Toxic heavy metal

From Wikipedia, the free encyclopedia

A 25-foot (7.6 m) wall of coal fly ash contaminated with toxic heavy metals, resulting from the release of 5.4 million cubic yards of coal fly ash slurry into the Emory River, Tennessee, and nearby land and water features, in December 2008. Testing showed significantly elevated levels of arsenic, copper, barium, cadmium, chromium, lead, mercury, nickel, and thallium in samples of slurry and river water. Cleanup costs may exceed $1.2 billion.

 

A toxic heavy metal is any relatively dense metal or metalloid that is noted for its potential toxicity, especially in environmental contexts. The term has particular application to cadmium, mercury, lead and arsenic, all of which appear in the World Health Organization's list of 10 chemicals of major public concern. Other examples include manganese, chromium, cobalt, nickel, copper, zinc, selenium, silver, antimony and thallium.

Heavy metals are found naturally in the earth. They become concentrated as a result of human caused activities and can enter plant, animal, and human tissues via inhalation, diet, and manual handling. Then, they can bind to and interfere with the functioning of vital cellular components. The toxic effects of arsenic, mercury, and lead were known to the ancients, but methodical studies of the toxicity of some heavy metals appear to date from only 1868. In humans, heavy metal poisoning is generally treated by the administration of chelating agents. Some elements otherwise regarded as toxic heavy metals are essential, in small quantities, for human health.

Contamination sources

Tetraethyl lead is one of the most significant heavy metal contaminants in recent use.

 

Heavy metals are found naturally in the earth, and become concentrated as a result of human activities. Common sources are mining and industrial wastes; vehicle emissions; lead-acid batteries; fertilisers; paints; treated woods; aging water supply infrastructure; and microplastics floating in the world's oceans. Arsenic, cadmium and lead may be present in children's toys at levels that exceed regulatory standards. Lead can be used in toys as a stabilizer, color enhancer, or anti-corrosive agent. Cadmium is sometimes employed as a stabilizer, or to increase the mass and luster of toy jewelry. Arsenic is thought to be used in connection with coloring dyes. Regular imbibers of illegally distilled alcohol may be exposed to arsenic or lead poisoning the source of which is arsenic-contaminated lead used to solder the distilling apparatus. Rat poison used in grain and mash stores may be another source of the arsenic.

Lead is the most prevalent heavy metal contaminant. As a component of tetraethyl lead, (CH
₃CH
₂)
₄Pb, it was used extensively in gasoline during the 1930s–1970s. Lead levels in the aquatic environments of industrialised societies have been estimated to be two to three times those of pre-industrial levels. Although the use of leaded gasoline was largely phased out in North America by 1996, soils next to roads built before this time retain high lead concentrations. Lead (from lead(II) azide or lead styphnate used in firearms) gradually accumulates at firearms training grounds, contaminating the local environment and exposing range employees to a risk of lead poisoning.

Entry routes

Heavy metals enter plant, animal and human tissues via air inhalation, diet and manual handling. Motor vehicle emissions are a major source of airborne contaminants including arsenic, cadmium, cobalt, nickel, lead, antimony, vanadium, zinc, platinum, palladium and rhodium. Water sources (groundwater, lakes, streams and rivers) can be polluted by heavy metals leaching from industrial and consumer waste; acid rain can exacerbate this process by releasing heavy metals trapped in soils. Plants are exposed to heavy metals through the uptake of water; animals eat these plants; ingestion of plant- and animal-based foods are the largest sources of heavy metals in humans. Absorption through skin contact, for example from contact with soil, or metal containing toys and jewelry, is another potential source of heavy metal contamination. Toxic heavy metals can bioaccumulate in organisms as they are hard to metabolize.

Detrimental effects

Heavy metals "can bind to vital cellular components, such as structural proteins, enzymes, and nucleic acids, and interfere with their functioning". Symptoms and effects can vary according to the metal or metal compound, and the dose involved. Broadly, long-term exposure to toxic heavy metals can have carcinogenic, central and peripheral nervous system and circulatory effects. For humans, typical presentations associated with exposure to any of the "classical" toxic heavy metals, or chromium (another toxic heavy metal) or arsenic (a metalloid), are shown in the table.

Element	Acute exposure usually a day or less	Chronic exposure often months or years
Cadmium	Pneumonitis (lung inflammation)	Lung cancer Osteomalacia (softening of bones) Proteinuria (excess protein in urine; possible kidney damage)
Mercury	Diarrhea Fever Vomiting	Stomatitis (inflammation of gums and mouth) Nausea Nephrotic syndrome (nonspecific kidney disorder) Neurasthenia (neurotic disorder) Parageusia (metallic taste) Pink Disease (pain and pink discoloration of hands and feet) Tremor
Lead	Encephalopathy (brain dysfunction) Nausea Vomiting	Anemia Encephalopathy Foot drop/wrist drop (palsy) Nephropathy (kidney disease)
Chromium	Gastrointestinal hemorrhage (bleeding) Hemolysis (red blood cell destruction) Acute renal failure	Pulmonary fibrosis (lung scarring) Lung cancer
Arsenic	Nausea Vomiting Diarrhea Encephalopathy Multi-organ effects Arrhythmia Painful neuropathy	Diabetes Hypopigmentation/Hyperkeratosis Cancer

History

The toxic effects of arsenic, mercury and lead were known to the ancients but methodical studies of the overall toxicity of heavy metals appear to date from only 1868. In that year, Wanklyn and Chapman speculated on the adverse effects of the heavy metals "arsenic, lead, copper, zinc, iron and manganese" in drinking water. They noted an "absence of investigation" and were reduced to "the necessity of pleading for the collection of data". In 1884, Blake described an apparent connection between toxicity and the atomic weight of an element. The following sections provide historical thumbnails for the "classical" toxic heavy metals (arsenic, mercury and lead) and some more recent examples (chromium and cadmium).

