Determination of Water Resistance
In vivo sunscreen testing involves photo-testing human subjects after applying sunscreen at an internationally agreed thickness of 2 mg/cm2 body surface. In vivo photo-testing is used to determine water resistance. A “water-resistant” claim can be made for a sunscreen if the SPF is maintained after two 20-minute water immersions. Remember, this SPF rating refers only to protection against UVB radiation. Sunscreens that do not meet the above criteria following water immersion are not considered “water-resistant”.
The current (new) FDA regulations address confusion in this area of water resistance. For years, sunscreen labels have contained the words “waterproof” or “sweatproof” which overstate the effectiveness of these products and leave the consumer with a false sense of security. New labeling requirements only allow the use of the term “water resistant”, and must clearly state the duration of effectiveness until reapplication is required. “Water-resistance” labeling must indicate whether the sunscreen is effective for 40 minutes or 80 minutes while swimming or sweating. Sunscreens which are not water resistant must include a recommendation advising consumers to use a “water-resistant” sunscreen if they are swimming or sweating.
Important note: Several studies indicate that in real-world conditions the application thickness of sunscreen more likely approximates 0.5-1.0 mg/cm2 (as opposed to the recommended 2 mg/cm2), thereby lowering the effective SPF of the product. Thus, if a SPF 30 sunscreen is applied at half the recommended thickness, the resulting effective SPF is only 15.