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Understanding Sunscreen: What You Need to Know About SPF and PA


I have written several articles on sun protection, and I believe that by now everyone has some knowledge about the basics. Today, I would like to discuss sun protection factors (SPF). Even if you understand the ingredients in sunscreens, it is important to know the actual benefits of a particular product.

Most people have heard of SPF and believe that the higher the number, the better the protection. However, SPF only measures the protection against UV-B rays and does not necessarily relate to UV-A protection. Furthermore, a high SPF does not guarantee 100% protection from UV-B rays. In ideal conditions, an SPF 30 sunscreen should block 96.6% of UV-B rays. It is important to note that most people apply less than the recommended 2mg/cm2 standard amount of sunscreen (averaging only 0.5-1.25mg/cm2), which leads to products not achieving their expected results. Therefore, applying the recommended amount of sunscreen is crucial. Additionally, all sunscreens (regardless of SPF) need to be reapplied every two to three hours, as their effectiveness does not last all day.

SPF refers to the sun protection factor. In simple terms, it refers to how long it takes for the skin to be damaged by the sun’s UV rays on a sunny day (excluding factors such as the intensity of the sun, time, and skin color, which clearly affect how long it takes to get sunburned). For example, if you use an SPF 15 sunscreen, it will take 15 hours for sunburn to appear if it usually takes an hour without any protection. Some people believe that an SPF 30 sunscreen provides twice the protection of an SPF 15 sunscreen. However, the actual difference in protection is only about 3%, as SPF 15 blocks 93% of UV-B rays, while SPF 30 blocks 97%.

Regarding the current protection level of sunscreens against UV-A rays, there is no internationally recognized standard as there is for UV-B with SPF. Different countries have different methods of calculation, mainly because UV-A rays can penetrate the dermis and cause chronic and cumulative damage to the skin. Therefore, it is difficult to establish a universally recognized standard. However, there are three general measurement methods: in vitro, in vivo, and skin immune cell suppression capacity evaluation.

The most commonly used measurement method is PA value, which is a Japanese Ministry of Health, Labour and Welfare (MHLW) standardized method for evaluating the degree of protection against UV-A rays. The PA value is divided into three levels: PA+, PA++, and PA+++, with the more plus signs indicating a higher level of protection. PA+ delays the time for skin tanning by 2-4 times, PA++ delays the time by 4-8 times, and PA+++ delays the time by approximately 8 times or more.

When selecting sunscreen products, it is important to consider both the UV-A and UV-B protection levels.