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 provide some coverage. 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 sun protection protection. 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, as well as adequate levels of antioxidant protection (if a separate product is not used).
Question: How do I mix sunscreens?
Answer: The requirements necessary for a good sunscreen are:
- Adequate UVA and UVB protection;
- Adequate antioxidant protection (if a separate product is not used); and is
- Aesthetically pleasing.
Before getting to the logistics, it’s important to note that it’s probably best to not mix sunscreens that contain organic UV filters, meaning the ones that contain ingredients like avobenzone, oxybenzone, or homosalate. This is because these ingredients are quite sensitive and can give largely 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 be rather 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.
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, organic sunscreens are a bit more temperamental compared to their inorganic counterparts. 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, it’s easier to reliably calculate the level of sun protection.
Step 2: 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.
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. Inorganic UV filters tend to always be coated, relegating them to the oil phase. There are exceptions, but that’s the general case.
For our scenario today, we’ll be using the oil phase to calculate the amount of sun protection you would be getting.
- Protection of SUN3 in the oil phase AND using the Real-Life rather than Clinical Calculations: SPF = 15.9, UVA-PF = 6.1.
Note that the amount applied should be about 2.0 mg/cm^2, the international standard.
I hope that helped everyone and if you guys have any questions, as always, I’m here to answer them!