Why is the patient with lead intoxication not light sensitive?

Ever since the 1930s, when it was found that the concentration of protoporphyrin (PP) in the red blood cells is markedly increased in lead workers, it has been keenly debated why these persons, like patients with certain types of porphyria, are not hypersensitive to light. The answer came in the 1970s when it was realised that erythrocyte-PP in lead poisoning is not free, as it is in porphyria, but is bound to zinc (ZPP). Free PP can pass through the erythrocyte membrane into the tissues, while ZPP remains confined in the red blood cells. On exposure of PP in the skin to sun shine, the molecule is stimulated to release energy, which causes blisters, reddening, swelling and other symptoms of hypersensitivity to light.