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For the purposes of the Ocean Health Index, ‘chemical’ refers to a  compound or substance that has been purified or manufactured by humans. It does not include the many chemicals that marine plants, animals and microbes produce as part of their normal life processes.  

More than 100,000 chemicals are used commercially (Daly 2006), and many enter the marine environment via atmospheric transport, runoff into waterways, or direct disposal into the ocean.

Three general categories of chemicals are of particular concern in the marine environment: oil, toxic metals and persistent organic pollutants. Although some countries possess good data on the occurrence and concentrations of such chemicals in their waters, such data are not available globally. Therefore, proxy methods were used to estimate the intensity of chemical pollution based on the amount of pesticides used by each country.     

How Was It Measured?

It is not yet, and may never be, possible to measure actual concentrations of the numerous substances found throughout the ocean. Because of limited data availability, we measured the chemicals component as the average of land-based organic pollution, land-based inorganic pollution and ocean-based pollution from commercial shipping and port, using the following proxy measures:.

Land-based inorganic chemical pollution used raw data were drawn from modeled plumes of land-based inorganic pollution, as developed in Halpern et al. (2008) that produced intensity of pollution at 1km2 resolution. The model used impervious surface area within watersheds as a proxy measure for the likely amount of this pollution runoff that reached river mouths. Plumes from the rivers were distributed using a diffusive model. Each ocean pixel was then rescaled 0-1 based on the global maximum pixel value.

Land-based organic chemical pollution was modeled from data on agricultural pesticide use. Data on the amounts (tons) of pesticides used in each country were obtained from FAO. For each country, the amount of pesticide used was distributed onto land use classes and then summed by watershed as a proxy measure for the likely amount of pollution that reached river mouths. Plumes from those rivers were then distributed using a diffusive model that produced intensity of pollution at 1 km2 resolution. Each ocean pixel was then rescaled 0-1 based on the global maximum pixel value.

Ocean-based chemical pollution is assumed to come from commercial and recreational ship activities, but since no global data for the latter are available a combination of commercial shipping and ports and harbors data were used.  Data on the density of commercial shipping for 1 km2 cells were gathered from World Meteorological Organization Voluntary Observing Ships Scheme. The amount of ship traffic is used as an estimator of the amount of ship-based pollution. Port-derived pollution is modeled as a diffusive plume from each port out to a maximum distance of 100 km. Data for 618 global ports comes from World Port Ranking, U.S Port Ranking, American Association of Port Authorities, Australia ports database and Lloyds List database.  

These models only provide rough estimates of pollution intensity. They do not represent all chemicals and they do not distinguish between chemicals that are more or less toxic. Focal studies at the country level may be able to use more detailed data, but they do not yet exist at the global level. Details about the models used are provided in online Supplementary Information for Halpern et al. (2012).

The Clean Water goal is unusual because its four components--Chemical Pollution, Nutrient Pollution, Pathogen Pollution and Trash Pollution--indicate both Status and Pressure.  Low levels of those factors produce a high goal score, but high levels produce a low score.  For example, perfectly clean water has no chemical pollution, so Status for this component is expressed as 1 - Chemical Pollution. Status for the other components is similarly expressed.  Input data for calculating Status and Pressure for each component is listed in Table S23 of Halpern et al. 2015.  The overall goal score is the geometric mean of the scores for the four components, which are weighted equally.    

Use of the geometric mean magnifies the importance of a very bad score for any one of the components, matching public perception that very high levels of a single pollutant would make waters seem ‘too dirty’ to enjoy for recreational or aesthetic purposes.

Chemical pollution is a pressure for nearly all of the Ocean Health Index goals. All pressures, including chemical pollution, have different effects on different goals. For each goal, the effect of each pressure is weighted 'low' (1), 'medium' (2) or 'high' (3). The actual data-derived value of the pressure is then multiplied by the weight assigned to it for that goal. That process is repeated for each pressure-goal combination.  The sum of those values divided by 3 (the (the maximum pressure-goal value) expresses the total affect of that pressure on the goal. 

