The editorial of this week’s Science issue puts the focus on mercury pollution, ahead of the Minamata convention that will take place in October in Japan. The convention will bring together some 140 nations to sign a treaty aiming at controlling and reducing human uses and releases of mercury. The Science editorial and accompanying articles discuss the issues associated with anthropogenic releases of mercury into the environment, and highlight the importance of drastically reducing these emissions while improving both the monitoring and the understanding of the consequences of local and global mercury pollution.
- Mercury life cycle and methylmercury
Mercury (chemical symbol Hg) is a naturally-occurring element present in heavy-metal-rich geologic deposits and in fossil fuels. It can be released into the atmosphere from natural processes (mainly volcanic activity), however humans largely contribute to releasing mercury from these stable reservoirs through mainly gold mining and coal burning.
Most mercury present in the atmosphere is in a gaseous form (Hg0) that can be transported over long distances before being deposited. This gaseous form can be oxidized by atmospheric chemicals to generate a highly water-soluble form (HgII) that is then deposited to terrestrial and aquatic ecosystems. Some of this mercury is transformed back into Hg0 and reemitted into the atmosphere, while the rest cycles through soils and oceans for decades to centuries before being sequestered back in the lithosphere.
Methylation of inorganic mercury occurs in aquatic systems, leading to the formation of methylmercury, which is a neurotoxic compound. Methylmercury is the only form of mercury that becomes concentrated with each successive level in the food chain, accumulating in fish and marine mammals consumed by humans. Importantly, the aquatic environment is the main route of exposure to mercury for both wildlife and humans.
- Sources of mercury emissions
Currently, human activities account for about 30% of annual atmospheric mercury emissions. The two main sources are gold mining and coal burning in power plants, followed by production of ferrous and non-ferrous metals, and cement production. Natural sources account for about 10% of annual atmospheric mercury emissions, and the remaining 60% actually come from mercury being reemitted from surface soils and oceans. Although the original source of this reemitted mercury cannot be ascertained for sure, the fact that for the past 200 years (since the start of the industrial age) mercury emissions from human activities have been larger than natural emissions implies that most of the mercury deposited in soils and oceans and that is now reemitted comes from anthropogenic sources in the first place.
Besides emissions to air, mercury can be released directly into water, for example from industrial sites (see the story of Minamata, the town in which the international convention on mercury pollution will take place) or from contaminated sites such as old mines, landfills or waste disposal locations.
In their Perspective article in Science, Krabbenhoft and Sunderland (from the US Geological Survey and Harvard University, respectively) point to studies showing that global atmospheric mercury concentrations have increased by 3- to 5-fold since the mid-1800s and that human activities have led to changes in mercury concentration in seawater.
- Reducing mercury use and release, now
Both Krabbenhoft and Sunderland’s article and the UN Environmental Programme’s Global Mercury Assessment 2013 highlight the importance of drastically reducing current anthropogenic mercury emissions in order to stabilize the levels of mercury in the atmosphere and aquatic systems. Given the large contribution of mercury reemitted from soils and oceans to total mercury atmospheric emissions, the effects of regulations aiming at cutting current mercury emissions will be difficult to see in the short term, however they are of major importance in the long term. Unless emissions are dramatically reduced now, the amount of mercury going into terrestrial and oceanic reservoirs will continue to grow, and so will the quantity of mercury reemitted from these reservoirs. Ultimately, reducing global mercury levels is also crucial to decreasing human and wildlife exposure to methylmercury in the long term.
Both the Science articles and the UN Environmental Programme report recognize that several factors, such as global economy, penetration of new technology and climate change, will affect future mercury emissions and deposition patterns, as well as the effectiveness of the efforts made by the nations signing the Minamata treaty. Climate change-associated events such as warmer temperatures, increased precipitation intensity or increased incidence of severe storms will affect the rates and patterns of mercury deposition into the aquatic systems, and will likely exacerbate methylmercury production and bioaccumulation in these systems. Increased thawing of frozen peatlands in the Arctic regions may also lead to the release of significant amounts of mercury into Arctic water systems.
Alongside a pledge to reduce mercury emissions, it will be important to improve the monitoring of mercury releases into the environment as well as our understanding of mercury life cycle and impact on marine and terrestrial wildlife and human health. Most people know that mercury is poisonous, so it may come as surprising to read that the effects of mercury pollution on human health are poorly known. The fact that high doses of mercury are extremely toxic (acute poisoning) is indeed fairly well known; by contrast, the long-term effects of low-level mercury exposure on animal health, including human health, are still poorly understood. Studies involving experts in fields such as toxicology, ecology, medicine and public health will be needed to improve our understanding of these effects and help inform future policies and regulations.
Mercury and health. McNutt M. Science. 2013 Sep 27;341(6153):1430. doi: 10.1126/science.1245924
Global change and mercury. Krabbenhoft DP, Sunderland EM. Science. 2013 Sep 27;341(6153):1457-8. doi: 10.1126/science.1242838
United Nations Environment Programme (UNEP): Global mercury assessment 2013
See Science (27 September 2013) for more articles on the topic of mercury pollution and the Minamata convention.