The status of the Baltic Sea marine environment in terms of contamination by hazardous substances is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of the status of the Baltic Sea in terms of concentrations of metals in the marine environment, this indicator also contributes to the overall hazardous substances assessment along with the other hazardous substances core indicators.
The core indicator on metal concentrations addresses the Baltic Sea Action Plan's (BSAP) hazardous substances segment's ecological objectives 'Concentrations of hazardous substances close to natural levels' and 'All fish safe to eat'. Mercury and cadmium are included in the HELCOM list of substances or substance groups of specific concern to the Baltic Sea.
The core indicator also addresses the following qualitative descriptors of the MSFD for determining good environmental status (European Commission 2008b):
and the following criteria of the Commission Decision (European Commission 2017):
All three metals are included in the EU WFD (Pb and Cd in water, Hg in biota) and EU Shellfish directive (in shellfish) (European Commission 2000, 2006b). Part of the EU food directives set limits in a range of fish species, shellfish and other seafood. In the OSPAR Coordinated Environmental Monitoring Programme (CEMP), metals are to be measured on a mandatory basis in fish, shellfish and sediment (OSPAR 2010).
Article 3 of the EU directive on environmental quality standards states that also long-term temporal trends should be assessed for substances that accumulate in sediment and/or biota (European Commission 2008a).
Metals are naturally occurring substances that have been used by humans since the Iron Age. The metals cadmium (Cd), lead (Pb) and mercury (Hg) are the most toxic and they have no known essential biological function. Furthermore, mercury and cadmium biomagnify, implying that the toxic effect may be enhanced at higher levels in the food web. For mercury, the organic form methyl-mercury (MeHg) is more toxic than elemental mercury and this form is readily bioaccumulated, i.e. activity transferred to lipid containing organs. Due to its high evaporation pressure, the net transport is from soils in the tropics up to Scandinavia to the Baltic Sea, and further north until concentrating in the Arctic due to the low temperatures and the resulting low evaporation - a process known as global distillation or the grasshopper effect.
Lead and mercury have been connected to impaired learning curves for children, even at small dosage. Lead can cause increased blood pressure and cardio-vascular problems in adults. Acute metal poisoning generally results in vomiting. Long term exposures of high levels of lead and mercury can affect the neurological system. Mercury can lead to birth defects as seen in Minamata Bay among fishermen in a mercury polluted area, and also after ingestion of methylmercury treated corn in Iran. Cadmium is concentrated in the kidney, and can result in impaired kidney function, and cadmium can exchange for calcium in bones and produce bone fractures (Itai-Itai disease).
Substances, litter and energy
The main source of all three metals is burning of fossil fuels. The atmospheric deposition to the Baltic Sea mainly originates from long range transport of the metals from outside the Baltic Sea catchment area (details available through the Baltic Sea Environment Fact Sheet Atmospheric deposition of heavy metals on the Baltic Sea). All three metals have been used for centuries, but in the last decades EU or world-wide legislation has been put in place banning most uses.
Current legal use of cadmium and lead includes rechargeable Ni-Cd batteries and for lead car batteries. For mercury current legal use includes low energy light sources. Sources of mercury include use in amalgams for dentistry (the loss from this use have been reduced by installing mercury traps in sinks and generally reducing the use of amalgams in dental works), as electrodes in paper bleaching, in thermometers and mercury switches and a range of other products that have been phased out. For lead, the main source was leaded fuels until their ban in Europe in the 1990s. Both cadmium and lead have pollution hotspots in connection with metal processing facilities, and cadmium coexists with all zinc ores, and is typically present at levels of 0.5–2% in the final products. Weathering of outdoor zinc-products thus leads to cadmium pollution.