Skin Deep

Sunscreen sun protection factor (SPF) testing methods explained

May is skin cancer month, so you will see reports from various sources about sunscreens that may confuse you. This year, I decided to try to educate you before these reports are released, so you can be a better judge of what they are saying.

This is the first of a two-part series on sunscreen safety. This column will discuss sun protection factor (SPF) testing methods, and my next column will discuss “hazardous sunscreen ingredients.”

Environmental Working Group ( is perhaps best known for its annual sunscreen report, which is released each May around Memorial Day. This report attracts media attention and sometimes scares consumers, resulting in a flurry of emails to me. For example, in May 2016, the big story was that nearly 75 percent of the sunscreens they tested failed to deliver the level of protection claimed on the label or contained “worrisome” ingredients.

In May 2017, Consumer Reports came out with a report that said, “Of the 62 lotions, sprays, sticks, and lip balms in our ratings this year, 23 tested at less than half their labeled SPF number.” According to the CR website, the study was performed as follows: “To check for UVB protection, a standard amount of each sunscreen is applied to six places on our panelists' backs. Then they soak in a tub of water. Afterward, each of those areas is exposed to six intensities of UVB light from a sun simulator for a set time. About a day later, the six spots are examined for redness. The resulting UVB protection ratings reflect each product's actual effectiveness after water immersion and are based on an average of our results for each sunscreen.”

Dr. Leslie Baumann.jpg
Dr. Leslie Baumann

It is important to understand how the FDA requires that SPF testing be done when interpreting these reports. The FDA requires that 2 mg/cm2 of SPF be applied to a subject’s back, and the size of the test site must be at least 30 cm2. It is required that 10 different test sites are used. After application of the SPF, there is a 15-minute waiting period. The skin is exposed to six sessions of UVB exposure (the dose depends on the patient’s Fitzpatrick Skin Type). The minimal erythema dose (MED) is the smallest UV dose that produces perceptible redness of the skin (erythema) with clearly defined borders. In the FDA studies, the MED is measured at 16 to 24 hours after UV exposure. The person who evaluates the test subsites should not be the same person who applied the sunscreen drug product to the test site or administered the UV doses. This prevents bias. The MED is used to determine the SPF.

If the company wants to claim that the sunscreen is water-resistant after 40 minutes, the following procedure is used:

  • Step 1: Apply the SPF 2mg/cm2 as described above

  • Step 2: Perform moderate activity in water for 20 minutes.

  • Step 3: Rest out of water for 15 minutes. Do not towel test site.

  • Step 4: Perform moderate activity in water for 20 minutes.

  • Step 5: Allow test sites to dry completely without toweling.

  • Step 6: Perform the UV testing as described above.

As you can see, there are very specific steps required by the FDA for SPF testing. This exact protocol should be followed when others are determining the SPF on the label of sunscreens so that they are consistent with the FDA testing. Otherwise, you are comparing apples to oranges.

The problem with the Consumer Reports 2017 study is that they immersed people in a tub of water before testing the sunscreen’s SPF, and they did not state how long the subjects were in the water and whether or not they toweled dry. Using a towel can greatly diminish SPF protection, as can water immersion.

Only water-resistant sunscreens should have their SPF values tested after water immersion. Any sunscreens that do not claim to be water-resistant should have been tested for their SPF value without prior water immersion as described above.

The Consumer Reports study design was misleading and made people distrust their sunscreen for no reason. Some sunscreens are just not designed to survive immersion in water.