Engineering Controls Database
Testing for Mercury Vapor in Herbarium Cabinets
| Mercuric chloride solutions have been used as pesticide and/or fungicide treatments for botanical specimens at least since the 18th century, were still in use in at least one major herbarium in 1982, and may continue to be used in some herbaria. Mercuric chloride is fairly soluble in water and very soluble in ethyl alcohol at 30oC. Alcohol or water solutions were used to immerse or spray specimens, pressing papers, and mounting sheets. Mercuric chloride, mercuric sulfide, and metallic mercury all constantly emit elemental mercury vapor into the surrounding air and the transition from solid compound to gaseous element is enhanced by light. | |
| Elemental mercury vapor and inorganic mercury compounds pose a threat to human health by targeting the central nervous system, kidneys, respiratory system, eyes and skin, and constant generation or sublimation means that untreated specimens may become contaminated when housed in cabinets with treated specimens. | |
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To detect levels of mercury vapor in herbarium cabinets, a microscope slide is coated with a thin layer of mercury indicator powder mixed with deionized water. The mercury indicator is composed of a mixture of cuprous iodide, sulfur, amorphous silica, and starch in proprietary proportions. The dry, yellow powder becomes reddish-brown on exposure to mercury vapor, and becomes gray in color when wetted and then exposed to the vapor. This technique can be used to screen empty cabinets to determine if they have become contaminated by mercury vapor over time and to test incoming shipments of specimines to determine whether they are likely to cause cross-contamination if housed with specimens that were never treated with mercury salts. The sensitivity of the indicator was shown to be greater than that of the Jerome 431-X Mercury Vapor Analyzer and comparable to that of the Lumex RA-915+ Analyzer, suggesting that the indicator is an especially useful tool when screening for very low concentrations of mercury vapor in closed environments. Once the presence of mercury vapor is detected within a particular cabinet, it must be presumed that vapor will re-accumulate inside the cabinet after it has been closed again. Safe work practices must be implemented that will minimize the initial, most acute exposure to mercury vapor upon opening a cabinet. Timed aeration of cabinets (i.e., opening the cabinet door and walking away) as a work practice can be effective in significantly decreasing ambient concentrations to within established standards. It is also recommended that cases where mercury levels are above 25 µg/M3 should be located in well-ventilated spaces, with sufficient space between rows to allow for adequate supply and return air circulation patterns.  |
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