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Reading Strategies


These graphic organizers are provided to help students locate and analyze information from the articles. Student understanding will be enhanced when they explore and evaluate the information themselves, with input from the teacher if students are struggling. Encourage students to use their own words and avoid copying entire sentences from the articles. The use of bullets helps them do this. If you use these reading and writing strategies to evaluate student performance, you may want to develop a grading rubric such as the one below.


Score

Description

Evidence

4

Excellent

Complete; details provided; demonstrates deep understanding.

3

Good

Complete; few details provided; demonstrates some understanding.

2

Fair

Incomplete; few details provided; some misconceptions evident.

1

Poor

Very incomplete; no details provided; many misconceptions evident.

0

Not acceptable

So incomplete that no judgment can be made about student understanding



Teaching Strategies:


  1. Links to Common Core Standards for Reading:

ELA-Literacy.RST.9-10.1: Cite specific textual evidence to support analysis of science and technical texts, attending to the precise details of explanations or descriptions.


ELA-Literacy.RST.9-10.5: Analyze the structure of the relationships among concepts in a text, including relationships among key terms (e.g., force, friction, reaction force, energy).
ELA-Literacy.RST.11-12.1: Cite specific textual evidence to support analysis of science and technical texts, attending to important distinctions the author makes and to any gaps or inconsistencies in the account.
ELA-Literacy.RST.11-12.4: Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context relevant to grades 11-12 texts and topics.


  1. Links to Common Core Standards for Writing:

ELA-Literacy.WHST.9-10.2F: Provide a concluding statement or section that follows from and supports the information or explanation presented (e.g., articulating implications or the significance of the topic).
ELA-Literacy.WHST.11-12.1E: Provide a concluding statement or section that follows from or supports the argument presented.


  1. Vocabulary and concepts that are reinforced in this issue:

Personal and community health

Reactive oxygen species

Fuel production and use

Molecular structures

Polymers



  1. Some of the articles in this issue provide opportunities, references, and suggestions for students to do further research on their own about topics that interest them.




  1. To help students engage with the text, ask students which article engaged them most and why, or what questions they still have about the articles. The Background Information in the ChemMatters Teachers Guide has suggestions for further research and activities.




  1. In addition to the writing standards above, consider asking students to debate issues addressed in some of the articles. Standards addressed:


WHST.9-10.1B Develop claim(s) and counterclaims fairly, supplying data and evidence for each while pointing out the strengths and limitations of both claim(s) and counterclaims in a discipline-appropriate form and in a manner that anticipates the audience’s knowledge level and concerns.
WHST.11-12.1.A Introduce precise, knowledgeable claim(s), establish the significance of the claim(s), distinguish the claim(s) from alternate or opposing claims, and create an organization that logically sequences the claim(s), counterclaims, reasons, and evidence.

Directions: As you read the article, complete the graphic organizer below to describe the problem with mercury in fish.


3

New things you learned




2

Ways a knowledge of chemistry can help you choose safe fish to eat




1

Question you have about mercury in fish




Contact!

What would you like to tell others about mercury in fish?






Background Information


(teacher information)

More on biomagnification and bioaccumulation
Many people confuse the terms bioaccumulation and biomagnification. These terms do both refer to the accumulation of toxic substances in the bodies of organisms, but they have distinctly different meanings.
Bioaccumulation refers to how pollutants enter the food chain. Bioaccumulation is the increase in the concentration of a chemical in a biological organism over time when compared to the chemical’s concentration in its environment. Compounds accumulate in living organisms when they are taken in and stored faster than they are broken down (metabolized) or excreted. Compound uptake, the entrance of a chemical in an organism, can occur by breathing, swallowing or absorbing it through the skin. One factor affecting bioaccumulation involves the time between the uptake and the eventual elimination. Other factors affecting bioaccumulation are the duration of the exposure and concentration of the chemical.
Biomagnification, also known as bioamplification, refers to the tendency of pollutants to concentrate as they move up a food chain. The process results in the accumulation of a chemical in an organism at higher levels than are found in its food. As reported by Dave McShafrey from Marietta College in Ohio:
In order for biomagnification to occur, the pollutant must be:

  1. long-lived

  2. mobile

  3. soluble in fats

  4. biologically active

If a pollutant is short-lived, it will be broken down before it can become dangerous. If it is not mobile, it will stay in one place and is unlikely to be taken up by organisms. If the pollutant is soluble in water it will be excreted by the organism. Pollutants that dissolve in fats, however, may be retained for a long time. It is traditional to measure the amount of pollutants in fatty tissues of organisms such as fish. In mammals, we often test the milk produced by females, since the milk has a lot of fat in it and because the very young are often more susceptible to damage from toxins (poisons). If a pollutant is not active biologically, it may biomagnify, but we really don't worry about it much, since it probably won't cause any problems.


(http://w3.marietta.edu/~biol/102/2bioma95.html)
The illustration below succinctly shows the difference between bioaccumulation and biomagnification.

(http://learn.anaee.com/wp-content/uploads/2015/01/Bioacc-VS-Biomag1.png)

More on mercury
History
Mercury is the only metal that is liquid at room temperature. The symbol, Hg, comes from the Greek word hydrargyrum, which means “water-silver” or quicksilver. Alchemists gave it the name mercury because of the element’s rapid liquid flow, which was like the Roman god, Mercury, known for his speed. Mercury is the only element to retain its alchemical name as its modern common name.
Mercury is one of a few elements that have been known since antiquity. It was discovered stored in a glass container in a 3500 year old Egyptian tomb in Kuma. Mercury and cinnabar, the most common ore of mercury, are mentioned in ancient manuscripts of the Chinese, Hindus, Egyptians, Greeks and Romans. Each had its own beliefs about mercury, and it was used as everything from a medicine to a talisman.
The Egyptians and the Chinese may have been using cinnabar as a red pigment for centuries before the birth of Christ. In many civilizations mercury was used to placate or chase away evil spirits. The alchemists thought that mercury, which they associated with the planet Mercury, had mystical properties, and they used it in their attempts to transmute base metals into gold. The Greeks knew of mercury and used it as a medicine. Mercury and mercury compounds were used from about the 15th century to the mid-20th century to cure syphilis. Because mercury is extremely toxic and its curative effect is unproven, other syphilis medicines are now used.
(http://nature.berkeley.edu/classes/eps2/wisc/hg.html)
Although mercury was believed to be an all-important substance, many of the ancients also knew it to be toxic. The mining of mercury presented the first evidence of its poisonous effects. People working in these mines started to have tremors and progressed to severe mental derangement. The Romans sent slaves, criminals and other undesirables to work in their mercury mines. They realized that it was likely that the prisoners would become poisoned and it would spare them the need for a formal execution.
Mercury(II) chloride, HgCl2, was used from the 15th to the 20th centuries to treat syphilis. It had terrible side effects, causing neuropathies (problems with the nerves), kidney failure, severe mouth ulcers, loss of teeth, and even death.
The saying “Mad as a Hatter” is related to the early production of felt hats that used a mercury compound.
To make felt, hatters separated fur from the skin of small animals in a process called carroting. In this process, the secondary nitrous gas released from mercury(II) nitrate caused the fur to turn orange, lose shape, and shrink. The fur also then became darker, coiled, and more easily removed. Prior to the use of mercury to remove fur, hatters used camel urine, which is mostly water but also contains nitrogen waste in the form of urea. When applied to fur, urea disrupts chemical bonds and causes protein denaturation. During the expansion of hat-making into 19th-century France and England, hatters frequently replaced camel urine with their own. Subsequently, it was noticed that an individual workman treated with mercury(II) chloride for syphilis consistently produced superior felt. As a result, mercury(II) nitrate came into wide use to obtain the same effects as the workman's mercury-contaminated urine.
(https://www.cas.org/news/insights/science-connections/mad-hatter)
By the 18th century the mercury(II) nitrate, Hg(NO3)2, was widely used in making felt hats. After absorbing the mercury compound for years, hat makers often suffered mercury poisoning, with symptoms that included staggered walk, tunnel vision, and brain damage which made them appear crazy. The phrase “mad as a hatter” became popular as a way to refer to someone that appeared insane. This gave rise to the character, the Mad Hatter, in Lewis Carroll’s Alice’s Adventure in Wonderland.
Properties


