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His reaction was more like a photoallergic dermatitis, in which sunlight causes hives — raised red patches that are intensely itchy and last less than 24 hours.

But that didn’t quite fit either; photoallergic reactions aren’t immediate.

This was a photoallergic reaction.

In one 20-year study of 69 people with documented photoallergic dermatitis, doctors at New York University found that antibiotics and ingredients in sunscreens each caused 23 percent of the reactions, other medications 20 percent, fragrances 13 percent and substances from plants 11 percent.

Unlike phototoxic reactions, which require a rather strong exposure to the offending substance, a photoallergic response can result from relatively small amounts of the allergen.