Sunscreen needs to be applied repeatedly to be more effective. Because the sunscreen effect of sunscreen can't last for a day once. Sunscreen needs to be replenished every two hours. When it is outdoors for a long time, ultraviolet radiation will oxidize sunscreen and air. Once the sunscreen is completely oxidized, the skin will not be protected. At this time, you will naturally get a tan or sunburn. So sunscreen must be applied every few hours, otherwise it won't work. The frequency with which you apply sunscreen will be related to the present environment. If you are an office worker who works indoors, you can usually make it up again at noon. If you are an outdoor person, you need to rub it every 2-3 hours, because sunscreen will oxidize more ultraviolet rays and gradually lose its sunscreen effect. People who are in the swimming pool or need to be active at the seaside usually have to make up for it every time they come out of the swimming pool or the sea. Although most sunscreens are waterproof now, they will be washed once after coming out of the water, which will reduce the effect of sunscreens.