What does SPF 50 sunscreen mean?

On today's sunscreens often listed with an SPF index indicating the level of protection against the harmful effects of UVB rays. The lowest SPF is 15 and the highest is 100, the indicators are equivalent to the time of sun protection.

1.1. What is SPF? SPF stands for Sun Protection Factor and is a measure of the protection against UVB rays used in sunscreens. UVB rays are a type of ultraviolet light that burns the skin and causes some changes in the skin. This norm is calculated according to the number of hours and percentage when applying sunscreen on the skin.
SPF in sunscreen is at least 15 and the highest is 100. According to international standards, 1 SPF is capable of protecting skin and limiting the harmful effects of UVB rays for about 10 minutes.
1.2. Meaning of SPF index on sunscreen The meaning of SPF index is understood in two ways as follows:
Protection time: That is, multiply the SPF index on sunscreen by 10 to calculate the time (minutes) of protection. protects the skin against the harmful effects of UVB rays. For example, a sunscreen with an SPF of 30 will protect the skin from UVB rays for about 300 minutes. By percentage blocking UVB rays from reaching the skin: When placed in perfect conditions, a sunscreen with an SPF of 15 will block about 93.4% of damage from UVB rays, SPF 30 is about 96.7 %, SPF 50 is about 98%. However, this ratio is achieved under perfect conditions, so it can vary with environmental conditions.

According to the interpretation of the meaning of SPF, SPF 50 sunscreen means:
The time to resist the harmful effects of UVB rays when used on the skin is about 500 minutes. The ability to block the penetration of UVB rays into the body is up to 98%. It is possible, however, that time works. However, this rate can be relative over a certain period of time. It may not take up to 500 minutes, depending on environmental conditions... Usually on sunscreens not only an SPF number but often with a PA index. For example, sunscreen SPF50 pa++ +
PA is an index that measures the ability of a sunscreen to protect the body against UVA rays published by the Japan Cosmetic Association. UVA rays can cause heatstroke, tanning, eye damage and skin changes that cause aging and skin cancer. PA index is denoted on sunscreen by the "+" sign and there are 4 levels of PA index.
Sunscreen SPF50 PA+++ means: UVB protection rate is 98%, effective time when used on skin is 500 minutes and can block 90% of UVA rays.
So sunscreen with an SPF of 50 means that when used on the skin, it can protect the skin against the harmful effects of UVB rays for about 500 minutes and has the ability to block the penetration of UVB rays into the body. up to 98%.