Chemical pollution has high effect (weight = 3) for Tourism & Recreation, Sense of Place (Iconic Species) and Clean Waters (where it is also a Status component). It has medium effect (weight = 2) on Natural Products (Aquarium Trade, Fish Oil and Seaweed), Carbon Storage (Seagrass), Coastal Protection (Seagrass), Livelihoods & Economies (fishing, Mariculture and Aquarium Trade), and Sense of Place (Lasting Special Places). Its effects on other goals are low (weight = 1).

Chemical Pollution Infographic


The total amount of oil entering the ocean has been estimated, but global data on the size and geographic distribution of oil spills are not available, so oil pollution could not be included as a separate category within the Ocean Health Index.  However, oil would be among the substances contained in runoff from impervious surfaces and released by shipping and ports.

‘Oil’ is the general term for any thick, viscous, typically flammable liquid that is  insoluble in water but soluble in organic solvents. Plants and animals produce a variety of natural oils, but the Clean Waters goal is primarily concerned with oil derived from geological deposits of petroleum (crude oil) for use as a fuel or lubricant.

Natural oil makes up 47% of the oil in the ocean. About 600,000 metric tonnes of oil enters the ocean naturally each year by seepage through many cracks in the seafloor (NRC 2003), but input from each is typically slow (Wells 1995) and natural seepage is not considered to be pollution.

The other half of the oil comes from anthropogenic sources, including boats, land-based runoff and, to a lesser degree, oil spills. These sources pose a greater threat to marine environments as the oil enters the ocean in concentrated areas at a high rate of flow.

The largest sources of human oil pollution are urban-based runoff and operational discharge of fuel from boating traffic and port operations. Discharge associated with boats constitutes 24% of the total amount of oil in the ocean (UNEP/GPA 2006).

Only 8% of overall oil ocean pollution is a result of spills during transportation or production. However, the toxicity levels of these spills tend to persist over time and have been linked to highly visible local and regional disasters.

After 20 years, oil pollution from the 1989 Exxon Valdez spill in Alaska persists, in some areas, is nearly as toxic as initial levels (Exxon Valdez Trustee Council 2009; Raloff 2009).  Events such as the Exxon Valdez spill and the large spill resulting from the explosion of BP's Deepwater Horizon offshore drilling platform in the Gulf of Mexico have substantial effects that should be visible in regional assessments, but are probably not large enough to be seen in global-scale studies.    

What Are The Impacts?


Oil pollution can degrade or destroy marine ecosystems.

Oil pollution can elevate concentrations of toxic elements (e.g. arsenic).

Oil pollution can kill marine life through ingestion, inhalation and absorption. Oil pollution can kill marine life through ingestion, inhalation or absorption. Because oil sticks to fur and feathers, seals, sealions and birds lose the insulation that those structures normally provide.  Oiled birds are also unable to fly. 

Oil pollution can have long-term effects on spawning grounds and recovery of fish stocks and populations of most other marine animals. .


Oil pollution can harm those who consume contaminated water or seafood or have contact with polluted waters through recreation and clean-up activities.

Symptoms can include chest pain, coughing, dizziness, headaches, respiratory distress and vomiting.


Responding to the ecological impacts of oil pollution can result in significant economic costs.

Response to the Deepwater Horizon oil spill cost BP US $13 billion dollars. Litigation and compensation for claims cost BP an additional US $15 billion (Telegraph 2011). The company recently agreed to a claims settlement totalling $18.7 billion. (New York Times 2015)

Local economies have to deal with costs resulting from contaminated or diminished fish stocks.

The BP Deepwater Horizon oil spill caused Louisiana to lose 50 percent of its seafood production, a US $2.4 billion dollar industry in Louisiana that supplied as much as 30 percent of the domestic seafood for the continental U.S. (Nawaguna-Clemente 2011).

Get More Information 

Woods Hole Oceanographic Institution [WHOI]

A diagram detailing the route of oil as it travels from the seafloor through the water column. 

