With all of the interest in nanosilver generated by the recent EPA petition filed by the International Center for Technology Assessment, I thought I would post some background material on EHS issues surrounding silver. A couple of disclaimers: the material is not comprehensive, and you might see parts of it again in "Nanotechnology Law and Policy" which should be published by Thomson-West legal publishers sometime in 2009 (if I can keep pace with the production schedule).
Silver (CASRN 7440-22-4) is a naturally occurring metal. It is usually found in extremely low concentrations in natural waters. “Humans are exposed to small amounts of silver from dietary sources.” “Silver levels of less than 0.000001 mg silver per cubic meter of air (mg/m3), 0.2-2.0 parts silver per billion parts water (ppb) in surface waters, such as lakes and rivers, and 0.20-0.30 parts silver per million (ppm) in soils are found from naturally occurring sources.” A 50 year old person has “an average retention of 0.23-0.48 g silver.”
Silver production in 1999 was estimated at 15.5 million kilograms world-wide, with Mexico and the US leading the list of producers. It is estimated that approximately 2.5 million kgs of silver in various forms is lost to the environment in the US every year, and that 29% of that amount is released to water and 68% to land. The most prevalent release routes are purportedly from smelting operations, photographic processing supplies, manufacturing of electrical components and wires, coal combustion, electroplating operations, and cloud seeding. NIOSH estimates that 70,000 people are exposed to silver in the workplace each year and inhalation is the most important route of exposure.
People and Animals. Silver has exhibited no known toxic effects to humans. According to the EPA, human health effects from breathing, eating, and/or drinking silver are "unknown." However, if you eat, drink, or breathe enough of it, your skin may turn a blue-gray color. This permanent cosmetic condition called “argyria” is not harmful to health. It results from silver depositing in the dermis layer of skin. Breathing high levels of silver dust may cause breathing and respiratory problems, throat irritation, or stomach pain – as with other types of particulate matter. Silver is not a known human carcinogen, but has been shown to cause cancer when inserted in lab animals under certain conditions. There are few, if any, toxicity animal studies based on oral or respiratory silver intake. “Tests in animals show that silver compounds are likely to be life-threatening for humans only when large amounts (that is, grams) are swallowed and that skin contact with silver compounds is very unlikely to be lifethreatening.” Some occupational studies intimate that exposure to silver may cause kidney problems, although more research is needed on this issue.
Silver Ions. Monovalent silver ions are very rare in the natural environment. “The acute toxicity of silver to aquatic species varies drastically by the chemical form and correlates with the availability of free ionic silver.” “For freshwater fish, the acute toxicity of silver is caused solely by silver ion interacting with the gills . . .” “On the basis of available toxicity test results, it is unlikely that bioavailable free silver ions would ever be at sufficiently high concentrations to cause toxicity in marine environments.” “About 95% of the total silver [lost to water in the environment] is removed in publicly owned treatment works from inputs containing municipal sewage and commercial photprocessing effluents, and effluents contain less than 0.07 ug ionic silver/litre.”
Drinking Water. The federal government has issued guidelines concerning the maximum level of silver allowed in drinking water (Maximum Contaminant Level – MCL): long term exposure is limited to 0.1 mg/L (previously 0.05mg/L), and short term exposure (1-10 days) is limited to 1.142 mg/L. The silver MCL was first promulgated by the United States Public Health Service in 1962 before the Environmental Protection Agency was ever formed. Silver was included on the original list on the basis of epidemiological data and the fact that it was used as an antimicrobial. The epidemiological data was based on exposures to medicinal silver and exposures through mining and metalworking. In 1989 EPA proposed changing the MCL for silver from 0.05 mg/L to 0.09 mg/L because the only potential human health concern was from argyria. “The proposal was finalized, using an CML of 0.1 mg/L, on January 30, 1991.”
