How many hours can spf20 protect you? How to read spf and pa values ​​and how to choose

There are relatively few spf20 sunscreens on the market, but fortunately they are cheaper than spf50. So how many hours can spf20 provide sun protection? How much UV light can spf20 filter? How to choose the appropriate SPF value and PA value and how to look at PA? Let’s take a look together below.

How many hours does spf20 protect

According to a widely circulated algorithm, SPF20 means that the product can provide sun protection for 5 hours. Because this is a widely circulated saying. SPF20 means that the skin's ability to protect against the sun's rays is extended 20 times. If you have average Asian skin color, you will be sunburned in about 15 minutes. Applying SPF 20 sunscreen will extend your skin's ability to protect against sunburn by 20 times. According to the algorithm, that is: 20*15 minutes=300 minutes=5 hours. However, since this theory is subject to many external factors, SPF 20 sunscreen products cannot provide protection for 5 hours.

How much UVB rays can spf20 absorb

SPF 20 = (20-1)/20*100% = 95%, which means about 5% of UVB can reach the skin;

Compared with sunscreen products with an SPF value of 50, SPF 50 = (50-1)/50*100% = 98%, which means that only 2% of UVB can reach the skin;

From this point of view, in fact, the ability of spf20 and spf50 to protect against UVB rays is similar.

What does the PA value of sunscreen mean?

In addition to the SPF value, the PA value on the packaging of sunscreen products is also confusing. In fact, PA value is the abbreviation of Protection Grade of UVA, and “+” indicates the product’s ability to defend against UVA. The more “+” signs there are, the better the protection effect is, and it is less likely to get tanned or photoaged. Studies in recent years have shown that UVA can penetrate into the dermis of the skin, causing greater damage to the skin. Therefore, when choosing sunscreen, you must look at the PA value. PA+: Delays tanning time by 2 to 4 times. PA++: Delays tanning time by 4 to 8 times. PA + + +: Delays tanning time by more than 8 times. China has adopted this method to express the sun protection ability of UVA ultraviolet rays. Currently, most qualified sunscreen products on the market are marked with these two sun protection indicators: SPF and PA.

How to choose the appropriate SPF value and PA value

Generally speaking, if it is cloudy or you are indoors, you can choose SPF15, PA+ products. When doing outdoor sports or being exposed to strong sunlight for a short period of time, such as when commuting to get off work, it is necessary to choose products with SPF30, PA++. When exercising outdoors for long periods of time in the summer or in plateau areas, it is best to choose a sunscreen with the strongest possible sun protection, such as SPF30+, PA+++, and apply it every 1 or 2 hours to effectively protect your skin from UV damage.

How often should I reapply spf 20 sunscreen?

Reapply every 2 hours indoors

When using SPF 20 sunscreen indoors, reapply every 2 hours. This can prevent the ultraviolet rays from fluorescent lamps and windows from damaging your skin.

Reapply every 40 minutes for outdoor use

If you really use SPF 20 sunscreen indoors, it is best not to spit every 40 minutes. If you find it troublesome, you can change to a sunscreen with a slightly higher SPF value and more PA values. Therefore, the intensity of ultraviolet rays outdoors is relatively high, and only reapplication will ensure that the sunscreen's sun protection effectiveness continues to be online.