Orpiment, a toxic arsenic mineral used in the tanning industry to remove hair from hides.

Arsenic

Arsenic, as realgar (As
₄S
₄) and orpiment (As
₂S
₃), was known in ancient times. Strabo (64–50 BCE – c. AD 24?), a Greek geographer and historian, wrote that only slaves were employed in realgar and orpiment mines since they would inevitably die from the toxic effects of the fumes given off from the ores. Arsenic-contaminated beer poisoned over 6,000 people in the Manchester area of England in 1900, and is thought to have killed at least 70 victims. Clare Luce, American ambassador to Italy from 1953 to 1956, suffered from arsenic poisoning. Its source was traced to flaking arsenic-laden paint on the ceiling of her bedroom. She may also have eaten food contaminated by arsenic in flaking ceiling paint in the embassy dining room. Ground water contaminated by arsenic, as of 2014, "is still poisoning millions of people in Asia".

Mercury

Saint Isaac's Cathedral, in Saint Petersburg, Russia. The gold-mercury amalgam used to gild its dome caused numerous casualties among the workers involved.

The first emperor of unified China, Qin Shi Huang, it is reported, died of ingesting mercury pills that were intended to give him eternal life. The phrase "mad as a hatter" is likely a reference to mercury poisoning among milliners (so-called "mad hatter disease"), as mercury-based compounds were once used in the manufacture of felt hats in the 18th and 19th century. Historically, gold amalgam (an alloy with mercury) was widely used in gilding, leading to numerous casualties among the workers. It is estimated that during the construction of Saint Isaac's Cathedral alone, 60 workers died from the gilding of the main dome. Outbreaks of methylmercury poisoning occurred in several places in Japan during the 1950s due to industrial discharges of mercury into rivers and coastal waters. The best-known instances were in Minamata and Niigata. In Minamata alone, more than 600 people died due to what became known as Minamata disease. More than 21,000 people filed claims with the Japanese government, of which almost 3000 became certified as having the disease. In 22 documented cases, pregnant women who consumed contaminated fish showed mild or no symptoms but gave birth to infants with severe developmental disabilities. Since the industrial Revolution, mercury levels have tripled in many near-surface seawaters, especially around Iceland and Antarctica.

Dutch Boy white lead paint advertisement, 1912.

Lead

The adverse effects of lead were known to the ancients. In the 2nd century BC the Greek botanist Nicander described the colic and paralysis seen in lead-poisoned people. Dioscorides, a Greek physician who is thought to have lived in the 1st century CE, wrote that lead "makes the mind give way". Lead was used extensively in Roman aqueducts from about 500 BC to 300 AD. Julius Caesar's engineer, Vitruvius, reported, "water is much more wholesome from earthenware pipes than from lead pipes. For it seems to be made injurious by lead, because white lead is produced by it, and this is said to be harmful to the human body." During the Mongol period in China (1271−1368 AD), lead pollution due to silver smelting in the Yunnan region exceeded contamination levels from modern mining activities by nearly four times. In the 17th and 18th centuries, people in Devon were afflicted by a condition referred to as Devon colic; this was discovered to be due to the imbibing of lead-contaminated cider. In 2013, the World Health Organization estimated that lead poisoning resulted in 143,000 deaths, and "contribute[d] to 600,000 new cases of children with intellectual disabilities", each year. In the U.S. city of Flint, Michigan, lead contamination in drinking water has been an issue since 2014. The source of the contamination has been attributed to "corrosion in the lead and iron pipes that distribute water to city residents". In 2015, drinking water lead levels in north-eastern Tasmania, Australia, were reported to reach over 50 times national drinking water guidelines. The source of the contamination was attributed to "a combination of dilapidated drinking water infrastructure, including lead jointed pipelines, end-of-life polyvinyl chloride pipes and household plumbing".

Chromium

Potassium chromate, a carcinogen, is used in the dyeing of fabrics, and as a tanning agent to produce leather.

 

Chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known since at least the late 19th century. In 1890, Newman described the elevated cancer risk of workers in a chromate dye company. Chromate-induced dermatitis was reported in aircraft workers during World War II. In 1963, an outbreak of dermatitis, ranging from erythema to exudative eczema, occurred amongst 60 automobile factory workers in England. The workers had been wet-sanding chromate-based primer paint that had been applied to car bodies. In Australia, chromium was released from the Newcastle Orica explosives plant on August 8, 2011. Up to 20 workers at the plant were exposed as were 70 nearby homes in Stockton. The town was only notified three days after the release and the accident sparked a major public controversy, with Orica criticised for playing down the extent and possible risks of the leak, and the state Government attacked for their slow response to the incident.

99.999% purity cadmium bar and 1 cm³ cube.

Cadmium

Cadmium exposure is a phenomenon of the early 20th century, and onwards. In Japan in 1910, the Mitsui Mining and Smelting Company began discharging cadmium into the Jinzugawa river, as a byproduct of mining operations. Residents in the surrounding area subsequently consumed rice grown in cadmium-contaminated irrigation water. They experienced softening of the bones and kidney failure. The origin of these symptoms was not clear; possibilities raised at the time included "a regional or bacterial disease or lead poisoning". In 1955, cadmium was identified as the likely cause and in 1961 the source was directly linked to mining operations in the area. In February 2010, cadmium was found in Walmart exclusive Miley Cyrus jewelry. Wal-Mart continued to sell the jewelry until May, when covert testing organised by Associated Press confirmed the original results. In June 2010 cadmium was detected in the paint used on promotional drinking glasses for the movie Shrek Forever After, sold by McDonald's Restaurants, triggering a recall of 12 million glasses.

Remediation

A metal EDTA anion. Pb displaces Ca in Na
₂[CaEDTA] to give Na
₂[PbEDTA], which is passed out of the body in urine.

