Substances that enter the environment should be categorized and ranked using hazard assessment criteria. This would not only ensure that truly pressing environmental issues are identified and prioritized, but would also maximize the use of limited resources. In the case of soluble organic substances, surrogate data such as persistence and bioaccumulation have been used, in combination with toxicity, for the purpose of hazard categorization. However, for insoluble or sparingly soluble substances such as metals and metal compounds, persistence and bioaccumulation are neither appropriate nor useful. Unfortunately, this is not always recognized by regulators or even by scientists.

The use of PBT was developed over three decades ago to address the hazards posed by synthetic organic chemicals. In fact, the criteria and test methods to evaluate persistence ("lack of degradability") and bioaccumulation ("water-octanol partition coefficient") were developed to be used in combination with toxicity in order to reduce the importance given to the use of toxicity data alone. These test methods were based on an understanding of the chemistry of organic compounds of concern at the time (e.g., DDT) and of the biological interactions that the synthetic organic chemicals would have with the surrounding biota. Specifically, it was realized that if synthetic organic chemicals exerted high intrinsic toxicity under standardized laboratory test conditions but did not persist or bioaccumulate, their environmental hazard would be lower. They should therefore be categorized and ranked at a lower priority. Although this approach remains true today for many organic chemicals, it is certainly not applicable to inorganic metals and metal compounds, and was never intended for this application.

Persistence
As mentioned above, persistence is measured by determining the lack of degradability of a substance from a form that is biologically available and active to a form that is less available — i.e. the substance remains bioavailable. This applies to synthetic organic substances. Inorganic metals and metal compounds tend to be in forms that are not bioavailable. Only under specific conditions would inorganic metals or metal compounds transform into a bioavailable form. Rather than persistence, then, the key criterion for classifying inorganic metals and metal compounds should be their capacity to transform into bioavailable form(s). It should also be kept in mind that, although bioavailability is a necessary precursor to toxicity, it does not inevitably lead to toxicity. It is in recognition of this process that the OECD is developing a procedure for determining the rate and extent of transformation (dissolution) of inorganic metals and metal compounds into bioavailable form(s) in aquatic environments. Although inorganic metals and metal compounds stay in the environment for long periods of time, the risk they may pose generally decreases over time. For example, metals introduced into the aquatic environment are subject to removal/immobilization processes (e.g., precipitation, complexation and absorption). The science is clear: persistence as scientifically defined for synthetic organic compounds does not apply to inorganic metals or metal compounds.

Bioaccumulation
Similarly, the use of bioaccumulation has significant limitations for predicting hazard for inorganic metals and metal compounds. Generally, either bioconcentration factors (BCFs) or bioaccumulation factors (BAFs) are used for this purpose. A BCF is the ratio of the concentration of a substance in an organism, following direct uptake from the surrounding environment (water), to the concentration of the same substance in the surrounding environment. A BAF considers uptake from food as well. In contrast to organic compounds, uptake of inorganic metals is not based on lipid partitioning. Further, organisms have internal mechanisms (homeostasis) that allow them to regulate (bioregulate) their uptake of essential metals and to control the presence of other metals. Thus, if the concentration of an essential metal in the surrounding environment is low and the organism requires more, it will actively accumulate that metal. This will result in an elevated BCF (or BAF) value which, while of concern in the case of organic substances, is not an appropriate measure in the case of metals. Again, the science is clear: bioaccumulation as used for assessing the hazard of synthetic organic chemicals does not apply to the hazard assessment of inorganic metals or metal compounds.

Toxicity
The primary determining factor of hazard for inorganic metals and metal compounds is therefore toxicity, which requires consideration of dose (indeed, the fundamental tenet of toxicology is "the dose makes the poison"). Historically, it has been the practice to measure the toxicity of soluble metal salts, or indeed the toxicity of the free metal ion. However, recent research has demonstrated that, in different media, metal ions compete with different types or forms of organic matter (e.g. fish gills, suspended solids, soil particulate material) to reduce the total amount of metals present in bioavailable form. Toxicity of the bioavailable fraction (i.e. as determined through transformation processes) is the most appropriate and technically defensible method for categorizing and ranking the hazard of inorganic metals and metal compounds.

A possible way forward
This would include the concept of transformation relative to the potential release of forms of metal that are bioavailable, as well as a better understanding of complexation and the effect of bioregulation on metal levels within organisms.

About the author: Peter M. Chapman, PhD, (pchapman@ibm.net) is a Senior Partner/Director of EVS Environment Consultants, an international environmental consulting firm located in North Vancouver, British Columbia, Canada. His publication list is extensive, including, in particular, research and projects related to: aquatic ecology, ecotoxicology and environmental risk assessment.