Middle-aged people die of melanoma. The "effect" of ultraviolet rays is beyond your imagination.

The benefits of ultraviolet light to people go far beyond the synthesis of vitamin D, and the harms go far beyond causing skin cancer. Balancing the beneficial and adverse effects of ultraviolet light in sunlight may be more important than you think.

Written by Wang Chenguang (PhD in Biology, former professor at Peking Union Medical College)

The news that the 37-year-old famous singer Zhang Hengyuan died of melanoma has once again made skin cancer a hot topic. It has also made ordinary people, who are already afraid of cancer, once again struggle with whether to go outdoors to bask in the sun: if they don't bask in the sun enough, they worry about insufficient vitamin D synthesis; if they bask in the sun too much, they worry about skin cancer.

No matter which kind of worry they have, their thinking is not comprehensive: the benefits of ultraviolet light to people are far more than synthesizing vitamin D, and the harm is far more than causing skin cancer. We need to have a more comprehensive understanding of ultraviolet light and the relationship between skin cancer and sun exposure.

Four types of skin cancer and three types of ultraviolet rays

On June 18, Father's Day, I participated in a mountain cross-country race in Jizhou, Tianjin. The race started at 7 a.m., when the temperature was 24 degrees Celsius and there was a slight breeze under the shade of the trees, which made the body feel comfortable. But after 10 a.m., the temperature rose sharply and soon reached 34 degrees Celsius. As the shadow of the human body on the ground became shorter and shorter, and sweating increased, the burning sensation of the sun shining on the exposed skin gradually increased. At this time, my running partner said that my face was sunburned.

What factors cause these changes in how we feel and in our bodies?

First, the infrared rays in sunlight are absorbed by the skin, which makes us feel hot, just like feeling warm by a fire in winter.

Then the skin turns red. This is because the ultraviolet rays (UV) in the sun irradiate the skin, causing the blood vessels under the epidermis to dilate, causing the skin to redden. This is also a warning. If you avoid sun exposure in time after this happens, the red skin will usually return to normal the next day. If you continue to be overexposed to ultraviolet rays, it will cause skin damage, sun spots or sunburn.

The skin tends to darken in the summer because ultraviolet rays stimulate the melanocytes in the skin to produce more melanin. This is a protective mechanism formed when the skin is exposed to ultraviolet rays. Once this protective mechanism goes wrong, the consequences may be serious. Melanoma, the most malignant skin cancer, is the result of the melanin-producing cells becoming cancerous after being frequently exposed to ultraviolet rays.

There are four main types of skin cancer (outdated classifications used to have three). The most common is basal cell carcinoma, accounting for about 80% of skin cancers. Basal cell carcinoma most often develops on the head and neck, mainly due to sun exposure; a few cases are caused by radiation therapy during childhood. This type of skin cancer usually grows slowly and rarely spreads to other parts of the body and causes death.

The second most common type of skin cancer is squamous cell carcinoma, which occurs in the surface layer of the skin and accounts for about 20% of all skin cancers. Squamous cell carcinoma is also mainly caused by sun exposure. The above two types of skin cancer are sometimes classified as one, namely keratinocyte carcinoma of the skin.

Melanoma is also a type of skin cancer, named after the melanocytes in the skin. Although it accounts for less than 1% of all skin cancers, melanoma is the most malignant type of skin cancer and is most likely to metastasize. Two-thirds of skin cancer deaths are caused by this type.

There is another type of skin cancer that is not often mentioned - Merkel cell carcinoma. This is an extremely rare, highly invasive or fast-growing malignancy. The occurrence of this tumor is related to a type of hormone-secreting cell in the skin, so it is also called neuroendocrine skin cancer. Recent studies have found that this tumor is closely related to a viral infection and is the type of skin cancer that is least closely related to sun exposure.

Melanoma is most closely related to ultraviolet rays. Sunlight is the most common source of ultraviolet rays that people are exposed to, so sun exposure is the main risk factor for melanoma.

Before we learn more about how to protect ourselves from sun-induced skin cancer, we need to first understand UV rays and how they damage the skin.