U.S. Senate Committee on Environment & Public Works

A policy brief on the Oil Pollution Act of 1990 by David Lungren 

National Oceanic and Atmospheric Administration [NOAA]

Six fact sheets evaluating the effect of the Gulf oil spill on Marine Mammals and Sea Turtles, Seafood Safety, Hurricane, South Florida, Restoration Efforts, and Cumulative Impacts on Wildlife.

Toxic Metals

Metals are chemical elements that are typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity. Metals are toxic if they change the structure and function of proteins and enzymes (GESAMP 2001).

Metals found in the ocean that are highly toxic on their own include mercury, cadmium, lead, arsenic, tin, copper, nickel, selenium, and zinc. Mercury, cadmium, and lead can become even more highly toxic in combination with organic compounds. For example, mercury can form neurotoxic compounds such as methylmercury (CH3Hg), when combined with carbon.

Arsenic, copper, nickel, selenium, tin, and zinc are not highly toxic by themselves but are able to react with organic materials, creating very toxic compounds (UNEP 2006).

Many metals occur naturally in the environment, but anthropogenic emissions from industrial and mining activities can increase concentrations of many to toxic levels.

96% of mercury enters the ocean via atmospheric input (GESAMP 2001).

While some metals are deliberately dumped in the ocean, most are found downstream from their sources, including waste dumps, industrial areas, mining operations and metal processing areas.

Although global databases describing the concentration of toxic metals throughout the ocean do not yet exist, such data are available in some countries and can be used in regional assessments.  For example, Halpern et al.'s (2015) Ocean Health Index assessment for the U.S. west coast used data from NOAA's collaborative Mussel Watch Contaminant Monitoring Program as an indicator of toxic metals along the coasts of California, Oregon and Washington. By filter feeding on plant plankton and small organic particles, mussels bioaccumulate toxic metals and other pollutants very effectively. The Mussel Watch program analyzes those contaminant loads and provides data for various regions in the U.S.  

What Are The Impacts?

Mercury Pollution Cycle


Results of laboratory studies can demonstrate the effects of one or several pollutants on growth, reproduction or other physiological processes in test organisms. Chemical analyses can also reveal the concentrations of pollutants in the tissues of marine organisms collected in the wild. However, limited information is available on how wild marine plants, animals, microorganisms and ecosystems respond to sub-lethal exposure to the many pollutants they encounter, and how other factors such as temperature or pH affect those responses. 


Certain metals, such as zinc, are essential to life in very small amounts, but are toxic in higher concentrations. Others, such as mercury or cadmium, are not used in normal metabolism and are harmful when taken into the body. Ingesting toxic metals can have serious effects on the kidney, liver, immune system, central nervous system and other organs.

Over 90% of methylmercury exposure occurs through the ingestion of contaminated fish and shellfish (USGS 2 2009).

In the past 20 years, mercury concentrations in the Pacific Ocean have increased 30 percent due to increases in human atmospheric emissions from industries and coal-burning power plants and are estimated to rise 50 percent by the year 2050 (Sunderland 2009).


In 2004, the U.S. Environmental Protection Agency (EPA) released recommendations for weekly fish consumption so that high levels of mercury ingestion could be avoided (EPA 2010).

Expenses can be incurred from health problems attributed to mercury ingestion.

An estimated 637,233 children in the United States are born each year with cord blood mercury levels greater than 5.8 mg/L, a level linked to decreased IQ and other birth defects (Trasande et al. 2005).

What Has Been Done?

Mercury in bluefish declines as power plants emit less. Coal usually contains about 0.1 to 0.2 parts per million of mercury by weight. Burning releases the mercury into the air.  Coal-fired power plants are the main source of the mercury that ends up in lakes, rivers and the ocean, where it bioaccumulates up the food chain.  Now, cleaner bluefish are showing the benefits of reductions in coal-fired mercury emissions over the past four decades. Bluefish today contain 40% less mercury, a decrease that  occurred much faster than scientists had initially anticipated. Reducing environmental mercury is good for all aquatic predators, including us.

