silver toxicity
A natural or man-made chemical present in the environment does not always lead to human or animal exposure. Exposure requires contact with substances containing the chemical. Exposure itself is not necessarily harmful; several other factors determine whether contact leads to harmful effects and the type and severity of these effects. These factors include the dose (how much), the duration (how long), the timing (when in the life cycle), the route of exposure (injection, inhalation, ingestion, contact) and individual characteristics and lifestyles (sex, age, health, habits, fitness, genetic predisposition).
Populations that are unusually susceptible to toxic effects from silver are those with dietary deficiencies of vitamin E or selenium, or those with genetically-based deficiencies in the metabolism of these essential nutrients. Those populations with damaged livers and those exposed to very high selenium levels in their diet are also at higher risk. Some people may exhibit an allergic response to silver.
silver speciation
Most toxicological studies have been conducted with silver in the free, elemental or 0 oxidation state, and with the +1 monovalent silver ion. The rarer +2 and +3 oxidation states have not been studied adequately. The majority of silver resulting from photo-processing occurs in an insoluble form. Theoretical calculations of organic and inorganic silver complexes indicate that, due to the low solubility of silver sulphide and the high affinity of silver for sulphide, little free silver would occur at equilibrium, in effluents or surface waters that contained any sulphide.
The Aquatic Ecosystem Objectives Committee of IJC agrees that 'free' silver is a better measure of toxicity than total silver, The committee does not recommend the adoption of an objective based on 'free' silver for the following reasons:
-near an effluent, silver may not yet be in equilibrium with all the available complexing agents.
-sulphide, which is the only reactant likely to reduce silver adequately, is readily oxidized to sulphate when oxygen is available. The half-life of sulphide is about 50 hours but oxidation rates may be increased five-fold by metals such as calcium. Sulphide has not been found, even at very low detection limits, in the Great Lakes.
-weaker organic complexing agents for silver still permit 'free' silver to exist at concentrations near the objective level.
-the method presented by Kodak for measuring silver ion activity is not adequate for low environmental concentrations; there are difficulties with reproducibility, dependability and comparability. The method is only reliable at levels of 'free' silver higher than the criteria.
Until a reliable method is developed to measure 'free' silver at concentrations below 0.1 micrograms per litre, silver objectives should be expressed as total silver. This level is the routine total or dissolved silver detection limit for water samples analyzed in labs used by the British Columbia Ministry of Environment for analysis of ambient water samples.