A study made at the University of North Carolina, Greeley, US, revealed that tattoos slightly decrease skin sensitivity to touch, although the way they do it remains unclear.

Psychologist Todd Allen tested tattooed and non tattooed people to compare skin sensibility, using a device called aesthesiometer (a sort of caliper with two plastic points). When the two points are closely assembled, the subject feels just one point of pressure, but, when the points have moved far enough apart, at a certain distance, a person will sense them as two distinct points. The smaller this distance, the more sensitive the skin portion is. Five body parts were tested: the lower back, the back of the calf muscle, the inner forearm, the tip of the index finger and a cheek.

Allen found a similar sensitivity on the untattooed body parts of tattooed participants and those of their non tattooed counterparts; but the corresponding tattooed regions performed weaker. E.g., in one tattooed subject, the tattooed portion on the right leg was less sensitive than the corresponding non tattooed portion on the left leg. "There is a relatively small but significant decline in sensitivity," says Allen.

Tattooed zones had an average sensitivity limit of 32 mm compared to 28 mm of the non tattooed portions. "The findings are particularly relevant given the increased popularity of tattoos," Allen says.

There may be three possible explanations for this decreased sensitivity, but they are difficult to test. One "possibility is that the ink injected into the skin may be interfering with the pressure of touch," Allen says. This means that ink could disrupt pressure transmission from the skin surface to receptors deep in the skin. Another possibility is that, during tattooing process, the tattoo gun could over-stimulate the receptors decreasing their sensitivity. A third explanation is that "somehow the tattooing process has just damaged or killed the receptors in the skin.".

"But these receptors generally can regenerate, so that's the least likely of the explanations," says Allen.

To check the third possible cause, researchers would have to investigate tattooed cadavers and stain the nerve endings under the tattoo. But tattooing does not affect in any way individuals, because most of them are made in places of least importance in body's touching sense. "It's not a big deal in reality," Allen said.

He proposes to investigate the effect of the number and size of the tattoos on general sensitivity.