Immunologic contact urticaria due to airborne methylhexahydrophthalic and methyltetrahydrophthalic anhydrides.

Acid anhydrides are low-molecular-weight chemicals known to cause respiratory irritancy and allergy. Skin allergy has on rare occasions been reported. 2 workers contracted hives and itching on uncovered skin after 2 months exposure to methyltetrahydrophthalic anhydride (MTHPA) and methylhexahydrophthalic anhydride (MHHPA), to which they had airborne exposure. Later, the patients also developed conjunctivitis, rhinitis, sore throat, cough or asthma. In addition to MTHPA, 1 worker was also exposed to unsaturated polyester resin (UP). Both patients' immediate allergy to MTHPA and MHHPA was verified by positive prick tests to MTHPA and MHHPA, conjugated with human serum albumin (HSA), and positive radioallergosorbent tests (RASTs) to these anhydrides. On prick testing, both patients also reacted to a phthalic anhydride (PA)-HSA-conjugate and 1 of the patients to UP-HSA-conjugate. Specific immediate allergy to UP was shown by RAST. RAST inhibition with MTHPA, MHHPA and UP-resin conjugates confirmed IgE-mediated allergy and cross-reactivity between anhydrides. Our patients had developed airborne contact urticaria caused by phthalic anhydrides, in addition to respiratory allergy. Phthalic anhydride contained in the UP resin was possibly responsible for the immediate reaction of the skin.