The facts on silver

What is silver?

A lustrous, soft white metal, silver is one of the elements that make up the Earth. Silver is found in nature as an elemental metal in its metallic form and combined with other elements such as sulfide, chloride and nitrate. Pure silver has a bright metallic white-gray color; silver nitrate and silver chloride are powdery white in color, while silver sulfide and silver oxide are dark gray to black.

Silver is stable in pure air and water, though it tarnishes quickly when exposed to air that contains elevated levels of ozone, hydrogen sulfide or sulfur. (In the past 200 years, the amount of sulfur in the atmosphere has increased, so silver tarnishes more quickly than it did in pre-Industrial times.) Tarnish can easily be removed, however, and does not destroy the metal the way oxidation process known as rust destroys iron. The fact that silver is otherwise impervious to the elements helps define it as a precious metal.

Silver is a rare metal that has long been valued for its versatility. Slag dumps in Asia Minor and the Aegean Sea islands show that our ancestors were mining silver over 5,000 years ago.

Where is silver found?

Concentrated deposits of silver are found in ores along with other metals including lead, zinc, copper and gold in diverse regions of the world including Mexico, Peru, and the United States. The natural process of rain and wind pounding repeatedly on silver-bearing rocks and soil also disperses silver into the environment.

Natural processes account for about 18 percent of the estimated 2,430 tons of silver entering the environment each year. The remaining 82 percent is released through human industry; almost half of that amount is produced by the photographic industry. Of the total silver released into the environment each year almost 4 percent enters the atmosphere, 28 percent enters aquatic environments, and 68 percent enters terrestrial ecosystems. Silver and silver compounds released into the environment can travel long distances in air and water, including groundwater.

Silver compounds can concentrate or accumulate to elevated levels in the environment in several ways: mixed with soil or water at hazardous waste sites; as a by-product from the mining of copper, lead, zinc or gold ores; or as a by-product from the production of photographic film. Such by-products can enter the environment directly, from a factory pipe draining into water for example, or indirectly, through water or sewage treatment plants. The growth of small photo processing units in a range of retail outlets has increased the amount of silver-contaminated wastes entering municipal sewage treatment systems. These wastes, which are not regulated, can overwhelm the capacities of these systems to keep treated water within acceptable environmental safety guidelines.

According to United States Agency for Toxic Substances and Disease Registry (ATSDR) the average proportion of silver in U.S. surface waters such as lakes and rivers is about 2 parts silver per billion parts water; the proportion in soil is around 20 to 30 parts silver per million parts soil. Public drinking water supplies in the United States have been found to contain up to 80 parts per billion of silver (80 micrograms per liter).

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What are the uses of silver?

Humans have valued silver since antiquity. Commonly crafted for its beauty into fine jewelry and silverware, silver’s value, rarity and durability inspired its use in coins throughout history. Sterling silver (an alloy of 92 percent silver, blended with copper and other metals) gives a lustrous appearance to coins, jewelry, and silverware. The fact that pure silver is slightly harder than gold, but still very ductile and malleable, makes it a natural choice for dental fillings. Silver has also been used to make equipment that processes foods and beverages. Silver makes a beautiful coating for mirrors that reflects visible light almost perfectly.

In a modern version of the ancient “rain dance”, silver iodide can be dropped or “seeded” onto clouds to produce or increase rainfall. Scientists are now conducting studies in desert regions of the United States and Mexico, to quantify and validate this silver cloud-seeding technology, and to determine whether it would increase rainfall in drought-ridden areas. Normally, clouds produce rain when tiny particles of dust attract moisture and grow into raindrops. To mimic this natural effect, planes fly through the updraft under clouds using pyrotechnic flares to discharge “seeds” (tiny grains of silver) upward. Cloud-seeding technology raises some concerns that adding chemicals to clouds would pollute the air, water or earth. Groups supporting the practice, including the National Weather Modification Association, claim that the amounts of silver are far too small to cause harm.

Silver's antibacterial properties have been exploited in a number of applications. For many years, silver drops were placed into newborn babies’ eyes to protect against blindness caused by maternal gonorrhea. Antibiotics such as erythromycin are now used for similar infections. Silver has also been used in salves for burn victims and to purify water. Although these uses of silver are less common now than a century ago, health experts are considering using silver again, in place of chlorine, in water-purification systems such as those that service swimming pools, because of concerns that chlorine can react with other elements in the environment to form carcinogenic by-products.

Pure silver has the highest electrical and thermal conductivity of any metal, along with having the lowest contact resistance. Because of its electrically conductive and reflective properties, silver is used industrially for photography (silver nitrate), soldering alloys, electrical and printed circuit board contacts, and high-capacity batteries made from silver-zinc and silver-cadmium alloys. Silver is a true “darling” of the modern high-tech world, and is distinguished from its sister metals by its industrial versatility.

Do we need silver for health?

Unlike other "essential" elements such as calcium, human bodies don’t need silver to function. Though silver was once used in medical applications, modern substitutes have largely superceded these uses, and there would be no ill health effects from going through life without ever contacting silver.

This does not happen, however. Trace amounts of silver are in the bodies of all humans and animals. We normally take in between 70 and 88 micrograms of silver a day, half of that amount from our diet. Humans have evolved with efficient methods of dealing with that intake, however. Over 99 percent is readily excreted from the body.