 

In humans, heavy metal poisoning is generally treated by the administration of chelating agents. These are chemical compounds, such as CaNa2 EDTA (calcium disodium ethylenediaminetetraacetate) that convert heavy metals to chemically inert forms that can be excreted without further interaction with the body. Chelates are not without side effects and can also remove beneficial metals from the body. Vitamin and mineral supplements are sometimes co-administered for this reason.

Soils contaminated by heavy metals can be remediated by one or more of the following technologies: isolation; immobilization; toxicity reduction; physical separation; or extraction. Isolation involves the use of caps, membranes or below-ground barriers in an attempt to quarantine the contaminated soil. Immobilization aims to alter the properties of the soil so as to hinder the mobility of the heavy contaminants. Toxicity reduction attempts to oxidise or reduce the toxic heavy metal ions, via chemical or biological means into less toxic or mobile forms. Physical separation involves the removal of the contaminated soil and the separation of the metal contaminants by mechanical means. Extraction is an on or off-site process that uses chemicals, high-temperature volatization, or electrolysis to extract contaminants from soils. The process or processes used will vary according to contaminant and the characteristics of the site.

Benefits

Some elements otherwise regarded as toxic heavy metals are essential, in small quantities, for human health. These elements include vanadium, manganese, iron, cobalt, copper, zinc, selenium, strontium and molybdenum. A deficiency of these essential metals may increase susceptibility to heavy metal poisoning.

Mercury in fish

From Wikipedia, the free encyclopedia

Nearby anthropogenic sources, such as coal burning and mining of iron, can contaminate water sources with methylmercury, which is efficiently absorbed in the bodies of fish. Through the process of biomagnification, mercury levels in each successive predatory stage increase.

 

Fish and shellfish concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organomercury compound. Fish products have been shown to contain varying amounts of heavy metals, particularly mercury and fat-soluble pollutants from water pollution. Species of fish that are long-lived and high on the food chain, such as marlin, tuna, shark, swordfish, king mackerel and tilefish (Gulf of Mexico) contain higher concentrations of mercury than others.

Mercury is known to bioaccumulate in humans, so bioaccumulation in seafood carries over into human populations, where it can result in mercury poisoning. Mercury is dangerous to both natural ecosystems and humans because it is a metal known to be highly toxic, especially due to its ability to damage the central nervous system. In human-controlled ecosystems of fish, usually done for market production of wanted seafood species, mercury clearly rises through the food chain via fish consuming small plankton, as well as through non-food sources such as underwater sediment.

The presence of mercury in fish can be a particular health concern for women who are or may become pregnant, nursing mothers, and young children.

Biomagnification

The consumption of fish is by far the most significant source of ingestion-related mercury exposure in humans and animals. Mercury and methyl mercury are present in only very small concentrations in seawater. However, they are absorbed, usually as methyl mercury, by algae at the start of the food chain. This algae is then eaten by fish and other organisms higher in the food chain. Fish efficiently absorb methyl mercury, but excrete it very slowly. Methyl mercury is not soluble and therefore not excreted. Instead, it accumulates, primarily in the viscera, although also in the muscle tissue. This results in the bioaccumulation of mercury, in a buildup in the adipose tissue of successive trophic levels: zooplankton, small nekton, larger fish, and so on. The older that such fish become, the more mercury they may have absorbed. Anything that eats these fish within the food chain also consumes the higher level of mercury that the fish have accumulated. This process explains why predatory fish such as swordfish and sharks or birds like osprey and eagles have higher concentrations of mercury in their tissue than could be accounted for by direct exposure alone. Species on the food chain can amass body concentrations of mercury up to ten times higher than the species they consume. This process is called biomagnification. For example, herring contains mercury levels at about 0.1 parts per million, while shark contains mercury levels greater than 1 part per million.

Legislation

Japan

Since the Minamata disaster, Japan has improved on its mercury regulation. During the 1970s Japan made strides to reduce mercury demand and production. Chief among these efforts was the reduction of inorganic mercury produced by mines. It was halted by 1974, and demand fell from 2,500 tons per year in 1964, its peak, to 10 tons per year in recent years. Since these initial strides, Japan has introduced a list of regulations governing the mercury content of a variety of materials.

Japanese Mercury Regulation

Category	Regulation	Result
Cosmetics	Pharmaceutical Affairs Act	Ban the use of mercury and its compounds
Agriculture	Agricultural Chemicals Control Act	Ban the use of mercury and its compounds as an active ingredient
Household Commodities	Act on Control of Household Products Containing Hazardous Substances	No mercury in household adhesives, household paints, household wax, shoe polish, shoe cream, diapers, bibs, undergarments, gloves, and socks
Pharmaceutical Products	Pharmaceutical Affairs Act	No use of mercury compounds in oral preparations. No use of mercury compounds, other than mercurochrome, as an active ingredient. Mercury as a preservative only if no other option is available.
Air	Air Pollution Control Law	No more than 40 ng/m3
Water	Basic Environment Law and Water Pollution Control Act	Environmental quality standard: no more than 0.0005 mg/L in waterway and ground water. Effluent standard: no more than 0.005 mg/L in effluence.
Soil	Basic Environment Law and Soil Contamination Countermeasures Act	Environmental quality standard: no more than 0.0005 mg/L sample solution. Elution standard: no more than 0.0005 mg/L. Content standard: no more than 15 mg/kg

Regulation of these potential sources of pollution reduces the amount of mercury that ends up in fish and, through biomagnification, in humans. In addition to enacting legislation controlling the mercury levels in potential pollutants, Japan has directly influenced the environment by issuing regulations setting acceptable levels of environmental mercury pollution. 

It is Japan's goal to promote international mercury legislation in hopes of preventing any country from experiencing what it did. Despite Japan's extensive regulation and experience with mercury-based disasters, there is still little information provided to the public. The Japanese Federal Fish Advisory's recommendations are less strict than those in America.

