Question

why does our body become tan whrn we go in sun at afternoon

Answer 1

In order to stimulate Vitamin D production in your body, you need to actually expose your skin to the sun; very little to none at all penetrates through heavy clothing like sweatshirts and pants. On the other hand, you need remarkably little exposure to the sun to produce the amount of Vitamin D your body needs; according to the Vitamin D Council (is there a Council or Study Group for everything these days?!?) you only need 15 minutes of daily exposure to sun to produce far more then the US RDA of Vitamin D. It would very hard to not get at least that much just walking to and from your car, living in CA, so I wouldn't worry about your Vitamin D levels. You can also ask your doctor to check it the next time you go for a routine exam as a part of your blood work. 

Answer 2

Protection from the Sun. Much of the damage to our skin caused by sunexposure can be prevented. Sunscreen must be applied 20 minutes beforegoing out in the sun and should be reapplied after two hours in the sunand after swimming or heavy sweating. You can also protect yourself with UV filtering sunglasses.