Another tattoo alternative is henna-based tattoos, which generally contain no additives. Henna is a plant-derived substance which is painted on the skin, staining it a reddish-orange-to-brown color. Because of the semi-permanent nature of henna, they lack the realistic colors typical of decal temporary tattoos. Due to the time-consuming application process, it is a relatively poor option for children. Dermatological publications report that allergic reactions to natural henna are very rare and the product is generally considered safe for skin application. Serious problems can occur, however, from the use of henna with certain additives. The FDA and medical journals report that painted black henna temporary tattoos are especially dangerous. Black Henna or Pre-Mixed Henna Temporary Tattoos May Be Harmful - see below for safety information.
Tattoos are also placed on animals, though rarely for decorative reasons. Pets, show animals, thoroughbred horses, and livestock are sometimes tattooed with identification and other marks. Tattooing with a 'slap mark' on the shoulder or on the ear is the standard identification method in commercial pig farming. Branding is used for similar reasons and is often performed without anesthesia, but is different from tattooing as no ink or dye is inserted during the process, the mark instead being caused by permanent scarring of the skin.[24] Pet dogs and cats are sometimes tattooed with a serial number (usually in the ear, or on the inner thigh) via which their owners can be identified. However, the use of a microchip has become an increasingly popular choice and since 2016 is a legal requirement for all 8.5 million pet dogs in the UK.[25]
In 2017, researchers from the European Synchrotron Radiation Facility in France say the chemicals in tattoo ink can travel in the bloodstream and accumulate in the lymph nodes, obstructing their ability to fight infections. However, the authors noted in their paper that most tattooed individuals including the donors analyzed do not suffer from chronic inflammation.[89]

"In the late eighteenth and early nineteenth centuries, tattoos were as much about self-expression as they were about having a unique way to identify a sailor's body should he be lost at sea or impressed by the British navy. The best source for early American tattoos is the protection papers issued following a 1796 congressional act to safeguard American seamen from impressment. These proto-passports catalogued tattoos alongside birthmarks, scars, race, and height. Using simple techniques and tools, tattoo artists in the early republic typically worked on board ships using anything available as pigments, even gunpowder and urine. Men marked their arms and hands with initials of themselves and loved ones, significant dates, symbols of the seafaring life, liberty poles, crucifixes, and other symbols."[64]

The American Academy of Dermatology distinguishes five types of tattoos:[9] traumatic tattoos, also called "natural tattoos", that result from injuries, especially asphalt from road injuries or pencil lead; amateur tattoos; professional tattoos, both via traditional methods and modern tattoo machines; cosmetic tattoos, also known as "permanent makeup"; and medical tattoos.
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