United States of America

Fish advisory chart issued by U.S. Environmental Protection Agency and Food and Drug Administration

The United States is a leader in mercury regulation. A key piece of mercury legislation in the United States is the Mercury and Air Toxics Standards (MATS). This policy was finalized by the Environmental Protection Agency (EPA) on December 16, 2011. This is a federal policy which directly influences mercury in fish, and is the first of its kind in the United States. The facilities targeted by this new policy are the chief sources of mercury in the air. The airborne mercury is dissolved in the oceans, where microorganisms convert waterborne mercury into methyl mercury; mercury thus finds its way into the food chain and into fish. MATS is legislated towards the aim of preventing about 90% of the emissions from power plants from reaching the air. In total the expected health benefits are estimated at $37 billion–$90 billion by 2016. In comparison, the expected economic cost is $9.6 billion annually. Another integral piece of legislation controlling the emission of mercury to the air is the Clean Air Act. Under this act, mercury is classified as a hazardous air pollutant, allowing the EPA to regulate emissions by establishing performance standards.

International

Legislation on a global scale is believed by some to be needed for this issue because mercury pollution is estimated to be so far-reaching. Pollution from one country does not stay localized to that country. Despite the need by some, international regulation has been slow to take off. The first forms of international legislation appeared in the 1970s, beginning as agreements about shared bodies of water. The next step was the Stockholm Declaration, which urged countries to avoid polluting the oceans by dumping. The 1972 Oslo Convention and the 1974 Paris Convention were adopted by parts of Europe. Both lessened polluting the ocean with mercury, the former by banning the dumping of ships and aircraft into the ocean and the latter by obligating participants to reduce land-based pollution on coastlines. The first real global legislation regarding mercury pollution was the Basel Convention of 1989. This convention attempts to reduce the movement of mercury across borders and primarily regulates the import and export of toxic chemicals, including mercury. In 1998 the Convention on Long-Range Transboundary Air Pollution was adopted by most of the European Union, the United States, and Canada. Its primary objective is to cut emissions of heavy metals. The convention is the largest international agreement on mercury established to date. In the early 21st century, the focus of mercury regulation has been on voluntary programs. The next phase in legislation is a global effort, and this appears to be what the Minamata Convention hopes to accomplish. The Minamata Convention, named after the Japanese city that suffered horribly from mercury pollution, has taken four years of negotiation but was finally adopted by delegates from over 140 countries. The convention will come into power after 50 countries have signed it. The Minamata Convention will require all participants to eliminate, where possible, the release of mercury from small-scale gold mining. It will also require a sharp reduction in emission from coal burning.

Levels of contamination

Most-contaminated fish species

The danger level from consuming fish depends on species and size. Size is the best predictor of increased levels of accumulated mercury. Sharks, such as the mako shark, have very high levels of mercury. A study on New Jersey coastal fish indicated that one third of the sampled fish had levels of mercury above 0.5 parts per million, a level that could pose a human health concern for consumers who regularly eat this fish. Another study of marketplace fish caught in waters surrounding Southern Italy showed that, undoubtedly, greater fish weight leads to additional mercury found in fish body tissues. Moreover, the concentration, measured in milligrams of mercury per kilogram of fish, steadily increases with the size of the fish. Anglerfish off the coast of Italy were found with concentrations as high as 2.2 milligrams of mercury per kilogram, higher than the recommended limit of 1 milligram of mercury per kilogram. Annually, Italy catches approximately a third of its fish from the Adriatic Sea, where these anglerfish were found.

Fish that consume their prey in a certain manner may contain much higher concentrations of mercury than other species. Grass carp off the coast of China hold far less internal mercury than do bighead carp. The reason for this is that bighead carp are filter feeders, while grass carp are not. Thus, bighead carp gather more mercury by eating large amounts of small plankton, as well as sucking up sediments that collect a sizable amount of methyl mercury.

US government scientists tested fish in 291 streams around the country for mercury contamination. They found mercury in every fish tested, according to the study by the U.S. Department of the Interior. They found mercury even in fish of isolated rural waterways. Twenty-five percent of the fish tested had mercury levels above the safety levels determined by the U.S. Environmental Protection Agency for people who eat the fish regularly.

Origins of mercury pollution

There are three types of mercury emission: anthropogenic, re-emission, and natural, including volcanoes and geothermal vents. Anthropogenic sources are responsible for 30% of all emissions, while natural sources are responsible for 10%, and re-emission accounts for the other 60%. While re-emission accounts for the largest proportion of emissions, it is likely that the mercury emitted from these sources originally came from anthropogenic sources.

Anthropogenic sources include coal burning, cement production, oil refining, artisan and small-scale gold mining, wastes from consumer products, dental amalgam, the chlor-alkali industry, production of vinyl chloride, and the mining, smelting, and production of iron and other metals. The total amount of mercury released by mankind in 2010 was estimated to be 1,960 metric tons. The majority of this comes from coal burning and gold mining, accounting for 24% and 37% of total anthropogenic output respectively.

Re-emission, the largest emitter, occurs in a variety of ways. It is possible for mercury that has been deposited in soil to be re-emitted into the mercury cycle via floods. A second example of re-emission is a forest fire; mercury that has been absorbed into plant life is re-released into the atmosphere. While it is difficult to estimate the exact extent of mercury re-emission, it is an important field of study. Knowing how easily and how often previously emitted mercury can be released helps us learn how long it will take for a reduction in anthropogenic sources to be reflected in the environment. Mercury that has been released can find its way into the oceans. A 2008 model estimated the total amount of deposition into the oceans that year to be 3,700 metric tons. It is estimated that rivers carry as much as 2,420 metric tons. Much of the mercury deposited in the oceans is re-emitted, however; as much as 300 metric tons is converted into methyl mercury. While only 13% of this finds its way into the food chain, that is still 40 metric tons a year.

