The ultraviolet radiation from the sun can be divided into three types according to their wavelengths, namely A, B and C. Ultraviolet C has the shortest wavelength and has a great impact on earth life. Fortunately, it can be absorbed by the ozone layer and cannot penetrate to the earth’s surface. Ultraviolet A has the longest wavelength and radiates to the earth’s surface with a stable intensity that can penetrate the ozone layer and ordinary glass. It can penetrate the surface layer of the skin and reach a deeper layer of the skin, and is the main ultraviolet ray for sun tanned skin. UVB has a lower penetrating power than UVA, can be partially absorbed by the ozone layer, and the intensity of radiation to the earth’s surface is influenced by the angle of incidence of the sun relative to the earth. As a result, the intensity of UV-B received by people at different latitudes and at different times of the day varies widely. UV-B has little penetrating power and can be blocked by ordinary glass, and when it can come into contact with the skin, it can only penetrate the surface layer of the skin, where it can cause skin burns and cause cancer, but it is the UV light that converts vitamin D. To make the most effective use of sunlight for vitamin D, the best time to get sun exposure is between 10 a.m. and 2 p.m. Sun exposure at other times will only darken the skin and increase the likelihood of skin cancer without getting the vitamin D we want. Can you get enough vitamins from regular sun exposure? The answer is no. Because, first, UV absorption is affected by many factors. Melanin in the skin can affect the absorption of UVB rays. Black and yellow people are less able to get vitamin D from the sun than white people. Also, the reduction in people’s outdoor activities, high latitude geography, long winters and atmospheric pollution have greatly reduced skin exposure to UVB. Second, as we age, the ability of the skin to synthesize vitamin D3 diminishes, and the ability of the kidneys to activate vitamin D decreases accordingly, making the possibility of vitamin D deficiency much more likely. According to a sample survey, 1/3 of the population in the United States is vitamin D deficient or lacking. Vitamin D deficiency can have no subjective symptoms and is diagnosed primarily by measuring blood levels of 25-hydroxylated vitamin D3. Textbooks in the past set normal values for vitamin D between 9 and 38 ng/ml. Nowadays, vitamin D experts believe that the normal value of vitamin D should be 30 to 60 ng/ml. if the vitamin D is between 20 and 30 ng/ml, it is called deficiency; if it is between 10-20 ng/ml, it is called vitamin D deficiency; if it is below 10 ng/ml, it is called severe deficiency. Some years ago, it was also widely believed that the main function of vitamin D was to maintain blood calcium and phosphorus levels and bone metabolism. Vitamin D deficiency can cause rickets, adult osteochondrosis, and osteoporosis. By 1980, two scientists found that mortality from colorectal cancer was higher in large cities and high latitudes with reduced natural sunlight, leading them to hypothesize that vitamin D may have an important role in preventing colorectal cancer. Since then, more and more studies on vitamin D have been reported. It has been found that coronary heart disease and cancer, two of the biggest killers of humans today, have also been linked to vitamin D deficiency.