Ultraviolet rays are part of the electromagnetic radiation that reaches the earth from the sun. They can be divided into UVA, UVB, and UVC according to the wavelength from long to short. UVA has the longest wavelength among the three types of ultraviolet rays, ranging from 320 to 400 nanometers, and UVB has a wavelength range of 290 to 320 nanometers. The shorter the wavelength, the higher the energy. As for the shorter wavelength UVC rays, humans living at low altitudes do not need to worry about them, as most of them will be absorbed by the ozone layer and are unlikely to reach the ground.

Both UVA and UVB can penetrate the atmosphere and are the main causes of premature skin aging, eye damage (including cataracts), skin cancer and other diseases. The International Agency for Research on Cancer (IARC) classifies UVB and UVA as Class I (sufficient evidence) and Class II (relatively sufficient evidence) carcinogens, respectively.

We spend a lot of our lives exposed to UVA rays, which penetrate clouds and window glass and reach our skin. 95% of the UV rays that reach the earth are of this type. Although it does not carry as much energy as UVB, it is dozens of times more than UVB. As long as the sun is out, there is UVA, and it maintains a relatively balanced intensity, so we should pay attention to UVA protection all year round.

UVA can penetrate deeper layers of the skin and is the culprit for skin darkening, aging, and wrinkling. The role of UVA in the development of skin cancer is still controversial. The current consensus is that UVA increases the risk of skin cancer, but not as seriously as UVB. Sunbathing uses artificial light, and the main component of the light is UVA, which is more than 10 times more intense than sunlight. People who often use sunbathing are 2.5 times more likely to develop squamous cell carcinoma and 1.5 times more likely to develop basal cell carcinoma than ordinary people. At the same time, frequent sunbathing also increases the incidence of melanoma by 75%.

UVB is also called outdoor ultraviolet rays because it cannot penetrate glass to reach indoors, and it is difficult to penetrate the surface of the skin to reach the deeper layers. It mainly damages the more superficial parts of the skin, but it is the main cause of dark spots and skin cancer. At the same time, it can also make the skin red and tanned after being exposed to the sun. The intensity of UVB changes with the time of day, location and season. Most parts of China have strong UVB from April to October every year, from 10 am to 4 pm every day. Ultraviolet rays are stronger in high-altitude areas, so it is easier to be exposed to larger doses of UVB radiation when off-roading or hiking in mountainous areas.

Another hazard of ultraviolet rays: damage to immune function

Since then, we have come to understand the negative impact of UV rays on health, especially their important role in the development of skin cancer. More than one million people worldwide suffer from skin cancer every year, and people with fair skin are more likely to develop melanoma after being exposed to UV radiation. So, how does UV rays induce skin cancer?

Almost all cancers are the result of gene mutations, and skin cancer is no exception. The energy of ultraviolet rays is enough to damage the genetic material (DNA) of human epidermal cells, causing DNA breaks. It is estimated that hundreds of thousands of human epidermal cells are damaged by ultraviolet rays every day.

Human cells have a powerful repair function. Partially damaged DNA is repaired, and those that cannot be repaired can hardly survive. The chance of developing cancer arises in the process of damage and repair. Incorrect repairs lead to changes in gene sequences (mutations), some of which cause changes in cell proliferation behavior. These changes accumulate over a long period of time, and finally induce the transformation of normal cells into tumor cells at a certain point in time.

The above are the "internal causes" of normal cell canceration. However, not all cells that change develop into tumors, because the human immune system constantly monitors these changes and eliminates the changed cells in a timely manner. In other words, the final occurrence of tumors often requires the cooperation of another force - mainly immune dysfunction. The reason why the incidence of tumors is higher in the elderly is, on the one hand, the accumulation of gene mutations in the above cells, and another important factor is that the immune function gradually decreases with aging.

Does this have anything to do with UV-induced skin cancer? Yes, and it is very relevant because UV rays can suppress immune function.