Physical

Chemical

Freezing point -38.8 oC

Not very reactive to acids

Boiling point 356.6 oC

Toxic

Density 13.5 g/cm3

Does not readily react with oxygen in the air

Heavy silvery metal

Common ions formed +1, +2
(+2 is more stable)

Only metal liquid at room temperature

Only rarely occurs free in nature

Poor conductor of heat (compared to other metals)




Good conductor of electricity




High surface tension




High vapor pressure and high volatility for metal




Dissolves metals like Ag, Au and Cu to form alloys called amalgams





Uses
The most prominent use of mercury today is in the industrial production of chlorine. The mercury cell process (see diagram below) was developed in 1892 by Hamilton Castner and Karl Kellner. It is also known as the Castner-Kellner process. Chlorine is produced from an aqueous sodium chloride solution using electrolysis. There was a problem with the process because it produced sodium metal which reacts violently with water. The scientists solved the problem by making a container with a layer of mercury. As the sodium forms it dissolves in the mercury. For years this was a popular method for making mercury, but today they are phasing out this method because of the harmful effects of mercury.

The mercury cell process
(http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Case_Study%3A_Industrial_Electrolysis)(http://learn.anaee.com/wp-content/uploads/2015/01/Bioacc-VS-Biomag1.png)

The mercury cell process


(http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Case_Study%3A_Industrial_Electrolysis)(http://learn.anaee.com/wp-content/uploads/2015/01/Bioacc-VS-Biomag1.png)
Mercury Cell Process

Anode Side

  • The anodes are placed in the aqueous NaCl solution, above the liquid mercury.

  • The reduction [Editors’ note: this is really oxidation] of Cl occurs to produce chlorine gas, Cl2 (g).

Cathode Side

  • A layer of Hg (l) at the bottom of the tank serves as the cathode.

  • With a mercury cathode, the reaction of H2O (l) to H2 has a fairly high over potential, so the reduction of Na+ to Na occurs instead. The Na is soluble in Hg (l) and the two combine to form the Na-Hg alloy amalgam. This amalgam can be removed and then mixed with water to cause the following reaction:

2 Na (in Hg) + 2 H2O → 2 Na+ + 2OH + H2 (g) + Hg(l)


  • The Hg (l) that forms is recycled back into the liquid at the bottom of the tank that acts as a cathode.

  • H2 gas is released.

  • NaOH is left in a very pure, aqueous form.

(http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Case_Study%3A_Industrial_Electrolysis)


A mercury switch
(https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Mercury_Switch_without_housing.jpg/220px-Mercury_Switch_without_housing.jpg)

The second major use of mercury is in the production of switches and other electrical applications. Mercury electrical switches consist of a small tube with two electrical contacts at one end. If the tube is tilted so that the mercury collects at this end, then contact is made and the circuit is complete. The circuit is broken when the mercury flows to the other end. These switches are used in such things as aircraft altitude indicators, fall alarms (alarm sounds if a worker falls), tilt alarms in vending machines, and old doorbells and thermostats.


