It is not difficult to work safely with mercury compounds, providing that sufficient care is taken to minimise spills and, should they occur, to make sure they are cleaned up immediately and thoroughly. It is also important to ensure that waste materials are disposed of safely. See this article for information about disposing of waste mercury compounds.
Mercuric chloride is also known as corrosive sublimate. This name is reportedly applied to it because it sublimes, and is corrosive to the mucosa of the mouth when administered orally. Today that seems ludicrous, knowing how poisonous it is, but in the past, before the advent of antibiotics, it was used as to treat some diseases, such as syphilis. It was administered in small doses and was sometimes effective, but mercury toxicity is cumulative, so the effect of these repeated doses was often tragic. At high enough levels the patient dies, but at lower levels neurological symptoms develop.
These symptoms are now collectively referred to as Minamata Disease, a reference to events at Minamata, Japan, where mercury contaminated fish formed a large part of the diet and resulted in many people developing these neurological symptoms. It was this tragedy which finally focussed attention on mercury contamination of the food chain and the dangers of eating too much contaminated fish. This is not just confined to Japan. Several years later there was a similar event in North-western Ontario, Canada, where fish were found to be contaminated with organic mercurial compounds which had found their way into the food chain, and the provincial government recommended limits on the amount that should be eaten.
It should not be surprising, then, that there is concern about mercury contamination of river systems and its effect on our health.
One of the well known characteristics of mercuric chloride is that it corrodes metal. When used as a component of fixative it is a standard comment that the tissue should be placed into a glass or plastic container, and that a metal lid should not be used. With the advent of plastic cassettes, this is no longer a major concern, but if a laboratory still uses the older metal cassettes, or metal cassette lids, then mercuric chloride containing fixatives should be used in glass or plastic jars with plastic lids. Placing a metal lid overnight in formal sublimate corrodes the metal so badly that it is no longer usable and, perhaps more importantly, covers the tissue in a thick grey sludge which interferes with sectioning. It should be noted that stainless steels are not resistant to mercuric chloride corrosion.
Latex, or similar, gloves should be worn when using mercuric chloride containing fixatives. While the likelihood is slight, repeated small spills onto the skin could result in toxic exposure due to the cumulative effects. Wearing gloves all but eliminates that possibility. Similarly, should a spill of a mercuric chloride containing fixative occur, gloves should be put on before it is wiped up, even if the spill is just a few millilitres.
Use disposable paper towels to absorb mercuric chloride solution spills. Then wash the bench well with water, several changes, absorbing each time with disposable paper towels. The towels should be collected as contaminated waste and included in the mercuric chloride disposal system. They must not just be discarded into the regular waste bins for transport to a landfill.
Wear latex gloves to weigh out mercuric chloride powder. If it is spilled it should first be collected with wet disposable paper towels by pressing the wet towel onto the powder. Collect all visible powder this way first. Then use quantities of water to dissolve any residual powder, and absorb this onto disposable paper towels. Repeated cleanings may be necessary. Always carefully check the sides of hands and the ends of coat sleeves. Once again, all of the paper towels used must be treated as mercury contaminated waste.
All glassware and plasticware used to contain a mercury fixative must be rinsed well with water before being discarded. Rinse thoroughly at least three times, the final time at least with fresh water, and save the rinse water. Do not discard it into a municipal, or other, waste system. The rinse water must be included in the materials for treatment as mercury contaminated waste. The glass or plastic ware may then be washed and reused, or discarded.
These simple precautions are usually all that are required to work safely with mercury compounds.
IHC World has a list of common histological chemicals and how they should be handled, including mercuric chloride, taken from "The ABC of Safe Practices for the Biological Sciences Laboratory", by Roy Ellis and Donald Perry"