Canadian mercury science assessment: executive summary

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Mercury is a metal released into the ecosystem through both natural events, such as forest fires and volcanic eruptions, and through human activities, such as coal burning and metal smelting. In its elemental form, mercury is stable in the air and can travel far from emission sources. When deposited in the environment, mercury may be transformed by natural processes to its organic and toxic form, methylmercury. Methylmercury can accumulate in biota (living organisms such as plants and animals) and in humans at levels that can pose serious health risks. Mercury is a persistent global pollutant that is a threat to human and environmental health, and global action is needed to mitigate exposure risks.

Mercury exposure at elevated levels can have effects on human neurological, immune, and reproductive systems. The primary route of exposure to mercury for humans is through the consumption of fish and certain wildlife species. The pathway from emission sources (where pollutants are discharged to the environment) to accumulation in fish is complex; therefore, predicting how changes in sources will affect the level of mercury in fish requires considerable knowledge from many different fields of expertise. In Canada, mercury levels in various media (e.g., lake sediments, some biota) and some human populations have increased several-fold since the onset of the industrial era. Over 90% of the fish advisories in Canada are due to mercury, and many fish and fish-eating birds and mammals are at risk from mercury exposure. In the Arctic, levels of mercury remain high in some wildlife, and exposure to mercury through the consumption of traditional foods may pose health risks to northern Canadians. Overall, mercury remains a concern in many regions of the country.

This assessment is a synthesis of scientific research that has been undertaken in Canada over the past 20 years to understand the status of mercury in the Canadian environment and the impact of mercury on Canadian ecosystems and population. While several Arctic and global assessments specifically related to mercury over the past 10 years have identified this metal as an important environmental and health issue, the Canadian Mercury Science Assessment, which includes up-to-date information on mercury in Canada as a whole, is the first comprehensive evaluation of mercury in the Canadian environment.

The Canadian Mercury Science Assessment presents science conducted under the Clean Air Regulatory Agenda (CARA) Mercury Science Program, led by Environment Canada, and the Northern Contaminants Program (NCP), led by Aboriginal Affairs and Northern Development Canada as well as scientific work funded by Health Canada, Natural Resources Canada, Fisheries and Oceans Canada, provincial and territorial governments, the Natural Sciences and Engineering Research Council, International Polar Year, and industry.

The CARA Mercury Science Program was created in 2007 to establish the scientific knowledge base to support regulatory decision-making on mercury. The intent of the program was to determine key indicators of environmental quality and risk to human health relevant to atmospheric emissions of mercury. Researchers were tasked with quantifying current and past levels of mercury in the environment as well as identifying gaps in the current state of knowledge of the transport routes from point of emission to exposure of wildlife, such as fish, and of humans.  Researchers were also challenged to develop the capacity to predict changes in indicators (e.g., mercury levels in fish) that are associated with changes in levels of atmospheric emissions of mercury or changes in the receiving environment. Contributions from over 230 researchers are included in this assessment.

Figure 1: The movement of mercury through the ecosystem (as indicated by the arrows).

The main processes that mercury undergoes in the ecosystem. The numbers indicate the chapters in which each process is discussed in the assessment report.

Chapter 2: Releases of Mercury Into Air and Water From Anthropogenic Activities in Canada; Chapter 3: Surface Fluxes; Chapter 4: Atmospheric Processes, Transport, Levels, and Trends; Chapter 5: Mercury Fate and Methylation in Terrestrial Upland and Wetland Environments; Chapter 6: Mercury Fate and Methylation in Freshwater Aquatic Ecosystems; Chapter 7: Mercury in the Marine Environment: Processes and Levels; Chapter 8: Influences of Anthropogenic Activities on Mercury Transport, Methylation, and Bioaccumulation; Chapter 9: Mercury Cycling in Ecosystems and the Response to Changes in Anthropogenic Mercury Emissions; Chapter 10: Mercury in Terrestrial and Aquatic Biota Across Canada: Geographic Variation;Chapter 11: Mercury in Terrestrial and Aquatic Biota Across Canada: Temporal Variation; Chapter 12: Health Effects of Mercury in Fish and Wildlife in Canada; Chapter 13: Assessment of Current Mercury Risks to Piscivorous Fish and Wildlife in Canada; Chapter 14: Mercury and Human Health

figure 1
Long description

This figure is a detailed schematic depicting the biogeochemical cycling of mercury in the environment. The numbers correspond to chapters in the full science assessment and indicate where that particular aspect of the cycle is discussed. The overall picture shows (from left to right) the natural environment with sunlight, clouds, rain, sea ice, ocean, mountains, sky, rivers, land, wetlands, forests and forest fires. Images of animals and humans are overlaid on the picture as follows:

  • a bird in the sky
  • birds and a polar bear on the sea ice
  • a seal and fish in the ocean
  • a moose, goose, loon, fish, hiker and fisherman near the land/wetland area.