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Is silver harmful to humans?

Unlike other metals such as lead and mercury, silver is not toxic to humans and is not known to cause cancer, reproductive or neurological damage, or other chronic adverse effects. Nor has normal day-to-day contact with solid silver coins, spoons or bowls been found to affect human health. This is because solid silver is almost completely biologically inert, and even if ingested, would pass through the human body without being absorbed into tissues.

In very high doses — such as those a factory worker might encounter in an accident — or from prolonged exposure to silver dust or fumes, silver can have some mostly mild effects on health. For example, inhaling silver fumes or dust may irritate mucous membranes or the upper respiratory tract.

Occasionally, sensitive individuals suffer allergic reactions — contact dermatitis or eye irritation — after exposure to powdered silver, silver solutions or dental fillings. Similarly, skin creams containing silver compounds (silver nitrate and silver sulphadiazine) cause local skin discoloration in certain sensitive individuals. Ingesting silver compounds, such as in medicines, can sometimes irritate the stomach.

Prolonged exposure to silver dust or to the silver compounds in medicines or supplements can also result in a permanent blue-gray staining of the eyes, nose, mouth, throat and skin. This blue-gray staining is known medically as “argyria.” The condition can make people look ill, as if they suffering from lack of oxygen. Once a person turns blue from argyria, the skin coloring is unfortunately permanent. Most medical professionals believe argyria is the most serious known health effect of silver on humans. Aside from its permanent cosmetic effect, argyria is not believed to pose any other risk to human health.

The mild, observed human health effects of silver exposure appear to be highly variable from one person or situation to another. Scientists have not identified exposure levels that can be generalized as harmful.

Who is at risk of harm from silver?

People who are most vulnerable to minor health effects from silver (including allergic contact dermatitis, or irritation of the eyes, mucous membranes, upper respiratory tract irritation, or the stomach) are those who work in factories where silver is manufactured into electrical or photography equipment, with fumes or dust resulting. Others who may be exposed to elevated levels of silver are those who work with silver in melting, casting, grinding, polishing or etching operations. In factory environments where high exposures to silver dust or fumes are likely, protective clothing, gloves, eye goggles and ventilators or respirator equipment can prevent ill effects.

How can I tell if I have been exposed to harmful levels of silver?

There are laboratory tests that can measure silver levels in blood, urine, feces or tissues accurately and reliably. Blood or urine samples are the easiest tests to conduct, and these tests measure recent exposures — those occurring within the past week or so. For past exposures, laboratory tests can measure how much silver has built up in the skin. Silver exposure tests are typically conducted outside of a doctor’s office because they require special equipment. Although these tests can show whether a person has been exposed to silver, they cannot predict whether any ill health effects will occur.

On a practical level, tests are not generally recommended except for industrial workers who inhale or ingest silver fumes or dust to the point of experiencing symptoms such as skin or eye irritation or discoloration, irritation of the stomach, mucous membranes or upper respiratory tract. Tests might also be recommended for someone who took silver-based medicines or silver “health supplements” and experienced any of the symptoms above, especially skin discoloration.

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Is silver in the environment a health risk?

In its pure metal form or in ores, silver does not dissolve and is not considered an environmental risk. But high doses of certain compounds of silver have been found to highly toxic to aquatic life forms, such as fish.

Scientists once believed that metals that existed as free ions were most likely to pose a risk to living things, since these forms tend to react more readily with biological molecules. Studies of fish and zooplankton exposed to high doses of silver nitrate (a form of the metal containing large quantities of free ions) confirmed that silver in this form is indeed highly toxic to aquatic creatures. This ionic form of silver interferes with an enzyme (sodium/potassium ATPase) that regulates the levels of potassium and sodium in fish. Disturbing the sodium/potassium equilibrium has fatal effects: Fish quickly lose ions from their blood, water seeps into their body tissues, and they die from cardiovascular collapse. Similar effects were found in tiny aquatic animals called zooplankton. Though these effects are dramatic, this ionic form of silver is rarely found outside a laboratory.

Scientists now suspect that lower doses of silver compounds over longer periods of time may have more subtle but equally worrisome effects on fish and other aquatic organisms — affecting the reproductive system in sensitive species. Researchers are investigating the effects of chronic silver exposure on aquatic life.

What are the government standards and guidelines on silver?

The Environmental Protection Agency (EPA) recommends that the concentration of silver in public drinking water supplies not exceed one milligram per liter of water — one part per million — because of the skin discoloration that may occur from chronic silver exposure. The agency also requires that spills or accidental releases of 1,000 pounds or more of silver be reported.

Workers in the United States are protected from excess silver exposure through regulations set by the Occupational Safety and Health Administration (OSHA). Silver in workplace air may not exceed .01 milligrams per cubic meter for an 8-hour workday and a 40-hour workweek. This is the same standard recommended by the National Institute of Occupational Safety and Health (NIOSH) and the American Conference of Government Industrial Hygienists (ACGIH).

Helpful links:

-- Agency for Toxic Substances and Disease Registry (ATSDR) Public Health Statement for Silver

-- Agency for Toxic Substances and Disease Registry (ATSDR) Short fact sheet for
Silver

-- Silver Metal Material Safety Data Sheet (pdf format)

-- Chemistry: Web Elements Periodic Table: Silver