1. SPF 50 Shows How Much UV Is Blocked, Not How Long You Can Stay Out
Many people might interpret "SPF 50" as meaning they can stay in the sun for 50 times longer. However, according to the Japan Cosmetic Industry Association, SPF is primarily an indicator of effectiveness against sunburn (reddening) caused by Ultraviolet B rays (UVB). A higher number indicates higher effectiveness [1]. Essentially, what is being measured is the amount of UV radiation one can be exposed to before turning red.
The reason this cannot be converted into time is that sunlight intensity changes with the season and time of day [2]. Even if you spend the same 10 minutes outside, the amount of UV you receive differs from day to day. Therefore, reading the number as a "multiplier of quantity" is more accurate.
2. How SPF Is Tested by Shining Lamps on People's Backs
The testing location is human skin on the back. UV light from a lamp is applied to areas with sunscreen and areas without, varying the intensity in 5 or 6 stages [2]. The next day, researchers look for the smallest amount of UV that causes faint redness. This is called the Minimal Erythema Dose (MED).
The SPF value is calculated by dividing the MED of the sunscreen-covered area by the MED of the uncovered area. In an example from the Ministry of the Environment's materials, the MED for the covered area was 14.1, and the uncovered area was 1.25. Dividing these gives 11.3, which is that person’s SPF. For actual product labeling, the average of results from more than 10 people is used [2].
3. What the "+" in PA Means: Protection Against Skin Darkening
Sunscreen containers often show "PA++++" alongside the SPF number. What does this "+" represent? PA is an indicator of effectiveness against Ultraviolet A rays (UVA), based on the reaction where skin darkens quickly. There are four levels, from PA+ to PA++++, with more plus signs indicating higher effectiveness [1].
PA++++ was a new level added in 2013; before that, PA+++ was the highest [5]. It is easier to organize this information by remembering that SPF measures the reaction of turning red, while PA measures the reaction of turning dark.
4. Who Sets the Rules for SPF and PA: Industry, Not the Government
Because these numbers seem so important, some might think they are regulations set by the government. In reality, the numerical values and testing methods for SPF and PA are voluntary industry standards established by the Japan Cosmetic Industry Association. The standard for measuring SPF took effect in January 1992, and the method for measuring and labeling UVA protection took effect in January 2000 [4]. Review papers also describe SPF values and PA labels as industry standards from this association [6].
Later, efforts moved toward aligning testing methods internationally. The international standard for measuring SPF took effect in 2007, and it was standardized as an ISO international standard in 2010 [4].
5. Why Applying Sunscreen Too Thinly Can Give Less Protection Than the Label
The SPF test uses a thick application of 2 mg per square centimeter of skin [2]. However, actual users tend to apply much thinner layers. An observational study during vacations (2012) reported that with an average application of 0.79 mg/cm², a product with SPF 15 was estimated to provide protection equivalent to about SPF 3 [7]. This is a researcher's estimate; the numbers do not necessarily drop in direct proportion to the amount applied.
6. How to Choose Sunscreen for Your Activity and Reapply Every 2-3 Hours
The Ministry of the Environment recommends choosing sunscreen based on how you will use it. For daily activities like hanging laundry or shopping, products with lower numbers are sufficient. For long periods outdoors, such as sports under direct sunlight or swimming at the seaside, choose products with high effectiveness. If you are entering water, products with high water resistance are suitable [2].
Apply sunscreen before going outside. For the face, take some on your palm and apply enough to cover an area roughly the size of a 1-yen coin placed flat, splitting it into two applications. Because sweat, hands, and clothing rub it off, reapplying every 2 to 3 hours is a good guideline [2]. As seen in the previous section, thin application reduces effectiveness, so amount and reapplication are just as important as the number on the label.
7. How to Read the Numbers on Your Own Sunscreen Bottle
If you have sunscreen at home, read the label. It may show the SPF value, the number of "+" signs for PA, and water resistance claims. Write these down and match them one by one with the "multiplier of redness amount" and "indicator of darkening reaction" explained in this article.
Comparing bottles within your family can help you think about what the differences in labels mean. However, there is no need to intentionally expose yourself to strong sunlight to test the sunscreen's effect. Read the containers at home and match the SPF and PA labels with the explanations in this text.