At the right side of the image are cartoons of a truck, a dam and industrial images depicting anthropogenic sources of mercury to the environment. Various arrows are shown on the image indicating biogeochemical processes of mercury with adjacent numbers indicating the corresponding chapter. Arrows facing upwards from the forest fire and the industrial emissions refer to Chapter 2: “Releases of Mercury into Air and Water from Anthropogenic Activities in Canada.” Arrows pointing both up and down over the sea ice, the ocean and the land refer to Chapter 3: “Surface Fluxes.” At the top of the image, a long arrow in the sky points to Chapter 4: “Atmospheric Processes, Transport, Levels, and Trends.” Circular arrows near the forest and wetlands refer to Chapter 5: “Mercury Fate and Methylation in Terrestrial Upland and Wetland Environments.”  The number 6 in the middle of the lake refers to Chapter 6: “Mercury Fate and Methylation in Freshwater Aquatic Ecosystems.” Circular arrows over the ocean refer to Chapter 7: “Mercury in the Marine Environment: Processes and Levels.” The number 8 (close to the truck and dam) refers to Chapter 8: “Influences of Anthropogenic Activities on Mercury Transport, Methylation, and Bioaccumulation.” An arrow facing downward from the cloud and sky refers to Chapter 9: “Mercury Cycling in Ecosystems and the Response to Changes in Anthropogenic Mercury Emissions.” Chapter 9 is also highlighted as part of the wetlands and lake images. Chapter 10: “Mercury in Terrestrial and Aquatic Biota Across Canada: Geographic Variation” and Chapter 11: “Mercury in Terrestrial and Aquatic Biota Across Canada: Temporal Variation” are both shown on the land surface. Circular arrows close to the land and lake beside the moose, goose and loon refer to Chapter 12: “Health Effects of Mercury in Fish and Wildlife in Canada” and Chapter 13: “Assessment of Current Mercury Risks to Piscivorous Fish and Wildlife in Canada.” The information contained in Chapter 14: “Mercury and Human Health” is indicated next to the two humans in the image.

Despite a decrease in mercury emissions from sources in Canada, the United States, and Europe, global emissions of mercury are on the rise, mainly due to contributions from Asia, which currently account for approximately 50% of the total anthropogenic emissions (those originating from human activity)Note de bas de page1 Globally, over 2 000 tonnes (t) per year of mercury is discharged into the air from anthropogenic sources. When emitted to the air, mercury can travel long distances before it is deposited onto the landscape. Reported Canadian anthropogenic atmospheric mercury emissions decreased 85% between 1990 and 2010Note de bas de page2 and currently account for a low percentage of the world’s anthropogenic emissions of mercury to the air (< 0.5%). Despite the decrease in domestic anthropogenic emissions of mercury, the concentrations of mercury in air and biota have not declined accordingly. Part of the reason for this discrepancy is that only a portion of the mercury measured in Canada is a result of Canadian emissions. As a whole, 95% of anthropogenic mercury deposition in Canada is derived from foreign emission sources. However, in areas close to point sources (such as coal-burning power plants and metal smelters) of mercury emissions, the local contribution of domestic emissions to mercury deposition can be much higher. Experimental studies and model projections indicate that, despite the relatively small contribution of domestic emitters to the atmospheric mercury burden in Canada, further reductions in mercury emissions from domestic sources would result in a decrease in mercury levels in biota such as fish and wildlife near these sources. These studies also show that emission reductions from foreign sources will further reduce mercury levels in biota in Canada. However, emissions reductions do not lower mercury levels in the Canadian environment following a linear relationship. Because of complex processes involved in the transport of mercury (see Figure 1) from emission sources to deposition, transformation, and bioaccumulation, reductions in environmental levels of mercury can be delayed, particularly in regions distant from the sources.

Mercury is generally found in the air in three forms: elemental, reactive (or oxidized), and bound to particles. Gaseous elemental mercury is the dominant form in the air, and it can travel long distances from emission sources. Depending on the chemistry of the atmosphere and the amount of sunlight present, this elemental form can be transformed to oxidized or particle-bound mercury, which are more readily deposited from the atmosphere to landscapes and water bodies. Once deposited on plants, soil, or into bodies of water, mercury can either be transformed back to its elemental form and be released, or enter the ecosystem as oxidized or particle-bound mercury. The properties of the receiving environment (such as lakes, wetlands, forests, and oceans) govern the next steps in the mercury cycle that can lead to the production of methylmercury.

Methylmercury is a potent neurotoxin that can accumulate in the tissues of living organisms (bioaccumulation) and be magnified as organisms higher in the food chain consume mercury-containing prey (biomagnification), posing exposure risks to human consumers and the health of the organisms themselves. Climate change, emissions of other pollutants (for example, acid gases), changes in land use, and the type of receiving environment can alter how mercury cycles within the environment and can determine whether it is converted to methylmercury and taken up by wildlife.

