Complaints such as swelling, itching, stinging, pain and redness are common, predominantly in black and red tattoos, but also frequent in blue tattoos, thus, not confined to one specific colour or chemical entity or class of pigment. Symptoms may switch on and off in seconds, typically not of the weal-and-flare type. Photochemical reactions to pigment or pigment-breakdown products in situ in the skin with induction of reactive oxygen species (ROS) is presumed to be one causative mechanism. Another possible mechanism especially relevant in black may be induction of ROS due to effects of aggregation of carbon black nanoparticles.