Why do some people get fairer the more they are exposed to sunlight

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The so-called tendency to stay fairer under sunlight is simply due to a more active metabolic mechanism and better skin recovery ability, which makes it relatively less easy for the skin to tan. And there’s no need to envy white people for not tanning; they have a much higher risk of developing skin cancer than people of Asian descent. The damage caused by sunlight to human skin primarily comes from ultraviolet A (i.e., UVA) and ultraviolet B (i.e., UVB). UVA belongs to the long-wave range; it acts on the deeper layers of the skin, its effect is slow, but it can cause immediate darkening. UVB belongs to the medium-wave range; it acts on the upper layer of the skin and produces quick results. It can stimulate skin keratinocytes, cause blood vessels to dilate and increase blood flow; it becomes red at first and then gradually turns brown. So, in simple terms, UVB causes the skin to turn redder with more exposure, while UVA causes it to darken with more exposure. SPF is the abbreviation for Sun Protection Factor, which indicates the level of sun protection provided by a sun protection product. It is determined based on the minimum erythema dose for the skin. Redness of the skin after sun exposure is medically known as “erythema”; it represents the mildest reaction of the skin to sunlight. The minimal erythema dose is the shortest exposure time to sunlight required to cause erythema on the skin. After using sun protection products, the minimum erythema dose for the skin increases. The SPF value of such sun protection products is calculated as follows: SPF = Minimum erythema dose (after using the product) / Minimum erythema dose (before using the product). The higher the SPF value, the longer the duration of protection. If a person stands exposed to the sun without any protection, redness appears on the skin after 15 minutes. If you use sunscreen with an SPF of 20, the time you can stay in the sun safely is 15×20=300 minutes. Sun exposure causes skin aging, leading to dark spots, freckles, and so on. Sunscreen products can effectively protect the skin from the damage caused by ultraviolet rays UVA and UVB. To find out how long sunscreen will provide you with protection, you first need to know how long it takes for your skin to turn slightly pink when exposed to the sun without any sunscreen at all ； Then, multiplying the required time by the SPF value gives the protection duration of the sunscreen. Although SPF is an important indicator for sun protection, it does not mean that the higher the SPF value, the stronger the protection. For example, SPF15 provides 93% protection, while SPF34 offers only 97% protection. However, the higher the SPF value, the worse the permeability of the product, which can hinder the skin’s normal secretion and breathing. According to research by dermatology experts, the most appropriate sun protection factor lies between SPF 15 and SPF 30.