Get More Information 

United States Environmental Protection Agency [EPA]    

Fact Sheet on mercury content in fish and shellfish.

Turtle Island Restoration Network

Fact Sheet on mercury in seafood.

European Health and Environment Alliance (HEAL)

Fact Sheet on fish and mercury consumption.

Persistent Organic Pollutants [POPs]

Persistent Organic Pollutants (POPs) are chemical compounds that are toxic to humans and wildlife.

POPs include pesticides such as DDT, herbicides, PCBs (a component found in many coolants, flame-retardants, adhesives), and BPA (a compound found in plastics – primarily in plastic bottles).

Though many measurements of the concentrations of various POPs in tissues of various marine organisms are available, they have not been combined into a comprehensive database suitable for use as a global indicator(s). Therefore, the Ocean Health Index developed the proxy described above based on the amount of pesticides used in each country as an estimate of chemical contamination in its coastal waters.  

What Are The Impacts?


The beluga whale population in Canada’s St. Lawrence estuary has declined from about 5,000 at the beginning of the 1900s to about 650 animals today. They have one of the highest rates of cancer known in any wild population, as well as some of the highest levels of POPs and toxic metals. (Lyons, 2008; Martineau et al. 2002).

POP concentrations increasingly accumulate at each stage in the food web. The highest concentrations are found in ‘apex’ predators that feed at the top of the food web (at a high trophic level).

Low temperatures cause POPs to break down more slowly and accumulate in higher concentrations than in more temperate zones.


POPs can cause birth defects, increase cancer risks, disrupt hormone functions and cause reproductive, behavioral, immune system, and neurological problems in humans.

Inuit populations that consume large amounts of whale and seal fat have health-threatening, heightened blood levels of POPs, including industrial chemicals such as PCBs and pesticides such as DDT, even though such products were made and used thousands of miles away (Kirby, 2008). 


Because POPs are versatile and inexpensive to manufacture, many countries continue to allow their use.  However, the economic costs to deal with the resultant pollution can be high.

An estimated 2.8 billion dollars has been spent on dredging and processing POP contaminants from PCB manufacture in the Rhine Delta since 1997.  

In the United States, the General Electric company (GE), which dumped polychlorinated biphenyl (PCB) used in manufacturing capacitors, will have to pay an estimated total of 1.4 billion dollars to remove PCB-contaminated sediments from the Hudson River (Greenpeace, 2011).

Get More Information

Blue Voice

Policy brief-fact sheet on Persistent Organic Pollutants (POPs).    

Webpage for the Stockholm Convention on Persistent Organic Pollutants

World Bank

Informational paper on Persistent Organic Pollutants Country Strategy Development: Experiences And Lessons Learned Under the Montreal Protocol


Banza, C. L. N. et al. High human exposure to cobalt and other metals in Katanga, a mining area of the Democratic Republic of Congo. Environ. Res. 109, 745–752 (2009).

Martineau, D. et al. (2002). Cancer in wildlife, a case study: beluga from the St. Lawrence estuary, Québec, Canada. Environ Health Perspect 110, 285–292 (2002).

National Research Council. Oil in the Sea III: Inputs, Fates, and Effects. National Research Council, (Washington, D.C.: National Academies Press. 2003)

Sunderland, E. M. (2007). Mercury Exposure from Domestic and Imported Estuarine and Marine Fish in the U.S. Seafood Market. Environ Health Perspect 115, 235–242 (2007).

Sunderland, E. M., Krabbenhoft, D. P., Moreau, J. W., Strode, S. A. & Landing, W. M. (2009). Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models. Global Biogeochem. Cycles 23, GB2010 (2009).

Trasande, L., Landrigan, P. J. & Schechter, C. (2005). Public Health and Economic Consequences of Methyl Mercury Toxicity to the Developing Brain. Environ Health Perspect 113, 590–596 (2005).