First, apply sunscreen 20 minutes before sun exposure.
Laboratory testing of sun protection factors involves applying sunscreen samples to artificial skin membranes and waiting 20 minutes before measurement.
This is because sunscreen agents must penetrate the skin's epidermal stratum corneum to effectively absorb and reflect UV rays over time. This penetration process typically requires approximately 20 minutes.
Second, apply at least 0.6ml of sunscreen to your face each time.
A sunscreen product's SPF rating is determined based on an application rate of 2μL/cm².
This means if you apply less than this amount, your skin won't receive the full protection indicated by the product's SPF label—the actual protection will be significantly reduced!
What does 2μL/cm² mean? Since we can't achieve the precision of a thin-layer applicator in practice, let's calculate a rough total volume for practical use. Assuming facial skin covers 300 cm² (you can measure this roughly with a ruler—don't forget the nose area has three-dimensional contours), 2μL/cm² × 300 cm² = 600μL = 0.6ml.
Thus, a 30ml bottle of sunscreen can cover the face approximately 50 times. If reapplied only once daily, it lasts about 25 days.