CR uses a testing protocol that is modeled on the one the Food and Drug Administration (FDA) requires sunscreen manufacturers to use. But as is the case with other products we test that have government or industry standards, we use those standards as benchmarks and develop our own methodology to identify differences in performance and give consumers a comparative evaluation.

We test only sunscreens with a listed SPF of at least 30 and are water-resistant (for 40 or 80 minutes, the two time periods the FDA permits water-resistance claims for). We buy the sunscreens for our tests off the shelf, the way consumers would and use three samples, preferably with different lot numbers, of each product.

Each sunscreen is rated on three criteria: SPF, variation from SPF, and UVA protection. In addition, CR's trained sensory panel evaluates the scent and skin feel of the products.

SPF is a relative measure of how long a sunscreen will protect you from sunburn. Usually the number is explained as the amount of time it takes an individual's skin to burn when it's covered in sunscreen compared with when it's not. For example, if you'd normally burn after 20 minutes in the sun without sunscreen, an SPF 30 protects for 600 minutes, or 10 hours (assuming you apply and reapply correctly). But intensity and wavelength distribution of UVB rays vary throughout the day and by location. And no matter what SPF you use, sunscreen needs to be reapplied every two hours, or immediately when you get out of the water.

We also calculate a score for variation from SPF. This is a measure of how closely a sunscreen's tested SPF matched the SPF on the label.

The SPF number gives you no information on how well a sunscreen protects against UVA rays. That's why you need to look for a broad-spectrum sunscreen, which protect against both UVA and UVB. However, no sunscreen blocks 100 percent of UVA or UVB rays. The breakdown: SPF 30 blocks 97 percent of UVB rays, SPF 50 blocks 98 percent, and SPF 100 blocks 99 percent.



Alas, there are no trips to Tahiti for our sunscreen panelists—they go to a lab. To check for SPF, a standard amount of each sunscreen is applied a 2x3-inch rectangle on our panelists' backs. Then they soak in a tub of water. Afterward, each of those areas is exposed to five or six intensities of ultraviolet light from a sun simulator for a set time. About a day later, the six spots are examined for redness. The resulting SPF ratings reflect each product’s effectiveness after water immersion and are based on an average of our results for each sunscreen.

The UVA test we use allows us to differentiate the degree of UVA protection among sunscreens. To test for UVA, we smear sunscreen onto plastic plates and pass UV light through and measure the amount of UVA and UVB rays that are absorbed. That information is then used to calculate our UVA score.