Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 8 pdf

Among the many heavy metals, lead Pb, mercury Hg, cadmium Cd, arsenic As, chromium Cr, zinc Zn, and copper Cu are of most concern, although the last three metals are essential nutrients

CHAPTER 8

Heavy Metals

Pollution caused by heavy metals is now a worldwide phenomenon Among the many heavy metals, lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), zinc (Zn), and copper (Cu) are of most concern, although the last three metals are essential nutrients in animal and human nutrition These metals are widely used

in industry, particularly in metal-working or metal-plating, and in such products as batteries and electronics They also are used in the production of jewelry, paint pigments, pottery glazes, inks, dyes, rubber, plastics, pesticides, and even in medi-cines These metals enter the environment wherever they are produced, used, and ultimately discarded

Heavy metals are very toxic because, as ions or in compound forms, they are soluble

in water and may be readily absorbed into living organisms After absorption, these metals can bind to vital cellular components such as structural proteins, enzymes, and nucleic acids, and interfere with their functioning In humans, some of these metals, even in small amounts, can cause severe physiological and health effects

In this chapter, we will consider Pb, Cd, and Hg, the three heavy metals widely recognized as the most toxic in our environment

LEAD

Lead (Pb) is one of the ancient metals and has been used by humans for several thousands of years Pb plays an important role in the economy of all industrialized countries in the world In the U.S., the industrial consumption of Pb is estimated to

be about 1.3 million tons per year, with a concomitant annual emission of about 600,000 tons of Pb into the environment (NAS 1980) Additional amounts are added through mining, smelting, manufacturing, disposal, and recycling processes Fur-thermore, until recently huge amounts of Pb and its compounds had been emitted into the atmosphere as a result of leaded gasoline combustion Consequently, Pb is ubiquitous in our environment

Because Pb is toxic to humans at high doses, levels of exposure encountered by some members of the population constitute a serious public health problem (NAS)

The importance of Pb as an environmental pollutant is apparent since the Environ-mental Protection Agency (EPA) has designated Pb as one of the six criteria air pollutants

Properties and Uses

Lead has a low melting point (326°C) It is a soft, malleable metal and it can

be easily formed into a variety of shapes It also can form alloys with many other metals Other important industrial products containing Pb include pipes, paints, solders, glass, pottery glazes, rubber, plastics, and insecticides

Exposure

Atmospheric Lead

Sources of atmospheric Pb include lead smelters, burning of coal and materials containing Pb, refining of scrap, wind blown from soils, and lead alkyls from gasoline Effluents from smokestacks and other gaseous emissions from smelters and refining processes can distribute significant quantities of Pb into the air, soils, and vegetation growing nearby However, the most common source of Pb contam-ination in ambient air until recently was the exhaust from automobiles Tetraethyl lead was introduced as an antiknock agent in gasoline in the 1920s and since then has played an increasingly important role as an atmospheric pollutant Following the mandatory use of unleaded gasoline and improved industrial emission control, atmospheric Pb emission has decreased dramatically According to an EPA report,

Pb emission from major emission sources in the U.S decreased from 56,000 to 7100 metric tons per year between 1981 and 1990 (EPA 1991) While the atmospheric

Pb pollution problem in other developed countries likewise has been significantly reduced, a similar trend has not occurred in many third-world countries

Waterborne Lead

Surface waters may contain significant amounts of Pb when subjected to some special contamination About 14% of representative drinking water supplies (i.e., piped drinking water) were found to contain more than 10 mg/l in a 1963–1965 survey Less than 1% was found to be in excess of 30 mg/l On the other hand, rainwater collected near a busy highway may contain as much as 50 mg/l

Another serious problem related to waterborne Pb is lead shot left in the North America’s lakes and ponds A large number of waterfowl in the U.S are poisoned

or killed following ingestion of the shot

Lead in Food

Food has long been a major source of Pb intake for animals and humans Animals may ingest Pb-contaminated vegetation and become intoxicated In humans, Pb may

be ingested through Pb-contaminated containers or Pb pottery glazes Researchers suggest that some Roman emperors might have become ill and even died from Pb poisoning by drinking wines contaminated with high levels of Pb

