Background: Contact allergy epidemics to chromate and nickel were addressed in Denmark in 1983 and 1990 by regulatory interventions.
Objectives: To evaluate whether regulatory interventions on nickel and chromate exposure have reduced the proportion of strong patch test reactions.
Methods: 22 506 patients with dermatitis aged 4–99 years were patch tested with nickel sulfate, potassium dichromate, or cobalt chloride between 1977 and 2009.
Results: The proportion of 3+ reactions to nickel sulfate was reduced and almost disappeared after the mid- and late 1980s (P-trend = 0.001). Today, 1+ and 2+ nickel reactions occur equally frequent. Cobalt chloride patch test reactivity reflected the nickel development to some degree. The proportion of 3+ reactions to potassium dichromate was reduced during the 1980s (P-trend = 0.13), whereas the proportion of 2+ reactions to potassium dichromate have increased in recent years.
Conclusions: The decrease in nickel sulfate and cobalt chloride 3+ patch test reactivity began long before the Danish nickel regulation came into effect. This could be because of research activity at the time as well as political attention in Northern Europe. The chromate content in cement regulation may have changed the epidemiology of patch test reactivity; however, in recent years, 2+ reactions to chromate have increased markedly, a development that should be carefully followed.