Where does mercury in the environment come from — and how can we get rid of it?
When most people think of mercury exposure, they imagine an old-fashioned thermometer shattering on the ground, spilling its silvery liquid metal. In reality, the primary mercury exposure Americans face today comes from eating a food they might think is benign: fish. Many people would also be surprised to learn that the source of most mercury in fish is the burning of fossil fuels like coal and oil.
Fossil fuel combustion is now the largest source of mercury to the environment, constituting ~70% of all emissions globally [1]. Mercury trapped in coal and oil deposits is released into the atmosphere when the fuels are burned, which has caused a tripling of the mercury content of the atmosphere since the beginning of the Industrial Revolution [2]. Atmospheric mercury is deposited on the land and in the water by precipitation, where some of it is converted by microbes into a form called methylmercury. Methylmercury accumulates in food webs, leading to greater concentrations in the tissues of top predators like tuna, sharks, and whales. Importantly, mercury in the atmosphere can travel all over the world before being deposited, so emissions anywhere present a risk to fish consumers globally.
Some fish, such as mackerel and swordfish, contain such high levels of mercury that the U.S. Environmental Protection Agency (EPA) recommends avoiding them entirely, while moderation is recommended for some more common species like tuna and salmon. Certain populations, including Asian Americans and Native Americans, are at higher risk of methylmercury exposure because they tend to eat more fish than the general population. Limiting mercury consumption is especially important for pregnant women because fetal mercury exposure can cause permanent neuro-developmental damage. Today, 5–10% of women in the US of child-bearing age have blood mercury concentrations that could impair fetal neurodevelopment. If current fossil fuel burning trends continue, that proportion could be 10–30% by 2100 [4]. This represents an urgent threat to public health. However, by implementing technology that already exists to reduce mercury emissions from power generation, and by increasing public awareness of the risks of fish consumption, this trend could be reversed.
Emissions from existing power plants can be reduced through technological enhancements, for instance by reacting flue gas with activated carbon to “scrub” mercury out of smokestacks (similar to technologies already used to reduce acid rain by controlling emissions of nitrogen and sulfur oxides). For a more detailed list of mitigation options that can be deployed today, check out the EPA’s list. Simply reducing total fossil fuel use would have co-benefits of reducing emissions of both mercury and greenhouse gases. Pursuing both of these approaches will make fish consumption safer, allowing more people to benefit from the nutritional values of fish, while supporting broader public health goals to reduce the risks of climate change. In fact, there is already evidence that this can work. Efforts over the past few decades to reduce mercury emissions in North American and Europe have already led to a detectable decrease in the mercury load of Atlantic bluefin tuna [5]. Finally, a public awareness campaign is critical to ensure that all Americans, especially women of child-bearing age, make use of the EPA guidelines for fish consumption.
Achieving meaningful reductions in mercury emissions requires global action. In 2011, the EPA adopted the Mercury and Air Toxics Standards to limit emissions of mercury from fossil fuel-burning power plants. This is a critical step to protecting the health of Americans from mercury and other toxic emissions, especially those living near power plants. But given that the US contributes only 5% of global mercury emissions [6], we must work with other nations to reduce worldwide emissions. This is why in 2013, the US was one of 140 nations that signed the Minamata Convention on Mercury under the United Nations Environment Program.
The continued implementation of the EPA’s mercury emission rules may be in jeopardy, given that the new head of the EPA, former Oklahoma Attorney General Scott Pruitt, has twice challenged the regulation in court, and has widespread support among Congressional Republicans to curtail environmental regulations ranging from water protections to climate mitigation. Therefore, supporters of this critical public health protection need to work to make sure it isn’t discarded to protect the utility and fossil fuel industries from having to upgrade outdated power plants or switch to cleaner energy sources. Standing firm on domestic mercury regulations will help the US maintain our leadership and authority in the Minamata Convention and encourage other countries to continue addressing this important issue.
References
[1] Sen, I. S., and B. Peucker-Ehrenbrink (2012), Anthropogenic Disturbance of Element Cycles at the Earth’s Surface, Environ. Sci. Technol., 46(16), 8601–8609, doi: 10.1021/es301261x.
[2] Streets, D. G., M. K. Devane, Z. Lu, T. C. Bond, E. M. Sunderland, and D. J. Jacob (2011), All-time releases of mercury to the atmosphere from human activities, Environ. Sci. Technol., 45(24), 10485–10491, doi: 10.1021/es202765m.
[3] Conceptual diagram illustrating the biomagnification of mercury within the aquatic food chain. Diagram from “Tropical Connections: South Florida’s marine environment” (pg. 138) — http://ian.umces.edu/press/publications/374/
[4] Mahaffey, K. R., R. P. Clickner, and R. A. Jeffries (2009), Adult women’s blood mercury concentrations vary regionally in the United States: Association with patterns of fish consumption (NHANES 1999–2004), Environ. Health Perspect., 117(1), 47–53, doi: 10.1289/ehp.11674. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2627864/
[5] Lee, C.-S., M. E. Lutcavage, E. Chandler, D. J. Madigan, R. M. Cerrato, and N. S. Fisher (2016), Declining mercury concentrations in bluefin tuna reflect reduced emissions to the North Atlantic Ocean, Environ. Sci. Technol., 50(23), 12825–12830, doi: 10.1021/acs.est.6b04328.
[6] Pacyna, E. G., J. M. Pacyna, K. Sundseth, J. Munthe, K. Kindbom, S. Wilson, F. Steenhuisen, and P. Maxson (2010), Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020, Atmos. Environ., 44(20), 2487–2499, doi: 10.1016/j.atmosenv.2009.06.009.