Vegetation growing near highways has been shown to accumulate high amounts

of Pb deposited from automobile exhaust (Lagerwerff et al 1973; Khalid et al 1996) Pica, children’s craving for unnatural foods, is thought responsible for the chronic

Pb poisoning among many poor urban children, as they eat flaking paint from the walls of old houses About 27 million housing units were built before 1940 when

Pb was in common use (Lin-Fu 1982) Lead paint poses a major threat for children and is one of the major public health problems that many communities face

Lead in Soils

Lead and other metals can impact soils and biota by deposition from polluted air Stack emission from smelters (Little and Martin 1972) and emission from automobile exhaust systems along highways are examples Pb contamination due to mine wastes also is an important problem in areas surrounding metal mines Earlier reports indicate that about 50% of the Pb liberated from motor vehicles in the U.S was deposited within 30 m of the roadways (Ryan 1976) and the remainder was scattered over large areas Lead accumulation in soils near roads varies with traffic volume and decreases rapidly with distance from the road For example, Pb con-centrations of 128 to 700 ppm are found in soil adjacent to 12 highways in the Minneapolis-St Paul area (Ryan 1976) These levels were much greater than the reported value of 10 to 15 ppm in unpolluted rural soils Grass collected near an intersection of two heavily traveled highways near Denver, CO contained as much

as 3000 ppm Pb, while vegetable samples from gardens less than 50 ft from roads

in Canandaigua, NY averaged 115 ppm Pb (range: < 10 ppm to 700 ppm)

In an attempt to assess the effect of the mandatory use of unleaded gasoline in new automobiles on Pb concentrations in highway soils, Byrd et al (1983) studied

Pb concentrations in soils along U.S Interstate 20 in northeast Louisiana and observed that the concentrations increased from 1973 to 1974 but decreased from

1975 to 1979 They concluded that the mandatory use of unleaded gasoline had significantly reduced the Pb concentrations in soils near highways

Lead Toxicity

Effect on Plants

Plants exposed to high levels of Pb from ambient air and soils can accumulate the metal and manifest toxicity The toxicity and presence of other trace metals vary greatly among plant species Based on in vitro studies, toxicity sequences have been determined for several species Barley plants were shown to be more sensitive to

Pb than to Cr, Cd, Ni, or Zn (Oberlander and Roth 1978), and exposure to relatively high levels of Pb was shown to inhibit seed germination (Koeppe 1977; Yu 1991) The effect of Pb on germination, however, was found to be less severe compared to

several other metals such as Cd, As, and Hg (Koeppe; Fargasova 1994) It is important

to note that, following plant uptake, Pb moves into the food chain and thus can affect animals and humans

Effect on Animals

The effect of Pb on freshwater fish varies depending on the species of fish Goldfish, for example, are relatively resistant to Pb, presumably due to their abundant gill secretion As mentioned above, following the ingestion of expended lead shot

in lakes or in the field, more than one million birds are estimated killed each year

in the U.S Lead absorbed by the bird paralyzes the gizzard leading to starvation, and death usually follows within several weeks after the exposure

Effect on Humans

Daily intake of Pb in humans is estimated to range from 20 mg to 400 mg per person The FAO/WHO Expert Committee established a Provisional Tolerable Weekly Intake (PTWI) of 3000 mg, approximately 500 mg/day Only one-half of this amount appears to be safe for children About 5 to 15% of ingested Pb is absorbed This amounts to 15 to 25 mg/day and represents two-thirds of the total absorbed Pb By contrast, about 20 to 40% of the inhaled Pb is absorbed, amounting

to about 8 mg/day, or one-third of the total absorbed Pb

The considerably higher blood Pb levels in industrial populations reflect wide-spread environmental Pb pollution However, data obtained from the Second National Health and Nutrition Examination Survey (NHANES II) indicate that there has been a reduction in the overall mean blood-lead level of the U.S population during the period 1976 through 1980, from 15.8 mg/dl to 10.0 mg/dl (Lin-Fu 1982)

It is suggested that an increased use of unleaded gasoline by the U.S population may be responsible for the observed decrease

