Up to 1980s,the most used preservative for herbaria specimens was HgCl2,sublimating at ambient air conditions;ionic Hg then reduces to Hg0 (gaseous elemental mercury,GEM) and diffuses throughout poor ventilated environments.High GEM levels may indeed persist for decades,representing a health hazard.In this study,we present new GEM data from the Central Italian Herbarium and Tropical Herbarium Studies Centre of the University of Florence (Italy).These herbaria host one of the largest collection of plants in the world.Here,HgCl2 was documented as plant preservative up to the 1920s.GEM surveys were conducted in July 2013 and July and December 2017,to account for temporal and seasonal variations.Herbaria show GEM concentrations well above those of extal locations,with peak levels within specimen storage cabinets,exceeding 50,000 ng/m3.GEM concentrations up to ～7800 ng/m3 were observed where the most ancient collections are stored and no ventilation systems were active.On the contrary,lower GEM concentrations were observed at the first floor.Here,lower and more homogeneously distributed GEM concentrations were measured in 2017 than in 2013 since the air-conditioning system was updated in early 2017.GEM concentrations were similar to other herbaria worldwide and lower than Italian permissible exposure limit of 20,000 ng/m3 (8-hr working day).Our results indicate that after a century from the latest HgCl2 treatment GEM concentrations are still high,i.e.,the treatment itself is almost irreversible.Air conditioning and renewing is probably the less expensive and more effective method for GEM lowering.