Sun Protection Factor, abbreviated as SPF, refers to the ratio of the energy required to produce small erythema on skin protected by sunscreen to the energy required to produce the same degree of erythema on unprotected skin. It is simple. In other words, it is the time multiple that the skin can resist ultraviolet rays.
Sun exposure can cause photoaging, causing dark spots, freckles, etc. on the skin. Redness of the skin after sun exposure is medically known as “erythema”, which is a mild reaction of the skin to sun exposure. A low erythema dose is a short period of sun exposure during which erythema appears on the skin. After using sunscreen, the low erythema dose of the skin will increase, then the sun protection index (SunProtection Factor, abbreviation: SPF) of the sunscreen is:
SPF = low erythema dose (after using sunscreen) / low erythema dose (before using sunscreen)
Ultraviolet rays in sunlight are invisible light and are divided into 3 sections according to different wavelengths: UVA area, long-wave ultraviolet rays, accounting for about 10% to 20%. It is the main cause of premature aging of the skin and can penetrate the dermis layer, causing Skin tanning causes damage to lipids and collagen, causing photoaging of the skin and even skin cancer. Its effect is slow, long-lasting, and cumulative, so it is called “age ultraviolet rays” and is not blocked by windows, parasols, etc. , even if it’s cloudy and rainy.
The value of SPF applies to everyone. The calculation method is: assuming that the intensity of ultraviolet rays does not change over time, a person without any sun protection measures will have his skin turn red after 20 minutes in the sun. When he uses SPF15 When using sunscreen, it means that the time can be extended 15 times, that is, the skin will not be red after 300 minutes. Sun protection factor SPF is a test index that measures the ability of sunscreen products to protect against UVB ultraviolet rays in the sun.
Sunscreen products can effectively protect against skin damage caused by ultraviolet rays UVA and UVB. To find out how long sunscreen protects you, first know how long you need to stay in the sun without using any sunscreen before your skin turns slightly red; then apply the required amount of sunscreen. Multiply the time with the SPF value to get the protection time of the sunscreen. The SPF value marked on the sunscreen is the protection index it provides.
For example, if you stay in the sun for 20 minutes without applying sunscreen, your skin will turn slightly red, and a product with an SPF of 15 can protect you for 5 hours.
20 (minutes) * 15 (SPF) = 300 minutes (5 hours)
The value of SPF sun protection factor is applicable to everyone. When using sunscreen with SPF 4, you can theoretically stay in the sun for 4 times (80 minutes) before your skin will appear reddish; if you use sunscreen with SPF 8 If you use SPF15 sunscreen, you can stay in the sun for 8 times longer (i.e. 160 minutes); when you use SPF15 sunscreen, it means you can stay in the sun for 15 times longer, that is, your skin will not become red after 300 minutes; and so on. .
Maintenance methods
1. For people with average skin types, the SPF value is 8 to 12;
2. For people who are sensitive to light, the sun protection index value is 12-20;
3. People with sensitive skin should choose plant-based sunscreens or physical sunscreens containing titanium dioxide.
4． For office workers who only come into contact with the sun on their way to and from get off work, the SPF value should be below 15, and they should mainly use facial sun protection;
5． When playing in the wild or swimming at the beach, the SPF value of sunscreen should be above 30. It is best to use waterproof sunscreen skin care products when swimming.
Sunscreen products
Sunscreen products mainly fall into two categories: chemical and physical. The sun protection principle of physical sunscreen is to reflect ultraviolet rays like a mirror, which is more suitable for sensitive skin. For example, children mostly use physical sunscreen. Chemical sunscreens absorb ultraviolet rays and then emit the ultraviolet energy in the form of heat.
Under the same SPF value, the sun protection effects of the two are almost the same. The higher the SPF value, the better. The higher the SPF value, the greater the irritation to the skin. In summer, you only need sunscreen with an SPF value of 30 to 40. Oily skin is recommended to use a slightly lower SPF value, dry skin can use a slightly higher SPF value. If you are going swimming or doing outdoor activities, you can also choose one with a higher SPF value. However, applying sunscreen once during the day is not foolproof when you go out. You must reapply it when you sweat. It is best to reapply chemical sunscreen every 4 hours.
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