Questions About the Environment
What is known about the transportation, fate and effects
(TF&E) of silver for given wastewater treatment scenarios?
What is known about the toxicity of silver-bearing waste?
What forms of silver are present in a sewage treatment plant discharge?
What happens to silver after it is discharged into natural waters?
Is silver in sediments released back into the water?
What is the mobility and stability of silver released into other environmental media such as soil or groundwater?
Questions About Silver & Human Health
What is known about silver and effects on human health ?
What have occupational studies shown regarding silver and human health ?
Why is there an EPA exposure limit (Reference Dose, RfD) for silver
if it has no associated adverse effects?
How are humans exposed to silver from the ambient environment ?
Why is silver so much different from most other metals in terms
of human health risk?
Questions the Economic Impact of Silver Regulations
What industries are affected by regulations for silver-bearing
waste?
What is the economic impact of these regulations?
What steps have the industries taken to engage in environmentally
sound management of silver-bearing waste?
Questions About the Environment
A: Silver is released from photoprocessing operations primarily as a thiosulfate complex. Any hydrated ion (Ag+) that may exist quickly (within seconds) binds to materials available at the point of release. Most photoprocessors discharge to municipal wastewater treatment facilities (also called "POTWs") where sulfides and other organic materials bind the silver to form insoluble compounds. Generally 90% or more of all incoming silver is deposited into sewage sludge.
Research has shown that less than 50% of the "total" silver in the water column of ambient and receiving waters is "dissolved" silver ("total" and "dissolved" are defined by EPA test methods). The ionic silver (Ag+) is found in the dissolved silver fraction, but a large proportion of this latter category is complexed with natural organic matter or bound to colloidal particles, so the concentration of ionic silver, and the resultant toxic effect, is very small indeed.
Q: What is known about the toxicity of silver-bearing waste?
A: Silver in wastes derived from the photoprocessing industry, because of the high rate of physical and chemical binding, is hundreds to thousands of times less toxic than "free" silver. This means that silver is not toxic to aquatic organisms in environmentally realistic concentrations, even close to POTW discharges. Silver thiosulfate, the dominant silver compound in photoprocessing effluents, has been demonstrated to have a toxicity in the range of 160,000 ug/L, while silver nitrate, which dissociates into ionic silver, has a toxicity range of 1-10 ug/L. Silver nitrate is not expected to be found in the environment.
The high concentration of organic and sulfur-based materials present in municipal wastewater treatment systems ensures that any active silver materials are quickly combined and sequestered into the sludge. Typically, 90% or more of the incoming silver load is removed in this fashion.
Q: What forms of silver are present in a sewage treatment plant discharge?
A: Silver that is present in POTW sludge and wastewater discharges is primarily silver sulfide and silver bound to particulate matter, with small amounts of metallic silver, silver chloride and silver organic complexes. The silver thiosulfate complex that is present in photographic processing solutions is biologically degraded to silver sulfide which is relatively non-toxic. Silver sulfide is one of the most stable and insoluble of the metal sulfides.
Q: What happens to silver after it is discharged into natural waters?
A: Sampling in natural waters near and downstream of POTW discharges indicates that the majority of the silver is found in the sediments, and is tightly bound within this matrix. Relatively non-toxic colloidal silver sulfide and silver organic matter complexes can be found at very low concentrations further downstream, and eventually these forms settle into the sediment where silver remains tightly bound.
Q: Is silver in sediments released back into the water?
A: While sediments may become re-suspended, the silver generally remains bound and is not released into the water column. Recent research has indicated that some sediments under aerobic and turbulent conditions release small amounts of silver; however, this silver is immediately re-complexed with sulfides and organic matter, and returned to sediments.
A: Silver thiosulfate complexes, at the concentrations typically found in photographic processes is not toxic to plants or soil organisms if it were spilled onto the ground. Silver thiosulfate is rapidly oxidized by air to form silver sulfide and sulfate. Silver sulfide is insoluble, very stable, not biologically available and not mobile in soils. Thus, silver sulfide does not migrate into groundwater.
Questions About Silver & Human Health
Q: What is known about
silver and effects on human health?
