Why are tattoos banned in Islam? No they don't affect health, nothing will happen to you if you get a little tattoo. Also, if it affects Taharah, that is not true. Does God Al-mighty care about a little centimeter of dotted skin?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

You have to know that Allaah is All-Wise in His Legislation and Decree; His Words are The Truth, and His Rulings are Just. Allaah says (which means): {And the Word of your Lord has been fulfilled in truth and in justice.}[6:115]. All that Allaah has Legislated is of Complete Wisdom. Allaah has made some things lawful because they are very beneficial, and has forbidden other things because they are very harmful. However, a human being might not know the reason behind all this. The evidence about the ignorance of a human being is that he constantly explores and invents things that he was unaware of in the past. Therefore, it is an obligation on the Muslim to submit himself to Allaah's Decree as Allaah says (which means): {It is not for a believer, man or woman, when Allaah and His Messenger have decreed a matter that they should have any option in their decision…}[33:36]. The first thing that can be said about the wisdom of forbidding tattoos is that it is confirmed that the Prophet forbade it and it is a revelation from Allaah as the Prophet does not speak of his own desires. There is no harm if after believing and acknowledging the truth and acting upon Allaah's Order and upon the narrations of the Prophet to seek the wisdom behind such prohibition. Imaam Al-Bukhari reported that Abu Juhayfah said: "The Prophet cursed those ladies who practice tattooing and those who get themselves tattooed."

Among the wisdoms that the scholars  have mentioned is that having tattoos is considered as changing the creation of Allaah, and in some narrations this has been mentioned "…whereby they change Allaah's creation." [Al-Bukhari and Muslim] In addition to this, having a tattoo subjects the body to pain without a preponderant need or benefit and this is forbidden.

Finally, tattoos do not affect the purity of a person as they are under the skin and do not prevent water from reaching the skin. 

Allaah knows best.