Much (an estimated 40%) of the mercury that eventually finds its way into fish originates with coal-burning power plants and chlorine production plants. The largest source of mercury contamination in the United States is coal-fueled power plant emissions. Chlorine chemical plants use mercury to extract chlorine from salt, which in many parts of the world is discharged as mercury compounds in waste water, though this process has been largely replaced by the more economically viable membrane cell process, which does not use mercury. Coal contains mercury as a natural contaminant. When it is fired for electricity generation, the mercury is released as smoke into the atmosphere. Most of this mercury pollution can be eliminated if pollution-control devices are installed.

Mercury in the United States frequently comes from power plants, which release about 50% of the nation's mercury emissions. In other countries, such as Ghana, gold mining requires mercury compounds, leading to workers receiving significant quantities of mercury while performing their jobs. Such mercury from gold mines is specifically known to contribute to biomagnification in aquatic food chains.

The farming of aquatic organisms, known as aquaculture, often involves fish feed that contains mercury. A study by Jardine has found no reliable connection between mercury in fish food affecting aquaculture organisms or aquatic organisms in the wild. Even so, mercury from other sources may still affect organisms grown through aquaculture. In China, farmed fish species, such as bighead carp, mud carp, and Siniperca chuatsi, carried 90% of total mercury content in all of the measured fish in a study by Cheng. This study also concluded that mercury bioaccumulates through food chains even in controlled aquaculture environments. Both total mercury and methyl mercury absorption was found to be derived from sediments containing mercury, not mainly from fish feed.

The Hawaii Institute of Marine Biology has noted that fish feed used in aquaculture often contains heavy metals such as mercury, lead, and arsenic, and has dispatched these concerns to organizations such as the Food and Agriculture Organization of the United Nations. 

Elemental mercury often comes from coal power plants, and oxidized mercury often comes from incinerators. Oil-fired power plants also contribute mercury to the environment. The energy industry therefore is a key player in the introduction of mercury into the environment. When addressing the issue of reducing seafood mercury bioaccumulation on a global scale, it is important to pinpoint major energy producers and consumers whose exchange of energy may be the root of the problem.

Controlling output of mercury pollution sources

A study that was led by scientists from Harvard University and U.S. Geological Survey has determined that in the next several decades there will be a 50 percent increase in mercury levels.^{[citation needed]} The study also shows that the increases are connected through industrial emissions and are not natural as previously thought. However, by decreasing emissions from industrial plants, the possibility of decreasing the high level of mercury remains plausible. Several nations are currently implementing systems that will detect and therefore later be able to control the output of mercury into the atmosphere. Air pollution control devices (APCDs) have been implemented in South Korea as the government is starting to take inventory of mercury sources. Mercury pollution can also be removed by electrostatic precipitators (ESPs). Bag-based filters are also used in factories that may contribute mercury to the environment. Flue-gas desulfurization, normally used to eliminate sulfur dioxide, can also be used in conjunction with APCDs to remove additional mercury before exhausts are released into the environment. Even so, countries such as South Korea have only begun to use inventories of mercury sources, calling into question how fast anti-mercury measures will be put into factories.

Health effects and outcomes

Disparate impacts

Mercury content in fish does not affect all populations equally. Certain ethnic groups, as well as young children, are more likely to suffer the effects of methyl mercury poisoning. In the United States, Wallace gathered data that indicated 16.9% of women who self-identify as Native American, Asian, Pacific Islander, or multiracial exceed the recommended reference dose of mercury. A study done on children of the Faroe Islands near Great Britain showed neurological problems stemming from mothers consuming pilot whale meat during pregnancy.

Regulation and health

While various studies have shown high concentrations of mercury accumulated in fish, medical cases often go unreported and pose a difficulty in correlating mercury in fish with human poisoning. Environmental issues cover a broad range of areas, but medical cases that are associated with pollutants released into the environment by factories or construction areas cause public health issues that affect not only the environment but also human well-being. Substances poisonous to the human body in a particular amount or dose may not cause any symptoms over time. While there are limits to how much of anything the body can have, mercury is a particular poison that produces immediate physical symptoms when the body has been accumulating it over a period of time.

In the United States, the Environmental Protection Agency measures the amount of mercury concentrated in human blood that does not pose fatal health outcomes. The agency is in charge of enforcing regulations and policies that cover a range of environmental topics. Analysis of blood mercury concentrations in childbearing women has proved that exposure to methyl mercury (MeHg) occurs primarily through the consumption of fish. The U.S. FDA highly recommends against pregnant woman and young children consuming raw fish. Pregnant women and young children often lack strong immune systems and are more at risk for foodborne illnesses.

Medical cases and exposure to mercury

In the United States, the EPA serves as an advisory organ to set the levels of mercury that are non-fatal in humans. Symptoms of exposure to high levels of methyl mercury include disturbed vision, hearing, and speech, lack of coordination, and muscle weakness. Medical studies have examined the correlation of fish consumption and health issues. American studies have presented evidence of fish consumption and its effects on child development. Longitudinal studies agree that human activities are what release and accumulate mercury in marine life. Addressing the issues of fish consumption forces health officials to recognize the sources of mercury in the human body. Specific Native American tribes are vulnerable to a high exposure of mercury. Studies have determined that these native peoples in the United States suffer more from mercury poisoning and illness than any other cohort group in the country. This is due to the fact that fish is a main source of protein. Exposure risk was assessed through a medical study, thus raising judicial issues of whether the public health of these groups is a priority in the United States.

Work and exposure

Most cases that arise are due to work exposure or medicinal poisoning. Environmental justice advocates can relate these mercury cases to the unregulated amount of mercury that enters the environment. Workers can be exposed to mercury through the manufacture of fluorescent tubes, chloralkali, or acetaldehyde among other products. Anthropogenic sources and places where mercury is released or used as a solid or vapor puts these has caused fatigue, dizziness, hyperhidrosis, chest congestion, and loss of motor skills. When taken to the hospital, the neurotoxicity levels had already exceeded the maximum amounts. Over-the-counter medicines have been shown to have traces of mercurous chloride. Medical research reported that the children who received doses of these medicines experienced physical symptoms such as "drooling, irregular arm movements, and impaired gait". Exposures to this result in severe physical impairments unregulated chemicals that are put in products. The intake of laxatives that contained about 120 mg of mercurous chloride has also been cases of mercury's toxicity.

