We show the first estimations of long-term (100 years, 1900 to 2000) total gaseous mercury concentrations (TGM) in the urban area of Almadén. The estimation was carried out by comparing data on known metallic mercury production with measured TGM concentrations. The estimated diurnal background level ranges from 60 to 120 ng m(-3) and corresponds to periods when the metallurgical complex (cinnabar roasting plant) was shut down. The average TGM concentration during the period from 1900 to 2000 was about 600 ng m(-3) (with peaks above 1,200 ng m(-3)). Additionally, a 24-h-based TGM monitoring program has highlighted significant differences between the diurnal and nocturnal concentrations, particularly during the warmer months. In this regard, given that the average nocturnal to diurnal ratio is 2.12, we suggest that the average nocturnal concentrations must have exceeded 1,200 ng m(-3), and peak nocturnal concentrations could have reached levels up to 2,400 ng m(-3). Our estimations indicate that most parts of the town of Almadén were generally exposed to TGM concentrations in air that exceed the World Health Organization air quality guideline for Hg (1,000 ng m(-3)) for countryside and urban areas.