 Once free of exposure, the rash will usually fade away within several hours; rare and extreme cases can take a day or two to normalize depending on severity of the reaction.Solar urticaria is an immunoglobulin E-mediated hypersensitivity that can be introduced through primary or secondary factors, or induced by exogenous photosensitization.Primary SU is believed to be a type I hypersensitivity (a mild to severe reaction to an antigen including anaphylaxis) in which an antigen, or substance provoking an immune response, is "induced by UV or visible radiation."