Even in countries, such as Sweden, that have phased out mercury in the dental industry and manufacturing, lingering quantities of mercury still exist in lakes and coastal areas. Moreover, global contributions of mercury to the environment also affect that country. A study in Sweden selected 127 women who had a high level of fish consumption. Around 20% of the women selected, after hair and blood samples, were found to have exceeded the EPA's recommended reference dose of 0.1 micrograms of methyl mercury per kilogram of body weight. Additionally, the study concluded that there was "no margin of safety for neuraldevelopmental effects in fetus[es]" without removing the offending species of fish from the diets of the women. This indicates that families intending to raise children should be especially careful about exposing their unborn babies to toxic mercury via fish. 

Children exposed to mercury are particularly susceptible to poisoning since the ratio of food, water, and air intake versus individual body weight is much higher than that of adults. Additionally, children undergo fast growth which causes them to be more susceptible to damaging exposure to methylmercury, as well as the long term consequences of such exposure during childhood development. Young age plays an important role in terms of damage caused by mercury, and much literature on mercury focuses on pregnant women and specific precautions designed to prevent youth mercury exposure. Prenatal methylmercury exposure does cause behavioral problems in infants and worsened cognitive test performance. Additionally, Hughner estimates that 250,000 women may be exposing their unborn babies to levels of methyl mercury above recommended federal levels.

Economically, there does not seem to be a difference in mercury exposure based on socioeconomic bracket and the ability to buy fish from the market. One study shows "no significant differences in mercury levels in tuna, bluefish, and flounder as a function of type of store or economic neighborhood".

By nation

Certain countries have cultural differences that lead to more fish consumption and therefore more possible exposure to seafood methylmercury. In Ghana, the local population traditionally consumes large quantities of fish, leading to potentially dangerous amounts of mercury in the bloodstream. In the Amazonian Basin, during the rainy season, herbivorous fish dominate the diet of 72.2% of the women selected from a particular Amazonian village. Analysis also shows increase of mercury content in the hair of humans who eat fish on a daily basis in the Amazon.

The most serious case of mercury poisoning in recent history was in the Japanese city of Minamata, in the 1950s. Minamata poisoning proves that significant prenatal and postnatal exposure to high levels of methylmercury causes serious neurological problems. Minamata victims also show higher than normal signs of psychiatric diseases, along with those diseases being caused by underlying neurological issues.

A 2014 USGS survey of mercury levels in the United States water system found that methylmercury concentrations in fish were typically highest in wetland areas including the coastal plain streams in the Southeast. Fish methylmercury levels were also high in the Western US, but only in streams that had been mined for mercury or gold.

Seafood consumption benefits

The American College of Obstetricians and Gynecologists note that, considering all the dangers and benefits, the overall result of eating fish in the United States is likely to improve personal health rather than damage it. The college argues that the omega-3 polyunsaturated fatty acids found in fish have a health benefit that outweighs the harm from mercury or polychlorinated biphenyls. Even so, the College also suggests limiting fish consumption for pregnant women. A risk-benefit study weighing the risks of mercury consumption against the benefits derived from fish in Alaska showed that the benefits outweigh the risks when consuming salmon for both cardiovascular health and infant neurological development, and that MeHg data for non-oily fish needs to be of high quality before relative risk can be reliably identified. The Seychelles Child Development Study traced more than seven hundred mother-child pairs for nine years, and found no neurological problems in the children resulting from both prenatal and postnatal methylmercury exposure. A study done with marketed fish in Oman concluded that, except in a few rare cases, the fish available for consumption had lower levels of mercury than limits defined by various health organizations. Clearly, these studies call into question whether or not normal everyday consumption of fish is dangerous in any way, and at very least justify the creation of place-based and culturally relevant consumption advisories. They do not take into account cases of severe mercury poisoning, such as that found in Minamata disease.

Selenium is an element that is known to counteract some of the dangers of ingesting mercury. Multiple studies have been done, such as those in New Jersey and Sweden, that take into account selenium as well as mercury levels. Fish often do contain selenium in conjunction with bioaccumulated mercury, which may offset some of the dangers associated with the mercury ingested.

Current advice

The complexities associated with mercury transport and environmental fate are described by USEPA in their 1997 Mercury Study Report to Congress. Because methyl mercury and high levels of elemental mercury can be particularly toxic to a fetus or young children, organizations such as the U.S. EPA and FDA recommend that women who are pregnant or plan to become pregnant within the next one or two years, as well as young children, avoid eating more than 6 ounces (170g, one average meal) of fish per week.

In the United States, the FDA has an action level for methylmercury in commercial marine and freshwater fish that is 1.0 parts per million (ppm). In Canada, the limit for the total of mercury content is 0.5 ppm. The Got Mercury? website includes a calculator for determining mercury levels in fish.

Species with characteristically low levels of mercury include shrimp, tilapia, salmon, pollock, and catfish (FDA March 2004). The FDA characterizes shrimp, catfish, pollock, salmon, sardines, and canned light tuna as low-mercury seafood, although recent tests have indicated that up to 6 percent of canned light tuna may contain high levels. A study published in 2008 found that mercury distribution in tuna meat is inversely related to the lipid content, suggesting that the lipid concentration within edible tuna tissues has a diluting effect on mercury content. These findings suggest that choosing to consume a type of tuna that has a higher natural fat content may help reduce the amount of mercury intake, compared to consuming tuna with a low fat content. Also, many of the fish chosen for sushi contain high levels of mercury.