Lead is one of the systemic poisons in that once absorbed into the circulatory system, it is distributed throughout the body where it causes serious health effects Manifested effects of Pb poisoning include nausea, anorexia, severe abdominal cramps, weight loss, anemia, renal tubular dysfunction, muscle aches, and joint pains Lead can pass the placental barrier and may reach the fetus, resulting in miscarriages, abortions, and stillbirths

Through interaction with cellular components of brain cells, Pb also adversely affects the central nervous system (CNS) Clinical symptoms such as encephalopa-thy, convulsions, and delirium may occur In severe cases coma and death may follow These injuries are often reflected by behavioral disturbances observed in Pb-poisoned victims

It is estimated that approximately 90% of Pb absorbed by humans is deposited

in the bone (Aufderheide and Wittmers 1992) Bone, however, is no longer consid-ered a sink for Pb in the body Rather, it is recognized as a two-way process of active influx and efflux of Pb to and from the bone and blood stream (Silbergeld

et al 1993) As a result, bone acts like a reservoir for Pb, thus influencing the exposure of the metal in the body

Although there is evidence that both inorganic and organic lead compounds are carcinogenic in experimental animals (Cherlewski 1979; Blake and Mann 1983), no conclusive evidence has been reported in humans

Biochemical Effect

Lead is taken up and transported in plants (Cannon and Bowles 1962) and can decrease cell division at very low concentrations Lead inhibits the electron transport

in corn mitochondria, especially when phosphate is present (Koeppe and Miller 1970)

Lead, as mentioned above, is a systemic poison and can induce deleterious effects

in living organisms The biochemical effect of Pb is complex and, in certain areas, its mode of action remains unclear Several well-established biochemical effects are discussed here First, as an electropositive metal, Pb has a high affinity for the sulfhydryl (SH) group Enzymes that depend on the SH group as the active site are, therefore, inhibited by Pb In this case, Pb reacts with the SH group on the enzyme molecule to form mercaptide, leading to inactivation of the enzyme The following reaction depicts such a relationship:

2RSH + Pb2+

→ R–S–Pb–S–R + 2H+ Examples of the sulfhydryl-dependent enzymes include adenyl cyclase and aminotransferases Adenyl cyclase catalyzes the conversion of ATP to cyclic AMP needed in brain neurotransmission Aminotransferases are involved in transamination and thus important in amino acid metabolism

Second, divalent Pb is similar in many aspects to Ca and may exert a competitive action on body processes such as mitochondrial respiration and neurological func-tions Lead can compete with Ca for entry at the presynaptic receptor Since Ca evokes the release of acetylcholine across the synapse, this inhibition manifests itself

in the form of decreased endplate potential The miniature endplate potential release

of subthreshold levels of acetylcholine has been shown to be increased (Barton et al 1978) The close chemical similarity between Pb and Ca may partially account for the fact that they seem interchangeable in biological systems and that 90% or more

of the total body burden of Pb is found in the skeleton

Third, Pb can interact with nucleic acids, leading to either decreased or increased protein synthesis Lead has been shown to reduce the ability of t-RNA to bind ribosomes The effect of Pb on nucleic acids, therefore, has important biological implications (Barton et al 1978)

Finally, it is widely known that Pb impairs the formation of red blood cells The mechanism involved in the impairment is that Pb inhibits both δ-aminolevulinic acid dehydratase (ALA-D)(Hernberg et al 1970) and ferrochelatase (Tephly et al 1978) These are two key enzymes involved in heme biosynthesis ALA-D catalyzes the conversion of δ-aminolevulinic acid into porphobilinogen (PBG), whereas ferroche-latase is responsible for catalyzing the incorporation of Fe2+ into protoporphyrin IX

to form heme (Figure 8.1) Lead inhibition of the two enzymes appears to be due

to its interaction with Zn and Fe required in the process

Cadmium (Cd) is a transition metal in Group IIb along with Zn and Hg It is frequently associated with Zn The U.S is the world’s largest producer of cadmium, with an annual output of about 5000 short tons Mexico is an important producer

of Cd-bearing dusts and fumes, but most of these are smelted in the U.S

Properties and Uses

Cadmium is a silver-white metal with an atomic weight of 112.4 and a low melting point of 321°C It is malleable and can be rolled out into sheets The metal unites with the majority of the heavy metals to form alloys It is readily oxidized to the +2 oxidation state, producing the colorless Cd2+ ion Cadmium persists in the environment with a half-life of 10 to 25 years

