The natural occurrence, behaviour and essentiality of metal substances need careful consideration when assessing risks of metal exposure to the environment. Metal concentrations vary widely over space and time in the environment (water, sediment, soil) due to differences in geology, anthropogenic (man-made) and natural loads from different sources and other factors. While for most organic substances there is no natural background concentration, for metals this is a fundamental point to include in risk assessments, and methods on how to account for background concentrations have been developed.
Unlike organic chemicals, metals are neither created nor destroyed by biological or chemical processes. The concept of “persistency”, therefore, needs to be treated differently for metals. In the environment, metals do not just remain in the same state but are often transformed from one chemical species to another which affects their behaviour in the environment. Environmental conditions affect these transformations, so metals behave differently in the environment depending on local conditions.
Availability of metal substances for absorption and their associated risk also vary widely according to the physical, chemical and biological conditions under which an organism is exposed. For instance, iron oxides in sediment reduce the toxicity of metal substances, so organisms living in a sediment containing a high level of iron oxides will show less toxicity to a metal than the same organisms exposed to the same metal and living in a sediment with low iron oxides. Likewise, soil toxicity is affected by the acidity of the soil which varies greatly between regions. Water chemistry is an important factor in terms of toxicity of the metal to aquatic organisms.
This means that even at equal exposure levels, an environmental risk assessment of a metal in one location may reach a different conclusion compared to the risk assessment for the same metal in another region because environmental conditions can vary hugely between different locations and regions.
Many metals are essential to at least some groups of organisms in the environment. Copper, iron, and zinc are broadly essential to animals and plants. Nickel is essential to plants and microorganisms. Some metals, like cadmium, appear to have no biological function. So, similar to human health essentiality, and as for essential metals this needs to be taken into account, it is necessary to have a certain concentration present in order for the organisms to function optimally.