Fluorescent lamps contain mercury, some of which is in the vapor form, and some of which vaporizes from heat generated when the lamp is turned on. Electric current passing through the mercury vapor excites electrons in the atoms, which causes them to emit ultraviolet radiation. The walls of the glass tube are coated with a phosphor, a substance that exhibits the property of luminescence—giving off visible light—when the ultraviolet radiation strikes it. The phosphor coating fluoresces, producing the visible light. Fluorescent lamps convert electricity into visible light much more efficiently than incandescent lamps, which rely on heat causing incandescence of the metal filament. Because of the hazards mercury presents, manufactures have reduced the amount of mercury in fluorescent lamps by 60 percent.
Mercury batteries were very popular, until the deleterious effects of mercury became widely known. These batteries have the advantage of having a long shelf-life and producing a steady voltage. In the 1980s, more than 1000 tons of mercury per year were used to make batteries. By 1996, less than 1 ton was used in batteries. The Mercury-Containing and Rechargeable Battery Management Act of 1996 prohibits the use of mercury in all batteries except for button cell batteries and mercuric oxide batteries. The mercury button cells are used in small portable devices such as watches, hearing aids, calculators and cameras. The mercuric oxide batteries are produced largely for the military and medical equipment that needs a stable current and long life.
(http://www.hk-phy.org/contextual/heat/tep/act_thermometers_e.html)

For decades mercury has been used in devices for scientific research, including thermometers and barometers. The mercury thermometer was invented by Daniel Fahrenheit in 1714. A mercury thermometer consists of a bulb containing mercury attached to a glass tube with a narrow diameter. Mercury works well in thermometers for many reasons. It is a good conductor of heat and has a high coefficient of expansion. As a result it takes little heat to cause it to expand, which makes it easy to measure its linear expansion in a thermometer in relation to temperature. Mercury has a high boiling point so it is suitable to measure higher temperatures. It is shiny and doesn’t stick to the glass.


A mercury barometer

(http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/section_14/8cec964659fd2bb7ec4dc6c2c78eb4f9.jpg)

Mercury was also widely used in barometers to measure atmospheric pressure. The invention of the mercury barometer is credited to Evangelista Torricelli, the first scientist to propose using mercury for this device. Several barometers had been made before this, but they used water as the liquid moved by the atmosphere. This resulted in huge changes in the height of the column (which was about 34 feet tall!). Torricelli suggested using mercury because the great density of mercury (13.7 g/cm3) resulted in relatively small, easily measurable differences in the height of the glass-encased mercury column as air pressure changed, sometimes rather drastically. The mercury column itself never rose more than about 760 mm (2-1/2 feet) high, compared to the 34 feet of water.