About two-thirds of all Cd produced is used in the plating of steel, Fe, Cu, brass, and other alloys to protect them from corrosion Other uses include solders and electrical parts, pigments, plastics, rubber, pesticides, galvanized iron, etc Special uses of Cd include aircraft manufacturing and semi-conductors Because Cd strongly absorbs neutrons, it is also used in the control rods in nuclear reactors

Exposure

General sources of exposure to Cd include air, water, and food Atmospheric emission of Cd may arise from such activities as mining and metallurgical process-ing, combustion of fossil fuel, textile printprocess-ing, application of fertilizers and fungi-cides, recycling of ferrous scraps and motor oils, disposal and incineration of Cd-containing products (e.g., plastics), and tobacco smoke

The major nonoccupational routes of human Cd exposure are through ingestion and inhalation Ambient air is not a significant source of Cd exposure for the majority

of the U.S population Nearly all airborne Cd is due to human activities and thus the highest concentrations are found in industrialized cities and in the vicinity of smelting operations (Fleischer 1974) While aerial deposition is an important route

of mobility for Cd, airborne routes of exposure are not as important as soil and water routes

Tobacco in all of its forms contains appreciable amounts of Cd, and tobacco smoke is one of the largest single sources of Cd exposure to humans Since the absorption of Cd in the lungs is much greater than that from the gastrointestinal tract, smoking contributes significantly to the total body burden Each cigarette on

Figure 8.1 Steps in heme synthesis inhibited by lead.

the average contains approximately 1.5 to 2.0 mg of Cd, of which 70% passes into the smoke

Waterborne Cd is probably the largest problem because it is common in the aquatic environment Many Cd-containing wastes end up in lakes and marine water Wastes from Pb mines, various chemical industries, motor oils, and rubber tires are some examples

Cadmium pollution of soils can occur from several sources; a major one being the deposition of municipal sewage sludge onto agricultural soils Other sources of Cd pollution are through rainfall and dry precipitation, as well as phosphate fertilizers Food consumption accounts for the largest sources of exposure to Cd by animals and humans, primarily because of the ability of plants to bioaccumulate Cd at high rates In addition, aquatic organisms can potentially accumulate large amounts of Cd

Cadmium Toxicity

Effect on Plants

Cadmium is accumulated by all plants The extent of Cd accumulation, however, varies markedly with species and variety Soil pH is the most important factor controlling Cd uptake by plants, with lower pH favoring its uptake Tobacco plants have been shown to absorb high levels of Cd from the soil (Bache 1985) Phytotox-icity of Cd is manifested by stunting, chlorosis, reduction in photosynthesis, wilting, and necrosis Like Pb, Cd inhibits seed germination under laboratory conditions (Koeppe 1977; Yu 1991; Fargasova 1994) Seedlings exposed to solutions of Cd salts exhibit decreased root elongation and development

Effects on Animals and Humans

Cadmium is toxic in small amounts and there is no evidence that Cd has any useful biological function Among the sources of exposure to Cd mentioned above, exposure through airborne Cd is minimal to the general population, with the excep-tion of tobacco smokers Cadmium in drinking water, although a major source, rarely becomes a serious problem On the average, potable waters contain about 10 ppb

Cd This amounts to an uptake of about 20 to 30 µg/day, based on daily water consumption of 2 to 3 liters (Friberg 1974)

Daily intake of Cd from food is estimated at 35 to 90 µg When dietary exposure reaches critical concentrations, estimated to be about 250 to 300 µg/day, toxicity symptoms are manifested Cadmium intakes of Japanese farmers suffering from the widely known “itai-itai” disease were reported to be from 600 to 1000 µg/day The disease was caused by ingestion of rice highly contaminated with Cd The rice paddies received water discharged from upstream Zn mines Many of the victims died as a result of the disease

