Sunscreen is one of those items you may buy casually and store in many places around your home—after all, it is a summertime essential that you don't want to be caught without. But sometimes, there comes a point where you realize you have a half-dozen nearly empty bottles across your closets, cabinets, bags, and car. You think there's probably a full bottle between them, so you ask yourself, "Can I mix two different sunscreens together?"

As it turns out, some experts caution that this may not be the safest option for skin protection. The ingredients in sunscreens can differ from product to product, and whether you mix them in the bottle or on your skin, combining certain ingredients can dilute the overall sun protection factor (SPF) of both sunscreens.

To understand why, it's important to know a little more about how sunscreens work and the types of ingredients that make them effective.

Chemical vs. Physical Sunscreen Ingredients

Many sunscreens contain at least some combination of two ingredient types:

  • Physical sunscreen ingredients include zinc oxide and titanium dioxide. These agents give sunscreens a physical barrier from the sun, which is why they typically rub on with a whitish color.
  • Chemical sunscreen ingredients include oxybenzone, octinoxate, and homosalate. These agents, among others, give sunscreens a different kind of sun protection power by helping the skin absorb and dissipate the sun's rays as heat.

If two products you want to mix both contain only physical ingredients, you'll probably be just fine using two sunscreens. But, if they each contain even a small amount of a chemical ingredient—as many sunscreens do—it's not advisable to combine the two.

When You Can't Mix Sunscreens...

Think back to your high school chemistry class. The science of chemistry involves a careful balance of substances to achieve desired results. If just one compound in a mixture is off, it could cause unexpected results—the same is true when it comes to the effectiveness of your sunscreen. So, if you mix two sunscreens that each have chemical ingredients together, they could interact and dilute each other, undermining the sun protection that each would give individually. Unfortunately, you probably won't be able to tell until your skin has already started to burn.

However, that doesn't mean you can't use up your half-empty bottles. Just keep them separate and use each leftover product for one sunscreen application (and remember to reapply sunscreen every two hours, or sooner if you get wet or sweaty). If you don't have enough in one bottle for a full application, consider using them on different body parts. For example, if you're going out and your clothes will mostly cover you, try using the last of a bottle to get your face. It's best to avoid blending separate sunscreens together on the same area of your body during an application.

...and When You Can

If the two sunscreens you're using contain the same ingredient profile—such as those that might come from the same manufacturer—it may be okay to layer products atop each other throughout the day. For example, you might use the EltaMD UV Shield Broad-Spectrum SPF 45 as a base layer, and then touch it up with the super handy EltaMD UV Stick Broad-Spectrum SPF 50+.

Again, multiple physical-only formulas should work together as long as they are all within their use-by dates.

Just keep in mind that SPF numbers don't compound, so adding two sunscreens with an SPF 45 won't double-up your protection power. Instead, expect to get no more protection than the lowest SPF number listed on the bottles—or potentially lower if you mix two sunscreens with chemical ingredients.

So, what's the verdict? Avoid mixing your sunscreens if possible, especially if those products contain chemical ingredients. It's the only way to ensure you're maintaining the careful chemical balance inside each bottle and getting the level of protection that the manufacturer promises.

The safest bet is to pick up a fresh bottle—you'll never regret having another to throw in a new bag or lend to a friend.