DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

Exposure. Inorganic mercury cations and metallic mercury are found
in the Earth’s crust, and mercury vapor is released naturally into the
environment through volcanic activity and off-gassing from soils.
Mercury also enters the atmosphere through human activities such as
combustion of fossil fuels. Once in the air, metallic mercury is photooxidized
to inorganic mercury, which can then be deposited in
aquatic environments in rain. Microorganisms can then conjugate
inorganic mercury to form methyl mercury. Methyl mercury concentrates
in lipids and will bioaccumulate up the food chain so that
concentrations in aquatic organisms at the top of the food chain, such
as swordfish or sharks, are quite high (Figure 67–6; ATSDR, 1999).
The primary source of exposure to metallic mercury in the
general population is vaporization of mercury in dental amalgam,
which often contains >50% Hg 0 mixed with silver and other metals.
This release is enhanced by chewing. There also is limited exposure
through broken thermometers and other mercury-containing devices.
Human exposure to organic mercury primarily is through the consumption
of fish. Other foods contain inorganic mercury at low levels
(ATSDR, 1999).
Workers are exposed to metallic and inorganic mercury, most
commonly though exposure to vapors. The highest risk for exposure
is in the chloralkali industry (i.e., bleach) and in other chemical
processes in which mercury is used as a catalyst. Mercury is a component
of many devices, including alkaline batteries, fluorescent
bulbs, thermometers, and scientific equipment, and exposure occurs
Hg 0
Hg 0
O 2 , light
Vaporization
(natural or
industrial)
Hg 2+
Rain
Hg 2+
during the production of these devices. Dentists also are exposed to
mercury from amalgam. Mercury can be used to extract gold during
mining, which results in substantial occupational exposure, because
the last step involves vaporization of the mercury. This process is
still commonly used in developing countries. Mercuric salts are used
as pigments in paints (ATSDR,1999).
Mercury salts were once found in a number of medications,
including antiseptics, antidiuretics, skin-lightening creams, and laxatives.
Most of these uses have been replaced by safer and more
effective drugs. Thimerosal is an antimicrobial agent used as a preservative
in vaccines. Its use is controversial because it releases ethyl
mercury, which is chemically similar to methyl mercury. Due to the
concerns of some parents that thimerosal might be a cause of autism,
the American Academy of Pediatrics and the U.S. Public Health
Service issued a call for its replacement in vaccines to improve the
prevalence of vaccination (Ball et al., 2001). However, studies have
not found an association between thimerosal use in vaccines and negative
outcomes, and it is still used in influenza vaccines (Heron and
Golding, 2004).
Chemistry and Mode of Action. There are three general forms of mercury
of concern to human health. Metallic, or elemental, mercury
(Hg 0 ) is the liquid metal found in thermometers and dental amalgam;
it is quite volatile, and exposure is often to the vapor form.
Inorganic mercury can be either monovalent (mercurous, Hg 1+ ) or
Micro-organisms
MeHg 2+
MeHg 2+
1865
CHAPTER 67
ENVIRONMENTAL TOXICOLOGY: CARCINOGENS AND HEAVY METALS
Figure 67–6. Mobilization of mercury in the environment. Metallic mercury (Hg 0 ) is vaporized from the Earth’s surface both naturally
and through human activities such as burning coal. In the atmosphere, Hg 0 is oxidized to form divalent inorganic mercury (Hg 2+ ).
Hg 2+ then falls to the surface in rain. Aquatic bacteria can methylate Hg 2+ to form methyl mercury (MeHg + ). MeHg + in plankton is
consumed by fish. Because of its lipophilicity, MeHg + bioaccumulates up the food chain.