According to the US Food and Drug Administration (FDA), the risk from mercury by eating fish and shellfish is not a health concern for most people. However, certain seafood might contain levels of mercury that may cause harm to an unborn baby (and especially its brain development and nervous system). In a young child, high levels of mercury can interfere with the development of the nervous system. The FDA provides three recommendations for young children, pregnant women, and women of child-bearing age:

1. Do not eat shark, swordfish, king mackerel, or tilefish (Gulf of Mexico) because they might contain high levels of mercury.
2. Eat up to 12 ounces (2 average meals of 170 g each) a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish and shellfish that are low in mercury are: shrimp, canned light tuna, salmon, pollock, and catfish. Another commonly eaten fish, albacore or ("white") tuna depending on its origin might have more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, it is recommended that you should not eat more than up to 6 ounces (one average meal) of albacore tuna per week.
3. Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal of 170 g) per week of fish you catch from local waters, but consume no other fish during that week.

Research suggests that selenium content in fish is protective against the toxic effects of methylmercury content. Fish with higher ratios of selenium to methylmercury (Se:Hg) are better to eat since the selenium binds to the methylmercury allowing it to pass through the body un-absorbed.

In 2012 the European Food Safety Authority (EFSA) reported on chemical contaminants they found in the food of over 20 European countries. They established that fish meat and fish products were primarily responsible for methylmercury in the diet of all age classes. Particularly implicated were swordfish, tuna, cod, pike, whiting and hake. The EFSA recommend a tolerable weekly intake for methylmercury of 1.3 μg/kg body weight.

Background

In the 1950s, inhabitants of the seaside town of Minamata, on Kyushu island in Japan, noticed strange behavior in animals. Cats would exhibit nervous tremors, and dance and scream. Within a few years this was observed in other animals; birds would drop out of the sky. Symptoms were also observed in fish, an important component of the diet, especially for the poor. When human symptoms started to be noticed around 1956 an investigation began. Fishing was officially banned in 1957. It was found that the Chisso Corporation, a petrochemical company and maker of plastics such as vinyl chloride, had been discharging heavy metal waste into the sea for decades. They used mercury compounds as catalysts in their syntheses. It is believed that about 5,000 people were killed and perhaps 50,000 have been to some extent poisoned by mercury. Mercury poisoning in Minamata, Japan, is now known as Minamata disease.

Gene flow

From Wikipedia, the free encyclopedia

Gene flow is the transfer of alleles from one population to another population through immigration of individuals.

 

In population genetics, gene flow (also known as gene migration or allele flow) is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population. It has been shown that it takes only "One migrant per generation" to prevent populations from diverging due to drift. Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants change the distribution of genetic diversity within the populations, by modifying the allele frequencies (the proportion of members carrying a particular variant of a gene). High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. For this reason, gene flow has been thought to constrain speciation by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to full speciation. In some cases migration may also result in the addition of novel genetic variants to the gene pool of a species or population. 

There are a number of factors that affect the rate of gene flow between different populations. Gene flow is expected to be lower in species that have low dispersal or mobility, that occur in fragmented habitats, where there is long distances between populations, and when there are small population sizes. Mobility plays an important role in the migration rate, as highly mobile individuals tend to have greater migratory prospects. Although animals are thought to be more mobile than plants, pollen and seeds may be carried great distances by animals or wind. When gene flow is impeded, there can be an increase in inbreeding, measured by the inbreeding coefficient (F) within a population. For example, many island populations have low rates of gene flow due to geographic isolation and small population sizes. The Black Footed Rock Wallaby has several inbred populations that live on various islands off the coast of Australia. The population is so strongly isolated that lack of gene flow has led to high rates of inbreeding.

Measuring gene flow

Decrease in population size leads to increased divergence due to drift, while migration reduces divergence and inbreeding. Gene flow can be measured by using the effective population size (${\displaystyle N_{e}}$) and the net migration rate per generation (m). Using the approximation based on the Island model, the effect of migration can be calculated for a population in terms of the degree of genetic differentiation(${\displaystyle Fst}$). This formula accounts for the proportion of total molecular marker variation among populations, averaged over loci. When there is one migrant per generation, the inbreeding coefficient (${\displaystyle Fst}$) equals 0.2. However, when there is less than 1 migrant per generation (no migration), the inbreeding coefficient rises rapidly resulting in fixation and complete divergence (${\displaystyle Fst}$ = 1). The most common ${\displaystyle Fst}$ is less than 0.25. This means there is some migration happening. Measures of population structure range from 0 to 1. When gene flow occurs via migration the deleterious effects of inbreeding can be ameliorated.

${\displaystyle Fst=1/(4N_{e}m+1)}$

The formula can be modified to solve for the migration rate when ${\displaystyle Fst}$ is known:

${\displaystyle Nm=(1/Fst-1)/4}$, Nm = number of migrants.

Barriers to gene flow

Allopatric speciation

Examples of speciation affecting gene flow.

 

When gene flow is blocked by physical barriers, this results in Allopatric speciation or a geographical isolation that does not allow populations of the same species to exchange genetic material. Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges, oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of China, which has hindered the gene flow of native plant populations. One of these native plants, Ulmus pumila, demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba, Heteropappus hispidus, and Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of China where Ulmus pumila grows. This is because Ulmus pumila has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects. Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools.

Sympatric speciation

Barriers to gene flow need not always be physical. Sympatric speciation happens when new species from the same ancestral species arise along the same range. This is often a result of a reproductive barrier. For example, two palm species of Howea found on Lord Howe Island were found to have substantially different flowering times correlated with soil preference, resulting in a reproductive barrier inhibiting gene flow. Species can live in the same environment, yet show very limited gene flow due to reproductive barriers, fragmentation, specialist pollinators, or limited hybridization or hybridization yielding unfit hybrids. A cryptic species is a species that humans cannot tell is different without the use of genetics. Moreover, gene flow between hybrid and wild populations can result in loss of genetic diversity via genetic pollution, assortative mating and outbreeding. In human populations, genetic differentiation can also result from endogamy, due to differences in caste, ethnicity, customs and religion.

