Inhibitory effect of zinc oxide on contact allergy due to colophony.

Contact allergy to a wound dressing with an adhesive mass consisting of colophony, zinc oxide and rubber (Mezinc®) was studied in 179 patients with a history of eczema. 12 patients were found to be allergic to colophony, whereas only 4 of these patients also showed a positive patch test reaction to the wound dressing. 14 patients with verified moderate contact allergy to colophony were patch tested with adhesive mass (10%). Portuguese colophony (10%), zinc oxide (10%), purified resin acids (10%) and Portuguese colophony (10%), in combination with zinc oxide. Only 3 patients reacted to the adhesive mass, whereas all patients showed a positive patch test reaction to Portuguese colophony. A combination of zinc oxide (10%) with Portuguese colophony (10%) provoked a positive patch test reaction in only 5 of these 14 patients. An allergic reaction to abietic acid (90–95% purity) was found in 7 patients and to neoabietic acid (99 +%) in 3 patients, whereas no reactions to dehydroabietic (99 +%), isopimaric (99 +%) or levopimaric acids (98 +%) were found.