In terms of risk, certain populations of wildlife and humans are more vulnerable than others to mercury exposure. For wildlife, top predators, particularly those associated with aquatic food chains, are at greatest risk from high dietary exposure to mercury because they accumulate mercury from their prey, which can lead to high levels over their lifetimes (biomagnification). In Canada, fish at the top of the food web and fish-eating birds and mammals are most vulnerable to high mercury exposure. In humans, consumption of contaminated fish is the primary source of methylmercury exposure. Although the average exposure of Canadians is low, methylmercury remains a potential public health issue for populations who rely heavily on the consumption of large predatory fish and marine mammals for food, and for potentially susceptible groups including developing fetuses, infants, and children. The developing nervous system is considered to be the most sensitive to potential harmful effects of methylmercury; thus, infants and children have a higher risk of developing adverse health outcomes from exposure before and after birth. Various human health effects, including impaired neurological development, cardiovascular disease, and immune system dysfunction, have also been linked to methylmercury exposure. In fish and wildlife, exposure to methylmercury is associated with impairment of reproduction, growth, and health.

The 2014 Canadian Mercury Science Assessment consists of 2 documents: the Summary of Key Results (including this Executive Summary) and the Science Assessment. This Executive Summary is a short document highlighting the results from the science assessment and prominent issues relating to mercury in Canada. The Summary of Key Results contains the most significant scientific results, recommendations for future work, and answers to policy-relevant science questions. The Science Assessmentis a comprehensive scientific review of knowledge of environmental mercury in Canada. It provides an in-depth knowledge baseline against which future changes in mercury levels in the environment can be attributed to changes in mercury emissions and climate. This assessment further identifies key gaps in our understanding of how mercury travels, where it ends up, the impact of human activities and changes in mercury pollution.

The information provided in this assessment is also intended to inform both national governments and international organisations on mercury science in Canada. Canada has identified mercury and its compounds as a toxic substance under the Canadian Environmental Protection Act, (1999). The Government of Canada’s actions to manage risks associated with mercury are summarized in the Risk Management Strategy for Mercury.Note de bas de page3 Canada has signed the United Nations Environment Programme’s Minamata Convention on Mercury (October 2013), which has as its primary goal the protection of human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds.

This assessment reflects the research undertaken in Canada within the past 20 years related to the following:

Highlights of Scientific Findings

In Canada:

Knowledge gaps

There are several overarching gaps identified in this science assessment, including gaps in knowledge of the impacts of climate change on the mercury cycle, in capabilities to predict future mercury levels, in knowledge of the impacts of mercury on the Arctic ecosystem and its vulnerability to mercury as well as on human health, and in information on mercury levels in the environment at sites across the country.

The most important knowledge gap is in our understanding of the impact of climate change on mercury cycling, methylation (i.e. how methylmercury is formed), and cumulative effects in Canada. Climate change can disrupt the physical characteristics and functions of the ecosystem, and these changes affect all of the processes in the biogeochemical cycle of mercury. Ecosystems in Canada that are especially vulnerable to climate change and mercury contamination include the Arctic, aquatic ecosystems, coastal regions, and wetlands. Other knowledge gaps that limit our understanding of the effects of climate change on mercury in the environment include the following: limited information on the processes driving emission of mercury from various surfaces and water bodies; lack of chemical identification of different species of atmospheric mercury and quantification of their deposition to surfaces; lack of knowledge of the impact of acidity, temperature, and organic matter collectively on methylmercury production and bioaccumulation in freshwater aquatic systems; insufficient information on methylmercury levels and production in the marine environment; insufficient knowledge of factors promoting methylmercury in food webs; and a lack of information on the fate and transport of mercury within terrestrial ecosystems.

Current predictive capabilities are limited by a lack of understanding of regional differences in the biogeochemistry of ecosystems and of emission sources other than those currently reported. As well, capacity to predict the effect of changes in anthropogenic emissions on fish mercury concentrations at a national scale is impeded by a lack of information on the physical characteristics of individual watersheds and the hydrodynamics of these watersheds across Canada.  

A significant portion of Canada is Arctic, and much is still unknown about mercury in this fragile ecosystem. Noteworthy gaps include an incomplete understanding of the disproportionate increase in mercury levels in biota in comparison with other regions, of the impact of sea ice conditions on the overlying atmosphere, and of the underlying ocean chemistry.   

In regard to human health, there is a lack of in-depth information to understand the balance between the benefits of nutrients from the consumption of fish and the risks of methylmercury exposure. As well, the relationship between methylmercury exposure and other diseases is not fully understood.

There remain insufficient monitoring data to expand the scale of information about mercury trends and predictions across the country. Scaling up to the national level from regional data has been a challenge given the geographic size and the diversity of ecosystems in Canada.  There is need for a national monitoring focus on targeted biota and abiotic (i.e. non-living) systems to identify areas at risk due to mercury exposure.  Furthermore, there is a lack of complete and fully characterized mercury emission data to enable accurate predictive capabilities in Canada. There is a lack of data on mercury exposure of Canadian children, and, in particular, of First Nations children.


Efforts in the last 20 years have greatly improved our understanding of the sources, transport, fate, and effects of mercury in the Canadian environment.  Environmental mercury pollution is complex, and future scientific work will require strong, coordinated, national leadership that engages multi-level partners from all aspects of human health, wildlife, and ecosystem research in an integrated approach.

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