Due to its toxicity and environmental problems, the trend has been away from mercury toward friendlier substitutes. If you have spent much time in a laboratory, you have most likely noticed alcohol thermometers and other such changes. However, the hazards are not only from elemental mercury. Read on to learn about the others.
Mercury, aka quicksilver, with the chemical symbol Hg and atomic weight of 200, is the 80th element of the periodic table. It takes three different forms—elemental, organic compounds, and inorganic compounds. Their properties, hazards, and symptoms of exposure are very different. We will also discuss safe handling, providing tips on everything from managing wastes to spill cleanups.
Forms of mercury
The most common form encountered in the lab is elemental mercury, found in thermometers, barometers, manometers, and sphygmomanometers, to name a few instruments found in laboratories. But you may have noticed less of these recently, as the potential hazards have led to phasing out mercury, especially in health care and clinical settings, for less harmful alternatives such as alcohol-filled thermometers and solid-state or infrared electronic instruments. (You might want to pass this article to your facility managers, physical plant personnel, and maintenance workers, as mercury is also found in batteries, float valves, mercury switches, and relays and fluorescent lamps, among other building items.)
Inorganic mercury compounds are formed when mercury combines with elements such as chlorine, sulfur, and oxygen. In the past, these compounds were found in laxatives and latex paints. However, the EPA canceled registration for mercury in interior paints in 1990 and this use was discontinued in 1991. In addition, most agricultural and pharmaceutical uses have also been discontinued in the US. One exception is mercuric chloride, which is still used in disinfectants and some pesticides.
Organic mercury compounds have little industrial value and few uses but are the most toxic. They are usually categorized under the generic term methyl mercury. They form primarily in nature through a metabolic process called biomethylation. Organomercury compounds are usually encountered as environmental pollution from contaminated fish or polluted waters but may also be used in chemical and research laboratories.
Exposure symptoms and effects
Elemental mercury, metallic mercury (nicknamed quicksilver), is the most common form found in labs and typical work environments. It is a silver, odorless liquid at normal temperatures and a major source of occupational exposures, primarily from inhalation of vapors following a spill or inadequate cleanup of one. Acute (short-term) exposures affect the central nervous system (CNS) producing tremors, mood change, and reduced cognitive, sensory, and motor nerve functions. Extremely high doses can affect the kidneys, causing renal failure. Chronic (long-term) low level exposures can cause the same CNS effects with increased irritability, insomnia, memory loss, headaches, and development of proteinuria.
Exposures to inorganic mercury compounds occur usually through ingestion with potential for dermal absorption. Acute exposures cause a metallic taste in the mouth and lead to nausea, vomiting, and severe abdominal pain. As little as one to four grams can be lethal for the average 70 kg person. Chronic exposures to inorganic mercury compounds produce kidney damage similar to that produced by metallic mercury vapors.
Organic mercury compounds produce the most severe effects. Exposures occur primarily from inhalation and ingestion, but dermal absorption is also significant. Acute inhalation exposures are rare, but lead to major CNS effects such as blindness, deafness, and impaired consciousness. Oral ingestion produces severe reproductive and developmental affects such as mental retardation, ataxia, blindness, and cerebral palsy. Chronic inhalation or ingestion damages the central nervous system with the earliest symptoms showing as blurred vision, malaise, and speech difficulties. Dermal exposures are very dangerous as it is easy to receive toxic doses. Low levels produce all the symptoms above, but a high dose (as little as 200 mg) can lead to death. (Recall the Dartmouth researcher who tragically received a fatal dose through protective gloves, albeit the wrong type.)
Safe handling and proper cleanup
Although OSHA does not have a specific standard covering mercury use in the workplace, there are established permissible exposure limits for mercury compounds. These are contained in the air contaminants table (29CFR1910.1000, Table Z-2). If any mercury compound is used in your workplace, an exposure assessment and hazard evaluation is required for those employees handling the material.
The OSHA permissible exposure limit (PEL) is 0.01 mg/M3 as an eight-hour time weighted average (TWA) for organic (alkyl) mercury compounds with a ceiling of 0.1 mg/M3 for all other mercury-containing materials. A ceiling limit is an exposure limit that you should never exceed during any part of the workday.
The most common and probable exposures will be from dealing with metallic mercury cleanups from accidental spills or broken equipment (thermometers, switches etc.). If you are performing specialized research with an organic mercury compound, then all employees must thoroughly review and understand the safety data sheets. We strongly recommend you develop and follow detailed safety protocols and SOPs. Perform all work in a fume hood and ensure employees are using the right personal protective equipment. Finally, have a qualified industrial hygienist review the SOPs and conduct the exposure assessments.
A few final words on mercury spill cleanups. First, secure the area and prevent any unnecessary foot traffic. Use a mercury-binding sulfur powder (or commercially prepared mixes) to dust the entire area where droplets are visible. A special mercury vacuum is needed to vacuum up the dust and free mercury. If the spill was onto carpet or porous materials, those should be removed and disposed (as hazardous waste) after vacuuming. Impervious surfaces should be dusted and vacuumed at least three times. Once you are reasonably sure all mercury has been collected and removed, have a qualified industrial hygienist conduct a clearance assessment, which should include air sampling for vapors. Once clearance testing passes, the area is ready for re-occupancy.