Mercuric chloride solutions have historically been used as pesticides to prevent bacterial, fungal and insect degradation of herbarium specimens. The University of Manchester museum herbarium contains over a million specimens from numerous collections, many preserved using HgCl 2 and its transformation to H g v 0 represents a health risk to herbarium staff. Elevated mercury concentrations in work areas (∼1.7 μg m −3) are below advised safe levels (<25 μg m −3) but up to 90 μg m −3 mercury vapour was measured in specimen boxes, representing a risk when accessing the samples. Mercury vapour release correlated strongly with temperature. Mercury salts were observed on botanical specimens at concentrations up to 2.85 wt% (bulk); XPS, SEM–EDS and XANES suggest the presence of residual HgCl 2 as well as cubic HgS and HgO. Bacterially derived, amorphous nanospheres of elemental selenium effectively sequestered the mercury vapour in the specimen boxes (up to 19 wt%), and analysis demonstrated that the H g v 0 was oxidised by the selenium to form stable HgSe on the surface of the nanospheres. Biogenic Se 0 can be used to reduce H g v 0 in long term, slow release environments.