I recently spent a few hours in beautiful, sunny Dolores Park in San Francisco. Being the responsible type (and wanting my tattoo to stay beautiful even though it was uncovered that day) I sprayed some sunscreen on my exposed shoulders, arms, and back.
But the next day I woke up with tight, itchy, burning red skin. I was so angry. I did everything right! I put on sunscreen, I even reapplied. How could this have happened? I began to suspect the spray-on sunscreen I used was somehow less effective than the lotion sunscreens I usually prefer.
So I did what I usually do when confronted with this type of question: I hopped on PubMed and got in touch with some experts. And, internet, I have answers for you.
Do spray-on sunscreens work differently from other formulations?
In a word: no.
Yes the sunscreen itself is just as useful as a lotion or roll-on. All sunscreens are tested to the same standard in the U.S., Stephen Ko Ma, president Mission of Kin, an Ontario-based skincare company, told me in an email. Under the FDA, the guideline is two milligrams of sunscreen per square centimeter of skin (2mg/cm). This standard was chosen because it provides the most reliable and reproducible results.
But most people dont put on nearly enough, Ko Ma said. Interestingly, one study found that participants got closest to the magic 2mg/cm number with spray-on sunscreen. Researchers had 52 participants use lotion, roll-on, and spray-on sunscreens. They got to an average of 1.6mg/cm with the spray-on types, 1.1mg with the lotion, and a measly 0.35mg with the roll-on.
And you might think that applying your SPF 30 or higher sunscreen at those ever-so-close to 2mg/cm levels is good enough, but another study says youre wrong. Researchers had 40 subjects use sunscreens at different application levels. They found that with higher SPF sunscreens (between 30-55), the protection went up exponentially as the subjects approached the 2mg mark. In other words, spraying at 1mg thickness conferred less than half the sunburn protection as making it to 2mg. Another study found that the protection followed a logarithmic curve, and yet another found a linear relationship between sun protection and thicknessso its not totally clear what the relationship is. One thing is clear though: If you are not applying 2mg/cm, youre not applying enough.
OK I get it, but how do I know when Ive hit 2mg/cm2?
Excellent question! Its not exactly easy to tell. Even Ko Ma was at a loss:
Spray sunscreens can be deceptive because there’s no guide to see how much sunscreen has been put on. Different nozzles, the texture of the sunscreen, and the propellant used will all affect the delivery rate – which is how much product is dispensed per time interval. So one can of sunscreen may deliver 0.2 mg of sunscreen per second, and another may deliver 0.5 mg of sunscreen per second. Manufacturers likely know the delivery rate, but I’ve never seen it on a consumer label. As well I’m not even sure if this information would be useful! Most people don’t know the area of their face, arm, etc…I certainly don’t.
He added that going through the trouble of actually discovering this information is so laborious that it probably negates the convenience of spray-on sunscreens. Lotion sunscreens are a bit easier since you can actually measure it out easily. His recommendations, based on a letter to the editor in the open access Dermatology Online Journal, are to use a beer bottle cap full of lotion. You should use half a capful should be enough to protect the face and neck, one capful per arm, two capfuls for the back, two capfuls for the front of your torso, and two capfuls for each leg.
I am stubbornly resolute in my commitment to the spray-on sunscreen, what should I do?
In short, according to the same study that found a linear relationship between coverage and sun protection: Buy a very high SPF (like 70+) and apply it twice, letting it dry between layers.
Ko Ma even recommends the double application of sunscreen of any type, just to be sure youre really covered.
Additionally, according to the American Academy of Dermatology (AAD), you should also be checking that your sunscreen is both broad spectrum (covers UVB and UVA rays) and water resistant. Water-resistant sunscreen is especially important when youll be sweating a lot or hanging out in a humid climatenot just for when you go swimming at the pool or beach. And, sorry, but water- and sweat-resistance usually only lasts 40 to 80 minutes. That information should be on the label.
To make it easier for you, the AAD recently reviewed the 10 most popular sunscreens on Amazon.com and found that four of them didnt make their cut for appropriate sun protection. (The article is freely available online. Check table 1 for their summarized review.) This should not be confused with not achieving the FDAs regulations for sunscreen, by the way. The AAD has their own standards that are separate from the governments regulations.
Lastly, I asked Ko Ma about combining makeup and sunscreen. Ive always been curious, if I want to wear foundation the same day Im wearing sunscreen, whats the best way to apply it?
There really is no good research on this topic, unfortunately. Applying the sunscreen first, and then the foundation may cause sunscreen to shift around on the skindisrupting protection, he said. Applying the foundation first, then the sunscreen may cause the sunscreen to dry unevenly on the skinagain disrupting protection.
He said when he applies foundation over sunscreen, he does so very delicately and only after the sunscreen has had plenty of time to dry and absorb into the skin. But you might want to consider just going without, or using a foundation that has SPF 30 or higher and being willing to re-apply it like you would sunscreen.
No matter what, be sure youre always checking your skin for new or changing moles. The AAD recommends doing it every year on your birthday, and be sure to talk to your doctor immediately about any rapidly changing, darkening, itchy, or bleeding moles. Skin cancer is extremely common, but also extremely treatable if you catch it early.
Have a burning science question? Ask me. firstname.lastname@example.org.