The skin is the largest immune organ in the human body. It not only provides a physical barrier between the body and the outside world to prevent microbial invasion and water loss, but also contains a large number of immune cells that participate in sensing and resisting pathogen invasion. The inhibition of immune function by ultraviolet rays is reflected in many aspects: for example, ultraviolet rays can directly inhibit the functions of various immune cells (such as T cells, B cells, natural killer cells and macrophages); long-term ultraviolet exposure may cause immune cell death, thereby reducing the number of immune cells; ultraviolet rays can also interfere with the regulatory mechanism of the immune system, inhibit the production of anti-inflammatory cytokines, increase the release of inflammatory cytokines, and lead to an imbalance in the immune response, thereby weakening the skin's immune function to monitor and clear tumor cells.

High-intensity UV exposure suppresses the immune function of the skin, which is the result of long-term evolution between humans and nature. It is the body's protective mechanism against high-intensity UV. Does it sound a bit contradictory? In fact, it is not contradictory. After ultraviolet rays kill skin cells, they will cause an excessive immune response in the body (this is why some people are allergic to ultraviolet rays). The secondary damage caused by this immune response to the body is even stronger than the ultraviolet rays themselves. As a result of evolution, the body has developed a mechanism to fight against this damage, that is, the immune response is suppressed while receiving ultraviolet rays.

Ultraviolet radiation suppresses immunity and may increase the risk of infection, weaken the surveillance of tumor cells, and aggravate the development of autoimmune diseases; however, moderate ultraviolet radiation is also necessary to maintain normal immune function, such as improving the symptoms of some skin diseases and autoimmune diseases. Ultraviolet radiation also has other positive effects, such as synthesizing vitamin D, regulating hormone production and secretion, and improving mood. Although UVA cannot promote the synthesis of vitamin D, it can promote the synthesis of nitric oxide. Nitric oxide can help lower blood pressure, relieve depression, and has many benefits for the body.

Therefore, balancing the beneficial and adverse effects of UV rays in sunlight is key to ensuring good health.

When should you avoid activities under the sun? Here is a simple method, also known as the "shadow rule" (applicable to low-altitude areas): If you are standing under the sun and your shadow is shorter than your height, the UVB, which is the most harmful to the skin, is very strong. This period is not suitable for long-term unprotected activities in the sun; if your shadow is longer than your height, the UVB in the light is reduced, and this period is suitable for outdoor activities, as you can synthesize vitamin D without getting sunburned too quickly.

For outdoor enthusiasts, the following general tips should serve as guidelines: Try to avoid outdoor activities during the strongest hours of the sun (usually 10 a.m. to 4 p.m.); spray sunscreen, choose a sunscreen with an SPF index of more than 30, and read the instructions carefully to ensure that it can block UVB; wear long-sleeved tops, long pants and wide-brimmed hats to minimize direct skin exposure to the sun; sunglasses can block almost all ultraviolet rays; try to stay in the shade or under an umbrella to reduce sun exposure.

Diagnose melanoma and avoid misunderstandings

So how do you determine whether a mole on the skin is melanoma? Many popular science articles in China have introduced a method for self-judgment, namely the ABCDE method.

Judging from the morphology, A (Asymmetry) refers to whether the mole is symmetrical. The more irregular and asymmetrical the edge is, the higher the risk of melanoma; B (Border) refers to the border. The risk is high if the border is unclear and jagged; C (Color) refers to the color. The risk is low if the color is single and uniform; D (Diameter) is the diameter. For moles with irregular edges, the risk is higher if the long diameter is greater than half a centimeter. Finally, E (Evolution) refers to the dynamic process, that is, whether the mole or spot has changed in the past few weeks or months.

It is necessary to emphasize the last point here. E here is the abbreviation of the English word evolution or evloving, which was changed to elevation in the simplified Chinese world. A single word difference can lead to a huge mistake. The former means "progress", which is the most important indicator for judging whether a mole on the skin has become cancerous. The latter refers to "elevation", and some so-called science experts in China have made a big mistake here, thinking that suspicious mole-like tissue on the skin will be higher than the skin. The actual situation is just the opposite. Melanoma, the most malignant skin cancer, usually appears flat and does not bulge on the skin surface.

Genetic factors play an important role in tumor occurrence, exceeding environmental influences, but today's science is almost unable to prevent or treat cancer from the perspective of changing genes. What we can do is to minimize the risk of environmental cancer.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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