Tanning is not only considered unsafe because it comes with the risk of getting a sunburn, which can directly contribute to skin cancer depending on the rate and time of exposure. Tanning is also problematic because the process of tanning involves ultraviolet rays piercing your skin and completely meddling with your DNA!
Tanning exposes the naked skin (when it is not protected by sunscreen or other skin care creams) to both UV-A and UV-B rays. Both these are damaging in multiple ways, but UV-A rays in particular break down the natural collagen present in the skin. Collagen is a component of the skin that aids in strengthening the skin, tightening it and providing the hydration it requires. Once collagen production is affected, the skin will start appearing unhealthy and lose its sheen. Wrinkles and blemishes can also form faster as a result, thereby completely affecting the quality of your skin.
Many people think that tanning actually helps them because it adds more melanin to the skin which can be protective in nature. This is a false and dangerous assumption. Yes, melanin acts as a natural sunscreen for the skin in normal cases; but it is produced in large quantities when tanning takes place on exposure to the sun. However, the striking difference is that melanin is produced after much damage is done to the skin cells. So over-production of melanin can do you only so much good.
Sun tanning for aesthetic reasons is not recommended for anybody in the long run as it can fasten the process of skin ageing. No amount of anti-ageing therapy or use of products can help reverse this after a point. The skin also has the chance of becoming dry and rough. Sun tanning also promotes pigmentation while darkening freckles present on the skin. These effects may not show up immediately, but when they do, there is nothing much one can do at that point.