GGR374H1 Chapter Notes - Chapter 4: Methylmercury, Primitive Reflexes, Intellectual Disability
Chapter 4Human Health and Heavy Metal Exposure:
Metals are notable for their wide environmental dispersion from such activity; their
tendency to accumulate in select tissue of the body; and their overall potential to be
toxic even at relatively minor levels of exposure.
Some metals such as iron and copper are essential life and play a big role.
However, some metals are xenobiotics they have no useful role in human
One of the reflection of the importance of metals relative to other potential hazards
in their making by the U.S agency for toxic substances and disease registry
(ATSDR) which lists the hazards present in toxic waste sites according to their
prevalence and severity of their toxicity
The first, second, third and sixth hazards respectively on the list are heavy metals:
lead, mercury, arsenic and cadmium.
Exposure to metals can occur through a variety of routes: metals may be inhaled as
dust or fumes. Some metals can be vaporized and inhaled. Metals can also be
ingested involuntarily from food and drink.
The amount of actually absorbed from the digestive tract can vary widely,
depending on the chemical form of the metal and the age and nutrional status of the
Once a metal is absorbed it distributes in tissues and organs.
Excretion typically occurs primarily through the kidneys and digestive tracts, but
metals tend to persist in some storage sties like the liver, bones and kidneys for
years or decades.
Toxicity of metals commonly involves the brain and the kidney, but other
manifestation occurs, and some metals such as arsenic are clearly capable of
An exposure remains high or is increasing in many developing countries through a
rapid increase in vehicles combusting leaded gasoline and in polluting industries.
Individuals will absorb more lead in their food if their diets are deficient in calcium,
iron or zinc.
Depending on the dose, lead exposure in children and adults can cause a wide
spectrum of health problems, ranging from convulsion, cona, renal failure and death
at the high end to subtle effects on metabolism and intelligence at the low end of
Children are particularly vulnerable to the neurotoxin effects of lead
Low level lead exposure in children less then five years of age, results in deficits in
intellectual development, as manifested by lost of IQ points.
Maternal bone lead stores are mobilized at an accelerated rate during pregnancy and
location are associated with decrements in birth weight, growth rate and mental
Mercury has been increasing in importantance as a widespread contaminant.
When deposited in soil, organic mercury compounds are slowly broke down into
inorganic compounds; conversely inorganic mercury can be converted by
microorganism in soil and water into the organic compound methyl mercury, which
is then biocentrated up the food chain.
At lower but more chronic level of exposure, a typical excitability, memory loos,
insomnia, timidity and sometimes delirium- that were once commonly seen to
workers exposed to mercury.
Mothers exposed to mercury in 1955 in minmata bay, Japan, gave birth to infants
with mental retardation, retention of primitive reflexes, cerebella symptoms, and
Even at much lower levels, mercury exposure on part of pregnant women through
deity intake of fish and whale meat, an important regional food staple is associated
with decrements in motor function, language, memory, and neural transmission in
Arsenic is a toxic and carcinogenic heavy metal, and human exposures are
Significant exposure to arsenic occurs through both anthropological and natural
Widespread dispersion of arsenic is a by-product of the combustion of fossil fuels
in which arsenic is a common contaminant.
The toxicity of an arsenic-containing compound depends on its valence state, its
form, and factors that modify its absorption and elimination.
Inorganic arsenic is generally more toxic than organic arsenic, and trivalent
arsenate is more toxic than pentavelent and zero-valent arsenic.
Chronic exposure to lower levels of arsenic results in somewhat unusual patterns
of skin hyper pigmentation, peripheral nerve damage manifesting as numbness,
tangling, and weakness in the hand and feet, diabetes and blood-vessel damage
resulting in gangrenous conditioning affecting extremities.
Exposure to this metal causes distinctive clinical syndromes.
Cadmium exposure is encountered in industries, dealing with pigment, metal
plating, some plastics, and batteries.
Can be exposed to humans through the ingestion of contaminated foodstuffs,
especially grains, cereals and leafy vegetables.
Airborne cadmium exposure is also a risk posed by the incineration of municipal
waste containing plastics and nickel-cadmium batteries.
Cigarette smoking constitutes an additional major source of cadmium exposure.
The health implications of cadmium exposure are exacerbated by the relative
inability of human beings to excrete cadmium.
Acute high-dose exposure can cause severe respiratory irritation.
Occupational levels of cadmium exposures are a risk factor for chronic lung disease
and testicular degeneration.
Even without causing kidney failure, however cadmium’s effect on the kidneys can
have metabolic effects with pathological consequences.
The loss of calcium caused by cadmium effect on the kidney can be severe enough
to lead to weakening of bones. ``itai-itai`disease, an epidemic of bone fractures in
Japan from gross cadmium contamination of rice stocks, has recently been shown
to happen in more subtle fashion among a general community living in an area of
relatively modest cadmium contamination.
Increased cadmium burden in this population was found to be predictive of an
increased risk of bone fractures in women, as well as decreased bone density and
height loss in both sexes.
Other metal concerns:
Manganese has become a metal of global concern because of the introduction of
methycyclopentadienyl magnese tricarbonyl (MMT) as a gaso-line additive.
Proponents of the use of MMT have claimed that the known link between
occupational magnese exposure and the development of a Parkinson disease-like
syndrome of tremor, postural instability, gait disorder, and cognitive disorder has
no implication for the relatively low levels of manganese exposure that would
ensue from its use in gasoline.
Aluminums contributes to the brain dysfunction of patients with sever kidney
disease who are undergoing dialysis.
High levels of aluminium have been found in neurofibrillary tangle, as well as in
the drinking water and soil of areas with unusually high incidence of Alzheimer’s
Experimental and epidemiologic evidence for a casual link between aluminium
exposure and Alzheimer’s disease is overall relatively weak.
Chromium, in its hexavalent form, which is the most toxic species of chromium.
Toxicity of chromium stems from its tendency to be corrosive and to cause allergic
Chromium is a carcinogen, particarully of the lung through inhalation.
Prescription to reduce human exposure to heavy metals:
Prescription 1: accelerate and complete the global phase-out of leaded gasoline.
Prescription 2: begin an effort to monitor levels and trends in metals pollution
Prescription 3: establish population based biomonitoring for selected metals.
Prescription 4: educate governments, scientists, and the general public about the
toxicity of metals
Prescription 5: declare a moratorium on the production, distribution, and use of
products likely to significantly increase global exposure to toxic metals.
Prescription 6: continue basic research into the impact of metals on human health.