A: Exposure to silver is not associated with adverse health effects in
humans, a fact that is supported through years of experience with the use of
silver in manufacturing and industrial operations, and which has been demonstrated
through epidemiological evidence (human occupational studies). There are
a few soluble forms of silver (e.g., silver nitrate) which can cause skin, eye,
and respiratory irritation, most notably at high airborne concentrations or upon
direct contact. However, most forms of silver, such as those found in nature
or used commercially, do not possess such irritant properties. The only
effect resulting from long-term chronic overexposure to soluble silver is argyria,
a condition characterized by bluish-gray pigmentation of the skin, mucous membranes,
and eyes. Argyria is considered a cosmetic effect by the EPA and is not
associated with compromised health status.
Q: What have occupational studies shown regarding silver and human health?
A: Several studies of workers who had chronic airborne exposures to various silver compounds have shown that the only potential long-term effect is argyria, a cosmetic effect. While chronic exposure may result in detectable levels of silver in the hair, blood, and feces of workers, adverse effects have not been detected or reported. It is important to note that, to our knowledge, argyria has never been reported in humans from ambient or everyday environmental exposures.
A: Even though silver is not associated with adverse health effects, the EPA establishes exposure limits for many materials based on various endpoints of concern. To our knowledge, silver is the only chemical or substance which has an RfD that is not based on an adverse health effect, but rather on a cosmetic effect (argyria). While an RfD for silver still exists, in 1991 the EPA deleted the maximum contaminant level, or MCL for silver in drinking water (an enforceable value) and replaced it with a secondary maximum contaminant level (SMCL) goal (nonenforceable) of 0.1 ppm (100 µg/L) that is twice the previous level, an action that was driven by the recognized low toxicity associated with silver.
Q: How are humans exposed to silver from the ambient environment?
A: Humans are exposed to trace amounts of silver in the environment through the air and water, and from ingestion of foods. It should be remembered that all metals are elements which occur naturally in the environment and thus trace amounts are present in most environmental media (air, water, soil). For humans, the primary source of exposure is from ingestion of foods that contain trace amounts of silver. Exposure via drinking water is very slight and inhalation of silver in the air, except in some occupational settings, is virtually nonexistent. Even assuming maximum exposure estimates from air, water, and food sources, human daily exposure is still well below recommended EPA exposure limits.
Q: Why is silver so much different from most other metals in terms of human health risk?
A: The two components that constitute the basis for a health risk, toxicity (hazard) and exposure, are both very low for silver: silver is not associated with adverse health effects and overall exposure to silver is very low as well. A recent EPA report on the presence of metals at some non-hazardous waste management sites revealed that silver was, by far, the least encountered metal, both in terms of concentration and frequency of detection. Thus, silver has neither of the critical aspects (toxicity potential or appreciable exposure) which can create a health risk.
Questions About the Economic Impact of Silver Regulations
Q: What industries are affected by regulations for silver-bearing waste?
A: There are approximately 360,000 photo processors affected by silver regulations, many of which can be classified as small businesses. In reality, the reach of overly burdensome silver regulations affects all areas of our society. The users of silver-based photographic imaging are numerous: hospitals, medical and dental offices, diagnostic clinics, police departments, newspapers, printers, banks, government agencies, motion picture studios, and amateur and professional photographers.
Q: What is the economic impact of these regulations?
A: Overly restrictive water quality standards could cost photo processors and other users of silver billions in capital costs and billions annually in established operating costs. At times, the costs of compliance associated with the management of silver-bearing wastes exceed the revenues of the operating business, which makes the business non-viable. In effect, nearly one million jobs could be affected due to business closures, personnel lay-offs, and consolidation or relocation of photo processing facilities.
A: The users and manufacturers of photographic materials have a long tradition of taking all reasonable steps to minimize the release of silver into the environment and to maximize the recovery of silver. To further this end, a Code of Management Practice (CMP) has been established to provide a consensus among the regulated and regulatory communities for controlling silver discharges from photographic processing facilities in a cost-effective and environmentally sound manner. This pollution prevention initiative will result in a net decrease in the amount of silver released to the environment and an increase in the amount of silver recycled and reused.
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