Even when a sunscreen is effective, it's often not used correctly. When the FDA tests sunscreens for SPF ratings (which the agency only guarantees to SPF 15) it slathers on the cream to an exact thickness of 2 grams per centimeter squared (2 g/cm2), but studies have documented that real-world application practices of sunscreen products typically are closer to 0.5 g/cm2 according to U.S. Pharmacist a national clinical journal for pharmacists.
If a less-than-recommended thickness is used (about 3 tablespoons of sunscreen for an adult in a bathing suit) the actual SPF value of the product may be drastically diminished.
A consumer could use an SPF 30 product, but if a lesser thickness is applied, the product may actually be closer to SPF 15. "When consumers apply too little sunscreen or reapply it infrequently – and that’s more common than not – sunscreens can cause more free radical damage than UV rays on bare skin," according to the EWG.
In other words, wearing too little sunscreen offers a false sense of protection and may be worse than wearing none at all.