Mercury (Hg) is a heavy, silvery-white metal, unique because it is the only elemental metal that remains liquid at standard room temperatures. Historically, this property led to its use in devices like thermometers, barometers, and electrical switches. The primary health concern associated with elemental mercury is its tendency to transform into an invisible, odorless gas. This vaporization allows mercury to enter the surrounding air, creating a potential exposure pathway.
Boiling Point Versus Room Temperature Evaporation
The temperature at which elemental mercury transitions entirely from a liquid to a gas is its boiling point, approximately 357°C (674°F) at standard atmospheric pressure. At this high temperature, the liquid’s vapor pressure equals the surrounding atmospheric pressure, causing the bulk liquid to boil. Evaporation, however, is distinct from boiling and occurs at any temperature above mercury’s freezing point.
Evaporation is driven by the liquid’s vapor pressure, which is the pressure exerted by its gaseous phase. Elemental mercury has a significant vapor pressure even at room temperature (20–25°C). This means a constant, measurable amount of mercury atoms has enough kinetic energy to escape the liquid surface and become gaseous vapor. Consequently, mercury is continuously evaporating, even when it appears stationary.
Factors Influencing Vaporization Rate
The rate at which elemental mercury vaporizes is influenced by environmental and physical parameters. Ambient temperature is a significant factor, as the vapor pressure of mercury increases logarithmically with heat. For example, the rate of vaporization roughly doubles for every 10°C increase in temperature. This means a spill in a warm room will release much more vapor than one in a cool basement.
The physical state of the spilled mercury, specifically its total exposed surface area, is also a determining factor. When liquid mercury spills, its high surface tension causes it to break into numerous tiny, scattered beads. This scattering dramatically increases the total surface area exposed to the air compared to a single large pool, accelerating the release of vapor. Furthermore, a lack of ventilation allows the released vapor to accumulate, leading to a higher concentration in the breathing zone. Proper airflow helps dilute the vapor concentration and move it away from the source.
Health Hazards of Inhaling Mercury Vapor
The main danger associated with elemental mercury comes from inhaling the invisible, gaseous vapor it constantly emits. Once inhaled, the vapor is readily absorbed through the lungs, with up to 80% entering the bloodstream. Because it is lipid-soluble, the vapor can easily cross the blood-brain barrier.
Inside the body, the elemental mercury is oxidized into an inorganic form that binds to proteins, leading to its accumulation in organs, primarily the brain and the kidneys. Acute exposure to high concentrations of vapor can cause immediate respiratory distress, including cough, chest tightness, and difficulty breathing, and can progress to severe lung damage. Chronic, low-level exposure is associated with neurotoxicity, presenting as a classic triad of symptoms: tremors, psychological disturbances (erethism), and inflammation of the gums. Long-term exposure also causes nephrotoxicity, a form of kidney damage characterized by protein in the urine.
Safe Handling and Cleanup of Mercury
Addressing a mercury spill requires a specific procedure to minimize vapor exposure and contamination. The immediate action should be to ensure good ventilation by opening exterior windows and closing interior doors to isolate the area. Everyone, including pets, should be removed from the contaminated space. No one should walk through the spilled material, as this spreads the beads and increases the surface area for evaporation.
It is imperative never to use a vacuum cleaner, mop, or broom on a mercury spill. A vacuum will heat the liquid and blow the vapor throughout the room. A broom or mop will break the beads into smaller, more numerous droplets, significantly worsening the contamination.
Cleanup involves using stiff paper or cardboard to gently push the beads together, collecting them with an eyedropper, and using sticky tape to pick up any remaining tiny specks. All cleanup materials, including contaminated gloves and clothing, must be placed in sealed, heavy-duty plastic bags or containers and disposed of as hazardous waste by contacting local authorities.