Core Viewpoint - The effectiveness and safety of sunscreen products decrease significantly after being opened for more than a year, with SPF values potentially dropping by 30% to 50% [2] Group 1: Sunscreen Stability and Expiration - Sunscreen has a shelf life of 2 to 3 years when unopened, but this changes once opened, as active ingredients degrade due to light, temperature, and oxygen exposure [2] - After being opened for over a year, the actual SPF value of a sunscreen labeled SPF 50 could be reduced to between 25 and 35 [2] - Sunscreens that have been opened for a long time may have an increased microbial count, leading to potential skin allergies [2] Group 2: Proper Usage and Application - The standard application amount for sunscreen is 2 mg/cm², with a typical adult face requiring about 1 to 1.2 grams to achieve optimal protection [4] - Sunscreen should be reapplied every 2 to 3 hours, especially after sweating or swimming, as even "water-resistant" products only maintain effectiveness for 40 to 80 minutes [4] - Choosing the right sunscreen based on skin type is crucial; sensitive skin should use physical sunscreens, while oily skin may prefer lighter formulations [4] Group 3: Storage and Cleaning - Sunscreen should be stored in a cool, dry place away from direct sunlight to maintain its effectiveness [3] - Proper evening cleansing is important; regular sunscreens can be washed off with face wash, while waterproof sunscreens require specialized makeup removers [5] Group 4: Interaction with Other Products - Using insect repellent in conjunction with sunscreen may significantly reduce the sunscreen's effectiveness against UVB rays [6] - Research indicates that the combination of sunscreen and insect repellent leads to increased absorption of certain chemicals, potentially compromising the protective effects of the sunscreen [6]