Human assisted gene-flow

Genetic rescue

Gene flow can also be used to assist species which are threatened with extinction. When a species exist in small populations there is an increased risk of inbreeding and greater susceptibility to loss of diversity due to drift. These populations can benefit greatly from the introduction of unrelated individuals who can increase diversity and reduce the amount of inbreeding, and thus increase overall fitness. This was demonstrated in the lab with two bottleneck strains of drosophila melanogaster, in which crosses between the two populations reversed the effects of inbreeding and led to greater chances of survival in not only one generation but two.

Genetic pollution

Human activities such as movement of species and modification of landscape can result in genetic pollution, hybridization, introgression and genetic swamping. These processes can lead to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal. Nonnative species can threaten native plants and animals with extinction by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones which can occur between island and mainland species. Interbreeding between the species can cause a 'swamping' of the rarer species' gene pool, creating hybrids that supplant the native stock. This is a direct result of evolutionary forces such as natural selection, as well as genetic drift, which lead to the increasing prevalence of advantageous traits and homogenization. The extent of this phenomenon is not always apparent from outward appearance alone. While some degree of gene flow occurs in the course of normal evolution, hybridization with or without introgression may threaten a rare species' existence. For example, the Mallard is an abundant species of duck that interbreeds readily with a wide range of other ducks and poses a threat to the integrity of some species.

Gene flow between species

Horizontal gene transfer

Horizontal gene transfer (HGT) refers to the transfer of genes between organisms in a manner other than traditional reproduction, either through transformation (direct uptake of genetic material by a cell from its surroundings), conjugation (transfer of genetic material between two bacterial cells in direct contact), transduction (injection of foreign DNA by a bacteriophage virus into the host cell) or GTA-mediated transduction (transfer by a virus-like element produced by a bacterium).

Viruses can transfer genes between species. Bacteria can incorporate genes from dead bacteria, exchange genes with living bacteria, and can exchange plasmids across species boundaries. "Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic 'domains'. Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes."

Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research". Biologists [should] instead use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of an intertwined net to visualize the rich exchange and cooperative effects of horizontal gene transfer.

"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT. Combining the simple coalescence model of cladogenesis with rare HGT events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times."

Hybridization

In some instances, when a species has a sister species and breeding capabilities are possible due to the removal of previous barriers or through introduction due to human intervention, species can hybridize and exchange genes and corresponding traits. This exchange is not always clear-cut, for sometimes the hybrids may look identical to the original species phenotypically but upon testing the mtDNA it is apparent that hybridization has occurred. Differential hybridization also occurs because some traits and DNA are more readily exchanged than others, and this is a result of selective pressure or the absence thereof that allows for easier transaction. In instances in which the introduced species begins to replace the native species, the native species becomes threatened and the biodiversity is reduced, thus making this phenomenon negative rather than a positive case of gene flow that augments genetic diversity. Introgression is the replacement of the native species genes with that of the invader species. It is important to note that hybrids are generally deemed less "fit" than their parental generation, and as a result is a closely monitored genetic issue as the ultimate goal in conservation genetics is to maintain the genetic integrity of a species and preserve biodiversity.

Examples

Marine iguana of the Galapagos Islands evolved via allopatric speciation, through limited gene flow and geographic isolation.

 

While gene flow can greatly enhance the fitness of a population, it can also have negative consequences depending on the population and the environment in which they reside. The effects of gene flow are context-dependent.

• Fragmented Population: fragmented landscapes such as the Galapagos Islands are an ideal place for adaptive radiation to occur as a result of differing geography. Darwin's Finches likely experienced allopatric speciation in some part due to differing geography, but that doesn't explain why we see so many different kinds of finches on the same island. This is due to adaptive radiation, or the evolution of varying traits in light of competition for resources. Gene flow moves in the direction of what resources are abundant at a given time.
• Island Population: The Marine Iguana is an endemic species of the Galapagos Islands, but it evolved from a mainland ancestor of land iguana. Due to geographic isolation gene flow between the two species was limited and differing environments caused the Marine Iguana to evolve in order to adapt to the island environment. For instance, they are the only iguana that has evolved the ability to swim.

• Human Populations: Two theories exist for the human evolution throughout the world. The first is known as the multiregional model in which modern human variation is seen as a product of radiation of Homo erectus out of Africa after which local differentiation led to the establishment of regional population as we see them now. Gene flow plays an important role in maintaining a grade of similarities and preventing speciation. In contrast the single origin theory assumes that there was a common ancestral population originating in Africa of Homo sapiens which already displayed the anatomical characteristics we see today. This theory minimizes the amount of parallel evolution that is needed.

• Butterflies: Comparisons between sympatric and allopatric populations of Heliconius melpomene, H. cydno, and H. timareta revealed a genome-wide trend of increased shared variation in sympatry, indicative of pervasive interspecific gene flow.
• Human-mediated gene flow: The captive genetic management of threatened species is the only way in which humans attempt to induce gene flow in ex situ situation. One example is the Giant Panda which is part of an international breeding program in which genetic materials are shared between zoological organizations in order to increase genetic diversity in the small populations. As a result of low reproductive success, artificial insemination with fresh/frozen-thawed sperm was developed which increased cub survival rate. A 2014 study found that high levels of genetic diversity and low levels of inbreeding were estimated in the breeding centers.

• Plants: Two species of Monkeyflowers, mimulus lewsii and mimulus cardinalis, were found to have highly specialized pollinators that acted on major genes resulting in a contribution to the floral evolution and reproductive isolation of these two species. The specialized pollination limited gene flow between the two species, eventually resulting in two different species.
• Sika deer: Sika deer were introduced into Western Europe, and they reproduce easily with the native red deer. This translocation of Sika deer has led to introgression and there are no longer "pure" red deer in the region, and all can be classified as hybrids.
• Bobwhite quail: Bobwhite quail were translocated from the southern part of the United States to Ontario in order to increase population numbers and game for hunting.The hybrids that resulted from this translocation was less fit than the native population and were not adapted to survived the Northern Winters.