Surface Water. Silver in surface water tends to settle down into the sediment. “Silver can remain attached to oceanic sediments for about 100 years under conditions of high pH, high salinity, and high sediment concentrations of iron, manganese oxide, and organics.” Silver levels in pristine surface water in unpolluted areas are approximately 0.01 μg /L and approximately 0.01 – 0.1 μg/L in urban and industrialized areas. The federal government regulates silver in surface water through the Federal Water Pollution Control Act (aka/ Clean Water Act) — 33 U.S.C. § 1251. “The silver criteria contains values to protect human health from ingestion of contaminated aquatic organisms and maximum acceptable concentrations to protect organisms that live in freshwater and salt water from toxic effects. The human health part of the silver criteria was drawn directly from the drinking water MCL. Criteria for the protection of aquatic life, on the other hand, were derived using a newly developed set of guidelines that called for extensive laboratory test data. The values are given as total recoverable silver.” The freshwater criteria maximum concentration (CMC) for silver is (3.2) 100mg/L, and the saltwater CMC is (1.9).
Air. Silver is not considered an air pollutant harmful to public health or environment under the National Ambient Air Quality Standards mandated by the Clean Air Act. Purportedly “[t]reatment of air emissions containing silver is not a concern as atmospheric emissions rarely approach the federal threshold limit value for occupational exposure of 0.01 mg/m3.”
Workplace. Workplace exposures to silver present unknown/unquantified health risks to humans. Most occupational exposures to silver are purportedly through photographic processing chemicals (dermal) or inhalation of silver dust particles from the ambient air. OSHA has set the maximum air quality standard for silver at 0.01 mg/m3 based on an 8 hour workday and 40 hour workweek.
Regulation of silver hazardous waste. Resource Conservation and Recovery Act (RCRA) is designed to (in part) prevent leaching of hazardous concentrations of particular toxic constituents into groundwater, and looks back to Primary Drinking Water Standards. Any waste that contains 100 times the amount of the relevant constituent is considered a hazardous waste. The “100 times” level was designed to compensate for the dilution of materials as they pass through soil when headed for ground water. Note, howeverm that the ACRA standard does not track the 1997 amendment to the drinking water standard. Since the original drinking water standard for silver was is 0.05mg/L, the maximum allowable limit is 5.0 mg/L for RCRA purposes. Wastes containing silver at this level or above are labeled as “hazardous wastes” under RCRA and are subject to further regulation under that Act. “Under CERCLA, silver-bearing hazardous wastes are designated as hazardous substances with a reportable quantity (RQ) equal to 1 pound (.454 kg).” Any release that exceeds the RQ in a 24-hour period must be reported to the National Response Center.
“Toxicological Profile for Silver,” Agency for Toxic Substances and Disease Registry, U.S. Public Health Service (December 1990).
P.D. Howe, et al., “Concise International Chemical Assessment Document 44: Silver and Silver Compounds: Environmental Aspects,” World Health Organization (2002).
US EPA Integrated Risk Management System (IRIS), Silver (CASRN 7440-22-4), http://www.epa.gov?IRIS/subst/0099.htm.
“25 Years of the Safe Drinking Water Act: History and Trends.”
Many states also regulate silver. Some state standards are more restrictive than EPA standards. See, e.g., “The Regulation of Silver in Photographic Processing Facilities,” Kodak Environmental Services, J-124 (1996).
T. Purcell, et al., “Historical Impacts of Environmental Regulation of Silver,” Environmental Toxicology and Chemistry, Vol. 18, No.1, pp. 3-8, 1999.
Aquatic life testing guidelines can be found at Fed. Reg. 45:79341 – U.S. EPA. 1980. “Guidelines for determination of ambient water quality for the protection of aquatic organisms and their uses.”
65 C.F.R. 31682
“The Regulation of Silver in Photographic Processing Facilities,” Kodak Environmental Services, J-124 (1996).
US EPA, Solid Waste and Emergency Response (5305W), RCRA Photo Processing, EPA530-K-99-002, January 1999.