This made it ideal for use in these instruments.
Because of the hazards of mercury, these thermometers and barometers are gradually being phased out. In fact, for that reason the National Institute of Standards and Technology will no longer calibrate mercury thermometers. (Note: If you are a relatively young teacher, you might not have ever even encountered a mercury thermometer in your own chemistry lab—and certainly not a mercury barometer.) Many alternatives exist, such as alcohol-filled thermometers and digital thermometers, and aneroid and digital barometers. The EPA has encouraged the reduction of use of these mercury-filled instruments. As reported by the EPA:
Some states and municipalities have passed laws or ordinances barring the manufacture, sale and/or distribution of mercury fever thermometers. This is to help remove the threat of thermometer breakage and the subsequent release of mercury vapor indoors. Thirteen states have passed such laws. They are: California, Connecticut, Illinois, Indiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, New Hampshire, Rhode Island, Oregon, and Washington.
(http://www.epa.gov/mercury/mercury-thermometers)
Mercury readily forms alloys with other metals such as gold, silver, zinc and copper. These alloys with mercury are called amalgams. Amalgams have been used throughout history. Mercury has been used to extract gold from ore since as early as 1000 A.D. The mercury dissolves the gold and then the mercury is boiled off. Silver is extracted in much the same way. Dental amalgams have been used for over 150 years in fillings. The dental amalgam consists of mercury combined with silver, tin, copper, and sometimes, zinc.
Compounds of mercury have been used since early times as well. Mercury(II) sulfide, HgS, also known as vermillion or cinnabar, has a red color which is used as a coloring agent. Mercury(I) chloride, Hg2Cl2, is an antiseptic. Mercury(II) fulminate, Hg(CNO)2, is an explosive and sensitive to impact. It is used in percussion caps for munitions.
Mercury in the environment
Mercury enters the environment through natural sources such as volcanoes, and the breakdown of minerals in rocks and soil. This release of mercury from natural sources has remained fairly constant over the years. The mercury concentration in the environment is increasing due to human activity.
During the 4000 years of man’s use of mercury, it is estimated that 350,000 tons of mercury have been released from the depths of Earth into the air, surface land and water. This is where its toxicity becomes problematic. The combustion of fossil fuels, especially from coal-fired power plants, releases large amounts of mercury into the air. Other human sources of mercury pollution arise from mining, smelting and solid waste combustion. The diagram below shows the many sources of mercury pollution in the environment.
http://i.livescience.com/images/i/000/057/351/original/mercury-pollution-sources.jpeg?1380219257
(http://i.livescience.com/images/i/000/057/351/i02/mercury-pollution-sources.jpeg?1380219257)
Once mercury enters the atmosphere, it can travel great distances and circulate for years. It eventually will either fall to the ground or be absorbed in water. Once there it can be methylated and enter the food chain. According to the U.S. Geological Survey:
Methylation is a product of complex processes that move and transform mercury. Atmospheric deposition contains the three principal forms of mercury, although inorganic divalent mercury, Hg(II), is the dominant from. Once in surface water, mercury enters a complex cycle in which one form can be converted to another. Mercury attached to particles can settle onto the sediments where it can diffuse into the water column, be resuspended, be buried by other sediments or be methylated. Methylmercury can enter the food chain, or it can be released back to the atmosphere by volatilization.
(http://www.usgs.gov/themes/factsheet/146-00/)
methlation cycle.
Mercury methylation and movement through ecosystems
(http://www.usgs.gov/themes/factsheet/146-00/fig5.gif)
Mercury poisoning and its effect on health
No level of mercury in the human body is normal. There is no specific function for mercury in our body. However, everyone is exposed to some level of mercury. Exposure may occur through inhalation of mercury vapor, ingestion or dermal contact (absorbed through the skin). Fortunately, most exposures are low level and often occur through chronic exposure (long term contact). Some people are exposed to high levels or acute exposure to mercury. An example of this would be an industrial accident involving mercury. According to the World Health Organization:
Factors that determine whether health effects occur and their severity include:

  • the type of mercury concerned;

  • the dose;

  • the age or developmental stage of the person exposed (the foetus is most susceptible);

  • the duration of exposure;

  • the route of exposure (inhalation, ingestion or dermal contact).

