Environmental Engineering Reference
In-Depth Information
. Elemental (or metallic) mercury—also an inorganic
form, which is mercury un combined with other
elements.
. Organic compounds, which refer to mercury com-
bined with methyl, ethyl, or other carbon groups.
High doses of methylmercury can cause severe,
potentially lethal damage to the nervous system, and
the developing nervous system is most sensitive (NRC,
2000). This was tragically demonstrated by the poison-
ings that occurred during the 1950s and 1960s, in and
around Minamata and Niigata, Japan, where several
chemical plants that produced acetaldehyde for the
manufacture of plastics were discharging methylmercury-
containing waste into the local waters (Ekino et al.,
2007). The fish from these waters—a dietary staple for
residents of the area—accumulated mercury at concen-
trations in the range of 6 to 36 parts per million (ppm)
(Harada, 1995). For comparison, most commercial fish
species sold in the United States (US) contain less than 1 ppm
(US FDA, 2006).
In Minamata, children exposed prenatally to high
doses of methylmercury through maternal fish con-
sumption were born with varying degrees of neurologic
damage, including cerebral palsy, mental retardation,
seizures, and sensory and reflex disturbances, some-
times resulting in death (Harada, 1995; NRC, 2000).
Symptoms in their mothers tended to be less severe,
and included numbness and tingling in the arms and
legs; lack of coordination or steadiness; disturbances in
speech, vision, and hearing; and tremor (Harada, 1995;
NRC, 2000). In some cases, symptoms in children and
adults improved upon cessation of exposure, while in
others they did not show up for months or years, or they
worsened over time (Kinjo et al. 1993; Harada, 1995;
US EPA, 2008a). Average hair mercury concentration in
mothers of affected children was estimated to be 41 ppm
(ranging from ~3.8 to ~133 ppm) (Akagi et al., 1998). In
the US, hair concentrations are typically under 1 ppm
(McDowell et al., 2004).
There is no question that exposure to methylmercury
at high doses is neurotoxic (International Program on
Chemical Safety [IPCS], 1990; NRC, 2000; US EPA, 2008a);
however, there is less agreement on the potential effects
of long-term exposure to lower levels (Myers et al., 2003;
Stern et al., 2004). In the past, attention has focused on
comparing two apparently confl icting studies of the associ-
ation between lower-level prenatal exposure and neurode-
velopmental outcomes in children from populations that
consume fi sh or seafood on a regular basis. One of the stud-
ies included 917 children from the Faroe Islands, where
prenatal exposure occurred primarily through maternal
consumption of the meat from pilot whales (Grandjean et
al., 1992). At ages 7 and 14 years, higher prenatal exposure
was associated with subtle defi cits in attention, language,
memory, and possibly visuospatial and motor functions
(Grandjean et al., 1997; Debes et al., 2006). The geometric
mean total mercury concentration in umbilical-cord blood
collected from this cohort of children was ~23 µg/L (inter-
quartile range, 13 to 41), and it was 4.3 ppm in maternal
hair at delivery (interquartile range, 2.6 to 7.7). In contrast,
a study conducted in the Republic of the Seychelles on
Elemental mercury in its silvery, liquid state is perhaps
most familiar, although its vapor has a far greater impact
on health. The toxicity of mercury varies according to the
form and route of exposure, the dose, and the age at which
a person is exposed (ATSDR, 1999).
Exposure Sources and Health Effects
Human exposure to mercury today may occur from a num-
ber of sources, including dietary fi sh and seafood, dental
amalgams, some vaccines, cosmetics and skin care prod-
ucts, traditional/herbal medicine products, ritualistic prac-
tices involving elemental mercury, some occupational set-
tings, and spills or breaks of mercury-containing articles
or devices. Methylmercury is arguably the most impor-
tant form for exposure to mercury worldwide because of
its pervasiveness in fi sh and its neurotoxicity (IPCS, 1990;
US EPA, 1997).
Methylmercury in Fish
Methylmercury makes its way into fi sh tissue through
a series of steps that begins with emission of elemental
mercury into the atmosphere, in part from natural pro-
cesses (e.g., volcanic activity, forest fi res, weathering of
rocks, and degassing from soil and surface water), and also
as a result of human activities (e.g., combustion of coal
and other fossil fuels, waste incineration, and recycling of
previously deposited anthropogenic mercury) (Lindberg
et al., 2007).
Atmospheric mercury may travel short or long distances
and deposit in the sediment beds of bodies of water, where
it can be methylated by microorganisms. Methylmercury
then travels up the food chain through a bio-accumulative
process when large fi sh eat smaller fi sh. The result is that fi sh
species highest on the food chain (e.g., shark and swordfi sh)
and those that have lived the longest have the highest lev-
els. Fish-eating birds and sea mammals can also accumulate
high levels of mercury (ATSDR, 1999; US EPA, 1997). Most
fi sh contain at least some mercury in their tissues, most of
which is methylated (US EPA, 1997; IPCS, 1976).
Upon ingestion, an estimated 95% of the methylmercury
in fi sh tissue is absorbed into the bloodstream and distributed
throughout the body (National Research Council [NRC],
2000; IPCS, 1990). Methylmercury crosses both the blood-
brain barrier and the placenta. In humans, it has a whole-
body half-life of about 70 days (50 days in blood) (Clarkson
et al., 2003; Clarkson and Magos, 2006). Exposure in the
general population is believed to occur almost entirely
through consumption of fi sh or seafood.
Search WWH ::




Custom Search