The incidence of hand dermatitis is high in the hairdressing trade; the individual disease burden is substantial, as are the costs of treatment, workers compensation and retraining programs. To evaluate the risk of developing occupational irritant hand dermatitis associated with several potential risk factors, namely constitution (atopy) and occupational exposure, an observational prospective population-based cohort study set in vocational training schools in northwest Germany, recruiting 2352 hairdressing apprentices, was performed. Unprotected wet work of more than 2 h per day is the major significant risk factor; low ambient absolute humidity is equally associated with a significantly increased risk. Compared to the youngest age group, risk diminishes with increasing age. Constitutional risk factors are difficult to evaluate because of selective dropout of atopic participants. For primary prevention, a population approach, improving working conditions and skin protection for all exposed, appears to be the most beneficial.