Sensitization to Nickel (Sulphate 5%) in North-Eastern Italy, 1997-2023: Prevalence, Trend Over Time and an Evaluation of the Occupational Risk

Background: Prevalence of nickel sensitization is heterogeneous worldwide, and in Europe, it is higher in Southern compared to Northern European countries, a likely reflection of delayed and less stringent application of the EU directive 94/27/CE. This multicentre study aimed at investigating prevalence and factors associated with positive patch test reactions to nickel in North-eastern Italy during 1997-2023. Methods: A cross-sectional study design was employed to estimate the yearly prevalence of positive patch test reactions to nickel sulphate 5% among 31 948 consecutive outpatients patch tested with the Triveneto series for suspected allergic contact dermatitis during 1997-2023 in four centres of Triveneto (North-Eastern Italy). Reading was performed at 48 h and 72 h/96 h. Multiple logistic regression separated by sex of patients was employed to investigate factors (birth year, atopic dermatitis, occupational dermatitis, body area affected and occupation) potentially associated with nickel sensitization, expressing the results as adjusted odds ratio (aOR) with 95% confidence intervals (95% CI). Results: Prevalence of nickel sensitization was 26.05% during 1997-2004 and 26.40% across 1997-2023 (excluding Trento-Bolzano-Rovigo), reducing over time in all centers combined. Prevalence of positive reactions significantly increased over the years during 1997-2004 among males yet reduced across the entire study period (1997-2023) among females Results (Logistic regression analysis). Regardless of the study period, nickel sensitization was significantly lower in males and followed an inverse U-shape with respect to birth year among females, increasing from 35.70% in those born during 1955-1964 to 46.24% in females born during 1965-1974, reducing to 41.36% in those born during 1975-1984. With regard to occupation, a significantly higher prevalence of positive patch test reactions to nickel was observed among sellers, whereas it was lower in retirees and housewives. Conclusions: Although decreasing over time, the prevalence of positive patch test reactions in this study is confirmed to be higher than that in Northern European countries. The latter pattern probably reflects delayed and less stringent application of the European directive. The U-trend of positive patch test reactions by birth year in female patients points to nickel exposure and sensitization in females aged 20-50 years before the enforcement of the European directive. Higher prevalence of positive patch test reactions in sellers could reflect prolonged exposure in coin handling occupations, whereas lower prevalence in retirees and housewives may be a result of reduced immune reactivity with increasing age.