Dutch dermatologists are on track to solve the mystery of reactions to red tattoos after collecting data from patients with adverse reactions to the red ink.

Their study shows that complications from the red ink are most common in tattoos on the distal parts of limbs and the face, regions that receive the most sunlight exposure.

“Additionally, a notable number of patients (32%) explicitly report sunlight as a trigger. These findings suggest that sunlight may play a role in forming the allergen,” the University of Amsterdam researchers say.

While 43% of the patients developed symptoms within one month of having the tattoo, some patients did not develop a reaction until months or years later, they noted.