(http://www.who.int/mediacentre/factsheets/fs361/en/)
There are three basic categories for mercury; elemental mercury, inorganic salt of mercury, and organic mercury. If elemental mercury is ingested, it is absorbed relatively slowly (less than 0.01% of a dose is absorbed) and may pass through the digestive system without causing much harm. There exists a greater risk if mercury vapor is inhaled. According to an article in the Journal of Preventative Medicine and Public Health:
Inhalation is a major exposure route of elemental mercury in the form of mercury vapor. Inhaled mercury vapor is readily absorbed, at a rate of approximately 80%, in the lungs, and quickly diffused into the blood and distributed into all of the organs of the body. Absorbed elemental mercury is oxidized to the mercuric form (Hg++) in the red blood cells and tissues, a process that takes several minutes. However, inhaled mercury vapor, in contrast to inorganic mercury salts, accumulates in the central nervous system.
(http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3514464/)
The inhalation of mercury vapors can cause nerve, brain and kidney damage, respiratory illness, skin rashes, tremors, and anemia.
Exposure to inorganic mercury salts generally occurs either through ingestion or absorption through the skin. Since mercury salts are nonvolatile solids, inhalation poisoning is rare. The more soluble the mercury salt is the more likely it will be absorbed. Inorganic mercury salts are not fat soluble, and they do not readily cross the blood-brain barrier or the blood-placenta barrier. They tend to be excreted in the urine and feces. The highest concentration is found in the kidney. Chronic inorganic mercury poisoning is rare, and the target organ affected is the kidney. Symptoms may include issues with the ability to urinate. Inorganic mercury salts are corrosive, which enhance gastrointestinal absorption. They are generally irritants to the skin, causing dermatitis and discoloration of the nails. Acute high doses of mercury salts cause chest pain and severe gastrointestinal symptoms that result from the corrosive damage to the gastrointestinal tract.
Organic mercury poisoning, especially due to methylmercury and dimethylmercury, is the most common form of mercury poisoning today. Methylmercury (CH3Hg+) has no industrial uses and is formed in the environment from the methylation of the mercury(II) ion. Methylmercury exposure primarily occurs through ingestion. Fish is the primary source for the ingestion of organic mercury. Organic mercury compounds are fat soluble and bioaccumulate in the body. These compounds are also able to cross the blood-brain and blood-placenta barriers. Developing embryos and infants are 5–10 times more sensitive to the effects of methylmercury than adults. Methylmercury damages the central nervous system and affects the immune system, by altering the immune and genetic systems. Symptoms of methylmercury poisoning in adults include visual field constriction (tunnel vision), behavioral changes, memory loss, headaches, tremors, loss of fine motor control, tingling or pricking of extremities and lips, and hair loss. Organic mercury damage tends to be irreversible.
There are many ways to reduce the mercury in the environment and thereby reduce human exposure to mercury. Promoting clean energy is essential in reducing environmental mercury. Coal contains mercury that is emitted when coal is burned in coal-fired power plants and industrial boilers. Eliminating the use of mercury in gold mining and promoting non-mercury extraction methods would reduce mercury contamination. The mining of mercury should not be needed if what is available now is recycled. Phasing out of non-essential mercury products would reduce the mercury entering the environment.
More on the advantages and disadvantages of eating fish
Eating fish and shellfish has many advantages for one’s health. According to the U.S. Food and Drug Administration, fish and shellfish should be an important part of a healthy diet. Fish and shellfish are a major source of omega-3 fatty acids, they are rich in vitamin D and they are a high-quality protein source that is low in saturated fat. Eating fish is good for the heart and blood vessels. In fact, it is reported that eating two three-ounce servings of fatty fish per week reduces the risk of dying from heart disease by 36%. According to the Harvard School of Public Health:
Eating fish fights heart disease in several ways. The omega-3 fats in fish protect the heart against the development of erratic and potentially deadly cardiac rhythm disturbances. They also lower blood pressure and heart rate, improve blood vessel function, and, at higher doses, lower triglycerides and may ease inflammation.
http://www.hsph.harvard.edu/nutritionsource/fish/
Omega-3 fatty acids cannot be synthesized by the human body so they must be consumed in one’s diet. For that reason they are called essential fatty acids. Omega-3 fatty acids are called omega-3s because they have a double bond at the third carbon atom from the end of the carbon chain.
http://images.tutorvista.com/cms/images/101/structure-of-omega-3-fatty-acids.png
Examples of Omega-3 Fatty Acids

