Silver Hazards to Fish, Wildlife, and Invertebrates:
A Synoptic Review (PDF)
(Excerpts from pages 1-16)
by Ronald Eisler
U.S. Department of the Interior
National Biological Service
Patuxent Wildlife Research Center
Biological Report 32 Contaminant Hazard Reviews
September 1996 Report No. 32
Ecological and toxicological aspects of silver (Ag) and silver salts in the environment are briefly summarized with an emphasis on natural resources.
Subtopics include sources and uses, chemistry and metabolism, concentrations in field collections, lethal and sublethal effects, and recommendations for the protection of natural resources.
Elevated silver concentrations occur in the vicinities of sewage outfalls, electroplating plants, mine waste sites, and silver-iodide seeded areas; in the United States, the photography industry is the major source of anthropogenic silver discharges into the biosphere.
Silver and its compounds are not known to be mutagenic, teratogenic, or carcinogenic.
Under normal routes of exposure, silver does not pose serious environmental health problems to humans at less than 50 μg total Ag/L drinking water or 10 μg total Ag/m3 air.
Free silver ion, however, was lethal to representative species of sensitive aquatic plants, invertebrates, and teleosts at nominal water concentrations of 1.2 to 4.9 μg/L; at sublethal concentrations, adverse effects were significant between 0.17 and 0.6 μg/L.
No data were found on effects of silver on avian or mammalian wildlife; all studied effects were on poultry, small laboratory animals, and livestock.
Silver was harmful to poultry at concentrations as low as 1.8 mg total Ag/kg whole egg fresh weight by way of injection, 100 mg total Ag/L in drinking water, or 200 mg total Ag/kg in diets; sensitive mammals were adversely affected at total silver concentrations as low as 250 μg/L in drinking water, 6 mg/kg in diets, or 13.9 mg/kg whole body.
Silver (Ag) found in the body of mammals (including humans) has no known biological purpose and is suspected of being a contaminant.
Silver is one of the most toxic metals known to aquatic organisms in laboratory testing, although large industrial losses to the aquatic environment are probably infrequent because of its economic value as a recoverable resource.
Silver, however, is of concern in various aquatic ecosystems because of the severity of silver contamination in the water column, sediments, and biota.
San Francisco Bay, for example, is impacted from discharges of silver in wastewater outfalls and from the remobilization of silver from contaminated sediments in the estuary. (note: for more on this issue, click here)
Emissions from smelting operations, manufacture and disposal of certain photographic and electrical supplies, coal combustion, and cloud seeding are some of the anthropogenic sources of silver in the biosphere.
Fallout from cloud seeding with silver iodide is not always confined to local precipitation; silver residuals have been detected several hundred kilometers downwind of seeding events.
In 1978, the estimated loss of silver to the environment in the United States was 2.47 million kg, mostly to terrestrial and aquatic ecosystems; the photography industry alone accounted for about 47% of all silver discharged into the environment.
In California, anthropogenic sources contributed 50% more silver to sediments of coastal basins than did natural sources, as judged by sedimentary basin fluxes of 0.09 μg/cm2 in anthropogenic sources of silver and 0.06 μg/cm2 in natural sources (Bruland et al. 1974).
Sometimes, liquid effluents from the nuclear industry contained significant quantities of radiosilver-110m.
In Lake Michigan, storms contribute a large fraction of the annual load of tributary-derived silver; concentrations of particle-bound silver in many rivers during storms were more than 0.1 μg/L.
The acute toxicity of silver to aquatic species varies drastically by the chemical form and correlates with the availability of free ionic silver. In natural aquatic systems, ionic silver is rapidly complexed and absorbed by dissolved and suspended materials that are usually present.
Thus, silver nitrate—which is strongly dissociated—is extremely toxic to rainbow trout.
In seawater, silver nitrate is less toxic than in fresh water.
This difference is probably due to the low concentration of free silver in seawater and to the high levels of chloride in seawater.
However, high levels of silver nitrate are toxic to marine invertebrates. Ionic silver interferes with calcium metabolism of frogs and marine worms.
Silver may enter the body of mammals through inhalation, ingestion, and movement across mucous membranes or broken skin.
In most cases [of silver miners studied], absorption occurs via the respiratory tract or at the eyes.
Silver is retained by all body tissues; tissue concentrations are related to the dose, form of administered silver, and route of exposure.
Silver also accumulates in mammalian tissues with increasing age of the individual, even if none is administered intentionally.
In mammals, the highest concentrations of silver are usually found in the liver and spleen and to some extent in the muscles, skin, and brain.
The primary sites of silver deposition in the human body are the liver, skin, adrenals, lungs, muscle, pancreas, kidney, heart, and spleen; silver is also deposited in blood vessel walls, the trachea, and bronchi.
Among mammals, low doses of ingested silver were eliminated from the body within 1 week [if no more exposure occurred].
The mean daily intake of silver in humans is about 88 μg; about 60 μg is excreted daily in the feces.
In humans, the whole-body effective half-time of persistence was 43 days.
The biological half-time of silver in the lungs of an exposed person was about 1 day; in liver it was 52 days.
In humans, 80% of the retained silver in lung was cleared in about 1 day; 50% of the remainder was usually cleared in 3 days[if not exposed to more].
Silver interacts competitively with selenium, vitamin E, and copper and induces signs of deficiency in animals fed adequate diets, or aggravates signs of deficiency, when diets lack one or more of these nutrients; antagonistic effects of silver have been described in dogs, pigs, rats, sheep, chicks, turkey poultry, and ducklings.
Conversely, the addition of selenium, copper, or vitamin E to diets of turkey poultry decreased the toxicity.
Silver can remain attached to oceanic sediments for about 100 years.
Smith and Carson report that sprays containing 9.8 mg dissolved Ag/L kill corn and sprays containing 100-1,000 mg dissolved Ag/L kill young tomato and bean plants.
[The scientists] planted seeds of corn, lettuce, oat , turnip, soybean, spinach , and Chinese cabbage in soils amended with silver sulfide and sewage sludge to contain 10, 50, or 100 mg Ag/kg DW soil.
All plants germinated and most grew normally at the highest soil concentration of silver tested.
But growth of Chinese cabbage and lettuce was adversely affected at 10 mg Ag/kg DW soil and higher.
In fish and amphibian toxicity tests with 22 metals and metalloids, silver was the most toxic tested element.
In solution, ionic silver is extremely toxic to aquatic plants and animals, and water concentrations of 1.2-4.9 μg/L killed sensitive species of aquatic organisms, including representative species of insects, daphnids, amphipods, trout, flounders, sticklebacks, guppies, and dace.
At nominal water concentrations of 0.5-4.5 μg/L, accumulations in most species of exposed organisms were high and had adverse effects on growth in algae, clams, oysters, snails, daphnids, amphipods, and trout; molting in mayflies; and histopathology in mussels.
Among all tested species, the individuals most sensitive to silver were the poorly nourished and young and those exposed to low water hardness or salinity.