water phase would show the bottom threshold, while the oil phase would dictate the upper.

• Protection of SUN3 in the water phase: SPF = 12.6, UVA-PF = 6.8
• Protection of SUN3 in the oil phase: SPF = 17.2, UVA-PF = 8.5

Note that the amount applied should be about 3.0 mg/cm^2 or 0.1 oz/ft^2, just to be safe.

I hope that helped everyone and if you guys have any questions, as always, I’m here to answer them!