An outbreak of dermatitis at one of the largest construction sites in the United States was evaluated. The evaluation started as a result of a request from one of the workers at the site for a Health Hazard Evaluation to be conducted by NIOSH in August of 1986. Two nuclear power facilities were under construction at the site, employing more than 5000 workers. The wood that was used for scaffolding and other temporary structures was treated with free retardant made by mixing dicyandiamide (461585), phosphorus-acid (13598362), and formaldehyde (50000) in water and applying it to the wood by a vacuum pressure process. Pruritic, maculopapular lesions were noted on the arms of some workers. Other parts of the body affected included the shoulders and flank. Workers reported the rashes began at work and lasted from days to weeks. Between February 2 and October 19, 1986 there was a total of 445 visits from 407 workers to the medical facility for skin related problems. Only 122 visits were made during the same time period the year before. Carpenters had the highest rate of skin related visits to the medical facility, following by laborers and then iron workers. Of all the carpenters who completed a questionnaires (92% of those eligible), 54% reported skin conditions, and 29% met the case definition of possible contact dermatitis. Total phosphate concentrations for the extracts of the fire retardant treated lumber ranged from 4.7 to 7.1 milligrams/gram of wood. Results indicated that no specific agent could be identified, nor was it conclusive that a causal role for the fire retardant lumber existed. The large number of workers afflicted suggested that the offending agent was more likely to have been an irritant than an allergen. The authors state that phosphates can leach from treated lumber by both water and sweat. The increased temperatures during the summer season suggests this possible course of events. Construction workers have been advised to handle this lumber with caution, particularly in high temperature and humidity conditions.