Final answer:
Tanning occurs as a defense mechanism against UV-B radiation by producing melanin. However, tanning indicates skin damage and poses health risks, including the potential for skin cancer. SPF-rated sun-tan lotions can partially protect the skin by blocking UV radiation.
Step-by-step explanation:
We become tanned after sitting in the sun because our body activates a defense mechanism to protect deeper skin layers from the damaging effects of UV-B radiation. This defense involves the production of the brown pigment melanin, which serves to absorb UV radiation, reduce its penetration, and prevent skin tissue damage. Sun-tan lotion, with various sun protection factor (SPF) ratings, can help block some of this radiation. However, tanning is still a sign of skin damage, and excessive tanning increases the risk of skin cancer due to DNA mutations caused by UV radiation. It takes about 10 days for melanin synthesis to peak after the initial sun exposure, which is why protection against sunburn may increase with gradual exposure. But melanin's protective effects are temporary, as melanin-filled cells in the skin's outer layer will eventually slough off.
While a tan can give the appearance of health, it is important to remember that tanning is actually a sign of skin damage and poses long-term health risks. Hence, moderate sun exposure with proper protection is advisable to balance the benefits of vitamin D production against the risks of sunburns and skin cancer.