Most sunscreen products are marked with SPF or PA, SPF(Sun Protection Factor) is a numerical value showing the sunscreen effect to prevent UVB damage, and PA(Protection UVA) is an indicator of the degree to which UVA is prevented. SPF is simply a sunscreen factor. After SPF20, 20 is a sunscreen coefficient, which means that the numerical value of SPF sunscreen coefficient is suitable for everyone. The calculation method is: assuming that the intensity of ultraviolet rays will not change with time, a person without any sun protection measures will turn red after 20 minutes in the sun. When he uses SPF20 sun protection products, it means that the time can be extended by 20 times, that is, his skin will turn red after 400 minutes. Therefore, the larger the number behind SPF, the longer the sun protection time. If it is a long-term outdoor activity, you should choose SPF30 or so. If you don't spend a long time outdoors in the sun, you can choose SPF20 or SPF 15 sunscreen. Because sunscreen with a large sun protection coefficient (such as SPF30 or higher) has a long sun protection time, However, the burden on the skin will also increase. PA++refers to the degree to which UVA is prevented. Previously, sunscreen products only had SPF value, but there was no PA value. Later, due to the constant warnings from dermatologists, it was emphasized that although UVA is not easy to sunburn the skin, it will cause skin aging and lesions, so PA labeling has attracted more and more attention. Today's sunscreen products are all marked with PA value. PA's defense effect is divided into three levels, namely, PA+, PA++, PA++,PA++ means effective, PA++means quite effective and PA++means very effective. Therefore, the SPA value is the value of sunburn and sunburn, and the PA value is the value of skin aging. If SPF20 and PA++ appear on the package of a bottle of sunscreen, it means that this bottle of sunscreen can effectively protect against the sun within 400 minutes, and its defense effect against UVA is quite effective.