Tattoos can cause allergic reactions, and tattoo needles, even clean ones, may be partly to blame, a new study found.

Previous studies have shown that tattoo inks contain high amounts of nickel, chromium, cobalt and mercury, which are transported to the lymph nodes and can cause allergic reactions. But now researchers have found that an additional source of metal is the needles themselves.

Scientists analyzed 12 steel tattoo needles and found that all contained chromium and nickel. They looked at the needles with an electron microscope before and after they were used. After a tattooing session, the needles were abraded, with microscopic bits of nickel and chromium worn off. The report is in Particle and Fibre Toxicology.

The problem, they discovered, was titanium dioxide, a chemical used to brighten the colors of tattoo inks. It is this abrasive additive that erodes the needles. The abrasion does not occur when black ink alone is used.