Let me start off by saying if you don't want any tattoos, by all means, leave your skin bare. However, if you wish to know if tattoos really are toxic as many people like to claim then keep reading.
Tattoos have been around for as long as humans have wanted to apply permanent body modifications. Tattoos on warriors could mark the most fearsome and save precious time that would have been spent on war paint. They can also mark differences in a social hierarchy, or which clan you may belong too. They could also be used as punishment or for a simple tracking system. In the Edo era, Japan would tattoo the forehead of non-violent criminals with a simple line, after 3 lines they were sentenced to death. The Nazis tattooed the prisoners to keep track of them much like farmers us tags on cattle. Tattoos could also be used for spiritual or medicinal purposes. Otzi the iceman is currently the oldest known preserved human body and he had 61 carbon tattoos. In ancient Egypt women were the ones getting most of the tattoos, there are some that feel it was prostitutes getting them, however, the nature of the tattoos can also point to being protection charms for pregnancy and other things that an average woman may be concerned about. The modern word "tattoo" actually comes from Polynesian culture, they have been heavily tattooed for thousands of years and as far as I can tell this is an important aspect of their cultural identity. No single group anywhere in the world can be traced as starting tattoos, every culture has tattoos in its history somewhere for some purpose. There seems to be an association with a desire for purity or separateness and shunning tattoos at various points in history.
The tools used for tattooing have changed over time. Traditional methods involved some sort of handheld tool that could cut the skin; bamboo, shark teeth, tortoiseshell, and metal are a few examples. The cutting portion was often attached to a handle of some sort and would be used to cut or scrape the design into the skin in portions. After the skin was broken the pigment could be put in using a tool or rubbed in by hand. One of the oldest pigments used in tattoos is soot, also known as carbon. The modern tattoo machine still breaks the skin but it uses fine needles and also delivers modern pigments into the skin at the same time. The first version of the tattoo machine was patented in the USA in 1891 by Sam O'Riley and was based on a design for an electric pen that Thomas Edison invented in 1876.
So what makes the ink stay in the skin for so long? There are multiple layers in our skin, the top layer we know as the epidermis which is constantly renewing, the second layer is the dermis. The dermis is where we find blood vessels, collagen, nerves, ect. It's also where we need the ink to go in order to make a tattoo permanent. When the tattoo is done the immune systems says "Oh shit we are under attack!!" and sends different types of cells to deal with the foreign invaders (the ink globules), when the cells swallow the ink they become too large to re-enter the bloodstream and end up staying in the dermis. The skin will be able to push a certain amount of ink out but the majority will remain. The ink that is pushed out will be seen in the scab that forms during the healing process, the scab is also made of a tiny amount of blood and some plasma (the yellow liquid you might see). Given enough time the body will VERY SLOWLY break down some of the ink, this contributes to a tattoo fading, UV light will speed this process up. Modern inks last much longer today than they did even 50yrs ago.
The whole process does create a permanent state of minor inflammation at the site of the tattoo but that isn't all bad. There is a study that shows getting more than one tattoo can actually boost immune function overall. Like working out, the first time can leave you feeling tired but the more you get, the better the immune system can deal with it. More studies are needed to verify this link further however.
There are different types of ink available and some shops even make theirs in house. Some inks do contain heavy metals which can carry some risks depending on the location of the tattoo and how heavy-handed the artist is. A heavier hand will place the ink deeper into the skin which may allow those ink globules to be transported into larger blood vessels. And if the tattoo is in a region where there are lots of lymph nodes then the lymphatic system can pick it up and transport any heavy metals to different organs like the liver or brain. Fun fact; scientists have discovered that there are lymph nodes in the brain! Ink manufacturers are not required to disclose their ingredients, however, due to some lawsuits they do have to place warning labels if there are any ingredients known to cause health problems. Some companies now make organic and vegan inks so I encourage you to ask what type of inks an artist uses. Even with the inks that have heavy metals reactions or allergies are not common, I personally still look for better inks just to avoid any issues. There is one colour that they cannot make yet with lower risk and that is red. All brands have this issue so you can just choose a design that doesn't need any red.
When it finally comes down to actual measurable risks there are really just a few things you should know. Always look for a physically clean tattoo shop, if the shop and/or artist has questionable hygiene get out of there before you have a terrible regret story. Next, you ask the artist what type of inks they use. Any good artist cares very much about the quality of the ink they use. If you have gotten past the first 2 points next you want to see that there are disposable gloves, disposable ink pots (tiny plastic cups), fully sealed needles, and distilled water (for diluting the ink, depending on the brand). After that, it's down to you to prevent infection if you chose to get a tattoo. Keep it clean and cover it when you go to bed. Dirty sheets can give some bad infections, so can clean ones for that matter.
Anything else comes down to making sure you like the artist's work, an ugly tattoo is a regretted tattoo.
Actually I have a funny anecdote from my days as a cashier in a grocery store; I noticed a male customer's tattoo on his forearm and part of the job is to make small talk so I said "I like your tattoo. Is it an alien?". The guy just looks at me for a second and says "No it's a koi." without skipping a beat (and before I've applied any internal censorship) I reply "What did he (the artist) do? Beat it with a stick?!". Needless to say, the guy wasn't pleased but I'm telling ya it was bad, I couldn't tell it was supposed to be a koi...
I hope this has helped with any questions or concerns you have. If you feel I missed anything please comment down below, or just say hi!
Reference material:
https://www.fda.gov/ForConsumers/ConsumerUpdates/ucm048919.htm
http://time.com/4725634/tattoo-ink-dangerous/
http://www.popsci.com/science/article/2013-06/fyi-what-makes-tattoos-permanent
http://www.tattooinfo.net/scripts/prodview.asp?idproduct=27
Tattooing to “Toughen up”: Tattoo experience and secretory immunoglobulin A
