It is not true that the longer of the mask is applied, the better. When the mask is applied, our skin and mask will form a closed environment. The hydration of the stratum corneum will be strengthened. The longer the mask is applied, the excessive hydration will result. The skin's barrier will be damaged then it lead the skin to become more sensitive, and it will pull dry after being applied a long time. Therefore, it must be operated according to the instructions, usually 15-20 minutes is enough.