Tattoos are dangerous for mental health. This is because when people expose these markings to others, judgments can be made against them. They might be turned down for a job or a loan or be frowned upon in a court of law. If these people lose out economically or socially from climbing the ladder, they can suffer emotionally from a lack of resources in their life.
Rather than saying tattoos themselves are dangerous, it's more accurate to say that getting a tattoo can be dangerous. The needles can be tainted, and there are various blood-carried diseases that can be transmitted through getting a tattoo, including hepatitis. It's important to choose a place that is highly reputable and knows how to properly handle the equipment and inks; or forgo them altogether.
The risks with tattoos are negative cosmetic effects, blood-borne pathogens, fungal infections, staph infections, and flesh eating bacteria(extreme cases). That sounds very scary but we are around those all the time. Hospitals and clinics have the highest rates of infections from those same diseases by just being there. Our kids are also exposed to those diseases in the gym locker rooms and gym classes. Tattoo parlors have low incidence of infection. Amateur artists however, that is a risk taking it to a level of when you were getting changed in the grimy gym locker room after gym class.
If a tattoo is done in a clean, professional setting and taken care of properly, the risk of infection or other dangers in minimal. However, there are risks included with the choice of a tattoo, such as: blood-transmitted diseases, infections, and depending on one's career path it could possibly hinder job offers and other possibilities. If you're going to get a tattoo, make sure you research the parlor and the artist, and make sure they are up to safety regulations and code. Also, never give yourself a tattoo unless you are professionally trained. Stick and poke, which is the most popular self-given tattoo, is extremely dangerous and easily infected and, like all infected tattoos, can lead to amputation, surgery, and in worse case scenarios death. Please think and research extensively on these sort of things.
I do not believe tattoos are dangerous. The only reason a tattoo could be dangerous is when the artist doesn't disinfect their equipment and replace the needles as they should be. I have two tattoo myself and I wouldn't describe the procedure as much different than getting your ears pierced.
Tattoo parlors have strict regulations and the measures taken while giving a tattoo are like those in a doctors office. Getting a tattoo is no more dangerous than getting a shot - provided the regulations are followed such as wearing gloves, using a new needle each time, and following other safety regulations.
There's a quote going around the Internet along the lines of "A tattoo used to mean you were a dangerous outlaw on the fringe of society. Now it means you make a delicious roast Brussels sprouts casserole with a balsamic finish." Everyone has them, they're fairly sanitary, and not dangerous in the least.