We happened to have a bag of zinc oxide powder on hand, so we decided to give it a go. Smith doesn’t share the exact proportions of each ingredient, but we eyeballed it and whisked the emollients together before adding a generous amount of zinc oxide powder — the only ingredient in the list that offers protection from the sun, according to Yolanda C. Holmes, a D.C.-based dermatologist and fellow at the American Academy of Dermatology.

“These ingredients can be good moisturizers for the skin, but that doesn’t mean they will protect you against the sun,” she said. “And using it could make you subject to sunburn.”

Holmes does not recommend homemade sunscreen because the ingredients could be irritants for some people, and “sunscreens should be scientifically tested in labs to make sure they are offering a certain level of sun protection.”

Most homemade sunscreens lack effective sun protection, said Daniel D. Bennett, a dermatologist and the secretary-treasurer of the American Academy of Dermatology. He said homemade sunscreens do not go through the rigorous testing required of commercial sunscreens by the Food and Drug Administration.

Making the sunscreen was simple enough. It took less than an hour to mix and an hour to set in the fridge. It was smooth to apply, smelled fresh and natural, and left as much of a white cast as most other sunscreens.

Once the homemade concoction set, we compared it to a lab-made SPF 50 sunscreen using purple-colored UV detection stickers. The round sticker, which is not a medical device, is stuck on skin and meant to turn transparent when covered with sunscreen that has sufficient UV protection.

The midmorning sun disappeared behind a cloud as soon as I stuck the stickers to my hand and arm. Then I applied the homemade sunscreen to my hand and the SPF 50 product to my arm.

When the sun returned, the sticker under the commercial sunscreen became transparent, while the one under the homemade creation remained purple.

The sticker’s packaging says that if it “doesn’t turn clear post-sunscreen application, SPF may be insufficient, expired, or not enough sunscreen has been applied.”

Holmes said that she can’t vouch for the science behind the stickers, but panned Smith’s recipe, saying “the ingredients in this homemade sunscreen are just not going to offer the sun protection you need.”