People in Asia and Africa where plenty of sunshine is available and weather is hot have a auburn or darker skin. This is because of production of a chemical called melanin in the skin and making it look brown or dark.
In Europe, particularly the Northern parts, there is hardly any Sun to be seen for most of period of the year. Due to the shortage of melanin, the color of the skin is white or what is called fair.
Most people are not satisfied with whatever God has given to them. For example, the people in India want to look fair like Europeans and vice versa. Europeans come to India and sprawl themselves on the sunny beaches in the hope of getting the tanned skin.
Tan is caused by the action of UV rays on the skin cells of the body. The UV rays have higher energy than the visible spectrum light. UV spectrum is further subdivided into three ranges namely UVA, UVB & UVC with decreasing wavelengths and increasing energies from UVA to UVC. UVC are so energetic that they react with the matter in atmosphere and never reach the Earth. UVA & B both break the chemical bonds. UVB stimulates the production of melanin by burning the skin and destroying DNA and gives the tan to the skin.
Interaction of UVB with skin produces vitamin D3 which is required to transport calcium to the bones. Additionally UV reduces the stress and eases the tension. There is scientific evidence that sunshine has a mood lifting effect and people in the places where the Sun is not seen for months show the symptoms of depression. Thus UV which is always seen in the negative terms has good side also.
But it is not healthy to expose your body directly to the UV rays. Instead sunscreens are beneficial and make the tanning in the Sun. How much time is safe for tanning is calculated by Sun Protection Factor (SPF) ratings on sunscreens. The SPF number is a “multiplier” and formula to calculate the safe time is = Minutes to burn without sunscreen x SPF number
For example, if a sunscreen has a SPF value of 20 and it takes 12 minutes for your skin to burn without sunscreen, then sunscreen will protect you for 240 minutes (12 min. x 20 SPF = 240 min.). Doctors recommend a water-resistant sunscreen rated 15 SPF or higher with UVA and UVB protection, generally labeled “broad spectrum.”
Maybe the best tan is sunless. “Tan in a Can” products use materials that generate a tan without UV exposure. Such creams, gels, lotions, and sprays contain dihydroxyacetone (DHA), a sugar that colors the outer skin cells. Unfortunately, this nice tan usually lasts only one week because you constantly shed dead cells.