The role of relative humidity in the control of gaseous elemental mercury emissions: Insights from herbaria museum collections

Natural history collections are irreplaceable archives of global biodiversity, yet many remain affected by chemical contamination originating from historical conservation practices. Among these, mercury-based pesticides extensively used in herbaria continue to pose risks to human health, indoor air quality, and the accessibility of biodiversity collections. In this paper, we investigate the role of relative humidity (RH) in regulating gaseous elemental mercury (GEM) emissions from mercury-contaminated herbarium specimens, i.e. plant samples mounted on paper supports. We combined controlled laboratory experiments, using a non-steady-state flux chamber, with in-situ monitoring of GEM, temperature, and RH in the herbaria halls of the Natural History Museum of the University of Florence (Italy), hosting one of the world’s most important botanical collections. Our results demonstrated a strong, non-linear dependence of GEM emissions on RH. Mercury fluxes from contaminated specimens were very low in dry conditions (30–60% RH) but increased exponentially at RH above c. 65%. In-situ herbaria halls monitoring confirmed that RH increases systematically coincide with higher GEM concentrations. These findings may suggest that moisture-driven physicochemical changes in cellulose-based materials strongly enhance mercury mobilisation and volatilisation. By explicitly linking microclimatic control to contaminant emissions, this study provides actionable evidence bridging scientific understanding and risk management in indoor environments. Maintaining low and stable RH (40–60%) emerges as a cost-effective strategy to reduce GEM emissions while preserving herbarium specimens. More broadly, our results highlight how environmental controls can mitigate chemical pollution in biodiversity repositories, supporting safer access to collections essential for ecological, taxonomic, and global change research.