Once absorbed, Cd readily shows up in the blood plasma, bound in albumin (Nordberg 1985) The bound Cd is shortly taken up by tissues, preferentially by the liver The Cd in the liver apparently cycles, bound with metallothionein (MT), through the blood, kidney, and, to a small amount, bone and muscle tissue

The excretion of Cd appears minimal under normal exposure Loss in the urine accounts for the major route of Cd excretion, whereas only minute amounts are excreted in the feces As mentioned above, absorbed Cd persists in body tissues The long-term excretion rate of Cd is only 0.005% per day beginning after about

50 years of age (Friberg 1974)

Although dietary intake is the means by which humans are most highly exposed

to Cd, inhalation of Cd is more dangerous than ingestion This is because, through inhalation, the body’s organ is directly and intimately exposed to the metal Further-more, 25 to 40% of inhaled Cd from the air is retained, while only 5 to 10% of ingested Cd is absorbed Inhaled Cd may cause emphysema and pneumonitis, while ingested Cd may result in disturbances in the gastrointestinal tract, vomiting, pro-teinuria, osteomalacia, liver dysfunction, kidney damage manifested by anemia, and hypertension Cadmium is also known to be embryotoxic

Biochemical Effect

Cadmium has been shown to impair many plant cellular functions, such as photophosphorylation, succinate oxidation, ATP synthesis, mitochondrial NADH oxidation, and electron transport (Nriagu 1980) Cadmium is a potent enzyme inhibitor, affecting a variety of plant enzymes such as PEP carboxylase, lipase, invertase (Yu 1998), and others Extensive reports are available concerning Cd-dependent inhibition of enzymes from animals and humans Alkaline phosphatase and ATPases of myosin and pulmonary alveolar macrophage cells are examples Two mechanisms appear to be involved in enzyme inhibition One is through binding

to SH groups on the enzyme molecule; another is through competing with Zn and displacing it from metalloenzymes Naturally, Cd also can bind with SH-containing ligands in the membrane and other cell constituents, causing structural and functional disruptions For instance, by inducing damage to mitochondria, Cd can uncouple oxidative phosphorylation and impair energy metabolism of the cell At moderate levels, Cd toxicity is related to its antimetabolite activities toward essential metals such as Zn, Cu, Se, and Fe In mammals, the impact caused by Cd is thus influenced

by the relative intakes of these and other metals and vice versa (Hamilton and Valberg 1974) In addition, dietary protein has been shown to be related to the toxicity of ingested Cd A low protein diet results in an increased absorption of Cd and thus increased toxicity

MERCURY

Mercury (Hg) is the only common metal that is liquid at room temperature It

is rare in the Earth’s crust (0.1 to 1 ppm) Although several forms occur, the principal ore is cinnabar, HgS Elemental Hg yields as cinnabar is “roasted” and the resulting

Hg vapor condensed Some inorganic and organic Hg compounds are extremely toxic A number of episodes leading to many fatalities occurred in different countries

in recent years as a result of exposure to the metal or its compounds

Properties and Uses

Mercury (atomic number 80, atomic weight 200.59) has a high specific gravity, 13.6 times that of water Its boiling point is 357°C, which is relatively low, and this property leads to easy separation from its ores and amalgams Its freezing point is –39°C, the lowest for any metal Mercury has a long liquid range of 396°C and it expands uniformly over this range This linear expansion, together with the fact that

Hg does not wet glass, makes the metal useful in thermometers Mercury has the highest volatility of any metal Its good electrical conductivity makes it exceptionally useful in electrical sealed switches and relays Many metals dissolve in mercury to form amalgams (alloys)

In the U.S the largest user of Hg is the chlor-alkali industry in which chlorine and caustic soda are produced by the electrolysis of a salt (NaCl) solution Mercury

is widely used in barometers, Hg batteries, and other electrical apparatus Many of its compounds are used as catalysts in industrial chemistry, and Hg vapor is utilized

in UV spectrophotometers High-pressure mercury-vapor lamps are now widely installed for street and highway lighting, and Hg compounds are added to paints as preservatives Formerly, certain Hg compounds were widely used as pesticides in agriculture Mercury has no known biological role and, as mentioned above, the metal and its compounds are toxic to all living organisms