(http://images.tutorvista.com/cms/images/101/structure-of-omega-3-fatty-acids.PNG)
Omega-3 fatty acids provide a number of health benefits. Besides helping to maintain cardiovascular health, they are important for prenatal and postnatal neurological development. There is some evidence that they may reduce tissue inflammation, alleviate symptoms of rheumatoid arthritis, reduce depression and slow the mental decline in older people.
The disadvantage of eating fish and shellfish is that they all contain traces of mercury. Mercury enters streams, rivers, lakes and oceans primarily from rain and surface water runoff. Bacteria convert the mercury to methylmercury. The methylmercury is taken up by plankton. The plankton bioaccumulate the mercury and then small fish consume the plankton. The older the fish the more methylmercury they absorb. As these fish are eaten the mercury moves up the food chain and biomagnifies. Predatory fish, like sharks and swordfish, have higher concentrations of mercury. When humans consume fish, the mercury accumulates in their bodies as well. The risk of consuming the mercury in fish is greatest for young children and women of childbearing age.
The benefits and risks of consuming a regular diet of fish is a controversial topic. According to a Harvard School of Public Health report:
At the levels commonly consumed from fish, there is also limited and conflicting evidence for effects of mercury in adults; thus, the Environmental Protection Agency, the Food and Drug Administration, the Institutes of Medicine report, and the analysis by Mozaffarian and Rimm all conclude that this evidence is insufficient to recommend limitations on fish intake in adults, given the established benefits of fish consumption for cardiovascular disease. In fact, the easiest way to avoid concern about contaminants is simply to eat a variety of fish and other seafood.
(http://www.hsph.harvard.edu/nutritionsource/fish/)
The U.S. Food and Drug Administration recommends following these three safety tips when considering eating fish or shellfish—especially women and young children
3 Safety Tips

  1. Do not eat

They contain high levels of mercury.

  • Shark

  • Swordfish

  • King Mackerel

  • Tilefish

  1. Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury.

  • Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.

  • Another commonly eaten fish, albacore ("white") tuna has more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week.

  1. 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) per week of fish you catch from local waters, but don't consume any other fish during that week.


Follow these same recommendations when feeding fish and shellfish to your young child, but serve smaller portions.
(http://www.fda.gov/food/resourcesforyou/consumers/ucm110591.htm)
More on trace measurement units
Health effects of any toxic substance, like mercury, partially depends on the level of exposure or the dose; the greater the dose, the more serious the effects. Some chemicals can be toxic in very low doses so it is important to understand the units commonly used for measuring these contaminants.
The most common units used to measure very small amounts of contaminants in our environment are parts per million (ppm) and parts per billion (ppb). These units are the ratio of the substance (contaminant) compared to the total solution.

http://www.chemtech.org/cn/cn1305/images/12-ppm.gif

(http://www.chemtech.org/cn/cn1305/images/12-ppm.gif)


Sometimes the weight of the contaminant is compared to the total weight.
Metric system units go in steps of 10, 100, and 1,000. For example, a milligram is a thousandth of a gram (moving the decimal point three places to the left) and a gram is a thousandth of a kilogram (again a difference of three places to the left on the decimal point). Thus, a milligram is a thousandth of a thousandth, or a millionth of a kilogram moving the decimal point six places. So, a milligram is one ppm of a kilogram; therefore, one ppm is the same as one milligram per kilogram.
(http://www.nesc.wvu.edu/ndwc/articles/ot/fa04/q&a.pdf)
In liquid solutions, measuring mass may not be feasible. Measurement of volume uses the liter. One liter of pure water has a mass of one kilogram. If the contaminant is a solid it is measured in milligrams; one milligram/liter, 1 mg/L, is 1 ppm. So, simply stated,

1 ppm = 1 mg/kg = 1 mg/L.


Parts per billion, ppb, is a measurement for even smaller concentrations. It can be represented by:
https://www.nauticus.org/chemistry/images/ppbg.jpg

(https://www.nauticus.org/chemistry/images/ppbg.jpg)


In terms of mass, a ppb is equal to a microgram/kilogram, 1 µg/kg, or a microgram/liter, 1 µg/L. It is important to know the difference between ppm and ppb, since a common mistake is reporting a concentration as ppm when it is actually ppb.

(http://www.abcwua.org/uploads/images/pool.gif)
http://www.abcwua.org/uploads/images/pool.gif

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