2 Chemicals without borders
Most chemicals find their way into the environment via millions of consumer, agricultural and industrial products and processes. Once in the environment, they can persist for long periods of time or break down into other chemicals with their own risks. They may also produce health or environmental effects when they act together with other natural or manufactured chemicals that are already in the environment.
Tracking the pathways, fate and exposure implications of chemicals is essential for effective risk management, but it is complex. It requires:
|1.||identifying the flows of each chemical and its by-products through the economy, from mining or synthesis to manufacture, marketing and use, and on to possible recycling and ultimate disposal;|
|2.||estimating emissions, pathways and depositions both to and from air, water, sediment and soil from the processes and products at each stage of their life cycle and identifying transformations of each chemical and resulting compounds;|
|3,||constructing an area pollution model (or "regional mass balance") for assessing the inputs, outputs, and fate of the chemicals on a geographic basis, and then estimating the likely exposures of people and ecosystems to the chemicals.|
This kind of analysis requires data and information which is only available for very few chemical substances (EEA, 1998a).
Some organic (carbon-based) substances persist in the environment, travel long distances and consequently circulate globally. This means that although these persistent organic pollutants (POPs) can be found almost anywhere, it is difficult to identify where they originated, let alone the pathways by which they travelled.
One of the main ways that the most volatile POPs travel is through the "grasshopper" effect (Fig. 2).
POPs released in one part of the world, via pesticides for example, can, through a repeated (and often seasonal) process of release, deposit, release, and deposit again, be transported to regions far away from their original source. This is why POPs can be found in the Arctic, thousands of kilometres from any major source of POPs.
Heavy metals such as lead, cadmium and more complex POPs like dioxins can also disperse over long distances. For example, cadmium in the Rhine basin in Germany has been on the increase for many years due to pollution from a number of sources , including oil combustion, steel production , zinc refining, cadmium plate manufactuing, and municipal waste disposal (Fig. 3).
Because cadmium accumulates in soils and groundwater, efforts to reduce cadmium pollution could take about 15 years to start reversing the upward trend. Inhabitants of the region may be exposed to cadmium greater than the World Health Organisation's recommended maximum acceptable levels, especially if the soil is acidified (Stigliani and Anderberg, 1994). Similarly, some pesticides can percolate slowly through soil and accumulate in groundwater and river sediments long after their use has stopped. For example, pyrethroid insecticides have been detected in river sediments at 10,000 times the level in the river water, where any monitoring of chemicals is usually focused. (Neal et al., 1997, 1998).
POPs can also travel through living organisms and can become increasingly concentrated in the tissues of animals at high levels of the food chain, such as predatory birds and mammals, including humans. This "bio-magnification" can, for example, increase concentrations of polychlorinated biphenyls (PCBs) to many million times their initial presence in the physical environment.
The ways in which humans and the environment are exposed to chemicals are thus multiple and complex, and exposure to mixtures, not just single substances, is common. However, enough is known about the exposures and effects of certain substances, including some POPs and heavy metals, to justify reducing exposure to them and to other chemicals that also persist and bioaccumulate. In addition, more research is needed in order to better understand the movements and metabolism of the thousands of other chemicals released and present in the environment.