Sources of Mercury Pollution

Mercury contamination of the environment is caused by both natural and man-made sources Natural sources include volcanic action and erosion of mer-cury-containing sediments Some of the ways humans contaminate the environment with Hg is through mining, transporting and processing mercury ores; dumping industrial wastes into rivers and lakes; combustion of fossil fuels (e.g., Hg content

of coal is about 1 ppm), pulp, and paper; use of mercury compounds as seed dressings

in agriculture; and exhaust from metal smelters

Toxicity

Effect on Plants

All plants appear to concentrate traces of Hg The concentration of Hg in plants depends on deposits in the soil, plant species, and locality Like Pb and Cd discussed previously, Hg can have a deleterious effect on different species of plants It is particularly toxic to barley plants, more so than Pb, Cr, Cd, Ni, and Zn (Oberlander and Roth 1978) Mercury, similar to Pb and Cd, impairs germination, as manifested

by depressed root elongation and shoot growth (Yu 1998)

Effect on Animals

Freshwater and marine organisms and their predators normally contain more Hg than terrestrial animals Levels in top predatory fish are higher Fish may accumulate

Hg in excess of the 0.5 mg/g FDA guideline depending on various factors This accumulation is part of a dynamic process in which an organism strives to maintain equilibrium between intake and elimination Numerous analyses have demonstrated that a majority of the tissue Hg in most fish is in the form of methylmercury (Westoo 1973) The Hg accumulated in fish comes primarily through absorption from the water across the gill or through the food chain, although some higher species may convert inorganic Hg into methylmercury Some Hg also is taken up through the mucous layer and/or skin

The metabolic rate of the fish and the mercury concentration in the aquatic ecosystem appear to be more important factors in bioaccumulation than age or exposure rate Since increased temperature enhances the metabolic rate, more Hg is concentrated in the summer than in the winter The toxicity of Hg and other heavy metals to fish is increased with increase in temperature The 96-h LC50 of Hg for freshwater crayfish (Procambarus clarkii, Girard) was found to be 0.79 mg/l at 20°C, 0.35 mg/l at 24°C, and 0.14 mg/l at 28°C (Del Ramo et al 1987)

Wild birds concentrate the highest levels of Hg in the kidney and liver with less

in the muscle tissues Swedish ornithologists observed the first Hg-related ecological problems during 1950s Many species of birds declined both in numbers and breeding success, while Hg levels increased in the feathers of several species of seed-eating birds In the US and Canada, elevated levels of Hg also were found in seed-eating birds and their predators, presumably through eating Hg-treated seed dressings In

1970 both countries banned alkylmercurial seed dressings, and the levels decreased

in game birds that do not feed on aquatic organisms However, where phenylmercuric seed dressings continue to be applied in the U.S., pheasants and other wild birds can still accumulate relatively high levels of Hg

Effect on Human Health

There is no indication that mercury compounds in the concentrations and forms found in either the atmosphere or drinking water supplies contribute significantly to the methylmercury burden in the human body The available data shows that almost all the methylmercury in the human diet comes from fish, other seafood, and possibly red meat The two major Japanese outbreaks of methylmercury poisoning in Minamata Bay and in Niigata were caused by industrial discharge of methylmercury and other mercury compounds into Minamata Bay and into the Agano River, resulting in accumulation of methylmercury in fish and shellfish The median total Hg level in fish caught in Minamata Bay at the time of the epidemic was estimated as 11 mg/g fresh weight More than 700 cases of methylmercury poisoning were identified in Minamata and more than 500 in Niigata (WHO 1975)

The critical organ concentration may differ for different stages of the human life cycle The developing fetal (and newborn) brain may be the most sensitive organ (i.e., critical organ) in terms of human methylmercury toxicity During the Japanese Minamata outbreak, 23 infants with severe psychomotor signs of brain damage were shown They were born to mothers who had consumed fish taken from waters known

to be heavily contaminated with effluent containing methylmercury